Dissertationen zum Thema „Unsteady simulation“

The design of slender structures such as longspan bridges, masts, offshore risers and cables is strongly influenced by their response behaviour when subjected to unsteady loads due to wind, waves and current. Simulation of the behaviour of a viscous flow past a structural cross section is of great importance to engineers concerned with the design of such structures. Offshore engineers are concerned with estimating the magnitude of structural forces induced by the most severe storm-induced wave events. Numerous studies have been conducted in an effort to estimate the structural forces induced by both regular and irregular waves. However, estimation of the maximum extreme wave-induced structural forces, particularly for relatively small diameter horizontal components, has received less attention. Since the most widely used method for estimating the force experienced by a bluff body subjected to wave loading is the empirical drag-inertia equation developed by Morison, O’ Brien, Johnson, and Schaaf (1950), it is important to determine whether this equation is adequate to describe the forces imposed by extremely large ocean waves. A method is presented for the simulation of incompressible viscous flow past acylinder using a stream function vorticity-transport formulation discretised on a cutcell quadtree mesh. A cut-cell technique is employed to provide accurate boundary representation and to facilitate the simulation of flow past a moving boundary. The finite volume discretisation consists of second-order accurate central difference approximations within uncut flow cells and a polynomial reconstruction technique within the cut-cells that are intersected by the solid boundary. Several preliminary validation tests concerned with flow past a circular cylinder are presented to confirm the accuracy of the numerical model. Firstly, the cut-cell discretisation is applied to the solution of the Euler equations and is shown to be almost second order accurate. Comparisons of wake geometry and force coefficients for steady and oscillatory flows at low Reynolds number are then made with existing results, and show satisfactory agreement. Preliminary tests are presented to assess the accuracy of a cut-cell based method for simulating flow past a circular body that moves across a background mesh. A series of experiments is also presented concerned with the measurement of theforce experienced by a circular cylinder undergoing a pre-defined two-dimensionalmotion within a still fluid. The cylinder trajectory is representative of the motionof a fluid particle beneath an idealised large ocean wave as defined by the NewWave formulation (Tromans et al. 1991). It is observed that, whilst the magnitude of high frequency vortex induced force fluctuations varies with the ratio of wave amplitude to cylinder diameter (A=D) and the wave spectrum shape, the overall shape of both x- and y-direction force time histories is very similar for all wave groups for which the underlying spectrum has the same shape. For all of the two-dimensional cylinder motions considered, the spectrum of both measured forces closely approximates the spectrum of uq (where u is a component of the velocity vector and q the absolute velocity) and, as a result, the vector form of the well known equation developed by Morison et al. (1950) is shown to provide a satisfactory estimate of the cartesian force components. The high frequency component of the force that is not captured by the Morison et al. equation is clearly identified as a lift-type force in the radial direction. For design purposes, a reasonable estimate of the magnitude of the peak force is obtained by neglecting inertial forces and employing a drag coefficient CD = 1.0.

There is currently a great deal of interest within the aviation community in the design of small, slow-flying but manoeuvrable uninhabited vehicles for reconnaissance, surveillance, and search and rescue operations in urban environments. Inspired by observation of birds, insects, fish and cetaceans, flapping wings are being actively studied in the hope that they may provide greater propulsive efficiencies than propellers and rotors at low Reynolds numbers for such Micro-Air Vehicles (MAVs). Researchers have posited the Strouhal number (combining flapping frequency, amplitude and forward speed) as the parameter controlling flapping wing aerodynamics in cruising flight, although there is conflicting evidence. This thesis explores the effect of flapping frequency and amplitude on forces and wake structures, as well as physical mechanisms leading to optimum propulsive efficiency. Two-dimensional rigid airfoils are considered at Reynolds number 2,000 ??? 40,000. A compressible Navier-Stokes simulation is combined with numerical and analytical potential flow techniques to isolate and evaluate the effect of viscosity, leading and trailing edge vortex separation, and wake vortex dynamics. The wake structures of a plunging airfoil are shown to be sensitive to the flapping frequency independent of the Strouhal number. For a given frequency, the wake of the airfoil exhibits ???vortex lock-in??? as the amplitude of motion is increased, in a manner analogous to an oscillating circular cylinder. This is caused by interaction between the flapping frequency and the ???bluff-body??? vortex shedding frequency apparent even for streamlined airfoils at low Reynolds number. The thrust and propulsive efficiency of a plunging airfoil are also shown to be sensitive to the flapping frequency independent of Strouhal number. This dependence is the result of vortex shedding from the leading edge, and an interaction between the flapping frequency and the time for vortex formation, separation and convection over the airfoil surface. The observed propulsive efficiency peak for a pitching and plunging airfoil is shown to be the result of leading edge vortex shedding at low flapping frequencies (low Strouhal numbers), and high power requirements at large flapping amplitudes (high Strouhal numbers). The efficiency peak is governed by flapping frequency and amplitude separately, rather than the Strouhal number directly.

Alterations along the Mississippi River, such as dams and levees, have greatly reduced the amount of freshwater and sediment that reaches the Louisiana coastal area. Several freshwater and sediment diversions have been proposed to combat the associated land loss problem. To aid in this restoration effort a 1-D numerical model was calibrated, validated, and used to predict the response of the river to certain stimuli, such as proposed diversions, channel closures, channel modifications, and relative sea level rise. This study utilized HEC-RAS 4.0, a 1-D mobile-bed numerical model, which was calibrated using a discharge hydrograph at Tarbert Landing and a stage hydrograph at the Gulf of Mexico, to calculate the hydrodynamics of the river. The model showed that RSLR will decrease the capacity of the Lower Mississippi River to carry bed material. The stage at Carrollton Gage is not significantly impacted by large scale diversions

Ces cinquantes dernières années, la majorité des paramètres de conception des moteurs cryotechniques ont été ajustés en l'absence d'une compréhension détaillée des phénomènes de combustion, en raison des limites des diagnostiques expérimentaux et des capacités de calcul. L'objectif de cette thèse est de réaliser des simulations numériques instationnaires d'écoulements réactifs transcritiques de haute fidélité, pour permettre une meilleure compréhension de la dynamique de flamme dans les moteurs cryotechniques et finalement guider leur amélioration. Dans un premier temps, la thermodynamique gaz-réel et son impact sur les schémas numériques sont présentés. Comme la Simulation aux Grandes Echelles (SGE) comporte des équations filtrées, les effets de filtrages induits par la thermodynamique gaz-réel sont ensuite mis en évidence dans une configuration transcritique type et un opérateur de diffusion artificiel, spécifique au gaz réel, est proposé pour lisser les gradients transcritiques en SGE. Dans un deuxième temps, une étude fondamentale du mélange turbulent et de la combustion dans la zone proche-injecteur des moteurs cryotechniques est menée grâce à la Simulation Numérique Directe (SND). Dans le cas non-réactif, les lâchers tourbillonnaires dans le sillage de la lèvre de l’injecteur jouent un rôle majeur dans le mélange turbulent et provoquent la formation de structures en peigne déjà observées expérimentalement dans des conditions similaires. Dans le cas réactif, la flamme reste attachée à la lèvre de l'injecteur, sans extinction locale, et les structures en peigne disparaissent. La structure de flamme est analysée et différents modes de combustion sont identifiés. Enfin, une étude de flamme-jet transcritique H2/O2, accrochée à un injecteur coaxial avec et sans retrait interne, est menée. Les résultats numériques sont d'abord validés par des données expérimentales pour l'injecteur sans retrait. Ensuite, la configuration avec retrait est comparée à la solution de référence sans retrait et à des données experimentales pour observer les effets de ce paramètre de conception sur l'efficacité de combustion
In the past fifty years, most design parameters of the combustion chamber of Liquid Rocket Engines (LREs) have been adjusted without a detailed understanding of combustion phenomena, because of both limited experimental diagnostics and numerical capabilities. The objective of the present thesis work is to conduct high-fidelity unsteady numerical simulations of transcritical reacting flows, in order to improve the understanding of flame dynamics in LRE, and eventually provide guidelines for their improvement. First real-gas thermodynamics and its impact on numerical schemes are presented. As Large-Eddy Simulation (LES) involves filtered equations, the filtering effects induced by real-gas thermodynamics are then highlighted in a typical 1D transcritical configuration and a specific real-gas artificial dissipation is proposed to smooth transcritical density gradients in LES. Then, a Direct Numerical Simulation (DNS) study of turbulent mixing and combustion in the near-injector region of LREs is conducted. In the non-reacting case, vortex shedding in the wake of the lip of the injector is shown to play a major role in turbulent mixing, and induces the formation of finger-like structures as observed experimentally in similar operating conditions. In the reacting case, the flame is attached to the injector rim without local extinction and the finger-like structures disappear. The flame structure is analyzed and various combustion modes are identified. Finally, a LES study of a transcritical H2/O2 jet flame, issuing from a coaxial injector with and without inner recess, is conducted. Numerical results are first validated against experimental data for the injector without recess. Then, the recessed configuration is compared to the reference solution and to experimental results, to scrutinize the effects of this design parameter on combustion efficiency

Numerical methods for the simulation of shock-induced turbulent mixing have been investigated, focussing on Implicit Large Eddy Simulation. Shock-induced turbulent mixing is of particular importance for many astrophysical phenomena, inertial confinement fusion, and mixing in supersonic combustion. These disciplines are particularly reliant on numerical simulation, as the extreme nature of the flow in question makes gathering accurate experimental data difficult or impossible. A detailed quantitative study of homogeneous decaying turbulence demonstrates that existing state of the art methods represent the growth of turbulent structures and the decay of turbulent kinetic energy to a reasonable degree of accuracy. However, a key observation is that the numerical methods are too dissipative at high wavenumbers (short wavelengths relative to the grid spacing). A theoretical analysis of the dissipation of kinetic energy in low Mach number flows shows that the leading order dissipation rate for Godunov-type schemes is proportional to the speed of sound and the velocity jump across the cell interface squared. This shows that the dissipation of Godunov-type schemes becomes large for low Mach flow features, hence impeding the development of fluid instabilities, and causing overly dissipative turbulent kinetic energy spectra. It is shown that this leading order term can be removed by locally modifying the reconstruction of the velocity components. As the modification is local, it allows the accurate simulation of mixed compressible/incompressible flows without changing the formulation of the governing equations. In principle, the modification is applicable to any finite volume compressible method which includes a reconstruction stage. Extensive numerical tests show great improvements in performance at low Mach compared to the standard scheme, significantly improving turbulent kinetic energy spectra, and giving the correct Mach squared scaling of pressure and density variations down to Mach 10−4. The proposed modification does not significantly affect the shock capturing ability of the numerical scheme. The modified numerical method is validated through simulations of compressible, deep, open cavity flow where excellent results are gained with minimal modelling effort. Simulations of single and multimode Richtmyer-Meshkov instability show that the modification gives equivalent results to the standard scheme at twice the grid resolution in each direction. This is equivalent to sixteen times decrease in computational time for a given quality of results. Finally, simulations of a shock-induced turbulent mixing experiment show excellent qualitative agreement with available experimental data.

A hybrid model for simulating rogue waves in random seas on a large time and space scale is proposed in this thesis. It is formed by combining the derived fifth order Enhanced Nonlinear Schrödinger Equation based on Fourier transform (ENLSE-5F), the fully nonlinear Enhanced Spectral Boundary Integral (ESBI) method and its simplified version. The numerical techniques and algorithm for coupling three models on time scale are provided. Using them, and the switch between the three models during the computation is triggered automatically according to wave nonlinearities. Numerical tests are carried out and the results indicate that this hybrid model could simulate rogue waves both accurately and efficiently. In some cases showed, the hybrid model is more than 10 times faster than just using the ESBI method.

The scope of this PhD thesis is the simulation of turbulence in time-dependent, separated and suddenly-expanded channel flows. High-resolution and very high-order numerical methods have been employed in the framework of Implicit Large Eddy Simulation (ILES) to elucidate open questions about the physics in flows with sudden expansion. It is well known that the planar sudden expansion (PSE), despite its simple and symmetric geometry it produces a very complex behaviour and a distinctly asymmetric flow pattern ascribed mainly to the Coanda effect. Such flows are encountered in a wide range of practical engineering applications, such as combustion, hydraulic and fluidic devices, air ducts, and mixing equipments. It is of great importance, therefore, to understand the mechanisms that dominate flows with separation and reattachment of the shear layers, as well as flows with regions of strong reversed motion. This thesis has for the first time analysed in detail the turbulent kinetic energy budget (TKEB) for the PSE. This analysis has been extended to examine the influence of Mach number on each individual component of the TKEB. The resulting data can be used as reference for further development of turbulence models capable of accurately resolving the flow behaviour in suddenly-expanded flows. Cont/d.

This thesis provides a numerical and theoretical investigation of transitional and turbulent enclosed rotating flows, with a focus on the formation of macroscopic coherent flow structures. The underlying processes are strongly threedimensional due to the presence of boundary layers on the discs and on the walls of the outer (resp. inner) cylindrical shroud (resp. shaft). The complexity of these flows poses a great challenge in fundamental research however the present work is also of importance for industrial rotating machinery, from hard-drives to space engines turbopumps - the design issues of the latter being behind the motivation for this thesis. The present work consists of two major investigations. First, industrial cavities are modeled by smooth rotor/stator cavities and therein the dominant flow dynamics is investigated. For the experimental campaigns on industrial machinery revealed dangerous unsteady phenomena within the cavities, the emphasis is put on the reproduction and monitoring of unsteady pressure fluctuations within the smooth cavities. Then, the LES of three configurations of real industrial turbines are conducted to study in situ the pressure fluctuations and apply the diagnostics already vetted on academic problems.

[ES] Está fuera de toda duda que la industria del automóvil está viviendo una profunda transformación que, durante los últimos años, ha progresado a un ritmo acelerado. Debido a la crecientemente estricta regulación sobre emisiones contaminantes y la necesidad de satisfacer la siempre creciente demanda de movilidad sostenible, es necesario que los motores de combustión modernos reduzcan su consumo y emisiones manteniendo el rendimiento del motor. Para enfrentarse a este desafío, los ingenieros de investigación y desarrollo han redoblado sus esfuerzos a la hora de diseñar y mejorar los modelos unidimensionales, hasta el punto en el que el desarrollo de modelos 1D así como la simulación juegan un papel fundamental en los primeras etapas de diseño de nuevos motores y tecnologías. Al mismo tiempo, la tecnología de turbosobrealimentación se ha consolidado como una de las más efectivas a la hora de construir motores de alta eficiencia, lo que ha hecho evidente la importancia de comprender y modelar correctamente los efectos asociados a los turbogrupos. Particularmente, los fenómenos que ocurren en la turbina en condiciones de flujo fuertemente pulsante han demostrado ser complicadas de modelar y sin embargo decisivas, ya que los códigos de simulación son especialmente útiles cuando son diseñados para trabajar en condiciones realistas. Este trabajo se centra en mejorar los modelos unidimensionales actuales así como en desarrollar nuevas soluciones con el objetivo de contribuir a una mejor predicción del comportamiento de la turbina sometida a condiciones de flujo pulsante. Tanto los esfuerzos realizados en los trabajos experimentales como en los de modelado se han producido para poder proporcionar métodos que sean fáciles de adaptar a las diferentes configuraciones de turbogrupo usadas en la industria, por ello, pueden ser aplicados por ejemplo en turbinas de entrada simple y también en las cada vez más usadas turbinas de entrada doble. En cuanto al trabajo de modelado en la parte de turbina de entrada simple, el foco se ha puesto en presentar una versión mejorada de un código quasi-2D. La validación del modelo se basa en los datos experimentales que están disponibles de trabajos enteriores de la literatura, proporcionando una comparación completa entre los modelos quasi-2D y el clásico modelo 1D. La presión a la entrada y salida de la turbina se ha descompuesto en ondas que viajan hacia delante y hacia atrás por medio de la descomposición de presiones, empleando la componente reflejada y transmitida para verificar la bondad del modelo. El trabajo experimental de esta tesis se centra en desarrollar un nuevo método para ensayar cualquier turbina de doble entrada sometida a condiciones de flujo fuertemente pulsante. La configuración del banco de gas se ha diseñado para ser suficientemente flexible como para realizar pulsos en las dos ramas de entrada por separado, así como para usar condiciones de flujo caliente o condiciones ambiente con mínimos cambios en la instalación. La campaña experimental se usa para validar un modelo integrado unidimensional de turbina tipo twin scroll con especial foco en las componentes reflejada y transmitida para analizar el desempeño del modelo su capacidad de predicción de la acústica no lineal. Finalmente, después de desarrollar el trabajo experimental y de modelado, se presenta un procedimiento para caracterizar el sonido y ruido de la turbina por medio de matrices de transferencia acústica que es comparado con el código unidimensional completo. En este sentido, el método proporciona una herramienta útil y fácil de implementar para simulaciones en tiempo real que aplica de una manera práctica el trabajo de modelado expuesto a lo largo de esta tesis.
[CAT] Està fora de tot dubte que la indústria de l'automòbil està vivint una profunda transformació que, durant els últims anys, ha progressat a un ritme accelerat. A causa de la creixentment estricta regulació sobre emissions contaminants i la necessitat de satisfer la sempre creixent demanda de mobilitat sostenible, és necessari que els motors de combustió moderns reduïsquen el seu consum i emissions mantenint el rendiment del motor. Per a enfrontar-se a aquest desafiament, els enginyers de recerca i desenvolupament han redoblat els seus esforços a l'hora de dissenyar i millorar els models unidimensionals, fins al punt en el qual el desenvolupament de models 1D així com la simulació juguen un paper fonamental en les primeres etapes de disseny de nous motors i tecnologies. Al mateix temps, la tecnologia de turbosobrealimentación s'ha consolidat com una de les més efectives a l'hora de construir motors d'alta eficiència, la qual cosa ha fet evident la importància de comprendre i modelar correctament els efectes associats als turbogrupos. Particularment, els fenòmens que ocorren en la turbina en condicions de flux fortament polsant han demostrat ser complicades de modelar i no obstant això decisives, ja que els codis de simulació són especialment útils quan són dissenyats per a treballar en condicions realistes. Aquest treball se centra en millorar els models unidimensionals actuals així com a desenvolupar noves solucions amb l'objectiu de contribuir a una millor predicció del comportament de la turbina sotmesa a condicions de flux polsant. Tant els esforços realitzats en els treballs experimentals com en els de modelatge s'han produït per a poder proporcionar mètodes que siguen fàcils d'adaptar a les diferents configuracions de turbogrupo usades en l'indústria, per això, poden ser aplicats per exemple en turbines d'entrada simple i també en les cada vegada més usades turbines d'entrada doble. Pel que fa al treball de modelatge en la part de turbina d'entrada simple, el focus s'ha posat a presentar una versió millorada d'un codi quasi-2D. La validació del model es basa en les dades experimentals que estan disponibles de treballs anteriors de la literatura, proporcionant una comparació completa entre els models quasi-2D i el clàssic model 1D. La pressió a l'entrada i eixida de la turbina s'ha descompost en ones que viatgen cap avant i cap enrere per mitjà de la descomposició de pressions, emprant la component reflectida i transmesa per a verificar la bondat del model. El treball experimental d'aquesta tesi se centra en desenvolupar un nou mètode per a assajar qualsevol turbina de doble entrada sotmesa a condicions de flux fortament pulsante. La configuració del banc de gas s'ha dissenyat per a ser prou flexible com per a realitzar polsos en les dues branques d'entrada per separat, així com per a usar condicions de flux calent o condicions ambient amb mínims canvis en la instal·lació. La campanya experimental s'usa per a validar un model integrat unidimensional de turbina tipus twin-scroll amb especial focus en les components reflectida i transmesa per a analitzar l'acompliment del model la seua capacitat de predicció de l'acústica no lineal. Finalment, després de desenvolupar el treball experimental i de modelatge, es presenta un procediment per a caracteritzar el so i soroll de la turbina per mitjà de matrius de transferència acústica que és comparat amb el codi unidimensional complet. En aquest sentit, el mètode proporciona una eina útil i fàcil d'implementar per a simulacions en temps real que aplica d'una manera pràctica el treball de modelatge exposat al llarg d'aquesta tesi.
[EN] It is beyond all doubt that the automotive industry is living a deep transformation that, during the last years, has progressed at an ever accelerating rate. Due to the increasingly stringent pollutant emission regulations and the necessity to fulfil an ever growing demand for sustainable mobility, the modern internal combustion engines are required to strongly reduce the fuel consumption and emissions, while keeping the engine performance. In order to confront this challenge, engine research and development engineers have redoubled their efforts in designing and improving one-dimensional codes, to the point that the development of 1D models and simulation campaigns play a major role in the early steps of designing new engines or technologies. At the same time as the turbocharging technology has arisen as one of the most effective and extended solutions for building high efficient engines, the importance of understanding and modelling correctly the turbocharger effects has become evident. In particular, the phenomena that occurs in the turbine under highly pulsating conditions have proven to be challenging to model and yet decisive, as simulation codes are especially useful when they are designed to work under realistic conditions. This work focusses on the improvement of current one-dimensional models as well as in the development of new solutions with the aim of contributing to a better prediction of the turbine performance under pulsating conditions. Both experimental and modelling efforts have been made in order to provide methods that are easily adaptable to different turbocharger configurations used in the industry, so they can be applied for example in single turbines and also in the increasingly used two-scroll turbine technology. Regarding the modelling work of the single entry turbine part, the work has been focused in presenting an improved version of a quasi-2D code. The validation of the model is based on the experimental data available from previous works of the literature, providing a complete comparison between the quasi-2D and a classic 1D model. By means of a pressure decomposition, the pressure at the turbine inlet and outlet has been split into forward and backward travelling waves, employing the reflected and transmitted components to verify the goodness of the model. The experimental work of the thesis is centred in developing a new method in order to test any two-scroll turbine under highly pulsating flow conditions. The gas stand setup has been designed to be flexible enough to perform pulses in both inlet branches separately as well as to use hot or ambient conditions with minimal changes in the installation. The experimental campaign is used to fully validate an integrated 1D twin-scroll turbine model with special focus in the reflected and transmitted components for analysing the performance of the model and its non-linear acoustics prediction capabilities. Finally, after the experiment and modelling work is developed, a procedure to characterise the turbine sound and noise by means of acoustic transfer matrices is presented and tested against the fully one-dimensional code. In this sense, this method provides a useful and easily-implementable tool for fast and real time simulations that applies in a practical way the modelling work exposed along this thesis.
Soler Blanco, P. (2020). Simulation and modelling of the performance of radial turbochargers under unsteady flow [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/141609
TESIS

Turbocharging the internal combustion (IC) engine is a common technique to increase the power density. If turbocharging is used with the downsizing technique, the fuel consumption and pollution of green house gases can be decreased. In the turbocharger, the energy of the engine exhaust gas is extracted by expanding it through the turbine which drives the compressor by a shaft. If a turbocharged IC engine is compared with a natural aspirated engine, the turbocharged engine will be smaller, lighter and will also have a better efficiency, due to less pump losses, lower inertia of the system and less friction losses. To be able to further increase the efficiency of the IC engine, the understanding of the highly unsteady flow in turbochargers must be improved, which then can be used to increase the efficiency of the turbine and the compressor. The main objective with this thesis has been to enhance the understanding of the unsteady flow in turbocharger and to assess the sensitivity of inflow conditions on the turbocharger performance. The performance and the flow field in a radial turbocharger turbine working under both non-pulsatile and pulsatile flow conditions has been assessed by using Large Eddy Simulation (LES). To assess the effects of different operation conditions on the turbine performance, different cases have been considered with different perturbations and unsteadiness of the inflow conditions. Also different rotational speeds of the turbine wheel were considered. The results show that the turbine cannot be treated as being quasi-stationary; for example,the shaft power varies for different frequencies of the pulses for the same amplitude of mass flow. The results also show that perturbations and unsteadiness that are created in the geometry upstream of the turbine have substantial effects on the performance of the turbocharger. All this can be summarized as that perturbations and unsteadiness in the inflow conditions to the turbine affect the performance. The unsteady flow field in ported shroud compressor has also been assessed by using LES for two different operational points. For an operational point near surge, the flow field in the entire compressor stage is unsteady, where the driving mechanism is an unsteadiness created in the volute. For an operational point far away from surge, the flow field in the compressor is relatively much more steady as compared with the former case. Although the stable operational point exhibits back-flow from the ported shroud channels, which implies that the flow into the compressor wheel is disturbed due to the structures that are created in the shear layer between the bulk flow and the back-flow from the ported shroud channels.
QC20100622

Thesis (Ph. D.)--Aerospace Engineering, Georgia Institute of Technology, 2007.
Yeung, Pui-Kuen, Committee Member ; Lieuwen, Tim, Committee Member ; Menon, Suresh, Committee Chair ; Seitzman, Jerry, Committee Member ; Syed, Saadat, Committee Member.

Ce travail de thèse, menée dans le cadre d'une convention CIFRE entre TURBOMECA et le CERFACS, s'inscrit dans un contexte d'amélioration des performances des turbines de type axial équipant les turboréacteurs d'hélicoptère. L'une des principales difficultés rencontrée dans cette démarche concerne la maîtrise de la température que voient les pales de ce composant, notamment la roue haute pression. Les travaux de cette thèse s'articulent autour de deux axes principaux: - Le premier traite l'analyse de la Simulations aux Grandes Echelles (SGE) autour de pales. Une approche numérique SGE sur des maillages non-structurés est comparée aux résultats Reynolds Averaged Navier-Stokes (RANS) sur des maillages structurés, usuels dans ce type de configuration, ainsi qu'à une approche SGE sur maillages structurés. La SGE sur maillage non-structuré démontre sa capacité à prendre en compte les phénomènes qui ont un impact sur les flux de chaleur pariétaux. - Le second axe de recherche a pour objectif de développer un outil numérique de couplage pour assurer le transfert d'information entre un code SGE réactif sur maillage non-structuré, employé dans les chambres de combustion, et un code non-réactif en RANS, utilisé par les industriels pour modéliser l'étage turbine. Cet outil a été validé sur plusieurs cas tests qui montrent le potentiel de cette méthodologie pour le couplage multi-composant
This PhD dissertation, conducted as part of a CIFRE research project between TURBOMECA and CERFACS, deals with improving performance of axial turbines from helicopter engines. One of the main difficulties with such an objective is the control of the temperature prediction around the blades, especially the temperature of the high pressure rotor. The work of this thesis focusses on two axes: - First concerns the analysis of Large Eddy Simulation (LES) predictions around blades: a numerical LES approach on unstructured meshes is compared to Reynolds Averaged Navier-Stokes (RANS) results on structured meshes as well as to LES on structured meshes. LES on unstructured meshes demonstrates its capacity of taking into account the phenomena which have an impact on wall heat flux around blades. - The second axis deals with the development of a numerical tool for coupling and transferring information between a reactive LES code, used in combustion chambers, and a non-reactive RANS solver, employed by industrial actors for modeling the turbine stage. This tool is validated on a number of test cases which show the potential of this methodology for multi-component predictions

The flow produced by a rotating propeller is inherently unsteady and three-dimensional. Conventional design of propellers uses blade-element theory but becomes inaccurate in capturing three-dimensional vortical and compressible effects at the tips, as well as the effect on downstream bodies. A propeller is always attached to a fixed component that affects its performance, thus the need arises to couple a fixed domain to a rotating domain in an unsteady aerodynamic simulation. A finite element formulation for the simulation of propellers is presented in terms of the Reynolds-averaged Navier-Stokes equations for unsteady, three-dimensional, viscous, compressible flows. The first step consists of preparing a mesh containing two separate domains interfacing at a virtual surface. Then, simulation is run to obtain an initial solution. This step highlights the live/dead interfacing scheme between the fixed and rotating domains without mesh movement. Finally the unsteady simulation performs interpolations at each time step with node movement until a periodic steady state is reached. Mesh movement can be treated by either an ALE formulation or a rotating frame of reference correction. Two test cases are used to validate the code: a two-dimensional pitching airfoil in transonic flow and a 3-bladed 5868-9 propeller with a liquid cooled nacelle.

In many areas of aerodynamics the technique of Large Eddy Simulation (LES) has proved a practical way of modelling the unsteady phenomena in numerical simulations. Few applications are as dependent on such an approach as the prediction of flow within a gas turbine combustor. Like any form of Computational Fluid Dynamics (CFD), LES requires specification of the velocity field at the inflow boundary, with much evidence suggesting the specification of inlet turbulence can be critical to the resultant accuracy of the prediction. While a database of time-resolved velocity data may be obtained from a precursor LES calculation, this technique is prohibitively expensive for complex geometries. An alternative is to use synthetic inlet conditions obtained from experimental data High-speed Particle Image Velocimetry (PIV) is used here to provide planar velocity data at up to 1kHz temporal resolution in two test cases representative of gas turbine combustor flows (a vortex generator in a duct and an idealised combustor). As the data sampling rate is approaching a typical LES time-step it introduces the possibility of applying instantaneous experimental data directly as an inlet condition. However, as typical solution domain inlet regions for gas turbine combustor geometries cannot be adequately captured in a single field of PIV data, it is necessary to consider a method by which a synchronous velocity field may be obtained from multiple PIV fields that were not captured concurrently. A method is proposed that attempts to achieve this by a combined process of Linear Stochastic Estimation and high-pass filtering. The method developed can be generally applied without a priori assumptions of the flow and is demonstrated to produce a velocity field that matches very closely that of the original PIV, with no discontinuities in the velocity correlations. The fidelity and computational cost of the method compares favourably to several existing inlet condition generation methods. Finally, the proposed and existing methods for synthetic inlet condition generation are applied to LES predictions of the two test cases. There is shown to be significant differences in the resulting flow, with the proposed method showing a marked ii reduction in the adjustment period that is required to establish turbulent equilibrium downstream of the inlet. However, it is noted the presence of downstream turbulence generating features can mask any differences in the inlet condition, to the extent that the flow in the core of the combustor test case is found to be insensitive to the inlet condition applied at the entry to the feed annulus for the test conditions applied here.

For high-altitude cruising unmanned aerial vehicles (UAVs), the aero-engine components operate at low Reynolds number condition, which has a significant impact on the running of the engines and the biggest negative effects are on low pressure turbines (LPTs). An in-depth understanding of blade boundary layer spatio-temporal evolution is crucial for the effective management and control of boundary layer transiton or separation, especially the open separation, which is a key technology for the design of low Reynolds number LPT. Focusing on the blade boundary layer spatio-temporal evolution of LPT under unsteady environments, a series of research works were conducted through large-eddy simulation (LES), during my Ph.D. study.Under low Reynolds number conditions, the complex flow phenomena on LPT blade surface make conventional Reynolds-averaged Navier-Stokes (RANS) method is difficult to meet the requirements of mechanism study. As a compromise between RANS and direct numerical simulation (DNS), LES is thought suitable for dealing with this problem. Then a multi-block parallel LES code was developed, which possesses the following features: the governing equations are compressible Navier-Stokes equations and the subgrid-scale (SGS) model is dynamic Smagorinsky model. The finite volume method was used to discretize the equations, the convective terms are fourth order skew-symmetric-like centered schemes, to remove the spurious odd-even oscillations, artifical viscosity terms were added to the equations or explicit filtering operations were used, viscous terms are second order centered scheme and time integration is third-order three-stage compact Runge-Kutta method. The code can deal with arbitrary multi-block grid with matching interfaces, which has also the ability of high-performance parallel computing through domain decomposition and message processing interface (MPI). Inflow boundary conditions for free-stream turbulence, periodic wakes are provided in the code. Numerical tests indicate that the new code is of high order accuracy and able to deal flow problems with complex geometry or physical boundary conditions, so it is suitable for the applications of complex flow phenomena in turbomachinery.Fully developed turbulent channel flow and sub-critical flow around circular cylinder were used to validate the new code. Through changing calculation parameters, a wide range of tests were conducted. Test results indicate that, to ensure the stability of the calculation, the isotropic parts of SGS stress tensor should be set to zero, and values of artificial viscosity would influence the numerical results obviously, which should be adjusted according to the flow conditions of certain problems.A LPT cascade flow was simulated under conditions of Reynolds number 60154 and Mach number 0.402. Referring the experiment data available, computations for four cases with different inflow boundary conditions were carried out, they are C1 – steady inflow, C2 – steady inflow with background turbulence, C3 – periodic wakes inflow and C4 – periodic wakes inflow with background turbulence. For steady inflow cases C1 and C2, numerical results indicated that, large separation regions all appeared in the suction side rear part of the blade, the scale of separation region of C1 was bigger than C2. The transition in laminar separated shear flows of C1 and C2 were all dominated by Kelvin-Helmholtz (K-H) instability, for case C2, the background turbulence promoted the destabilization and transition process of separated shear layer, so a smaller separation region appeared. For periodic wake inflow cases C3 and C4, numerical results indicated that, because of the high passing frequency and high intensity of the wakes, flow phenomena in cascade were dominated by the effects of periodically sweeping wakes and, in contrast with case C2, the effects of background turbulence were small, so the results of C3 and C4 are similar. Under the sweeping of periodic wakes, large separation regions were replaced by small scale separation bubbles, and the total pressure loss of the cascade significantly decreased. K-H instability and turbulent spots are all effective factors in the transition process, the turbulent spots may appear before the separation point, or appear in the separated free shear layer, and the structure of the spots looks like a series of vortex loops.

Dissertation (Ph.D. in Aeronautical and Astronautical Engineeirng) Naval Postgraduate School, Sept. 2001.
Dissertation supervisor: Platzer, Max F. "September 2001." Includes bibliographical references (p. 121-126). Also Available online.

Solidification during horizontal continuous casting of low carbon steel billets with cyclic withdrawal was simulated and the wavy profile of the solidifying shell characteristic of this process was reproduced. Effects of rate of withdrawal cycle, superheat and casting speed were determined. In order to carry out this simulation in a personal computer, efficient numerical techniques had to be developed for mesh refinement by coordinate transformation, interfaces with temperature discontinuities and re-entrant corners. A flexible means of mesh generation involving polynomials was also developed. From the transient heat transfer model finite difference equations peculiar to each gridpoint in the solution field were derived and solved by the Alternating Direction Implicit (ADI) method. Graphics software were developed to view the results with 3-D as well as contour plots. The heat transfer model was verified with published results of vertical continuous casting of Mg alloys and steel. Due to its ability to deal with interfaces, unlike previous work, the present model could solve temperature at both casting and mold simultaneously. A model for the shell growth, rupture and healing at the break-ring of horizontal continuous casting molds was incorporated into the heat transfer model. An interesting result of this simulation was the presence of transient hot spots in the hot face of the mold. Elimination of such hot spots should aid shell strength and hence the casting rate. A semi-quantitative dependence of the depth of the primary witness mark on cycle rate was also established.

Les phénomènes instationnaires dans les cavités rotor/stator sont connus pour être la source de dangereuses vibrations dans les turbopompes spatiales. Bien que plusieurs mesures palliatives aient été prises en comptes durant les phases de conception, des campagnes d’essais ont mis en évidence de fortes oscillations des écoulements internes pouvant menacer le moteur cryogénique des lanceurs. Aujourd’hui, l’origine de ce phénomène, appelé «bandes de pression », est peu compris et difficile à prédire numériquement. L’objectif de cette thèse est d’analyser les mécanismes physiques de ce phénomène afin d’apporter des solutions pour le contrôler. Pour répondre à cette problématique, deux types de configuration sont étudiés: une cavité rotor/stator annulaire et une cavité de turbopompe spatiale. Les couches limites tournantes dans ces cavités sont connues pour être 3D et réceptives à plusieurs types d’instabilités prenant entre autre la forme de spirales ou d’anneaux. Les simulations basées sur la moyenne de Reynolds (RANS) ont par le passé échoué à prédire ce type d’écoulement. Cependant, les Simulations aux Grandes Echelles (SGE) se sont avérées être une alternative à ce problème et sont donc été utilisées tout au long de cette thèse. Des Densités Spectrales de Puissance (DSP) ainsi que des Décompositions modales dynamiques (DMD) appliqués aux résultats SGE, ont permis de montrer que le phénomène de bandes de pression est visible également dans une cavité annulaire de type académique et composé de trois modes dictant toute la dynamique du système. Afin d’étudier les interactions de ces modes, une nouvelle méthode appelée Dynamic Mode Tracking/Control (DMT/DMTC) a été proposée durant cette these. La DMT est construite pour extraire des structures cohérentes dans une simulation SGE. De plus, en ajoutant un terme de relaxation dans les équations de Navier-Stokes couplées avec la DMT, sa variante appelée DMTC permet de contrôler et de suivre en temps réelle un ou plusieurs modes et donc de pouvoir analyser de possibles interactions. Appliqué à la cavité académique annulaire, cette méthode a permis de montrer que le mode basse fréquence est généré dans l’écoulement par le mode dominant du système. Pour aller plus loin, des analyses de stabilité linéaire de type global (GLSA), sont effectuées sur la cavité académique. Grâce à des méthodes adjointes, la GLSA a permis de mettre en avant l’origine spatiale de chacun des trois modes. Deplus, afin de mettre en place des stratégies de contrôle, la sensibilité à la modification de l’écoulement de base, obtenue par GLSA, a permis d’identifier la région à modifier pour stabiliser un mode donné ou décaler sa fréquence. Appliqué au cas académique, il est montré et que contrôler la couche limite du stator est le moyen le plus efficace de supprimer le phénomène de bandes de pression à travers des injections/aspirations. Pour finir, le phénomène de bandes de pression est analysé dans une cavité de turbopompe spatiale. En particulier, la sensibilité de ce phénomène aux changements géométriques est abordée à travers deux configurations: une première sans les aubes du stator de la turbopompe et une deuxième avec. Bien que les aubes génèrent un écoulement complexe, des fréquences similaires de fluctuation de pression sont retrouvées dans les deux configurations avec cependant des nombres azimutaux caractéristiques différents. En se basant sur les études faites sur la cavité académique, une version adaptée de GLSA pour la dynamique de la turbopompe permet de mettre en avant que malgré que le phénomène de bandes pression soit particulièrement présent dans la veine de la turbopompe, la source de ces modes se situe dans les cavités inférieures entre le rotor et le stator. De plus les résultats de GLSA mettent en avant que deux moyens de contrôle pourraient être appliqués pour supprimer le phénomène de bandes de pression dans ce cas industriel: modifier le joint d’étanchéité ou modifier la fuite du moyeu
Unsteady phenomena in rotor/stator cavity are well known to be the source of dangerous vibrations in space turbopump. Even though many palliative measures have been taken during their design, experimental campaigns often reveal high flow oscillations that can jeopardize turbomachinery components and even the rocket engine. Today, the origin of such flow instabilities usually called ’pressure band phenomenon’(PBP) is not well understood and difficult to predict numerically. The main goal of this thesis is to investigate such phenomenon mechanism to find technical solutions so as to control it. This problematic is addressed here trough two types of configuration: an academic rotor/stator cavity and a space turbopump cavity. When it comes to cavity flows, their rotating boundary layers are known to be three dimensional and receptive to several instabilities taking the form of spirals or annuli. Reynolds Averaged Navier-Stokes Simulations (RANS)failed to predict such unsteady systems. However, Large Eddy Simulation (LES) proved to be a relevant alternative in many similar applications and is therefore chosen for the present work. Using Power Spectral Analysis (PSD) and Dynamic Mode Decomposition (DMD) on LES predictions, one shows that the PBP is retrieved in an annular smooth rotor/stator cavity and it is composed of three modes driving all the system dynamics. To investigate these mode organization and their possible interactions, a new tool called Dynamic Mode Tracking /Control (DMT/DMTC) is introduced. DMT is constructed so as to extract "on-thefly" flow coherent structures with a given frequency on the basis of LES. Furthermore, augmenting the Navier-Stokes equations with a relaxation term coupled to DMT, DMTC allows to control and follow the evolution of a controlled mode as well as non controlled ones and thereby observe interactions. This strategy after validation is applied to the annular rotating cavity and shows that the low frequency mode is generated by the dominant mode of the system. To go further, Global Linear Stability Analysis (GLSA) augmented with adjoint methods is used to shed some light on all mode origins and points out that the low frequency and dominant modes are coming from the stationary boundary layer. In order to set up control strategies, the GLSA framework is further developed introducing the concept of the sensitivity to base flow modifications which gives the location where the flow should be modified if one wants to stabilize or at least shift a frequency mode. Applied to the academic cavity, one shows that contrary to most studies in the literature, controlling the stator boundary layer is the more efficient way to damp the PBP through suction/injection devices. Finally, gathering all the previous understanding of this flow, the LES framework enables to validate the control strategies proposed and to stabilize the PBP for very low suction amplitudes. To finish, the PBP is analyzed in real space turbompump cavities. In particular, the sensitivity of this specific phenomenon to geometry changes is investigated through two configurations: one without and one with the blades of the stator of the turbopump. Even though the introduction of the blades in the LES creates a more complex flow with the presence of shocks, similar pressure fluctuation spectra are retrieved in both configurations but with azimuthal wavenumber modes that are shifted. Following the studies on the academic cavity, an adapted GLSA to the non-linear dynamics of the turbopump enables to point out that even though the PBP modes are particularly marked in the mainstream of the system, the source of these modes is located in the subcavity in the rotor-stator wheel space. In particular, GLSA results indicate that two possible ways to control the phenomenon are possible: modifying the flow around the seal rim and or modifying the leak around the hub

A general numerical simulation of closely coupled lifting surfaces in steady and unsteady ground effects was developed. This model was coupled with the equations of motion to simulate aerodynamic-dynamic interaction. The resulting model was then coupled with a feedback-control law to form a general nonlinear unsteady numerical simulation of control of an aircraft in and out of ground effect. The aerodynamic model is based on the general unsteady vortex-lattice method and the method of images. It is not restricted by planform, angle of attack, sink rate, dihedral angle, twist, camber, etc. as long as stall or vortex bursting does not occur. In addition, it has the versatility to model steady and unsteady aerodynamic interference. The present model can be used to simulate any prescribed flare and to model the effects of cross and/or head winds near the ground. The present results show the influences of various parameters on the aerodynamic coefficients for both steady and unsteady flows. Generally, the ground increases the aerodynamic coefficients; the greater the sink rates, the stronger the effects. Increasing the aspect ratio increases both the steady and unsteady ground effects. An exception is a large aspect-ratio wing with large camber. The present results are generally in close agreement with limited exact solutions and experimental data. In the aerodynamic-dynamic simulation, the equations of motion were solved by Hammlng's predictor-corrector method. The aircraft, air stream, and control surfaces were treated as a single dynamic system. The entire set of governing equations was solved simultaneously and interactively. The aerodynamic-dynamic model was used to study a configuration that resembles a Cessna 182 airplane. The ground lowers the effectiveness of the tail in controlling pitch, increases the lift and drag, and makes the hinge-moment less negative. Proportional and rate control laws were used in a feedback system to control pitch. One set of gains was used in and out of ground effect. For the same control input, the pitch angle responds faster and overshoots more near the ground than it does far from the ground. The present results demonstrate the feasibility of using the current simulation to model more complicated motions and the Importance of including the unsteady ground effects when analyzing the performance of an airplane during a landing maneuver.
Ph. D.

The unsteady aerodynamic and aeroelastic analysis of flapping flight under various kinematics and flow parameters is presented in this dissertation. The main motivation for this study arises from the challenges facing the development of micro air vehicles. Micro air vehicles by requirement are compact with dimensions less than 15-20 cm and flight speeds of around 10-15 m/s. These vehicles operate in low Reynolds number range of 10,000 to 100,000. At these low Reynolds numbers, the aerodynamic efficiency of conventional fixed airfoils significantly deteriorates. On the other hand, flapping flight employed by birds and insects whose flight regime coincides with that of micro air vehicles offers a viable alternate solution. For the analysis of flapping flight, a boundary fitted moving grid algorithm is implemented in a flow solver, GenIDLEST. The dynamic movement of the grid is achieved using a combination of spring analogy and trans-finite interpolation on displacements. The additional conservation equation of space required for moving grid is satisfied. The solver is validated with well known flow problems such as forced oscillation of a cylinder, a heaving airfoil, a moving indentation channel, and a hovering fruitfly. The performance of flapping flight is analyzed using Large Eddy Simulation (LES) for a wide range of Reynolds numbers and under various kinematic parameters. A spiral Leading Edge Vortex (LEV) forms during the downstroke due to the high angle of attack, which results in high force production. A strong spanwise flow of the order of the flapping velocity is observed along the core of the LEV. In addition, the formation of a negative spanwise flow is observed due to the tip vortex, which slows down the removal of vorticity from the LEV. This leads to the instability of the LEV at around mid-downstroke. Analysis with different rotation kinematics shows that a continuous rotation results in better propulsive efficiency as it generates thrust during the entire flapping cycle. Analysis with different angles of attack shows that a moderate angle of attack which results in complete shedding of the LEV offers high propulsive efficiency. The analysis of flapping flight at Reynolds numbers ranging from 100 to 100,000 shows that higher lift and thrust values are obtained for Re?100. The critical reasons are that at higher Reynolds numbers, the LEV is closer to the surface and as it sheds and convects it covers most of the upper surface. However, the Reynolds number has no or little effect on the lift and thrust as identical values are obtained for Re=10,000 and 100,000. The analysis with different tip shapes shows that tip shapes do not have a significant effect on the performance. Introduction of stroke deviation to kinematics leads to drop in average lift as wing interacts with the LEV shed during the downstroke. A linear elastic membrane model with applied aerodynamic load is developed for aeroelastic analysis. Analysis with different wing stiffnesses shows that the membrane wing outperforms the rigid wing in terms of lift, thrust and propulsive efficiency. The main reason for the increase in force production is attributed to the gliding of the LEV along the camber, which results in a high pressure difference across the surface. In addition, a high stiffness along the spanwise direction and low stiffness along the chordwise direction results in a uniform camber and high lift and thrust production.
Ph. D.

Two-dimensional flow in a viscous incompressible fluid, generated by a circular cylinder executing large-amplitude rectilinear oscillations in a plane perpendicular to its axis and parallel to one of the sides of a surrounding rectangular box filled with incompressible fluid is studied numerically. The circular cylinder moves back and forth through its own wake, resulting in an extremely complex flow field. For ease of implementing boundary conditions, a numerically generated body-fitted coordinate system is used. At each time step, the physical domain is doubly-connected, and a cut is introduced in order to map it into a rectangular computational domain. A body-fitted grid is generated by solving a pair of Laplace equations with a simple grid spacing control method which preserves the essential one-to-one property of the mapping. A finite difference/pseudo-spectral technique is used in this work to solve the Navier-Stokes equations in velocity-vorticity formulation. The time integration of the vorticity transport equation is handled by a fully explicit three-level Adams-Bashforth method. The two Poisson equations for the velocity components are 11-banded and block-diagonal in form, and are solved by a preconditioned biconjugate gradient routine. An integral constraint on the vorticity field is used to determine the boundary vorticity that simultaneously satisfies the no-slip and no-penetration conditions. The surface vorticity is uniquely determined by a general solution procedure developed in this study which is valid for flows over multiple solid bodies. With this approach, the physical process of vorticity generation on the solid boundary is properly simulated and the principle of vorticity conservation is satisfied. Results for various test cases and the complex vortex shedding phenomena generated by an oscillating circular cylinder are presented and discussed.

L’urbanisation croissante fait émerger des enjeux sociétaux et environnementaux relatifs à la pollution atmosphérique et au microclimat urbain. La compréhension des phénomènes physiques de transport de quantité de mouvement, de chaleur et de masse entre la canopée urbaine et la couche limite atmosphérique est primordiale pour évaluer et anticiper les impacts négatifs de l’urbanisation. Les processus turbulents spécifiques à la couche limite urbaine sont étudiés par une approche de simulation des grandes échelles, dans une configuration urbaine représentée par un arrangement de cubes en quinconce. Le modèle de sous-maille de type Smagorinsky dynamique est implémenté pour mieux prendre en compte l’hétérogénéité de l’écoulement et les retours d’énergie des petites vers les grandes structures. Le nombre de Reynolds basé sur la hauteur du domaine et la vitesse de l’écoulement libre est de 50000. L’écoulement est résolu dans les sous-couches visqueuses et le maillage est raffiné dans la canopée. Le domaine est composé de 28 millions de cellules. Les résultats sont comparés à la littérature et aux données récentes obtenues dans la soufflerie du LHEEA. Chaque contribution au bilan d’énergie cinétique turbulente est calculée directement en tout point. Cette information, rare dans la littérature, permet d’étudier les processus dans la sous couche rugueuse. Grâce à ces résultats 3D, l’organisation complexe de l’écoulement moyen (recirculations, vorticité, points singuliers) est analysée en relation avec la production de turbulence. Enfin, une simulation où les obstacles sont remplacés par une force de traînée équivalente est réalisée à des fins d’évaluation de cette approche
The rapid development of urbanization raises social and environmental challenges related to air pollution and urban climate. Understanding the physical processes of momentum, heat, and mass exchanges between the urban canopy and the atmospheric boundary-layer is a key to assess,predict and prevent negative impacts of urbanization. The turbulent processes occurring in the urban boundary-layer are investigated using computational fluid dynamics (CFD). The unsteady flow over an urban-like canopy modelled by a staggered arrangement of cubes is simulated using large eddy simulation (LES). Considering the highspatial and temporal in homogeneity of the flow, a dynamic Smagorinsky subgrid-scale model is implemented in the code to allow energyback scatter from small to large scales. The Reynolds number based on the domain height and free-stream velocity is 50000. The near-wall viscous sub-layers are resolved and the grid is refined in the canopy resulting in about 28 million grid cells. LES results are assessed by comparison with literature and data recently acquired in the wind tunnel of the LHEEA. The turbulent kinetic energy budget in which all contributions are independently computed is investigated. These rarely available data are used to analyse the turbulent processes in the urban canopy. By taking advantage of the three-dimensionality of the simulated flow, the complex 3D time-averaged organization of the flow (recirculation, vorticesor singular points) is analyzed in relation with production of turbulence. Finally a drag approach where obstacles are replaced by an equivalent drag force is implemented in the same domain and results are compared to obstacle-resolved data

In internal combustion engine (ICE), researchers have to face with stringent environment regulations concerning pollutants while improving engine thermal efficiency, making the engine design a complex task. To meet these requirements, an understanding of the salient features of all the engine processes are very important. Being the primitive process of engine operations, fuel injection influences whole engine cycle via fuel-air mixture preparation, thereby the combustion behavior and subsequently the emission performance. The inhospitable environment inside a combustion chamber makes the experimental investigations more complex and expensive. In contrast, a CFD based investigation can provide comprehensive insight about in-cylinder flow field, spray injection phenomena as encountered in IC-engine. In the present study, a CFD tool that enables to investigate the real unsteady behavior of realistic engine configuration is developed by coupling Large Eddy Simulation (LES) together with a spray module using the KIVA4-mpi Code. It is based on an Eulerian-Lagrangian framework to describe the spray evolution including primary and secondary atomization. A linear instability sheet atomization (LISA) based sub-model is integrated to represent the primary atomization. The secondary atomization is modeled by an available Taylor analogy break-up (TAB) model. In dense spray region, the droplet-droplet interaction considerably influences the overall spray dynamics. The first novelty of the proposed methodology is to include droplet-droplet interaction processes via an appropriate collision sub-model that is independent of mesh size and type. Thereby, taking account of different regimes, such as bouncing, separation, stretching separation, reflective separation and coalescence. The formation of wall film on hot cylinder surface is a critical process in an IC-engine, since it largely influences the engine performance and emission characteristics. The second novelty of this spray module is the implementation of an improved wall film model that includes the combined effects of droplet kinetic energy and wall temperature into KIVA4-mpi code. To perform an IC-engine simulation, a good quality mesh generation in ICEM-CFD for an engine geometry is challenging task. The KIVA4-mpi is compatible only with block structured mesh without any use of O-grid. Due to this reason, only certain degree of mesh refinement is possible. This makes it difficult to achieve a good quality fine mesh required for LES simulation. In the present study, a new meshing strategy is proposed to generate suitable mesh for real IC-engine configurations. The new method clearly demonstrates the improvement in resolving the in-cylinder flow structures. First, the simulated results for motored case (no fuel injection and no combustion) are compared with the experimental data for a transparent combustion chamber (TCC) engine configuration from Engine Combustion Network (ECN). Second, to demonstrate the importance of fuel injection sub-models, further simulations are carried out including the evolution of evaporating fuel spray with wall impingement. Third, using the new meshing strategy, simulations are also performed for a real complex canted 4-valve engine configuration. Simulated results are compared well with available experimental data.

Les aspirateurs de turbines hydrauliques jouent un rôle crucial dans l’extraction de l’énergie disponible. Dans ce projet, les écoulements dans l’aspirateur d’une turbine de basse chute ont été simulés à l'aide de différents modèles de turbulence dont le modèle DDES, un hybride LES/RANS, qui permet de résoudre une partie du spectre turbulent. Déterminer des conditions aux limites pour ce modèle à l’entrée de l’aspirateur est un défi. Des profils d’entrée 1D axisymétriques et 2D instationnaires tenant compte des sillages et vortex induits par les aubes de la roue ont notamment été testés. Une fluctuation artificielle a également été imposée, afin d’imiter la turbulence qui existe juste après la roue. Les simulations ont été effectuées pour deux configurations d’aspirateur du projet BulbT. Pour la deuxième, plusieurs comparaisons avec des données expérimentales ont été faites pour deux conditions d'opération, à charge partielle et dans la zone de baisse rapide du rendement après le point de meilleur rendement. Cela a permis d’évaluer l'efficacité et les lacunes de la modélisation turbulente et des conditions limites à travers leurs effets sur les quantités globales et locales. Les résultats ont montrés que les structures tourbillonnaires et sillages sortant de la roue sont adéquatement résolus par les simulations DDES de l’aspirateur, en appliquant les profils instationnaires bidimensionnels et un schéma de faible dissipation pour le terme convectif. En outre, les effets de la turbulence artificielle à l'entrée de l’aspirateur ont été explorés à l'aide de l’estimation de l’intermittence du décollement, de corrélations en deux points, du spectre d'énergie et du concept de structures cohérentes lagrangiennes. Ces analyses ont montré que les détails de la dynamique de l'écoulement et de la séparation sont modifiés, ainsi que les patrons des lignes de transport à divers endroits de l’aspirateur. Cependant, les quantités globales comme le coefficient de récupération de l’aspirateur ne sont pas influencées par ces spécificités locales.
Draft tubes play a crucial role in elevating the available energy extraction of hydroturbines. In this project, turbulent flows in the draft tube of a low-head bulb turbine were simulated using, among others, an advance hybrid LES/RANS turbulent model, called DDES, which can resolve portions of the turbulent spectrum. Providing appropriate inflow boundary conditions for such models is a challenging issue. In this regard, different inflow boundary conditions were tested, including axisymmetric 1D profiles, and unsteady 2D inflow profiles that take runner blade wakes and vortices into account. Artificial fluctuation at the inlet section of the draft tube was also included to mimic the turbulence existing after the runner. Simulations were conducted for two draft tube configurations of the BulbT project. For one of them, intensive comparisons with experimental data were done for two operating conditions, one at part load and another in the sharp drop-off portion of the efficiency hill after the best efficiency point. This allowed to assess the effectiveness and shortcomings of the adopted turbulence modeling and boundary conditions through their effects on the global and local quantities. The results showed that the runner-related vortical structures and wakes are appropriately resolved using stand-alone DDES simulation of the draft tube flows. This is achieved by applying unsteady 2D inflow profiles along with adopting low dissipation scheme for the convective term. Furthermore, the effects of applying artificial turbulence at inlet were explored using separation intermittency, two-point correlation, energy spectrum and Lagrangian coherent structure concepts. These analyses revealed that the type of inflow boundary conditions modifies the details of the flow and separation dynamics as well as patterns of the transport barriers in different regions of the draft tube. However, the global quantities such as recovery coefficient are not influenced by these local features.

Dans l'industrie aéronautique, la génération d'énergie dépend presque exclusivement de la combustion d'hydrocarbures. La meilleure façon d'améliorer le rendement de ces systèmes et de contrôler leur impact environnemental, est d'optimiser le processus de combustion. Avec la croissance continue du de la puissance des calculateurs, la simulation des systèmes complexes est devenue abordable. Jusqu'à très récemment dans les applications industrielles le rayonnement des gaz et la conduction de chaleur dans les solides ont été négligés. Dans ce travail les outils nécessaires à la résolution couplée des trois modes de transfert de chaleur ont été développés et ont été utilisés pour l'étude d'une chambre de combustion d'hélicoptère. On montre que l'inclusion de tous les modes de transfert de chaleur peut influencer la distribution de température dans le domaine. Les outils numériques et la méthodologie de couplage développés ouvrent maintenant la voie à un bon nombre d'applications tant scientifiques que technologiques
In the aeronautical industry, energy generation relies almost exclusively in the combustion of hydrocarbons. The best way to improve the efficiency of such systems, while controlling their environmental impact, is to optimize the combustion process. With the continuous rise of computational power, simulations of complex combustion systems have become feasible, but until recently in industrial applications radiation and heat conduction were neglected. In the present work the numerical tools necessary for the coupled resolution of the three heat transfer modes have been developed and applied to the study of an helicopter combustion chamber. It is shown that the inclusion of all heat transfer modes can influence the temperature repartition in the domain. The numerical tools and the coupling methodology developed are now opening the way to a good number of scientific and engineering applications

Flows over hydraulically smooth walls are predominant in turbulence studies whereas real surfaces in engineering applications are often rough. This is important because turbulent flows close to the two types of surface can exhibit large differences. Unfortunately, neither experimental studies nor theoretical studies based on conventional computational fluid dynamics (CFD) can give sufficiently accurate, detailed information about unsteady turbulent flow behaviour close to solid surfaces, even for smooth wall cases. In this thesis, therefore, use is made of a state of the art computational method “Direct Numerical Simulation (DNS)” to investigate the unsteady flows. An “in-house” DNS computer code is developed for the study reported in this thesis. Spatial discretization in the code is achieved using a second order, finite difference method. The semi-implicit (Runge-Kutta & Crank-Nicholson) time advancement is incorporated into the fractional-step method. A Fast Fourier Transform solver is used for solving the Poisson equation. An efficient immersed Boundary Method (IBM) is used for treating the roughness. The code is parallelized using a Message Passing Interface (MPI) and it is adopted for use on a distributed-memory computer cluster at University of Aberdeen as well as for use at the UK’s national high-performance computing service, HECToR. As one of the first DNS of accelerating/decelerating flows over smooth and rough walls, the study has produced detailed new information on turbulence behaviours which can be used for turbulence model development and validations. The detailed data have enabled better understanding of the flow physics to be developed. The results revealed strong non-equilibrium and anisotropic behaviours of turbulence dynamics in such flows. The preliminary results on the rough wall flow show the response of turbulence in the core and wall regions, and the relationship between the axial and the other components are significantly different from those in smooth wall flows.

This study presents a Non-Linear Frequency Domain (NLFD) method for the simulation of periodic unsteady flows for overset meshes. The main objective is to allow the simulation of unsteady flows for geometries that are too complex for traditional meshing techniques. The current research describes in detail both the formulation of the governing equations and the framework allowing overlapping meshes with the NLFD technique. The approach is firstly validated through the simulation of a vortex advection. Secondly, this work presents the steady flow simulation for a NACA-0012 airfoil in transonic flow. Lastly, pitching, forced flutter and flapping motions are presented to assess of the validity of the framework for complex unsteady flows.
Cette étude présente une méthode basée sur le domaine des fréquences non-linéaires (NLFD) permettant de simuler des écoulements périodiques instationnaires à l'aide de maillages superposés. L'objectif principal est de permettre la simulation d'écoulements instationnaires qui seraient trop complexes pour des techniques de maillage traditionnelles. Cette recherche décrit en détails à la fois la formulation des équations fondamentales de mécanique des fluides et le cadre de travail permettant l'utilisation des maillages superposés dans le domaine des fréquences non-linéaires. L'approche est tout d'abord validée à travers l'étude de l'advection d'un vortex. Ensuite, ce travail présente la simulation d'un écoulement transsonique stationnaire impliquant un profil d'aile de type NACA-0012. Finalement, des mouvements de variation de l'angle d'incidence, de flottement forcé et de battement d'aile sont présentés pour témoigner de la validité du cadre de travail pour la simulation d'écoulements complexes non-stationnaires.

Thesis (Ph. D.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2005.
Yoganathan, Ajit, Committee Member ; Sturm, Terry, Committee Member ; Webster, Donald, Committee Member ; Roberts, Philip, Committee Member ; Sotiropoulos, Fotis, Committee Chair ; Fritz, Hermann, Committee Member. Includes bibliographical references.

Wind energy industry thrived in the last three decades, environmental concerns and government regulations stimulate studies on wind farm location selection and wind turbine design. Full-scale experiments and high-fidelity simulations are restrictive due to the prohibitively high cost, while the model-scale experiments and low-fidelity calculations miss key flow physics of unsteady high Reynolds number flows. A hybrid RANS/LES turbulence model integrated with transition formulation is developed and tested by a surrogate model problem through joint experimental and computational fluid dynamics approaches. The model problem consists of a circular cylinder for generating coherent unsteadiness and a downstream airfoil in the cylinder wake. The cylinder flow is subcritical, with a Reynolds number of 64,000 based upon the cylinder diameter. The quantitative dynamics of vortex shedding and Reynolds stresses in the cylinder near wake were well captured, owing to the turbulence-resolving large eddy simulation method that was invoked in the wake. The power spectrum density of velocity components showed that the flow fluctuations were well-maintained in cylinder wake towards airfoil and the hybrid model switched between RANS/LES mode outside boundary layer as expected. According to the experimental and simulation results, the airfoil encountered local flow angle variations up to ±50 degrees, and the turbulent airfoil boundary layer remained attached. Inspecting the boundary layer profiles over one shedding cycle, the oscillation about mean profile resembled the Stokes layer with zero mean. Further processing the data through phase-averaging technique found phase lags along the chordwise locations and both the phase-averaged and mean profiles collapsed into the Law of Wall in the range of 0 < y+ < 50. The features of high blade loading fluctuations due to unsteadiness and transitional boundary layers are of interest in the aerodynamic studies of full-scale wind turbine blades, making the model problem a comprehensive benchmark case for future model development and validation.
Ph. D.

Le présent travail s'inscrit dans le cadre d'une convention CIFRE entre Liebherr-Aerospace Toulouse SAS et le Département d'Aérodynamique Energétique et Propulsion (DAEP) de l'ISAE. L’étude porte sur l'analyse des mécanismes responsables de la perte de la stabilité d'un étage de compression centrifuge. L'objectif industriel sous-jacent est l'élargissement de la plage de fonctionnement stable des compresseurs. Les travaux sont abordés par voie numérique à l'aide du code de calcul elsA, développé par l'ONERA et le CERFACS. Les simulations instationnaires sont effectuées sur la circonférence complète des roues.La topologie de l'écoulement est analysée dans un premier temps selon trois points de fonctionnement stables répartis sur la courbe caractéristique de l'iso-vitesse de design. Cette analyse permet de décrire l'évolution des phénomènes lorsque le débit est réduit. Au voisinage de la ligne de stabilité, l'excès d'incidence sur les aubes du rouet déclenche le décollement de la couche limite sur la face en d’expression. Le guide issu de la zone décollée migre en direction du carter et alimente l'écoulement de jeu. En conséquence, le déficit de vitesse au voisinage du carter s'intensifie et un lâcher périodique de structures tourbillonnaires apparaît à l'interface entre les écoulements secondaires et l'écoulement principal.La perte de la stabilité de l'étage s’établit suite à la naissance d'une perturbation de type « modal » au sein de l'espace lisse. Cette dernière induit de fortes distorsions circonférentielles dans l'étage de compression mais affecte plus particulièrement l'écoulement à l'entrée du rouet. Sur la moitié de la circonférence, l'interface entre les écoulements secondaires et l'écoulement principal ainsi que les structures tourbillonnaires sont déplacées en amont du front de grille. Le taux total-total du rouet chute de manière brutale et entraîne la perte de la stabilité de l'étage.Enfin, la dernière partie de ce travail est dédiée à la mise en place de critères adaptés au modèle stationnaire mono-canal utilisé chez LTS. Ces critères seront utilisés pour améliorer la prédiction de la ligne de pompage en phase de design. Dans un premier temps, la pertinence de l'utilisation du modèle stationnaire au voisinage de la ligne de stabilité est évaluée. Dans un second temps, les influences de la vitesse de rotation et de la géométrie de l’étage sur la topologie sont étudiées.Deux situations sont jugées critiques vis-à-vis de la stabilité. La première concerne l'alignement de l'interface entre l'écoulement de jeu et l'écoulement principal avec le front de grille. La deuxième concerne l'opération du compresseur avec un angle d'écoulement en sur-incidence sur les aubes du diffuseur qui s'établit sur toute la hauteur de veine
This work results from a CIFRE partnership between Liebherr-Aerospace Toulouse SAS and the Aerodynamics, Energetics and Propulsion Department (DAEP) of ISAE. The main objective is to investigate the mechanisms responsible of the stall onset in a centrifugal compressor operating at the nominal rotational speed. It is part of a larger work which aims at extending the stable operating range of compressors integrated in air conditioning system. The analyses are based on the results of unsteady simulations, in a calculation domain comprising all the blade passages. They are performed with the elsA software developed by ONERA and CERFACS.The investigations show the modifications of the unsteady flow pattern when the mass flow is reduced along the speed line. Near the stability limit, the high incidence angle on the impeller blade leads to a boundary layer separation on the suction side. The fluid in the separation zone moves toward the shroud and enlarges the low momentum flow zone generated by the leakage flow. The interface between the leakage flow and the main flow becomes unstable to the extent of a periodic vortex formation.The path to instability is driven by the growth of a small amplitude disturbance (modal wave) rotating in the vaneless space. The length scale of the wave is equal to the compressor circumference. This perturbation induces distortions and alters the flow characteristics in every location of this subsonic stage, and more specifically the impeller inlet flow structure: the unstable interface between the main flow and the leakage flow is periodically moved upstream of the leading edge plane causing a significant drop of the impeller total-to-total pressure ratio. The last part of this work concerns the definition of criteria which can improve the surge line prediction during the design process in an industrial environment. Therefore, they are adapted to the numerical steady model using the mixing plane approach. To do so, the capacity of the steady model to predict the flow structure when the compressor operates near stall is investigated. Then, the effects of the rotational speed and of the compressor geometry are evaluated. Theses two steps have permitted to define two critical situations regarding the stage stability. The first one is related to the alignment of the interface between the main flow and the leakage flow with the leading edge plane. The second one concerns the compressor operation with positive incidence on the diffuser vane, along the full span

The transport or the separation of solid particles or droplets suspended in a fluid flow is a common task in mechanical and process engineering. To improve machinery and physical processes (e.g. for coal combustion, reduction of NO_x and soot) an optimization of complex phenomena by simulation applying the fundamental conservation equations is required. Fluid-particle flows are characterized by the ratio of density of the two phases gamma=rho_P/rho_F, by the Stokes number St=tau_P/tau_F and by the loading in terms of void and mass fraction. Those numbers (Stokes number, gamma) define the flow regime and which relevant forces are acting on the particle. Dependent on the geometrical configuration the particle-wall interaction might have a heavy impact on the mean flow structure. The occurrence of particle-particle collisions becomes also more and more important with the increase of the local void fraction of the particulate phase. With increase of the particle loading the interaction with the fluid phase can not been neglected and 2-way or even 4-way coupling between the continous and disperse phases has to be taken into account. For dilute to moderate dense particle flows the Euler-Lagrange method is capable to resolve the main flow mechanism. An accurate computation needs unfortunately a high number of numerical particles (1,...,10^7) to get the reliable statistics for the underlying modelling correlations. Due to the fact that a Lagrangian algorithm cannot be vectorized for complex meshes the only way to finish those simulations in a reasonable time is the parallization applying the message passing paradigma. Frank et al. describes the basic ideas for a parallel Eulererian-Lagrangian solver, which uses multigrid for acceleration of the flow equations. The performance figures are quite good, though only steady problems are tackled. The presented paper is aimed to the numerical prediction of time-dependend fluid-particle flows using the simultanous particle tracking approach based on the Eulerian-Lagrangian and the particle-source-in-cell (PSI-Cell) approach. It is shown in the paper that for the unsteady flow prediction efficiency and load balancing of the parallel numerical simulation is an even more pronounced problem in comparison with the steady flow calculations, because the time steps for the time integration along one particle trajectory are very small per one time step of fluid flow integration and so the floating point workload on a single processor node is usualy rather low. Much time is spent for communication and waiting time of the processors, because for cold flow particle convection not very extensive calculations are necessary. One remedy might be a highspeed switch like Myrinet or Dolphin PCI/SCI (500 MByte/s), which could balance the relative high floating point performance of INTEL PIII processors and the weak capacity of the Fast-Ethernet communication network (100 Mbit/s) of the Chemnitz Linux Cluster (CLIC) used for the presented calculations. Corresponding to the discussed examples calculation times and parallel performance will be presented. Another point is the communication of many small packages, which should be summed up to bigger messages, because each message requires a startup time independently of its size. Summarising the potential of such a parallel algorithm, it will be shown that a Beowulf-type cluster computer is a highly competitve alternative to the classical main frame computer for the investigated Eulerian-Lagrangian simultanous particle tracking approach.

This diploma thesis deals with centrifugal pump running as a turbine. Basic working principles of a pump are included, both in pump and turbine regime. Experimental data obtained from laboratory test bed are compared with CFD simulation on slightly simplified geometry. Obtained results are then processed using spectrogram. Influence of time step and mesh size on results is also researched.

The study of the motion of manoeuvring aircraft has traditionally considered the aircraft to be rigid. This simplifying assumption has been shown to give quite accurate results for the flight dynamics of many aircraft types. As modern transport aircraft have developed however, there has been a marked increase in the size and weight of these aircraft. This trend is likely to continue with the development of future blended-wing-body and supersonic transport aircraft. This increase in size and weight has brought about a unique set of aeroelastic and handling quality issues. The aerodynamic forces and moments acting on an aeroplane have traditionally been represented using the aerodynamic derivative approach. It has been shown that this quasisteady aerodynamic model inadequately predicts the aircraft’s stability characteristics, and that the inclusion of unsteady aerodynamics “greatly improves the fidelity” of aircraft models. This thesis thus presents a novel numerical simulation of an aeroelastic aeroplane for real-time analysis. The model is built around the standard six degree-of-freedom equations of motion for a rigid aeroplane using the mean-axes system, and includes unsteady aerodynamics and structural dynamics. This is suitable for pilot-in-the-loop simulation, handling qualities and flight loads analysis, and control law development. The dynamics of the structure are modelled as a set of normal modes, and the equations of motion are realised in state-space form. The unsteady aerodynamic forces acting on the aeroplane are described by an indicial state-space model, including unsteady tailplane downwash and compressibility effects. An implementation of the model is presented in the MATLAB/ Simulink environment. The interaction between the flight control system, the aeroelastic system and the rigidbody motion of the aeroplane can result in degraded handling qualities, excessive actuator control, and fatigue problems. The introduction of load alleviation systems for the management of loads due to manoeuvres and gusts is also likely to result in the handling qualities of the aeroplane being degraded. This thesis presents a number of studies into the impact of structural dynamics, unsteady aerodynamics, and load alleviation on the handling qualities of a flexible civil transport aeroplane. The handling qualities of the aeroplane are assessed against a number of different handling qualities criteria and flying specifications, including the Neal-Smith, Bandwidth, and CAP criterion. It is shown that aeroelastic effects alter the longitudinal and lateral-directional characteristics of the aeroplane, resulting in degraded handling qualities. Manoeuvre and gust load alleviation are similarly found to degrade handling qualities, while active mode control is shown to offer the possibility of improved handling qualities.