Дисертації з теми "Reynolds Ranges"

In this thesis we describe the development of Reynolds- averaged Navier Stokes code for the flow past two- dimensional configuration. Particularly, emphasis has been laid on the study of low Reynolds number airfoil aerodynamics. The thesis consists of five chapters covering the back ground history, problem formulation, method of solution and discussion of the results and conclusion. Chapter I deals with a detailed background history of low Reynolds number aerodynamics, problem associated with it, state of the art, its importance in practical applications in aircraft industries. Chapter II describes the mathematical model of the flow physics and various levels of approximations. Also it gives an account of complexity of the equations at low Reynolds number regarding flow separation, transition and reattachment. Chapter III describes method of solution, numerical algorithm developed, description of various upwind schemes, grid system, finite volume discrieti-zation of the governing equations described in Chapter II. Chapter IV describes the application of the newly developed Navier Stokes code for the test cases from GAMM Workshop proceedings. Also it describes validation of the code for Euler solutions, Blasius solution for the flow past flat plate and compressible Navier Stokes solution for the flow past NACA 0012 Airfoil at low Reynolds number. Chapter V describes the application of the Navier Stokes code for the more test cases of current practical interest . In this chapter laminar separation bubble characteristics are investigated in detail regarding formation, growth and shedding in an unsteady environment. Finally the conclusion is drawn regarding the robustness of the newly developed code in predicting the airfoil aerodynamic characteristics at low Reynolds number both in steady and unsteady environment. Lastly, suggestion for future work has been highlighted.

Nous montrons dans cette thèse la capacité des schémas numériques modernes à simuler des écoulements turbulents sur des maillages totalement non-structurés générés automatiquement à l’aide de méthodes adaptatives. Nous détaillons le développement de différentes versions du modèle de Spalart-Allmaras ainsi que les choix numériques garantissant une robustesse suffisante du solver pour ne pas nécessiter de couche limite structurée. Nous introduisons en suite l’analyse d’erreur nécessaire pour proposer different estimateurs d’erreur à la base de l’optimisation de maillage. Cette méthodologie est testée sur différents cas tests d’aérodynamique externe et de turbomachines et comparée aux méthodes traditionnelles de géneration de maillage. Nous montrons ainsi la capacité des méthodes d’adaptation de maillage à générer automatiquement des maillages adaptés optimaux pour les simulations RANS autour de géométries réalistes et complexes
The fast and reliable simulation of turbulent flow using Reynolds Averaged Navier Stokes (RANS) models is a major financial issue for many industries. With the increasing complexity of geometries and simulated flows, as well as requirements in terms of fidelity, the generation of appropriate meshes has become a key link in the chain of computation. We show in this thesis the ability of modern numerical schemes to simulate turbulent flows on fully unstructured meshes generated automatically using mesh adaptation methods. We present the implementation of different versions of the Spalart-Allmaras model as well as the numerical choices guaranteeing a sufficient robustness of the solver in order to not require a structured boundary layer. We then introduce the error analysis necessary to propose different error estimators for mesh optimization. This methodology is tested on various external aerodynamic and turbomachinery test cases and compared to traditional mesh generation methods. We show the ability of mesh adaptation methods to automatically generate optimal mesh sizes for RANS simulations on realistic and complex geometries

Hydraulically induced scour of the streambed at the base of bridge piers is the leading cause of bridge failures. Despite the significant scientific efforts towards the solution of this challenging engineering problem, there are still no reliable tools for the prediction and mitigation of bridge scour. This shortcoming is attributed to the lack of understanding of the physics behind this phenomenon. The experimental studies that attempted the physical investigation of bridge scour in the past have faced two main limitations: i) The characterization of the dynamic interaction between the flow and the evolving bed that is known to drive scour, was not possible due to the limitations in the available instrumentation and the significant experimental difficulties; ii) Most of the existing literature studies are based on the findings of laboratory experiments whose scale is orders of magnitudes smaller compared to bridges in the field, while the scale effects on the scour depth have never been quantified. The objective of this research was to enhance the existing understanding of the phenomenon by tackling the aforementioned experimental challenges. To accomplish this, the first part of this work involved the development of a new underwater photogrammetric technique for the monitoring of evolving sediment beds. This technique is able to obtain very high resolution measurements of evolving beds, thus allowing the characterization of their dynamic properties (i.e. evolving topography and scour rates) and overcoming existing experimental limitations. Secondly, the underwater photogrammetric technique was applied on a bridge scour experiment, of simple geometry, and the dynamic morphological characteristics of the phenomenon were measured. The detailed measurements along with reasonable comparisons with descriptions of the flow, from past studies, were used to provide insight on the interaction between the flow and the bed and describe quantitatively the mechanisms of scour. Finally, the scale effects on scour were studied via the performance of two experiments under near-prototype conditions. In these experiments the effects of the Reynolds number on the flow and the scour were quantified and implications concerning existing small-scale studies were discussed.
Ph. D.

A three-dimensional unstructured incompressible RANS code has been developed using artificial compressibility and Spalart-Allmaras eddy viscosity model. A node-based finite volume method is used in which all flow variables are defined at the vertices of tetrahedrons in an unstructured grid. The inviscid fluxes are computed by using the Roe's flux difference splitting method, and higher order accuracy is attained by data reconstruction based on Taylor series expansion. Gauss theorem is used to formulate necessary gradients. For time integration, an implicit scheme based on linearized Euler backward method is used. A tetrahedral unstructured grid generation code has been also developed and applied to the tip clearance flow in a highly staggered cascade. Surface grids are first generated in the flow passage and blade tip by using several triangulation methods including Delaunay triangulation, advancing front method and advancing layer method. Then the whole computational domain including tip gap region is filled with prisms using the surface grids. Each prism is divided into three tetrahedrons. To accomplish this division in a consistent manner, connectivity pattern is assigned to each triangle in the surface grids. A new algorithm is devised to assign the connectivity pattern without reference to the particular method of triangulation. This technique offers great flexibility in surface grid generation. The code has been validated by comparisons with available computational and experimental results for several test cases: invisicd flow around NACA section, laminar and turbulent flow over a flat plate, turbulent flow through double-circular arc cascade and laminar flow through a square duct with 90° bend. For the laminar flat plate case, the velocity profile and skin friction coefficient are in excellent agreement with Blasius solution. For the turbulent flat plate case, velocity profiles are in full agreement with the law of the wall up to Reynolds number of 1.0E8, however, the skin friction coefficient is under-predicted by about 10% in comparison with empirical formula. Blade loading for the two-dimensional circular arc cascade is also compared with experiments. The results obtained with the experimental inflow angle (51.5° ) show some discrepancies at the trailing edge and severely under-predict the suction peak at the leading edge. These discrepancies are completely remedied if the inflow angle is increased to 53.5° . The code is also capable of predicting the secondary flow in the square duct with 90° bend, and the velocity profiles are in good agreement with measurements and published Navier-Stokes computations. Finally the code is applied to a linear cascade that has GE rotor B section with tip clearance and a high stagger angle of 56.9° . The overall structure of the tip clearance flow is well predicted. Loss of loading due to tip leakage flow and reloading due to tip leakage vortex are presented. On the end wall, separation line of the tip leakage vortex and reattachment line of passage vortex are identified. The location of the tip leakage vortex in the passage agrees very well with oil flow visualization. Separation bubble on the blade tip is also predicted. Mean streamwise velocity contours and cross sectional velocity vectors are compared with experimental results in the near wake, and good agreements are observed. It is concluded that Spalart-Allmaras turbulence model is adequate for this type of flow field except at locations where the tip leakage vortex of one blade interacts with the wake of a following blade. This situation may prevail for blades with longer span and/or in the far wake. Prediction of such an interaction presents a challenge to RANS computations. The effects of blade span on the flow structure have been also investigated. Two cascades with blades of aspect ratios of 0.5 and 1.0 are considered. By comparing pressure distributions on the blade, it is shown that the aspect ratio has strong effects on loading distribution on the blade although the tip gap height is very small (0.016 chord). Grid convergence study has been carried out with three different grids for pressure distributions and limiting streamlines on the end wall.
Ph. D.

Mieux comprendre les spécificités de la dynamique des couches limites à grand nombre de Reynolds malgré les contraintes métrologiques et son coût de simulation numérique est crucial. A titre d'exemple, cette dynamique peut déterminer plus de la moitié de la traînée d'un avion en croisière. Décrire la turbulence pariétale peut guider la résolution numérique d'une partie des fluctuations à un coût maîtrisé par des stratégies WMLES (simulation des grandes échelles avec modèle de paroi). Les présentes analyses physiques de couches limites turbulentes incompressibles à gradient de pression nul et à grand nombre de Reynolds s'appuient sur des simulations numériques avancées. Après validation d'une base de données, le frottement moyen pariétal est décomposé selon l'identité FIK (Fukagata et al. (2002)), dont l'application malgré le développement spatial est discutée. Une analyse spectrale montre que les grandes échelles (\lambda_x > \delta) contribuent à environ la moitié du frottement vers Re_\theta = 10^4. Les limitations de l'identité FIK motivent la dérivation d'une décomposition physique de la génération du frottement dont le comportement asymptotique est alors relié à la production d'énergie cinétique turbulente dans la zone logarithmique. Pour mieux reconstruire les spectres spatiaux, une nouvelle méthode d'estimation de la vitesse de convection turbulente en fonction de la longueur d'onde des fluctuations, adaptée au développement spatial et à des signaux temporels de durée finie, est dérivée, interprétée et évaluée à Re_\theta = 13000. Certaines des conclusions éclairent des modifications d'une stratégie WMLES, le mode III de la méthode ZDES
Better understanding the specificities of the dynamics of high-Reynolds number boundary layers despite metrological constraints and its numerical simulation cost is crucial. For instance, this dynamics can determine more than half of the drag of a cruising aircraft. Describing wall turbulence can guide the numerical resolution of some of the fluctuations at a limited cost by WMLES strategies (wall-modelled large eddy simulation). The present physical analyses of zero-pressure gradient incompressible turbulent boundary layers at high Reynolds number rely on advanced numerical simulations. After validating a database, mean skin friction is decomposed by means of the FIK identity (Fukagata et al. (2002)), whose application despite the spatial growth is discussed. A spectral analysis shows that the large scales (\lambda_x > \delta) contribute approximately half of the friction near Re_\theta = 10^4. The limitations of the FIK identity motivate the derivation of a physical decomposition of the generation of friction whose asymptotic behaviour is then related to turbulent kinetic energy production in the logarithmic layer. In order to better reconstruct spatial spectra, a new method to estimate the turbulent convection velocity as a function of the wavelength of the fluctuations, adapted to spatial growth and to temporal signals of finite duration, is derived, interpreted, and assessed at Re_\theta = 13000. Some of the conclusions enlighten modifications to a WMLES strategy, mode III of the ZDES method

The data-driven adaptive algorithms are explored as a means of increasing the accuracy of Reynolds-averaged turbulence models. This dissertation presents two new data-driven adaptive computational models for simulating turbulent flow, where partial-but-incomplete measurement data is available. These models automatically adjust (i.e., adapts) the closure coefficients of the Reynolds-averaged Navier-Stokes (RANS) k-ω turbulence equations to improve agreement between the simulated flow and a set of prescribed measurement data. The first approach is the data-driven adaptive RANS k-ω (D-DARK) model. It is validated with three canonical flow geometries: pipe flow, the backward-facing step, and flow around an airfoil. For all 3 test cases, the D-DARK model improves agreement with experimental data in comparison to the results from a non-adaptive RANS k-ω model that uses standard values of the closure coefficients. The second approach is the Retrospective Cost Adaptation (RCA) k-ω model. The key enabling technology is that of retrospective cost adaptation, which was developed for real-time adaptive control technology, but is used in this work for data-driven model adaptation. The algorithm conducts an optimization, which seeks to minimize the surrogate performance, and by extension the real flow-field error. The advantage of the RCA approach over the D-DARK approach is that it is capable of adapting to unsteady measurements. The RCA-RANS k-ω model is verified with a statistically steady test case (pipe flow) as well as two unsteady test cases: vortex shedding from a surface-mounted cube and flow around a square cylinder. The RCA-RANS k-ω model effectively adapts to both averaged steady and unsteady measurement data.

La difficulté principale de réaliser la simulation numérique d'un écoulement turbulent à haut nombre de Reynolds est de préserver la capture des effets instationnaires sans induire un coût de calcul prohibitif. Nous avons, tout d'abord, exhibé les principaux défauts des simulations des grandes échelles avec un modèle de paroi standard dans une configuration de canal plan bi-périodique dans un contexte de maillage grossier. Dans ce cadre, nous avons proposé deux approches basées sur une stratégie de couplage RANS/LES pour corriger pour corriger ces défauts. La première repose sur l'application du modèle de paroi TBLE à une simulations des grandes échelles, qui consiste à résoudre des équations de couche limite simplifiées et instationnaires avec une modélisation de type RANS dans la zone proche paroi. La seconde consiste à réaliser simultanément un calcul RANS et une simulation des grandes échelles dont le champ filtré moyen sera corrigé grâce au calcul RANS par l'intermédiaire d'un terme de forçage. Ces différentes méthodes de modélisations ont été implémentéesdanns le code de calcul TRIO_U du CEA Grenoble. Les configurations étudiées sont le canal plan bi-périodique et un écoulement pariétal dans une matrice d'obstacles cubiques. Les deux approches fournissent des résultats encourageants et permettent d'effectuer des simulations instationnaires à un coût numérique réduit.

This work evaluates whether the branch of Reynolds Averaging in Computational Fluid Dynamics can be used to, based on real field measurements, find the source of the measured gas in question. The method to do this is via the adjoint to the Reynolds Averaged Scalar Transport equation, explained and derived herein. Since the Inverse is only as good as the main equation, forward runs are made to evaluate the turbulence model. Reynolds Averaged Navier Stokes is solved in a domain containing 4 cubes in a 2x2 grid, generating a velocity field for said domain. The turbulence model in question is a union of two modifications to the standard two equation k-ε model in order to capture blunt body turbulence but also to model the atmospheric boundary layer. This field is then inserted into the Reynolds Averaged Scalar Transport equation and the simulation is compared to data from the Environmental Flow wind tunnel in Surrey. Finally the adjoint scalar transport is solved, both for synthetic data that was generated in the forward run, but also for the data from EnFlo. It was discovered that the turbulent Schmidt number plays a major role in capturing the dispersed gas, three different Schmidt numbers were tested, the standard 0.7, the unconventional 0.3 and a height dependent Schmidt number. The widely accepted value of 0.7 did not capture the dispersion at all and gave a huge model error. As such the adjoint scalar transport was solved for 0.3 and a height dependent Schmidt number. The interaction between measurements, the real source strength (which is not used in the adjoint equation, but needed to find the source) and the location of the source is intricate indeed. Over estimation and under estimation of the forward model may cancel out in order to find the correct source, with the correct strength. It is found that Reynolds Averaged Computational fluid dynamics may prove useful in source term estimation.
Detta arbete utvärderar hurvida Reynolds medelvärdesmodellering inom flödessimuleringar kan användas till att finna källan till en viss gas baserat på verkliga mätningar ute i fält. Metoden går ut på att använda den adjungerade ekvationen till Reynolds tidsmedlade skalära transportekvationen, beskriven och härledd häri. Då bakåtmodellen bygger på framåtmodellen, måste såleds framåtmodellen utvärderas först. Navier-Stokes ekvationer med en turbulensmodell löses i en domän, innehållandes 4 kuber i en 2x2 orientering, för vilken en hastighetsprofil erhålles. Turbulensmodellen som användes är en union av två olika k-ε modeller, där den ena fångar turbulens runt tröga objekt och den andra som modellerar atmosfäriska gränsskiktet. Detta fält används sedan i framåtmodellen av skalära transportekvationen, som sedan jämförs med körningar från EnFlo windtunneln i Surrey. Slutligen testkörs även den adjungerade ekvationen, både för syntetiskt data genererat i framåtkörningen men även för data från EnFlo tunneln. Då det visade sig att det turbulenta Schmidttalet spelar stor roll inom spridning i det atmosfäriska gränsskiktet, gjordes testkörningar med tre olika Schmidttal, det normala 0.7, det väldigt låga talet 0.3 samt ett höjdberoende Schmidttal. Det visade sig att det vanligtvis använda talet 0.7 inte alls lyckas fånga spridningen tillfredställande och gav ett stort modellfel. Därför löstes den adjungerade ekvationen för 0.3 samt för ett höjdberoende Schmidttal. Interaktionen mellan mätningar, den riktiga källstyrkan (som är okänd i den adjungerade ekvationen) samt källpositionen är onekligen intrikat. Över- samt underestimationer av framåtmodellen kan ta ut varandra i bakåtmodellen för att finna rätt källa, med rätt källstyrka. Det ter sig som Reynolds turbulensmodellering mycket möjligt kan användas inom källtermsuppskattning.

Le phénomène de décrochage est souvent décrit comme une chute soudaine de portance lorsque l'angle d'incidence augmente. Ce phénomène est préjudiciable aux avions et aux hélicoptères et limite leur enveloppe de vol. Plusieurs études numériques et expérimentales, particulièrement centrées sur le décrochage statique (i.e. pour des ailes fixes), ont révélé des phénomènes apparaissant proche de l'angle de décrochage : des oscillations basses fréquences et une hystérésis des coefficients aérodynamiques. Le premier phénomène se traduit par une oscillation de la portance entre une valeur maximale et une valeur minimale obtenues quand l'écoulement est respectivement attaché ou détaché. Le nombre de Strouhal associé (St ~ 0.02) est habituellement un ordre de grandeur plus faible que le nombre de Strouhal (St ~ 0.2) du lâcher tourbillonnaire qui apparaît pour de plus grandes incidences. Le second phénomène est caractérisé par l'existence de solutions moyennées en temps autour de l'angle de décrochage qui diffèrent selon que l'angle d'attaque est augmenté ou diminué.L'objectif de cette thèse est d'avoir une meilleure compréhension de l'origine du décrochage et de ces deux phénomènes grâce à des simulations numériques d'écoulements turbulents modélisés par une approche RANS (Reynolds-Averaged Navier-Stokes). Une combinaison de diverses approches numériques et théoriques (simulations instationnaires, continuation de solutions stationnaires, stabilité linéaire et analyse de bifurcation) est développée et appliquée dans le cas du décrochage d'un profil 2D de pale d'hélicoptère, le OA209, à bas nombre de Mach (M~0.2) et haut nombre de Reynolds (Re~1.8x10^6).Des solutions stationnaires sont calculées pour différents angles d'attaques en considérant le modèle de turbulence de Spalart-Allmaras et en utilisant des méthodes de continuation (continuation naïve et méthode du pseudo-arclength). Les résultats mettent en évidence une branche supérieure (à haute portance), une branche inférieure (à basse portance) et, entre les deux, une branche du milieu. Pour un même angle d'attaque, des solutions coexistent proche de l’angle de décrochage sur chacune des branches, ce qui est caractéristique d’un phénomène d'hystérésis. Des analyses de stabilité linéaire réalisées autour de ces états d'équilibres révèlent l'existence d'un mode instable basse fréquence associé au décrochage. L'évolution des valeurs propres associées à ce mode le long des branches stationnaires nous permet d'établir une première version du diagramme de bifurcation. Afin de le compléter, des calculs RANS instationnaires sont réalisés et des cycles limites basse fréquence sont identifiés sur une plage réduite d'angles d'attaque proches du décrochage. Ces solutions périodiques sont caractérisées par des valeurs de portance maximales et minimales plus grandes et plus petites que celles des solutions stationnaires à haute et basse portance associées, respectivement. Pour clarifier la formation et la disparition de ce cycle limite basse fréquence et permettre une meilleure compréhension du scénario de bifurcation, un modèle à une équation reproduisant les caractéristiques linéaires du phénomène est proposé. Ce modèle non-linéaire du décrochage statique est calibré sur les états stationnaires et leur comportement linéaire obtenus par calculs RANS. Une étude du comportement non-linéaire de ce modèle révèle un scenario possible qui pourrait conduire à l'apparition et à la disparition du cycle limite basse fréquence. Finalement, le cas d'un NACA0012 à nombre de Reynolds Re~1.0x10^6 est considéré pour valider la robustesse du scenario identifié
Airfoil stall is commonly described as a sudden drop of lift when increasing the angle of attack. This phenomenon is detrimental to aircrafts and helicopters, since it strongly limits their flight envelope. Past experimental and numerical investigations, specifically dedicated to static stall (i.e. for rigid wings), have clearly identified two phenomena which appear close to the stall angle: low-frequency oscillations and hysteresis of the lift coefficient. The first one is an oscillation of the lift between maximal and minimal values obtained when the instantaneous flow is attached and fully separated, respectively. The corresponding Strouhal number (St ~ 0.02) is usually an order of magnitude lower than the Strouhal number (St ~ 0.2) of the vortex-shedding that may appear for larger angles of attack. The second phenomenon is characterized by the existence of different time-averaged solutions around the stall angle depending on whether the angle of attack is increased or decreased.The objective of this thesis is to better understand the origin of stall and of these two phenomena using numerical simulations of turbulent flows modelled in the RANS (Reynolds-Averaged Navier-Stokes) framework. A combination of various numerical and theoretical approaches (unsteady simulations, continuation of steady solutions, linear stability and bifurcation analyses) have been developed and applied to the stall of a 2D helicopter blade airfoil OA209 at low Mach number (M~0.2) and high Reynolds number (Re~1.8x10^6).Steady RANS computations are performed using Spalart-Allmaras model to obtain steady states for several angles of attack taking advantage of continuation methods (naive continuation and pseudo-arclength method). The results highlight one upper branch (of high lift), one lower branch (of low lift) and, in between, a middle branch. Close to stall, for a same angle of attack, solutions coexist on each branch, characterizing a hysteresis phenomenon. Linear stability analyses performed around these equilibrium states reveal the existence of a low-frequency unstable mode associated to stall. The evolution of the corresponding eigenvalues along the branches of steady solutions allows us to establish a first sketch of the bifurcation scenario. Unsteady RANS computations are carried out to complete it. Low-frequency limit-cycle solutions have been identified in a narrow range of angles of attack close stall. These periodic solutions are characterized by maximal and minimal instantaneous values of the lift that are larger and lower than the associated high-lift and low-lift steady solutions, respectively. To clarify the formation and disappearance of this low-frequency limit cycle, and thus improve our knowledge about the bifurcation scenario, a one-equation model reproducing the linear characteristics of the phenomenon is proposed. This nonlinear static-stall model is calibrated on the steady states and their linear behavior obtained with RANS computations. A study of the nonlinear behavior of this model then reveals a possible scenario leading to the appearance and collapsing of the low frequency limit cycle. Finally, the case of a NACA0012 at Re~1.0x10^6 is considered to check the robustness of the scenario identified

La capacité de simuler la dynamique de la couche limite turbulente représente aujourd'hui un enjeu important pour la prévision de l'aérodynamique instationnaire et de l'aéroacoustique des aéronefs et des véhicules terrestres. Aussi, les travaux présentés dans ce manuscrit proposent une méthode originale de simulation de la dynamique des écoulements turbulents pariétaux, à haut nombre de Reynolds, sur des géométries curvilignes. L'approche ZDES, dans son mode " Wall Modelled Large Eddy Simulation ", est ici retenue. Dans un premier temps, une méthode de réactivation turbulente, par combinaison de l'injection de turbulence synthétique et de l'application de termes de forçage, est développée afin de permettre une résolution WMLES locale des régions d'intérêt au sein d'une simulation majoritairement RANS. Puis l'étude des interactions entre la physique et la méthode de résolution numérique (ZDES), sur le cas d'une couche limite turbulente sans gradient de pression en développement spatial jusqu'à Re13 000, a conduit à une généralisation des conditions d'emploi du mode WMLES de la ZDES. Des post-traitements instationnaires avancés ont permis de démontrer la capacité de la méthode à simuler la dynamique particulière de la zone externe à ces grands nombres de Reynolds. Enfin, les différentes méthodes développées au cours de cette étude ont été appliquées à la simulation d'une manche à air coudée de section rectangulaire. Cette nouvelle méthode a permis de simuler avec succès la dynamique multi-échelles de cet écoulement et des distorsions dynamiques associées, pour un coût environ 50 fois inférieur à celui d'une approche LES classique

The primary purpose of this thesis is to quantify the effects of RANS turbulence modeling on the resolution of free shear vortical flows. The simulation of aerodynamic wing-tip vortices is used as a test bed. The primary configuration is flow over an isolated finite wing with aspect ratio, , and Reynolds number, . Tip-vortex velocity profiles, vortex core and wake turbulence levels, and Reynolds stresses are compared with wind tunnel measurements. Three turbulence models for RANS closure are tested: the Lumley, Reece, and Rodi full Reynolds stress transport model and the Sparlart-Allmaras model with and without a proposed modification. The main finding is that simulations with the full Reynolds stress transport model show remarkable mean flow agreement in the vortex and wake due to the proper prediction of a laminar vortex core. Simulations with the Spalart-Allmaras model did not indicate a laminar core and predicted over-diffusion of the tip-vortex. Secondary investigations in this work include the study of wall boundary layer treatment and simulating the wake-age of an isolated rotorcraft in hover using a steady-state RANS solver. By comparing skin friction plots over the NACA 0012 airfoil, it is shown that wall functions are most effective in the trailing edge half of the airfoil, while high velocity gradient and curvature of the leading edge make them more vulnerable to discrepancies. The rotorcraft simulation uses the modified Spalart-Allmaras turbulence model and shows proper, qualitative, resolution of the interaction between the vortex sheet and the tip vortex.
Master of Science

Turbulent flows affected by buoyancy lie at the basis of many applications, both within engineering and the atmospheric sciences. A prominent example of such an application is the atmospheric boundary layer, the lowest layer of the atmosphere, in which many physical processes are heavily influenced by both stably stratified and convective turbulent transport. Modelling these turbulent flows correctly, especially in the presence of stable stratification, has proven to be a great challenge and forms an important problem in the context of climate models. In this thesis, we address this issue considering an advanced class of turbulence models, the so-called explicit algebraic models.In the presence of buoyancy forces, a mutual coupling between the Reynolds stresses and the turbulent heat flux exists, which makes it difficult to derive a fully explicit turbulence model. A method to overcome this problem is presented based on earlier studies for cases without buoyancy. Fully explicit and robust models are derived for turbulence in two-dimensional mean flows with buoyancy and shown to give good predictions compared with various data from direct numerical simulations (DNS), most notably in the case of stably stratified turbulent channel flow. Special attention is given to the problem of determining the production-to-dissipation ratio of turbulent kinetic energy, for which the exact equation cannot be solved analytically. A robust approximative method is presented to calculate this quantity, which is important for obtaining a consistent formulation of the model.The turbulence model derived in this way is applied to the atmospheric boundary layer in the form of two idealized test cases. First, we consider a purely stably stratified boundary layer in the context of the well-known GABLS1 study. The model is shown to give good predictions in this case compared to data from large-eddy simulation (LES). The second test case represents a full diurnal cycle containing both stable stratification and convective motions. In this case, the current model yields interesting dynamical features that cannot be captured by simpler models. These results are meant as a first step towards a more thorough investigation of the pros and cons of explicit algebraic models in the context of the atmospheric boundary layer, for which additional LES data are required.

QC 20150522

This thesis aims at analysing the predictive capabilities of statistical URANS and hybrid RANS-LES methods to model complex flows at high Reynolds numbers and carrying out a physical analysis of the near-region turbulence and coherent structures. This study handles configurations included in the European research programmes ATAAC (Advanced Turbulent Simulation for Aerodynamics Application Challenges) and TFAST (Transition Location Effect on Shock Wave Boundary Layer Interaction). First, the detached flow in a configuration of a tandem of cylinders, positionned behind one another, is investigated at Reynolds number 166000. A static case, corresponding to the layout of the support of a landing gear, is initially considered. The fluid-structure interaction is then studied in a dynamic case where the downstream cylinder, situated in the wake of the upstream one, is given one degree of freedom in translation in the crosswise direction. A parametric study of the structural parameters is carried out to identify the various regimes of interaction. Secondly, the physics of the transonic buffet is studied by means of time-frequency analysis and proper orthogonal decomposition (POD), in the Mach number range 0.70–0.75. The interactions between the main shock wave, the alternately detached boundary layer and the vortices developing in the wake are analysed. A stochastic forcing, based on reinjection of synthetic turbulence in the transport equations of kinetic energy and dissipation rate by using POD reconstruction, has been introduced in the so-called organised-eddy simulation (OES) approach. This method introduces an upscale turbulence modelling, acting as an eddy-blocking mechanism able to capture thin shear-layer and turbulent/non-turbulent interfaces around the body. This method highly improves the aerodynamic forces prediction and opens new ensemble-averaged approaches able to model the coherent and random processes at high Reynolds number. Finally, the shock-wave/boundary-layer interaction (SWBLI) is investigated in the case of an oblique shock wave at Mach number 1.7 in order to contribute to the so-called "laminar wing design" studies at European level. The performance of statistical URANS and hybrid RANS-LES models is analysed with comparison, with experimental results, of integral boundary-layer values (displacement and momentum thicknesses) and wall quantities (friction coefficient). The influence of a transitional boundary layer on the SWBLI is featured.

Thesis (MScEng)--Stellenbosch University, 2011.
ENGLISH ABSTRACT: The hydrodynamic characteristics of three typical ship-like hull sections, in different motions, are numerically investigated using FLUENT, 2009. These simple shapes, namely a v-bottom (triangle) hull, a at-bottom (square) hull and a round-bottom (semi-circle) hull, are investigated in uncoupled heave, sway and roll. The problem is described in two dimensions. A combination of numerical methods and models, found in literature, are used to conduct this investigation. Hull characterisation is achieved through the use of hull mass and damping coe cients. These numerically determined coe cients are compared to experimental work conducted by Vugts (1968). A good correlation between the numerical and experimental results exists for the heave and sway cases. By normalising the coe cients, different hulls are comparable to one another. The numerical models used are validated and veri ed. Roll motion remains largely unsolved for very large angles of roll (in excess of 11°). Different uid ow phenomena occurring around the hull sections have varying degrees of in uence on the motions of a hull. It is found that not one of the turbulence models investigated can be employed to globally solve each type of hull-motion case. Also, forced oscillations in computational simulations require considerably more computational time than free-decay oscillating hull simulations.
AFRIKAANSE OPSOMMING: Die hidrodinamiese karakteristieke van verskillende skeepsrompvorms, in verskeie bewegingswieë, is numeries ondersoek met behulp van FLUENT, 2009. Drie eenvoudige vorms ('n v-bodem (driehoek), plat-bodem (reghoek) en rondebodem (semi-sirkel) romp) is onderskeidelik ondersoek in opwieg, dwarswieg en rol. Die probleem is twee-dimensioneel. Daar is gebruik gemaak van 'n kombinasie van numeriese metodes en modelle, uit die literatuur, om die ondersoek uit te voer. Die rompe is gekarakteriseer met behulp van massa- en dempingskoëffi siënte. Hierdie numeries bepaalde koë ffisiënte is vergelyk met die eksperimentele werk van Vugts (1968). Daar bestaan 'n goeie korrelasie tussen die numeriese en eksperimentele resultate vir die opwieg en dwarswieg gevalle. Die koë ffisiënte is genormaliseer om die verskeie rompvorms te vergelyk. Die numeriese modelle is geverifi eer en valideer. Rolbewegings is onopgelos vir groot rolhoeke (groter as 11°). Die mate waartoe die romp se beweging beïnvloed word deur die verskillende vloei verskynsels wat om die rompe ontstaan, verskil. Daar is bevind dat geen van die turbulensie modelle gebruik kan word om alle skeepsbeweging-gevalle op te los nie. Gedwonge-ossilasie numeriese simulasies benodig meer berekeningstyd as vrye-verval ossilasie gevalle.

L'objectif de la présente étude est d'analyser la modélisation de la turbulence de simulations en moyenne de Reynolds (RANS) dans le cadre d'écoulements de type turbomachines, en utilisant des simulations aux grandes échelles (SGE) comme référence. L'étude porte sur deux cas test: un décollement de coin dans une grille d'aubes rectiligne, et un écoulement de jeu pour un aubage isolé dans un jet. Deux lois de comportement, la loi de comportement de Boussinesq et la loi de comportement quadratique (quadratic constitutive relation ou QCR), sont analysées, avec deux versions du modèle de turbulence k-omega de Wilcox. Les lois de comportement étudiées reposent sur deux hypothèses: une hypothèse d'alignement entre le tenseur de Reynolds et un tenseur construit à partir de l'écoulement moyen, et une hypothèse sur la viscosité turbulente. L'hypothèse d'alignement est étudiée à partir de la SGE, pour laquelle les deux tenseurs sont indépendamment connus, en utilisant un indicateur construit sur le produit scalaire des tenseurs. Les résultats sont présentés sous forme d'une fonction de répartition de la valeur de l'indicateur pour le domaine complet, puis pour trois sous-domaines d'intérêt: l'entrée, une région où l'écoulement interagit fortement avec les parois, et une région où l'écoulement est fortement tourbillonnaire. L'hypothèse d'alignement n'est que rarement valide pour la loi de comportement de Boussinesq. Pour la QCR, les résultats sont meilleurs en entrée, comparé à la loi de Boussinesq. Il ne sont cependant pas meilleurs pour les régions où l'écoulement est plus tourbillonnaire. Une amélioration de la loi de comportement est nécessaire pour pouvoir faire progresser la modélisation turbulente en RANS. En revanche, l'utilisation de l'énergie cinétique turbulente et du taux de dissipation spécifique semble correcte pour estimer la valeur de la viscosité turbulente. L'analyse de la modélisation de l'équation d'énergie cinétique turbulente (ECT) est réalisée au travers d'une comparaison terme à terme avec l'équation d'ECT résolue par la SGE. Les résultats SGE présentent une turbulence qui n'est pas à l'équilibre : la production et la dissipation ne sont pas superposées, et le terme de transport est important. Pour le RANS, la turbulence est à l'équilibre : la production et la dissipation sont superposées, et le terme de transport est de faible intensité. Un modèle de turbulence qui prend en compte le déséquilibre est nécessaire pour améliorer ce point. En dernier lieu, une nouvelle formulation hybride RANS/SGE est proposée, fondée sur la distance à la paroi en unités de paroi. La formulation est validée dans un canal bi-périodique et un premier essai est réalisé sur le cas de décollement de coin, mais d'autres analyses sont nécessaires avant que cette formulation ne soit fonctionnelle
The present study aims at analysing turbulence modelling in Reynolds-averaged Navier-Stokes (RANS) simulations, in the context of turbomachinery flows, using large-eddy simulations (LES) as references. Two test cases are considered: a corner separation (CS) flow in a linear compressor cascade, and a tip-leakage (TL) flow of a single blade in a jet. Two constitutive relations, the Boussinesq constitutive relation and the quadratic constitutive relation (QCR), are investigated, with two versions of Wilcox's $k-\omega$ turbulence model. The studied constitutive relations rely on two hypotheses: an alignment hypothesis between the Reynolds stress tensor and a mean flow tensor, and an hypothesis on the turbulent viscosity. The alignment hypothesis is investigated using LES, where both the tensors are known independently, with an indicator built on the inner product of the tensors. The results are presented as probability density functions of the indicator value for the entire domain first, and then for three specific areas of interest: the inlet area, similar to a boundary-layer flow, an area of strong interaction between the flow and the walls (CS: passage area, TL: tip clearance) and an area of highly vortical flow (CS: separation wake, TL: tip-leakage vortex). The alignment hypothesis is rarely verified in any area for the Boussinesq constitutive relation. For the QCR, the results are improved for the inlet areas compared to the Boussinesq constitutive relation, but no significant improvement is found in the highly vortical regions. An improvement of the constitutive relation is needed in order to improve the RANS turbulence modelling. In contrast, the use of the turbulent kinetic energy and the specific dissipation rate appears quite correct to estimate the turbulent viscosity. The modelling of the RANS turbulent kinetic energy (TKE) budget equation is investigated through a term to term comparison with the resolved LES TKE budget equation. The LES presents a turbulence that is not at equilibrium, with the production and the dissipation not superimposed, and an important amount of transport. This differs from the RANS models, at equilibrium: the production and the dissipation are superimposed, with a small amount of transport. The development of a non-equilibrium turbulence model for RANS simulations could improve this aspect of turbulence modelling. Finally, a new hybrid RANS-LES formulation, based on the wall distance in wall units, is also proposed. It is validated on a bi-periodical channel flow, and a first attempt is made on the corner separation case, but further investigations are still needed for the model to be fully operational

This thesis deals with the near-field of confluent jets, which can be of interest in many engineering applications such as design of a ventilation supply device. The physical effect of interaction between multiple closely spaced jets is studied using experimental and numerical methods. The primary aim of this study is to explore a better understanding of flow and turbulence behavior of multiple interacting jets. The main goal is to gain an insight into the confluence of jets occurring in the near-field of multiple interacting jets. The array of multiple interacting jets is studied when they are placed on a flat and a curved surface. To obtain the boundary conditions at the nozzle exits of the confluent jets on a curved surface, the results of numerical prediction of a cylindrical air supply device using two turbulence models (realizable 𝑘 − 𝜖 and Reynolds stress model) are validated with hot-wire anemometry (HWA) near different nozzles discharge in the array. A single round jet is then studied to find the appropriate turbulence models for the prediction of the three-dimensional flow field and to gain an understanding of the effect of the boundary conditions predicted at the nozzle inlet. In comparison with HWA measurements, the turbulence models with low Reynolds correction (𝑘 − 𝜖 and shear stress transport [SST] 𝑘 − 𝜔) give reasonable flow predictions for the single round jet with the prescribed inlet boundary conditions, while the transition models (𝑘 − 𝑘l − 𝜔𝜔 and transition SST 𝑘 − 𝜔) are unable to predict the flow in the turbulent region. The results of numerical prediction (low Reynolds SST 𝑘 − 𝜔 model) using the prescribed inlet boundary conditions agree well with the HWA measurement in the nearfield of confluent jets on a curved surface, except in the merging region. Instantaneous velocity measurements are performed by laser Doppler anemometry (LDA) and particle image velocimetry (PIV) in two different configurations, a single row of parallel coplanar jets and an inline array of jets on a flat surface. The results of LDA and PIV are compared, which exhibit good agreement except near the nozzle exits. The streamwise velocity profile of the jets in the initial region shows a saddle back shape with attenuated turbulence in the core region and two off-centered narrow peaks. When confluent jets issue from an array of closely spaced nozzles, they may converge, merge, and combine after a certain distance downstream of the nozzle edge. The deflection plays a salient role for the multiple interacting jets (except in the single row configuration), where all the jets are converged towards the center of the array. The jet position, such as central, side and corner jets, significantly influences the development features of the jets, such as velocity decay and lateral displacement. The flow field of confluent jets exhibits asymmetrical distributions of Reynolds stresses around the axis of the jets and highly anisotropic turbulence. The velocity decays slower in the combined regio  of confluent jets than a single jet. Using the response surface methodology, the correlations between characteristic points (merging and combined points) and the statistically significant terms of the three design factors (inlet velocity, spacing between the nozzles and diameter of the nozzles) are determined for the single row of coplanar parallel jets. The computational parametric study of the single row configuration shows that spacing has the greatest impact on the near-field characteristics.

La compréhension de l'aérothermique d'un véhicule durant sa phase de développement est une question essentielle afin d'assurer, d'une part, un bon refroidissement et une bonne efficacité de ses composants et d'autre part de réduire la force de traînée et évidement le rejet des gaz à effet de serre ou la consommation d'essence. Cette thèse porte sur la simulation numérique et la validation expérimentale de l'aérothermique d'un véhicule à trois roues dont deux, en avant et une roue motrice en arrière. La simulation numérique est basée sur la résolution des équations de conservation de la masse, de la quantité de mouvement et de l'énergie en utilisant l'approche RANS (Reynolds-Averaged Navier-Stokes). Le rayonnement thermique est modélisé grâce à la méthode S2S (Surface to Surface) qui suppose que le milieu séparant les deux surfaces rayonnantes, ici de l'air, ne participe pas au processus du rayonnement. Les radiateurs sont considérés comme des milieux poreux orthotropes où la perte de pression est calculée en fonction de leurs propriétés inertielle et visqueuse; leur dissipation thermique est modélisée par la méthode Dual flow. Une première validation de l'aérodynamique est faite grâce à des essais en soufflerie. Ensuite, une deuxième validation de la thermique est faite grâce à des essais routiers. Un deuxième objectif de la thèse est consacré à la simulation numérique de l'aérodynamique en régime transitoire du véhicule. La simulation est faite à l'aide de l'approche Detached eddy simulation (DES). Une validation expérimentale est faite à partir d'étude en soufflerie grâce à des mesures locales de vitesse à l'aide de sondes cobra.

In this dissertation, various VAT parameters are investigated to determine the effect of the overall efficiency of the turbine at a high Reynolds number. To increase the efficiency of the vertical axis turbines, 2D CFD simulations are completed in an effort to better understand the physics behind the operation of these turbines. Specifically, the effect of advance ratio, solidity, and wake interactions were investigated. Simulations were completed in OpenFOAM using the k-ω SST turbulence model at a nominal Reynolds number of 500,000 using a NACA 0015 airfoil. To simulate the motion of the turbine, Arbitrary Mesh Interfacing (AMI) was used. For all of the parameters tested, it was found that the geometric effective angle of attack seen by the turbine blades had a significant impact on the power extracted from the flow. The range of effective angles of attack was found to decrease as the advance ratio increased. In spite of this, a severe loss in the power coefficient occurred at an advance ratio of 2.5 during which the blade experienced dynamic stall. This effect was also seen when the number of turbine blades was changed to four, at a solidity of 1.08. This negative impact on performance was found to be due to the increase in the drag component of the tangential force when dynamic stall occurs. Results indicate that wake interactions between subsequent blades have a large impact on performance especially when the wake interaction alters the flow direction sufficiently to create conditions for dynamic stall. To improve the performance of the VAT in the presence of dynamic stall, calculations were completed of a static twisted blade profile using GenIDLEST and OpenFOAM. There was found to be no improvement in the lift coefficient when comparing the twisted blade profile with a 2D blade at the same median angle of attack as the twisted blade. To further see the effects of the twisted blade, an effective VAT pitching motion was given to the blade and again compared to a 2D blade with the same motion. In this case there was significant improvement seen in the performance of the twisted blade.
Master of Science

The present study considers the numerical simulation of the different flow characteristics involved in the conjugate heat transfer analysis of an internally cooled gas turbine blade. Conjugate simulations require full coupling of convective heat transfer in fluid regions to the heat diffusion in solid regions. Therefore, accurate prediction of heat transfer quantities on both external and internal surfaces has the uppermost importance and highly connected with the performance of the employed turbulence models. The complex flow on both surfaces of the internally cooled turbine blades is caused from the boundary layer laminar-to-turbulence transition, shock wave interaction with boundary layer, high streamline curvature and sequential flow separation. In order to discover the performances of different turbulence models on these flow types, analyses have been conducted on five different experimental studies each concerned with different flow and heat transfer characteristics. Each experimental study has been examined with four different turbulence models available in the commercial software (ANSYS FLUENT13.0) to decide most suitable RANS-based turbulence model. The Realizable k-&epsilon
model, Shear Stress Transport k-&omega
model, Reynolds Stress Model and V2-f model, which became increasingly popular during the last few years, have been used at the numerical simulations. According to conducted analyses, despite a few unreasonable predictions, in the majority of the numerical simulations, V2-f model outperforms other first-order turbulence models (Realizable k-&epsilon
and Shear Stress Transport k-&omega
) in terms of accuracy and Reynolds Stress Model in terms of convergence.

Une méthode haut ordre de résolution des équations de Navier-Stokes pour un écoulement incompressible basée sur une discrétisation éléments finis Galerkin discontinu hybride est présentée pour laquelle la stabilité énergétique est assurée et la masse et la quantité de mouvement sont conservées. La formulation calcule exactement des champs de vitesse solénoïdaux pour des types d'élément standard sans avoir recours à des opérateurs de post-traitement ou à des espaces fonctionnels \textit{H}(div) conformes. Ceci est réalisé en proposant une définition simple et nouvelle de l'espace fonctionnel pour la pression, de sorte qu'il contienne la divergence de la vitesse discrétisée. Une attention particulière est accordée à l'application de cette méthode à différentes formes d'éléments en introduisant le concept d'éléments d'ordre réduit pour toutes les formes standard en 2D et 3D. En outre, la contrainte d'incompressibilité est gérée via la condensation statique pour résoudre le problème du point selle. De plus, dans le but de simuler des écoulements à nombres de Reynolds élevés, la signification de la stabilisation de la diffusion dans le cadre discontinue de Galerkin hybride est analysée. Alors que dans la littérature, le terme de stabilisation de la diffusion est directement proportionnel à la diffusivité ou à la viscosité pour les équations de Navier-Stokes, la présente étude dérive mathématiquement une nouvelle expression pour le terme de stabilisation de diffusion où le terme est inversement proportionnel à la diffusivité ou à la viscosité. Son importance pour les écoulements dominés par la convection est soulignée et étayée par de nombreux exemples numériques. De plus, la formulation proposée pour les équations de Navier-Stokes en régime incompressible est étendue pour résoudre ces équations en moyenne de Reynolds (RANSE) pour les modèles de turbulence TNT, BSL et SST $ k- \omega $ pour des nombres de Reynolds jusqu'à $ 10 ^ 9 $. La résolution des équations en formulation RANSE est une tâche difficile pour les méthodes d'ordre élevé, en raison de profils non réguliers des quantités caractérisant la turbulence. Dans le cadre de la formulation Galerkin discontinu, l'approximation polynomiale de ces quantités conduit à de grandes oscillations qui impactent le solveur non linéaire. Compte tenu de la complexité des méthodes d'ordre élevé et des erreurs de modélisation assez importantes de la modélisation RANS, les méthodes d'ordre inférieur sont par conséquent, souvent considérées dans la littérature comme plus pragmatiques. Cependant, cette thèse montre que la résolution des équations RANSE avec la méthode d'ordre élevé proposée est robuste et conduit à des amplitudes d'erreur significativement plus faibles par rapport aux solveurs basés sur les volumes finis du second ordre. De plus, on observe une réduction remarquable du nombre d'itérations pour obtenir une solution convergée. Une attention particulière est portée au traitement du taux spécifique de dissipation de la turbulence $\omega$ dans le cadre des approximations d’ordre élevé. Les possibilités et les limites de la simulation d'écoulements incompressibles industriels à l'aide de cette formulation haut-ordre sont évaluées afin de tirer des conclusions générales pour les applications industrielles
A high-order energy-stable method for solving the incompressible Navier-Stokes equations based on hybrid discontinuous Galerkin method is presented for which the mass and momentum are conserved. The formulation computes exactly pointwise divergence-free velocity fields for standard element types without post-processing operators nor using \textit{H}(div)-conforming spaces. This is achieved by proposing a simple and novel definition to the functional space of the pressure, such that it contains the divergence of the approximate velocity. Specific focus is given on applying this method on different element shapes by introducing the concept of reduced-order elements for all standard shapes in 2D and 3D. Further, the incompressibility constraint is handled via the static condensation to solve the saddle point problem. Furthermore, with the aim to simulate high Reynolds numbers flows, the significance of the diffusion stabilization in the hybridizable discontinuous Galerkin framework is analyzed. Referring to literature, the diffusion stabilization term is directly proportional to the diffusivity or the viscosity for the Navier-Stokes equations. In this work, a new expression for the diffusion stabilization term is mathematically derived, where the term is inversely proportional to the diffusivity or viscosity. Its importance for convection dominated flows is emphasized and supported by numerical examples.Moreover, the proposed formulation for the incompressible Navier-Stokes is extended to solve the RANSE for the TNT, BSL, and SST $k-\omega$ models for Reynolds numbers up to $10^9$.Solving RANSE is a resilient task for high-order methods, due to the non-smooth profiles of the turbulence quantities. In the discontinuous Galerkin framework, the polynomial approximation for these quantities leads to large oscillations that obstruct the non-linear solver. Taking into account the complexity with high-order methods and the fairly large modeling errors of the RANS modeling, low-order methods are believed to be more pragmatic. However, it is illustrated that solving RANSE with high-order methods leads to significantly smaller error magnitudes compared with second-order finite volume based solvers. Additionally, there is a remarkable improvement regarding the number of iterations to obtain a converged solution. Attention is given to the treatment of the specific rate of turbulence dissipation $\omega$ in the high-order framework. The possibilities and limitations of simulating industrial incompressible flows using discontinuous Galerkin based methods are assessed in order to draw some general conclusions for industrial applications

Hot wire anemometers have been used in several wind velocity sensors deployed in Mars. They are based in keeping the temperature of a surface at a constant value, above the ambient. This is done by means of a heater controlled with an electronic system. The cooling rate of each point at the sensor surface can be used to calculate the wind velocity and direction. However, due to turbulent fluctuations, the cooling rate is not constant even in the case of constant velocity. Moreover, RANS simulations cannot estimate such fluctuations as they only provide an estimation of the averaged flow field. The goal of this work has been to estimate such fluctuations and the e↵ect they might have on the sensor readings. To do so, the turbulent cooling rate (Nusselt number) of a sensor with a generic shape, under the typical conditions to be find in Mars, has been simulated using high performance LES (Large Eddy Simulation) simulations and compared with RANS and URANS simulations.

The legislation has forced the vehicle industry to reduce tail-end emissions. The air pollutant nitrogen oxide (NOX) has been shown to have a negative impact on human health and the environment. One of the key technologies to reduce the levels of NOX emitted from a vehicle is by implementing an after-treatment system. The after-treatment system includes catalysts, a particle filter and an evaporation system. In the evaporation system a liquid jet containing a urea-water solution known as AdBlue is injected into the hot exhaust gases to evaporate into gaseous ammonia NH3 and water H2O. Then NH3 enters the Selective Catalytic Reduction (SCR) catalyst where it chemically reacts with NOX to form N2 and H2O. Problems can arise if an excessive amount of AdBlue is injected and a fluid film is formed on evaporation surfaces. At certain operating conditions the fluid film can crystallise and form solid deposits. The solid deposits can cause high back-pressure, material deterioration and ammonia slip. This project is done in collaboration with Scania CV AB. Scania is a world-leading manufacturer of heavy-duty vehicles, busses and engines. Scania works continuously to develop new simulation methods to capture the complex phenomena of AdBlue spray, wall film dynamics and risk of solid deposits, to use in the development process of new components. The aim of this project is to implement and evaluate a new method to predict the risk of crystallisation of urea (AdBlue) using the software Star-CCM+. Two different geometries are studied, a test rig and a Scania silencer. Different operating conditions, parameter settings and a speed-up method are analysed. During the project a base-line model has been created and the results have been compared with measurement results and the software AVL Fire. The results on the test rig show the effect of altering the mesh and important model parameters. Injected particles are grouped into parcels with the same properties. The number of parcels is a crucial factor for the wall film formation and should be sufficiently high to get a statistical representation of the droplet size distribution. The results from the real silencer show strong evaporation and thin wall film formation with the suggested method. The method is shown to be stable and the software is user-friendly. A speed-up method was investigated to decrease the computational time. The computational time was reduced by a factor 20. The outcome of this project is a guide for set-up of AdBlue spray and wall film simulations. Recommendations to future work includes further validation of the settings, investigation of the evaporation rate and droplet size distribution and the application to other cases. The next step is also to tune the critical thresholds for deposit risk assessment.
Lagstiftning har tvingat fordonstillverkare att minska avgasutsläppen. Luftföroreningen kväveoxid (NOX) har visat sig ha en negativ inverkan på människors hälsa och på miljön. En viktig teknik för att minska utsläppen av NOX ¨ar att implementera ett efterbehandlingssystem. Efterbehandlingssystemet tar hand om avgaserna genom substrat, filter och ett förångningssystem. I förångningssystemet sprutas en urea-vattenlösning, som kallas AdBlue, in i de heta avgaserna där den förångas till ammoniak NH3 och vatten H2O. Ammoniakgasen leds därefter in till SCR katalysatorn där den kemiskt reagerar med NOX och bildar kvävgas N2 och vattenånga. Problem kan uppstå om fel mängd AdBlue sprutas in, då kan vätska byggas upp på förångsningsytor, kristallisera och bilda avlagringar. Avlagringarna kan bygga upp en solid klump som kan orsaka ett högt mottryck, nedbrytning av material och ammoniakslip. Detta arbete är ett samarbete med Scania CV AB som är en världsledande producent av lastbilar, bussar och motorer. Scania arbetar kontinuerligt med att utveckla nya simuleringsvertyg för att beskriva uppkomsten av Urea avlagringar för att använda i utvecklingen av nya komponenter. Syftet med detta arbete är att implementera och utvärdera en ny metod för att prediktera klump mha simuleringsverktyget Star-CCM+. Två olika geometrier är använd i arbetet: en testrigg och en av Scanias ljuddämpare. Olika driftspunkter, parametrar och en uppsnabbad metod är studerade. Under projektets gång har en modell byggts upp och jämförts med mätningar och simuleringar från programvaran AVL Fire. Resultatet från simuleringarna på testriggen visar effekten av att variera olika parametrar. Partiklarna som sprutas in i systemet är grupperade i paket med liknande egenskaper. Antalet paket påverkar uppbyggnaden av väggfilm och det rekommenderas att denna parameter hålls hög för att statistiskt beskriva droppfördelningen av partiklar. Resultaten på ljuddämparen visar en stark förångning och en tunn väggfilm för samtliga driftspunkter. Den implementerade metoden har visat sig vara stabil och användarvänlig. En uppsnabbad metod har utvärderats för att minska beräkningstiden. Beräkningstiden kunde minskas med en faktor 20. Resultatet av arbetet är en guide för hur metoden implementeras och bör användas. Rekommendationer till framtida arbete är en fortsatt undersökning av parametrar, utvärdering av förångningsmodellen, validering av droppstorleksfördelningen och tillämpningen på andra geometrier. Nästa steg i utvecklingen skulle vara att kalibrera tröskelvärden för prediktering av klump.

L'objectif de ce travail est de prendre en compte les instationnarités naturelles à grande échelle dans les écoulements décollés et à un coût plus faible que la LES, tout en s'intéressant à la modélisation des effets de paroi par des modèles statistiques au second ordre. S'inspirant des approches de Durbin, le modèle à pondération elliptique EB-RSM reproduit l'effet non-local de blocage, en résolvant une équation différentielle sur le terme de pression. La limite à deux composantes de la turbulence est bien prédite en canal. Ce modèle est appliqué à la marche descendante, dans une approche URANS. Nous avons montré que les erreurs numériques peuvent être suffisantes pour exciter le mode le plus instable de la couche cisaillée, et aboutir à une solution instationnaire. La solution est stationnaire quand on raffine le maillage, rendant l'URANS peu fiable. Récemment, Schiestel \& Dejoan ont proposé le modèle hybride non-zonal PITM. Le coefficient $C_{\e_2}$ de l'équation de la dissipation devient fonction de la coupure dans le spectre, et la valeur $C_{\e_1}=3/2$ est déduite par ces auteurs. Nous avons donné une formulation plus générale où la valeur de $C_{\e_1}$ est quelconque. Pour offrir un formalisme plus cohérent aux modèles hybrides non-zonaux dans les écoulements de paroi, une approche basée sur un filtrage temporel est proposée. Enfin, l'adaptation du modèle EB-RSM dans un cadre hybride a été réalisée. Les résultats en canal sont encourageants : la transition continue d'un modèle RANS en proche paroi à une LES au centre du canal est mise en évidence. Le transfert d'énergie des échelles modélisées vers celles résolues est bien reproduit quand on raffine le maillage.

[ES] El principal desafío en los motores turbina de gas empleados en aviación reside en aumentar la eficiencia del ciclo termodinámico manteniendo las emisiones contaminantes por debajo de las rigurosas restricciones. Ésto ha conllevado la necesidad de diseñar nuevas estrategias de inyección/combustión que operan en puntos de operación peligrosos por su cercanía al límite inferior de apagado de llama. En este contexto, el concepto Lean Direct Injection (LDI) ha emergido como una tecnología prometedora a la hora de reducir los óxidos de nitrógeno (NOx) emitidos por las plantas propulsoras de los aviones de nueva generación. En este contexto, la presente tesis tiene como objetivos contribuir al conocimiento de los mecanismos físicos que rigen el comportamiento de un quemador LDI y proporcionar herramientas de análisis para una profunda caracterización de las complejas estructuras de flujo de turbulento generadas en el interior de la cámara de combustión. Para ello, se ha desarrollado una metodología numérica basada en CFD capaz de modelar el flujo bifásico no reactivo en el interior de un quemador LDI académico mediante enfoques de turbulencia U-RANS y LES en un marco Euleriano-Lagrangiano. La resolución numérica de este problema multi-escala se aborda mediante la descripción completa del flujo a lo largo de todos los elementos que constituyen la maqueta experimental, incluyendo su paso por el swirler y entrada a la cámara de combustión. Ésto se lleva a cabo través de dos códigos CFD que involucran dos estrategias de mallado diferentes: una basada en algoritmos de generación y refinamiento automático de la malla (AMR) a través de CONVERGE y otra técnica de mallado estático más tradicional mediante OpenFOAM. Por un lado, se ha definido una metodología para obtener una estrategia de mallado óptima mediante el uso del AMR y se han explotado sus beneficios frente a los enfoques tradicionales de malla estática. De esta forma, se ha demostrado que la aplicabilidad de las herramientas de control de malla disponibles en CONVERGE como el refinamiento fijo (fixed embedding) y el AMR son una opción muy interesante para afrontar este tipo de problemas multi-escala. Los resultados destacan una optimización del uso de los recursos computacionales y una mayor precisión en las simulaciones realizadas con la metodología presentada. Por otro lado, el uso de herramientas CFD se ha combinado con la aplicación de técnicas de descomposición modal avanzadas (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificación numérica de los principales modos acústicos en la cámara de combustión ha demostrado el potencial de estas herramientas al permitir caracterizar las estructuras de flujo coherentes generadas como consecuencia de la rotura de los vórtices (VBB) y de los chorros fuertemente torbellinados presentes en el quemador LDI. Además, la implementación de estos procedimientos matemáticos ha permitido tanto recuperar información sobre las características de la dinámica de flujo como proporcionar un enfoque sistemático para identificar los principales mecanismos que sustentan las inestabilidades en la cámara de combustión. Finalmente, la metodología validada ha sido explotada a través de un Diseño de Experimentos (DoE) para cuantificar la influencia de los factores críticos de diseño en el flujo no reactivo. De esta manera, se ha evaluado la contribución individual de algunos parámetros funcionales (el número de palas del swirler, el ángulo de dichas palas, el ancho de la cámara de combustión y la posición axial del orificio del inyector) en los patrones del campo fluido, la distribución del tamaño de gotas del combustible líquido y la aparición de inestabilidades en la cámara de combustión a través de una matriz ortogonal L9 de Taguchi. Este estudio estadístico supone un punto de partida para posteriores estudios de inyección, atomización y combus
[CA] El principal desafiament als motors turbina de gas utilitzats a la aviació resideix en augmentar l'eficiència del cicle termodinàmic mantenint les emissions contaminants per davall de les rigoroses restriccions. Aquest fet comporta la necessitat de dissenyar noves estratègies d'injecció/combustió que radiquen en punts d'operació perillosos per la seva aproximació al límit inferior d'apagat de flama. En aquest context, el concepte Lean Direct Injection (LDI) sorgeix com a eina innovadora a l'hora de reduir els òxids de nitrogen (NOx) emesos per les plantes propulsores dels avions de nova generació. Sota aquest context, aquesta tesis té com a objectius contribuir al coneixement dels mecanismes físics que regeixen el comportament d'un cremador LDI i proporcionar ferramentes d'anàlisi per a una profunda caracterització de les complexes estructures de flux turbulent generades a l'interior de la càmera de combustió. Per tal de dur-ho a terme s'ha desenvolupat una metodología numèrica basada en CFD capaç de modelar el flux bifàsic no reactiu a l'interior d'un cremador LDI acadèmic mitjançant els enfocaments de turbulència U-RANS i LES en un marc Eulerià-Lagrangià. La resolució numèrica d'aquest problema multiescala s'aborda mitjançant la resolució completa del flux al llarg de tots els elements que constitueixen la maqueta experimental, incloent el seu pas pel swirler i l'entrada a la càmera de combustió. Açò es duu a terme a través de dos codis CFD que involucren estratègies de mallat diferents: una basada en la generación automàtica de la malla i en l'algoritme de refinament adaptatiu (AMR) amb CONVERGE i l'altra que es basa en una tècnica de mallat estàtic més tradicional amb OpenFOAM. D'una banda, s'ha definit una metodologia per tal d'obtindre una estrategia de mallat òptima mitjançant l'ús de l'AMR i s'han explotat els seus beneficis front als enfocaments tradicionals de malla estàtica. D'aquesta forma, s'ha demostrat que l'aplicabilitat de les ferramente de control de malla disponibles en CONVERGE com el refinament fixe (fixed embedding) i l'AMR són una opció molt interessant per tal d'afrontar aquest tipus de problemes multiescala. Els resultats destaquen una optimització de l'ús dels recursos computacionals i una major precisió en les simulacions realitzades amb la metodologia presentada. D'altra banda, l'ús d'eines CFD s'ha combinat amb l'aplicació de tècniques de descomposició modal avançades (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificació numèrica dels principals modes acústics a la càmera de combustió ha demostrat el potencial d'aquestes ferramentes al permetre caracteritzar les estructures de flux coherents generades com a conseqüència del trencament dels vòrtex (VBB) i dels raigs fortament arremolinats presents al cremador LDI. A més, la implantació d'estos procediments matemàtics ha permès recuperar informació sobre les característiques de la dinàmica del flux i proporcionar un enfocament sistemàtic per tal d'identificar els principals mecanismes que sustenten les inestabilitats a la càmera de combustió. Finalment, la metodologia validada ha sigut explotada a traves d'un Diseny d'Experiments (DoE) per tal de quantificar la influència dels factors crítics de disseny en el flux no reactiu. D'aquesta manera, s'ha avaluat la contribución individual d'alguns paràmetres funcionals (el nombre de pales del swirler, l'angle de les pales, l'amplada de la càmera de combustió i la posició axial de l'orifici de l'injector) en els patrons del camp fluid, la distribució de la mida de gotes del combustible líquid i l'aparició d'inestabilitats en la càmera de combustió mitjançant una matriu ortogonal L9 de Taguchi. Aquest estudi estadístic és un bon punt de partida per a futurs estudis de injecció, atomització i combustió en cremadors LDI.
[EN] Aeronautical gas turbine engines present the main challenge of increasing the efficiency of the cycle while keeping the pollutant emissions below stringent restrictions. This has led to the design of new injection-combustion strategies working on more risky and problematic operating points such as those close to the lean extinction limit. In this context, the Lean Direct Injection (LDI) concept has emerged as a promising technology to reduce oxides of nitrogen (NOx) for next-generation aircraft power plants In this context, this thesis aims at contributing to the knowledge of the governing physical mechanisms within an LDI burner and to provide analysis tools for a deep characterisation of such complex flows. In order to do so, a numerical CFD methodology capable of reliably modelling the 2-phase nonreacting flow in an academic LDI burner has been developed in an Eulerian-Lagrangian framework, using the U-RANS and LES turbulence approaches. The LDI combustor taken as a reference to carry out the investigation is the laboratory-scale swirled-stabilised CORIA Spray Burner. The multi-scale problem is addressed by solving the complete inlet flow path through the swirl vanes and the combustor through two different CFD codes involving two different meshing strategies: an automatic mesh generation with adaptive mesh refinement (AMR) algorithm through CONVERGE and a more traditional static meshing technique in OpenFOAM. On the one hand, a methodology to obtain an optimal mesh strategy using AMR has been defined, and its benefits against traditional fixed mesh approaches have been exploited. In this way, the applicability of grid control tools available in CONVERGE such as fixed embedding and AMR has been demonstrated to be an interesting option to face this type of multi-scale problem. The results highlight an optimisation of the use of the computational resources and better accuracy in the simulations carried out with the presented methodology. On the other hand, the use of CFD tools has been combined with the application of systematic advanced modal decomposition techniques (i.e., Proper Orthogonal Decomposition and Dynamic Mode Decomposition). The numerical identification of the main acoustic modes in the chamber have proved their potential when studying the characteristics of the most powerful coherent flow structures of strongly swirled jets in a LDI burner undergoing vortex breakdown (VBB). Besides, the implementation of these mathematical procedures has allowed both retrieving information about the flow dynamics features and providing a systematic approach to identify the main mechanisms that sustain instabilities in the combustor. Last, this analysis has also allowed identifying some key features of swirl spray systems such as the complex pulsating, intermittent and cyclical spatial patterns related to the Precessing Vortex Core (PVC). Finally, the validated methodology is exploited through a Design of Experiments (DoE) to quantify the influence of critical design factors on the non-reacting flow. In this way, the individual contribution of some functional parameters (namely the number of swirler vanes, the swirler vane angle, the combustion chamber width and the axial position of the nozzle tip) into both the flow field pattern, the spray size distribution and the occurrence of instabilities in the combustion chamber are evaluated throughout a Taguchi's orthogonal array L9. Such a statistical study has supposed a good starting point for subsequent studies of injection, atomisation and combustion on LDI burners.
Belmar Gil, M. (2020). Computational study on the non-reacting flow in Lean Direct Injection gas turbine combustors through Eulerian-Lagrangian Large-Eddy Simulations [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/159882
TESIS

High speed planing hulls are currently widely used for example in recreational and emergency vessel applications. However, very little CFD research has been done for planing vessels, especially for those with stepped hulls. A validated CFD method for planing stepped hulls could be a valuable improvement for the design phase of such hulls. In this thesis, a CFD method for stepped hulls, with a primary focus on two-step hulls, is developed using STAR-CCM+. As a secondary objective, porpoising instability of two-step hulls is investigated. The simulations are divided into two parts: In the first part a method is developed and validated with existing experimental and numerical data for a simple model scale planing hull with one step. In the second part the method is applied for two two-step hulls provided with Hydrolift AS. A maximum two degrees of freedom, trim and heave, are used, as well as RANS based k-w SST turbulence model and Volume of Fluid (VOF) as a free surface model. The results for the one-step hull mostly corresponded well with the validation data. For the two-step hulls, validation data did not exists and they were first simulated with a fixed trim and sinkage and compered between each other. In the simulations with free trim and heave both hulls experienced unstable porpoising behavior.

This thesis focuses on ingestion of foreign objects into standard turboprop engine GE H80 situated in aircraft Let L-410 Turbolet. Aim of this study is to create methodology of numerical simulation of particle movement inside the engine, which could be used during design process of Inertial Particle Separator device. Thesis consists of backward-facing step benchmark study which validates used methodology. Second part describes flow field calculation and numerical setup. The last part is dedicated to particle tracking analysis. Simulated trajectories are visually investigated, and coordinates of particle impacts at 1st rotor of a compressor are correlated to position of real observed damage.

Ce travail de thèse concerne l'étude d'extracteurs d'air hybrides éoliens. Il se décompose en des investigations amont et appliquées. Dans la partie amont, des simulations fines ont été effectuées en conduite carrée avec et sans rotation, pour des nombres de Reynolds turbulents de l'ordre de 600, afin d'analyser l'impact de la rotation sur la turbulence. Elles ont montré que la rotation rompt la symétrie de l'écoulement. La partie appliquée est dédiée à la conception d'une nouvelle géométrie d'extracteur d'air. Cette géométrie a été proposée à partir de l'analyse de simulations RANS. Ses performances ont été confirmées par des mesures expérimentales sur banc d'essais. Les tests en soufflerie d'un système de captage d'énergie éolienne, conçu pour l'extracteur, ont mis en évidence son adéquation au régime de fonctionnement de l'extracteur. Les essais expérimentaux de l'extracteur complet, montrent que le système de captage apporte une part significative de l'énergie. Des essais en soufflerie ont permis d'observer le comportement global de l'extracteur
This PhD work concerns the study of hybrid air extractors. It is composed of upstream and applied investigations. In the upstream part, fine simulations are realized in square duct flow with and without rotation to analyse the impact of rotation on turbulence. It is found that rotation removes symmetry property of the flow with turbulent Reynolds number of 600. The applied part is dedicated to the conception of a new air extractor geometry. This geometry is proposed from the analyse of RANS simulations. Its performances are confirmed by experimental measurements on test rig. Wind tunnel tests of a wind power capturing system, designed for the extractor, show a good adequation to the operating regime of the extractor. Experimental investigations on the complete air extractor, show the wind power capturing system brings a significant part of the energy. Wind tunnel tests allow to observe the complete air extractor behaviour

Early in the ship design process, naval architects must often evaluate and compare multiple hull forms for a specific set of requirements. Analytical tools are useful for quick comparisons, but they usually specialize in a specific hull type and are therefore not adequate for comparing dissimilar hull types. Scale model hydrodynamic testing is the traditional evaluation method, and is applicable to most hull forms. Scale model tests are usually performed on the largest model possible in order to achieve the most accurate performance predictions. However, such testing is very resource intensive, and is therefore not a cost effective method of evaluating multiple hull forms. This thesis explores the testing of small scale models. It is hypothesized that although the data acquired by these tests will not be accurate enough for performance predictions, they will be accurate enough to rank the performance of the multiple hull forms being evaluated.

This item is only available electronically.
The Arunta Region of Central Australia contains Paleoproterozoic granites extremely enriched in high heat producing elements, in comparison to a global upper crustal average of 1.69 μWm-3. This study uses geochemistry, geochronology, and zircon saturation thermometry to investigate the source and tectonic environment of emplacement of the ca. 1780 Ma Napperby Gneiss. The Napperby Gneiss is peraluminous, suggesting a metasedimentary source. Samples have negative Eu anomalies ranging from 0.10 to 0.57, and show further evidence of fractionation in negative correlations of Ba and Sr with increasing SiO2. Initial εNd values are similar to surrounding exposed metasedimentary rocks and suggest a strong influence of an evolved crustal source but indicate a necessary juvenile component. Matches of inherited xenocrystic zircons from the gneiss with detrital patterns from the regional metasedimentary Lander Formation indicate that sediments similar to the Lander Formation are the source of the protolith granite. Zircon saturation temperatures suggest the granites were emplaced at 790°C – 872°C. Heat production is less than the slightly older ca 1800 ma suites of the Aileron province, and zircon saturation temperatures are higher. The Napperby was produced by dehydration melting rather than fluid flux melting, possibly in a back arc extensional environment with heat provided by upwelling mantle.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2016

This item is only available electronically.
Zircon and monazite U–Pb isotope geochronology combined with structural mapping in the Mt Boothby region in the central Aileron Province in Central Australia has constrained the timing of two tectonically distinct phases of high-grade deformation and metamorphism. The first event (D1/M1) occurred at around 1790 Ma and was associated with the emplacement of a bimodal magmatic suite that underwent high-grade deformation prior to the emplacement of voluminous granite also at around 1790 Ma. The timing of D1/M1 coincides with the early stages of the Yambah Event, which is widely recognised in the southern Aileron Province, but has not previously been unequivocally shown to be associated with deformation . Subsequent pervasive reworking occurred over the interval 1600-1570 Ma, and was associated with long-lived granulite-grade metamorphism. The timing of this event coincides with the Chewings Orogeny which largely shaped the tectonic geology further west in the Reynolds and Anmatjira Ranges. During the Chewings Orogeny the c.1790 Ma D1 structures were transposed into a composite S1/S2 fabric. Map scale F2 folding is interpreted to have a shallow plunge suggesting that the S1 fabric may have originally been shallow dipping, raising the possibility that deformation was extensional in nature, and coeval with deposition of the nearby Reynolds Range Group which is constrained to the interval 1806-1785 Ma. Although inferred here to be Yambah aged, the timing constraints for D1 /M1 also overlap with the c. 1800 Ma Stafford Event which was associated with voluminous felsic magmatism, mafic magmatism and extreme geothermal gradient magmatism. This suggests that an extended period of extension, sedimentation, magmatism and deformation may have occurred at around 1800 Ma in the central Aileron Province.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Earth and Environmental Sciences, 2012

abstract: Reynolds-averaged Navier-Stokes (RANS) simulation is the industry standard for computing practical turbulent flows -- since large eddy simulation (LES) and direct numerical simulation (DNS) require comparatively massive computational power to simulate even relatively simple flows. RANS, like LES, requires that a user specify a “closure model” for the underlying turbulence physics. However, despite more than 60 years of research into turbulence modeling, current models remain largely unable to accurately predict key aspects of the complex turbulent flows frequently encountered in practical engineering applications. Recently a new approach, termed “autonomic closure”, has been developed for LES that avoids the need to specify any prescribed turbulence model. Autonomic closure is a fully-adaptive, self-optimizing approach to the closure problem, in which the simulation itself determines the optimal local, instantaneous relation between any unclosed term and the simulation variables via solution of a nonlinear, nonparametric system identification problem. In principle, it should be possible to extend autonomic closure from LES to RANS simulations, and this thesis is the initial exploration of such an extension. A RANS implementation of autonomic closure would have far-reaching impacts on the ability to simulate practical engineering applications that involve turbulent flows. This thesis has developed the formal connection between autonomic closure for LES and its counterpart for RANS simulations, and provides a priori results from FLUENT simulations of the turbulent flow over a backward-facing step to evaluate the performance of an initial implementation of autonomic closure for RANS. Key aspects of these results lay the groundwork on which future efforts to extend autonomic closure to RANS simulations can be based.
Dissertation/Thesis
Masters Thesis Aerospace Engineering 2017

This item is only available electronically.
The southern margin of central Australia is characterised by anomalous heat production, 3–5 times higher than global averages. Paleoproterozoic voluminous granitoid complexes in the region are important in the study of this anomalous heat flow. Ca.1800 Ma high-heat producing granites in Mt Boothby have A/NCK (molecular Al2O3/(CaO+Na2O+K2O)) ratios > 1, indicating a predominant origin from partial melting of metasedimentary rocks. The Boothby Orthogneiss is characterised by moderately negative Eu anomalies (Eu/Eu*: 0.03–0.43) and strong depletion in Ba, Rb, Nb and Sr. The enrichment of Ba and Rb relative to Sr and high K2O contents also support a metasedimentary source. The heat production values calculated for the Boothby Orthogneiss and the surrounding Lander formation show that the region is enriched in heat producing elements. The U-Pb zircon age data of inherited zircons in these granites are similar to the detrital zircons of the widespread outcropping; Lander formation. Nd values of -3.5 to 1.3 of the granites infer an evolved crustal source coupled with mixing of a newly mantle-derived component through lower crust assimilation. Zircon saturation temperatures calculated suggest that the Boothby intrusive complex was emplaced at 688–845oC, with a maximum temperature of 776oC, implying an arc environment with associated fluid-flux melting in the mantle wedge, ultimately controlled by subduction dynamics.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2016

This item is only available electronically.
Melt loss during regional high-grade metamorphism has important consequences for interpreting the metamorphic evolution of the lower crust and for understanding processes leading to the chemical differentiation of the crust. However, melt loss typically modifies the protolith; making it difficult to reconstruct the conditions of prograde metamorphism and the extent to which melt loss modified the rock composition. The Reynolds Range in central Australia preserves a rare example where a single melt-prone stratigraphic unit can be traced from greenschist to granulite grade conditions. Using this as a natural laboratory, P–T mineral equilibria forward models have been calculated to explore melt loss and melt reintegration where both the protolith and the residuum compositions are preserved. Incremental melt loss modelling from the protolith composition along an isobaric heating path at 5 kbar shows that the residual granulite facies rock composition is consistent with around 18% melt loss from the protolith. Large-scale, one-step melt loss from a closed rock system that had built up 18% melt resulted in a similar residual composition to incremental melt loss. The fertility of the open (incremental) system and the closed system showed the closed system produced 5.4% more melt along a heating path from 700–800 °C. Determination of the concentrations of K–U–Th with increasing metamorphic grade shows that K and U concentrations decreased with increasing metamorphic grade. Conversely, Th concentrations increased, resulting in a slight overall increase in heat production from the protolith to the residuum, despite around 18% volume loss associated with melt extraction. An implication for this is that for melt prone rocks such as metapelites, melt loss during granulite facies metamorphism does not deplete the concentration of heat producing elements in the lower crust as is typically assumed.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2015

In a typical modern gas turbine engine, the nozzle guide vanes (NGVs) endure the highest operating temperatures. There exists a great drive in the turbine industry to increase the turbine inlet temperatures leading to higher thermal efficiency. This has led to a drive to increase turbine vane- and blade-cooling. Numerical modelling has to a large degree replaced empirical codes and models as the main research tool regarding simulation of blade-cooling. Outdated empirical solvers have made way for commercial CFD solvers such as FLUENT, a Reynolds-averaged Navier-Stokes (RANS) solver. One such empirical solver, TACT1, has until recently still proved to yield acceptable results. A comparative study has been done using the T56 NGV blade to establish the differences, advantages and disadvantages of these 2 codes. The engine and subsequent NGV blade were analysed using NREC, STAN5, LOSS3D and TACT1. RANS simulations were found to be computationally expensive. TACT1 yielded acceptable results compared with computational cost. For modern-day designers, RANS would be the preferred tool.
Dissertation (MEng)--University of Pretoria, 2009.
Mechanical and Aeronautical Engineering
unrestricted

A computational approach to analyze the hydrodynamic performance of a hydrofoil or a marine propeller undergoing unsteady motion has been developed. In order to simulate heave and pitch motion of a hydrofoil, an unsteady boundary element method based modeling is performed. The wake of the hydrofoil is modeled by a continuous dipole sheet and determined in time by applying a force-free condition on its surface. An explicit vortex core model is adapted in this model to capture the rolling up shape and to avoid instability due to roll-up deformation of the wake. The numerical results of the developed model are compared with analytical results and those from the commercial Reynolds-Averaged Navier-Stokes solver (ANSYS/FLUENT). The results show close level of agreement with each other. The problem of flow around a marine propeller performing surge, roll and heave motion in an unbounded fluid is formulated and solved using both a vortex-lattice method and a boundary element method. A fully unsteady wake alignment algorithm is implemented into the vortex-lattice method in order to satisfy the force-free condition on the propeller wake surface. Finally, a comparative study of transient propeller forces on a propeller blade obtained from BEM and VLM (with or without fully aligned wake) is carried out and results are presented. In some cases, results from the presented methods are compared with those from RANS or other numerical methods available in the literature.
text

This thesis is concerned with the study of parallel mixing of two dissimilar gases under compressible conditions in the confined environment. A number of numerical studies are reported in the literature for the compressible mixing of two streams of gases where (1) both the streams are of similar gases at the same temperatures, (2) both the streams are at different temperatures with similar gases, and (3) dissimilar gases are with nearly equal temperatures. The combination of dissimilar gases at large temperature difference, mixing under compressible conditions, as in the case of scramjet propulsion, has not been adequately addressed numerically. Also many of the earlier studies have used two dimensional numerical simulation and showed good match with the experimental results on mixing layers that are inherently three dimensional in nature. In the present study, both two-dimensional (2-d) and three dimensional (3-d) studies are reported and in particular the effect of side wall on the three dimensionality of the flow field is analyzed, and the reasons of the good match of two dimensional simulations with experimental results have been discussed. Both two dimensional and three dimensional model free simulations have been conducted for a flow configuration on which experimental results are available. In this flow configuration, the mixing duct has a rectangular cross section with height to width ratio of 0.5. In the upper part of the duct hydrogen gas at a temperature of 103 K is injected through a single manifold of two Ludweig tubes and in the lower part of the duct nitrogen gas at a temperature of 2436 K is supplied through an expansion tube, both the gases are at Mach numbers of 3.1 and 4.0 respectively. Measurements in the experiment are limited to wall pressures and heat flux. The choice of this experimental condition gives an opportunity to study the effect of large temperature difference on the mixing of two dissimilar gases with large molecular weights under compressible conditions. Both two dimensional and three dimensional model free simulations are carried out using higher order numerical scheme (4th order spatial and 2nd order temporal) to understand the structure and evolution of supersonic confined mixing layer of similar and dissimilar gases. Two dimensional simulations are carried out by both SPARK (finite difference method) and OpenFOAM (finite volume method based open source software that was specially picked out and put together), while 3D model free simulations are carried out by OpenFOAM. A fine grid structure with higher grid resolution near the walls and shear layer is chosen. The effect of forcing of fluctuations on the inlet velocity shows no appreciable change in the fully developed turbulent region of the flow. The flow variables are averaged after the attainment of statistical steady state established through monitoring the concentration of inert species introduced in the initial guess. The effect of side wall on the flow structure on the mixing layer is studied by comparing the simulation results with and without side wall. Two dimensional simulations show a good match for the growth rate of shear layer and experimental wall pressures. Three dimensional simulations without side wall shows 14% higher growth rate of shear layer than that of two dimensional simulations. The wall pressures predicted by these three dimensional simulations are also lower than that predicted using two dimensional simulations (6%) and experimental (9%) results in the downstream direction of the mixing duct. Three dimensionality of the flow is thought of as a cause for these differences. Simulations with the presence of side wall show that there is no remarkable difference of three dimensionality of the flow in terms of the variables and turbulence statistics compared to the case without side walls. However, the growth rate of shear layer and wall surface pressures matches well with that predicted using two dimensional simulations. It has been argued that this good match in shear layer growth rate occurs due to formation of oblique disturbances in presence of side walls that are considered responsible for the decrease in growth rate in 3-d mixing layers. The wall pressure match is argued to be good because of hindrance from side wall in the distribution of momentum in third direction results in higher wall pressure. The effect of dissimilar gases at large temperature difference on the growth rate reduction in compressible conditions is studied. Taking experimental conditions as baseline case, simulations are carried out for a range of convective Mach numbers. Simulations are also carried out for the same range of convective Mach numbers considering the mixing of similar gases at the same temperature. The normalized growth rates with incompressible counterpart for both the cases show that the dissimilar gas combination with large temperature difference shows higher growth rate. This result confirms earlier stability analysis that predicts increased growth rate for such cases. The growth rate reduction of a compressible mixing layer is argued to occur due to reduced pressure strain term in the Reynolds stress equation. This reduction also requires the pressure and density fluctuation correlation to be very near to unity. This holds good for a mixing layer formed between two similar gases at same temperature. For dissimilar gases at different temperatures this assumption does not hold well, and pressure-density correlation coefficient shows departure from unity. Further analysis of temperature density correlation factor, and temperature fluctuations shows that the changes in density occur predominantly due to temperature effects, than due to pressure effects. The mechanism of density variations is found to be different for similar and dissimilar gases, while for similar gases the density variations are due to pressure variations. For dissimilar gases density variation is also affected by temperature variations in addition to pressure variations. It has been observed that the traditional k-ε turbulence model within the RANS (Reynolds Averaged Navier Stokes) framework fails to capture the growth rate reduction for compressible shear layers. The performance of k-ε turbulence model is tested for the mixing of dissimilar gases at large temperature difference. For the experimental test case the shear layer growth rate and wall pressures show good match with other model free simulations. Simulations are further carried out for a range of convective Mach numbers keeping the mixing gases and their temperatures same. It has been observed that a drop in the growth rate is well predicted by RANS simulations. Further, the compressibility option has been removed and it has been observed that for the density and temperature difference, even for incompressible case, the drop in growth rate exists. This behaviour shows that the decrease in growth rate is mainly due to the interaction of temperature and species mass fraction on density. Also it can be inferred that RANS with k-ε turbulence model is able to capture the compressible shear layer growth rate for dissimilar gases at high temperature difference. The mixing of heat and species is governed by the values of turbulent Prandtl and Schmidt numbers respectively. These numbers have been observed to vary for different flow conditions, while affecting the flow field considerable in the form of temperature and species distribution. Model free simulations are carried out on an incompressible convective Mach number mixing layer, and the results are compared with that of a compressible mixing layer to study the effect of compressibility on the values of turbulent Prandtl / Schmidt numbers. It has been observed that both turbulent Prandtl and Schmidt numbers show an almost constant value in the mixing layer region for incompressible case. While, for a compressible case, both turbulent Prandtl and Schmidt numbers show a continuous variation within the mixing layer. However, the turbulent Lewis number is observed to be near unity for both incompressible and compressible cases. The thesis is composed of 8 chapters. An introduction of the subject with critical and relevant literature survey is presented in chapter 1. Chapter 2 describes the mathematical formulation and assumptions along with solution methodology needed for the simulations. Chapter 3 deals with the two and three dimensional model free simulations of the non reacting mixing layer. The effect of the presence of side wall is studied in chapter 4. Chapter 5 deals with the effect of compressibility on the mixing of two dissimilar gases at largely different temperatures. The performance of k-ε turbulence model is checked for dissimilar gases in Chapter 6. Chapter 7 is concerned with the effect of compressibility on turbulent Prandtl and Schmidt numbers. Finally concluding remarks are presented in chapter 8. The main aim of this thesis is the exploration of parallel mixing of dissimilar gases under compressible conditions for both two and three dimensional cases. The outcome of the thesis is (a) a finding that the presence of sidewall in a mixing duct does not make flow field two dimensional, instead it causes the formation of oblique disturbances and the shear layer growth rate is reduced, (b) that it has been shown that the growth rates of dissimilar gases are affected far more by large temperature difference than by compressibility as in case of similar gases, (c) that the growth rates of compressible shear layers formed between dissimilar gases are better predicted using k-εturbulence model and (d) that for compressible mixing conditions the turbulent Prandtl and Schmidt numbers vary continuously in the mixing layer region necessitating the use of some kind of model instead of assuming constant values.

Multiphase flows are a common phenomenon. Rains, sediment transport in rivers, snow and dust storms, mud slides and avalanches are examples of multiphase flows occurring in nature. Blood flow is an example of multiphase flow in the human body, which is of vital importance for survival. Multiphase flows occur widely in industrial applications from hydrocarbon extrac-tion to fuel combustion in engines, from spray painting to spray drying, evaporators, pumps and pneumatic conveying. Predicting multiphase flows is of vital importance to understand natural phenomenon and to design and improve industrial processes. Separated flows and dispersed flows are two types of multiphase flows, which occur together in many industrial applications. Physical features of these two classes are different and the transition from one to another involves complex flow physics. Experimental studies of multiphase flows are not easy, as most real world phenomenon cannot be scaled down to laboratory models. Even for those phenomenon that can be demonstrated at lab-oratory scale, rescaling to real world applications requires mathematical models. There are many challenges in experimental measurements of multiphase flows as well. Measurement techniques well suited for single phase flows have constraints when measuring multiphase phenomenon. Un-certainty in experimental measurements poses considerable difficulties in validating numerical models developed for predicting these flows. Owing to the computational effort required, direct simulation of multiphase flows, even for small scale real world applications is out of present scope. Numerical methods have been developed for dealing with each class of flow separately, that in-volves use of models for phenomenon that is computationally demanding. Reynolds Averaged Navier-Stokes (RANS) methods for predicting multiphase flows place strong requirements on turbulence models, as information about fluctuating quantities in the field, that have significant effects on dispersed phase, is not available. Large Eddy Simulation (LES) gives better predictions than RANS as the instantaneous field data is available and large scale unsteadiness that effects the dispersed phase can be captured. Recent LES studies of multiphase flows showed that the sub-grid-scale (SGS) model used for the continuous phase has an effect on the evolution of the dispersed phase. In this work, LES of multiphase flows is performed using Explicit Filtering Large Eddy Sim-ulation method. In this method, spatial derivatives are computed using higher order compact schemes that have spectral-like resolution. SGS modeling is provided by the use of a filter with smoothly falling transfer function. This method is mathematically consistent and converges to a DNS as the grid is refined. It has been successfully applied to combustion and aero-acoustics and this work is the first application of the method to multiphase flows. Study of dispersed multiphase flows was carried out in this work. Modeling of the dispersed phase is kept simple since the in-tention was to evaluate the capability of explicit filtering LES method in predicting multiphase flows. Continuous phase is solved using a compressible formulation with explicit filtering method. Spatial derivatives are computed using fourth and sixth order compact schemes that use derivative splitting method proposed by Hixon & Turkel (2000a) and second order Runge-Kutta (RK2) time stepping. The grid is stretched as needed. Non-reflecting boundary conditions due to Poinsot & Lele (1992) are used to avoid acoustic reflections from boundaries. Buffer zones (Bogey & Bailly (2002)) are employed at outflow and lateral boundaries to damp vortical structures. The code developed for continuous phase is evaluated by studying round jets at Re =36,000 and comparing with experimental measurements of Hussein et al. (1994) and Panchapakesan & Lumley (1993). Simulations showed excellent agreement with experimental results. Rate of decay of axial velocity and the evolution of turbulence intensities on the centerline matched very well with measurements. Radial profiles of mean and fluctuating components of velocities exhibit self-similarity. A set of studies were then performed using this code to assess the effect of numerical scheme, grid refinement & stretching and simulation times on the predictions. Results from these simulations showed good agreements with experiments and established the code for use in multiphase flows under various simulation conditions. To assess the prediction of multiphase flows using this LES method, an evaporating spray ex-periment by Chen et al. (2006) was simulated. The experiment uses a nebuliser for generating a finely atomized spray of acetone, which avoids complex breakdown phenomenon associated with air blast atomizers and provides well defined boundary conditions for model evaluation. The neb-uliser sits upstream in a pipe carrying air and droplets travel along with air for a distance of 10 diameters before exiting into a wind tunnel with co-flowing air. Droplet breakdown, if any, takes place inside the pipe and the spray is finely atomized by the time it reaches pipe exit. One of the experimental cases at Re =31,600, with a mass loading of 1.1% and a jet velocity of 56 m/s is simulated. Particle size has a χsquared distribution with a Sauter mean diameter of 18µm. In the self-similar region, decay of centerline velocity and turbulence intensities matched well with ex-perimental results. Continuous phase exhibits self-similar behavior. A series of simulations were then performed to match the initial region of the spray by altering the inflow conditions in the sim-ulation. Simulation that matched the breakdown location of the experiment revealed the presence of a relaxation zone with a higher initial spreading rate, followed by a lower asymptotic spreading rate. Studies were performed to understand the effect of various phenomenon like evaporation and droplet size on this behavior. A study of breakdown region of particle-laden jets was performed to understand the presence of relaxation zone post breakdown. Flow conditions were similar to evaporating spray experiment except that particles do not evaporate, mass loading is 2% and jet Reynolds number Re =2000. A series of grid refinements were performed and on the largest grid, gird spacing Δy =7.5η, where ηis an estimate of the Kolmogorov length scale based on flow conditions. Decay of axial velocity on the centerline showed variations with grid refinement, tending to the experimentally measured value as the grid is refined. Variation of turbulence intensities along the centerline revealed a jump in axial velocity fluctuations at the breakdown location, while radial and azimuthal velocities showed a smooth increase to their asymptotic value. This jump was resolved on grid refinement and on fine grids axial velocity fluctuations followed the other two quantities closely in their rise to asymptotic state. Comparison of these quantities with a jet without particles revealed that the flow features are same for a jet with and without particles, and at the mass loading studied, particles have negligible effect on jet breakdown. Another study performed at a higher Reynolds number of Re =11,000, under similar flow conditions showed similar behavior. To assess the ability of predicting dispersed phase, simulations of particle-laden flows at low Stokes number were performed and compared against an experiment by Lau & Nathan (2014). The experiment studies variation of velocity and particle concentration along the centerline, and half widths of a jet velocity and concentration. Particles are injected into a pipe along with air, and the two phase flow is fully developed by the time it exits the pipe into a wind tunnel along with a co-flow. Particles are mono-disperse with a density of 1200 kg/m3. Mass loading is 40% so that particles have a significant effect on the continuous phase. Two cases at particle Stokes number of 1.4, one with Re =10,000, bulk velocity of 12 m/s and particle diameter of 20µm and another with Re =22,500, bulk velocity of 36 m/s and particle diameter of 10µm were simulated. Simulations of both the cases showed good match with experimental measurements of centerline decay for the continuous phase. For the dispersed case, simulations with larger particles showed good match with experimental results, while smaller particles showed differences. This was understood to be the effect of lateral migration which is prominent in case of smaller particles, the models for which have not been used in the present simulation study.

Convergent-Divergent (C-D) nozzles are used in rocket engines to produce thrust as a reaction to the acceleration of hot combustion chamber gases in the opposite direction. To maximize the engine performance at high altitudes, large area ratio, bell-shaped or contoured nozzles are used. At lower altitudes, the exit pressure of these nozzles is lower than the ambient pressure. During this over-expanded condition, the nozzle-internal flow adapts to the ambient pressure through an oblique shock. But the boundary layer inside the divergent portion of the nozzle is unable to withstand the pressure rise associated with the shock, and consequently flow separation is induced. Numerical simulation of separated flows in rocket nozzles is challenging because the existing turbulence models are unable to correctly predict shock-induced flow separation. The present thesis addresses this problem. Axisymmetric, steady-state, Reynolds-Averaged Navier-Stokes (RANS) simulations of a conical nozzle and three sub-scale contoured nozzles were carried out to numerically predict flow separation in over-expanded rocket nozzles at different nozzle pressure ratios (NPR). The conical nozzle is the JPL 45◦-15◦ and the contoured nozzles are the VAC-S1, the DLR-PAR and the VAC-S6-short. The commercial CFD code ANSYS FLUENT 13 was first validated for simulation of separated cold gas flows in the VAC-S1 nozzle. Some modeling issues in the numerical simulations of flow separation in rocket nozzles were determined. It is recognized that compressibility correction, nozzle-lip thickness and upstream-extension of the external domain are the sources of uncertainty, besides turbulence modeling. In high-speed turbulent flows, compressibility is known to affect dissipation rate of turbulence kinetic energy. As a consequence, a reduction in the spreading rate of supersonic mixing layers occurs. Whereas, the standard turbulence models are developed and calibrated for incompressible flows and hence, do not account for this effect. ANSYS FLUENT uses the compressibility correction proposed by Wilcox [1] which modifies the turbulence dissipation terms based on turbulent Mach number. This, as shown in this thesis, may not be appropriate to the prediction of flow separation in rocket nozzles. Simulation results of the standard SST model, with and without the compressibility correction, are compared with the experimental data at NPR=22 for the DLR-PAR nozzle. Compressibility correction is found to cause under-prediction of separation location and hence its use in the prediction of flow separation is not recommended. In the literature, computational domains for the simulation of DLR subscale nozzles have thick nozzle-lips whereas for the VAC subscale nozzles they have no nozzle-lip. Effect of nozzle-lip thickness on flow separation is studied in the DLR-PAR nozzle by varying its nozzle-lip thickness. It is found that nozzle-lip thickness significantly influences both separation location and post-separation pressure recovery by means of the recirculation bubbles formed at the nozzle-lip. Usually, experimental values of free stream turbulence are unknown. So conventionally, to minimize solution dependence on the boundary conditions specified for the ambient flow, the computational domain external to the nozzle is extended in the upstream direction. Its effect on flow separation is studied in the DLR-PAR nozzle through simulations conducted with and without this domain extension. No considerable effect on separation location and pressure recovery is found. The two eddy-viscosity based turbulence models, Spalart-Allmaras (SA) model and Shear Stress Transport (SST) model, are well known to predict separation location better than other eddy-viscosity models, but with moderate success. Their performances, in terms of predicting separation location and post-separation wall pressure distribution, were compared with each other and evaluated against experimental data for the conical and two contoured nozzles. It is found that they fail to predict the separation location correctly, exhibiting sensitivity to the range of NPRs and to the type of nozzle. Depending on NPR, the SST model either under-predicts or over-predicts Free Shock Separation (FSS). Moreover, it also fails to capture Restricted Shock Separation (RSS). With compressibility correction, it under-predicts separation at all NPRs to a greater extent. Even though RSS is captured by using compressibility correction, the transition from FSS to RSS is over-predicted [2]. Early efforts by few researchers to improve predictions of nozzle flow separation by realizability corrections to turbulence models have not been successful, especially in terms of capturing both the separation types. Therefore, causes of turbulence modeling failure in predicting nozzle flow separation correctly were further investigated. It is learnt that limiting of the shear stress inside boundary layer, due to Bradshaw’s assumption, and over-prediction of jet spreading rate are the causes of SST model’s failure in predicting nozzle flow separation correctly. Based on this physical reasoning, values of the a 1 parameter and the two diffusion coefficients σk,2 and σω,2 were empirically modified to match the predicted wall pressure distributions with experimental data of the DLR-PAR and the VAC-S6-short nozzles. The results confirm that accurate prediction of flow separation in rocket nozzles indeed depends on the correct prediction of spreading rate of the supersonic separation-jet. It is demonstrated that accurate RANS simulation of flow separation in rocket nozzles over a wide range of NPRs is feasible by modified values of the diffusion coefficients in turbulence model.