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Cocconi, Giacomo. "Numerical investigation of turbulent/non-turbulent interface." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2013. http://amslaurea.unibo.it/5237/.
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The subject of this work is the diffusion of turbulence in a non-turbulent flow. Such phenomenon can be found in almost every practical case of turbulent flow: all types of shear flows (wakes, jet, boundary layers) present some boundary between turbulence and the non-turbulent surround; all transients from a laminar flow to turbulence must account for turbulent diffusion; mixing of flows often involve the injection of a turbulent solution in a non-turbulent fluid. The mechanism of what Phillips defined as “the erosion by turbulence of the underlying non-turbulent flow”, is called entrainment. It is usually considered to operate on two scales with different mechanics. The small scale nibbling, which is the entrainment of fluid by viscous diffusion of turbulence, and the large scale engulfment, which entraps large volume of flow to be “digested” subsequently by viscous diffusion. The exact role of each of them in the overall entrainment rate is still not well understood, as it is the interplay between these two mechanics of diffusion. It is anyway accepted that the entrainment rate scales with large properties of the flow, while is not understood how the large scale inertial behavior can affect an intrinsically viscous phenomenon as diffusion of vorticity. In the present work we will address then the problem of turbulent diffusion through pseudo-spectral DNS simulations of the interface between a volume of decaying turbulence and quiescent flow. Such simulations will give us first hand measures of velocity, vorticity and strains fields at the interface; moreover the framework of unforced decaying turbulence will permit to study both spatial and temporal evolution of such fields. The analysis will evidence that for this kind of flows the overall production of enstrophy , i.e. the square of vorticity omega^2 , is dominated near the interface by the local inertial transport of “fresh vorticity” coming from the turbulent flow. Viscous diffusion instead plays a major role in enstrophy production in the outbound of the interface, where the nibbling process is dominant. The data from our simulation seems to confirm the theory of an inertially stirred viscous phenomenon proposed by others authors before and provides new data about the inertial diffusion of turbulence across the interface.
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Er, Sarp. "Structure interne, transfert turbulent et propriétés de cascade de l'interface turbulent/non-turbulent d'un jet turbulent." Electronic Thesis or Diss., Université de Lille (2022-....), 2023. http://www.theses.fr/2023ULILN048.
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L'interface turbulent/non-turbulent (TNTI) est une couche très fine entre les régions turbulentes et non turbulentes de l'écoulement. Cette étude vise à mieux comprendre le bilan d'énergie cinétique au voisinage de l'interface turbulent/non-turbulent. L'équation de Kármán-Howarth-Monin-Hill (KHMH) est utilisée pour caractériser le bilan énergétique cinétique local, y compris les transferts d'énergie dans l'espace et entre les échelles. L'analyse est effectuée à l'aide de données obtenues par simulation numérique directe (DNS) finement résolue d'un jet plan turbulent se développant avec le temps. Les lois d'échelles de vitesse et de longueur du jet plan turbulent en evolution temporelle sont différentes de celles de son homologue en développement spatial, dans le sens où ces lois sont indépendantes de l'échelle de dissipation turbulente, qu'elle soit à l'équilibre ou hors équilibre. Il est montré que la variation de la vitesse moyenne de propagation à travers l'épaisseur de la TNTI est fonction de la dimension fractale de la surface à chaque position. Une méthodologie basée sur une opération de moyennage le long de la TNTI est utilisée pour l'analyse de l'écoulement local à proximité de la TNTI. L'analyse du vecteur normal associé à l'orientation locale de la TNTI fournit des informations précieuces sur les caractéristiques géométriques prédominantes de l'interface. Les statistiques moyennes de l'interface sont ensuite conditionnées par sa courbure moyenne et sa vitesse de propagation locale afin de caractériser la variation locale de l'écoulement et le bilan de l'équation KHMH dans les différentes couche de l'interface. Il est démontré que l'épaisseur de la TNTI et de ses sous-couches diminuent de manière significative dans les régions de fort entraînement. Les transferts entre échelles et en espace sont décomposés en une partie solénoïdale et une partie irrotationnelle, ce qui montre l'importance, au niveau de la TNTI, des transferts irrotationnels d'énergie cinétique entre échelles et en espace, associés à la corrélation pression-vitesse. Des phénomènes de compression et d'étirement sont observés en moyenne à proximité de la TNTI, dans les directions respectivement normale et tangentielle à l'interface. L'étude du terme de transfert inter-échelles montre la présence d'une cascade directe dans la direction normale et d'une cascade inverse dans la direction tangentielle. Dans les régions d'entraînement inverse, les statistiques locales montrent un étirement dans la direction normale et de la compression dans la direction tangentielle, ce qui contraste avec les statistiques observées pour l'ensemble de la TNTI et les régions d'entraînement locales. Près de la TNTI, du côté turbulent, un équilibre inattendu ressemblant à celui de Kolmogorov est observé entre le transfert inter-échelle et le taux de dissipation pour une large gamme d'échelles. Pour ces échelles, contrairement à l'équilibre de Kolmogorov habituel pour la turbulence homogène, le transfert inter-échelle est constitué uniquement de la partie irrotationnelle qui est directement associée aux corrélations pression-vitesseThe turbulent/non-turbulent interface (TNTI) is a very sharp interface layer between turbulent and non-turbulent regions of the flow. This study aims to gain insight into the kinetic energy balance in the vicinity of the TNTI. The K'arm'an-Howarth-Monin-Hill equation (KHMH) is used to characterize the local kinetic energy balance including interscale/interspace energy transfers. The analysis is conducted by using a data set obtained by highly resolved direct numerical simulation (DNS) of a temporally developing turbulent planar jet. The scalings for the velocity and length scales of the temporally developing turbulent planar jet are shown to be different from its spatially developing counterpart in the sense that these scalings are independent of the turbulent dissipation scaling, whether equilibrium or non-equilibrium. The variation of the mean propagation velocity across the thickness of the TNTI is shown as a function of the fractal dimension of the surface at each location. Furthermore, a methodology based on a TNTI-averaging operation is used for the analysis of the local flow field in the vicinity of the TNTI. The analysis of the normal vector associated with the local facing direction of the TNTI provides valuable insights into the predominant geometric characteristics of the interface. The TNTI-averaged statistics are further conditioned on the mean curvature and the local propagation velocity of the interface, in order to characterize the variation of the local flow field and KHMH balance in various regions of the interface. The thickness of the TNTI and its sublayers are shown to reduce significantly in regions of fast entrainment. The interscale/interspace transfer terms are decomposed into solenoidal/irrotational parts showing the central importance at the TNTI of the irrotational interscale/interspace transfers of kinetic energy associated with pressure-velocity correlation. Compression and stretching are observed on average at the TNTI location, in the normal and tangential directions of the interface respectively. Investigation of the interscale transfer term shows the presence of a forward cascade in the normal direction and an inverse cascade in the tangential direction. In regions of detrainment, the local statistics display stretching in the normal direction and compression in the tangential direction, which is in contrast with the statistics observed for the entire TNTI and the local entrainment regions. Close to the location of TNTI, on the turbulent side, an unexpected Kolmogorov-like balance is observed between the interscale transfer and the dissipation rate for a wide range of scales. For these scales, unlike the usual Kolmogorov balance for homogeneous turbulence, the interscale transfer consists solely of the irrotational part which is directly associated with the pressure-velocity correlations
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Zhang, Huangwei. "Extinction in turbulent swirling non-premixed flames." Thesis, University of Cambridge, 2015. https://www.repository.cam.ac.uk/handle/1810/254974.
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This thesis investigates the localized and global extinction in turbulent swirling non-premixed flames with Large Eddy Simulation (LES) and sub-grid scale Conditional Moment Closure (CMC) model. The first part of this thesis describes the derivations of the three dimensional conservative CMC governing equations and their finite volume discretization for unstructured mesh. The parallel performance of the newly developed CMC code is assessed. The runtime data coupling interface between the 3D-CMC and LES solvers is designed and the different solvers developed during the course of this research are detailed. The aerodynamics of two swirling non-reacting flows from the Sydney and Cambridge burners are first simulated. The main ow structures (e.g. the recirculating zones) in both cases are correctly predicted. The sensitivity analysis about the influences of turbulent inlet boundary, computational domain and mesh refinement on velocity statistics is conducted. This analysis acts as the preparatory investigation for the following flame simulations. The Sydney swirl diluted methane flame, SMA2, is then simulated for validating the LES/3D-CMC solvers. Excellent agreements are achieved in terms of velocity and mixture fraction statistics, averaged reactive scalars in both physical and mixture fraction space. The local extinction level from the increased central fuel velocity is reasonably predicted. At the experimental blow-off point, the LES/3D-CMC modelling does not obtain the occurrence of complete extinction, but severe extinction occurs at the flame base, qualitatively in line with experimental observations. Localized extinction features of a non-premixed methane flame in the Cambridge swirl burner are investigated and it is found that the occurrence of local extinction is typically manifested by low heat release rate and hydroxyl mass fraction, as well as low or medium temperature. It is also accompanied by high scalar dissipation rates. In mixture fraction space, the CMC cells undergoing local extinction have relatively wide scatter between inert and fully burning solutions. The PDFs of reactedness at the stoichiometric mixture fraction demonstrate some extent of bimodality, showing the events of local extinction and reignition and their relative occurrence frequency. Local extinction near the bluff body in the Cambridge swirl burner is also studied. The convective wall heat loss is included as a source term in the conditionally filtered total enthalpy equation. It shows a significant influence on the mean flame structures, directly linked to the changes of the conditional scalar dissipation near the wall. Furthermore, the degree of local extinction near the bluff body surface is intensified because of the wall heat loss. However, the wall heat loss shows a relatively small influence on the statistics of lift-off height. Finally, the blow-off conditions and dynamics in the Cambridge swirl burner are investigated. The blow-off critical air bulk velocity from LES/3D-CMC is over-predicted, greater than the experimental one by at most 25%. The predicted blow-off transient lasts finitely long duration quantified by the blow-off time, in good agreement with the experimental results. The reactive scalars in both physical and mixture fraction space demonstrate different transient behaviors during blow-off process. When the current swirling flame is close to blow-off, high-frequency and high-amplitude fluctuations of the conditionally filtered stoichiometric scalar dissipation rate on the iso-surfaces of the filtered stoichiometric mixture fraction are evident. The blow-off time from the computations is found to vary with different operating conditions.
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Pater, Sjoerd Gerardus Maria. "Acoustics of turbulent non-premixed syngas combustion." Enschede : University of Twente [Host], 2007. http://doc.utwente.nl/58039.
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Hossain, Mamdud. "CFD modelling of turbulent non-premixed combustion." Thesis, Loughborough University, 1999. https://dspace.lboro.ac.uk/2134/12230.
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The thesis comprises of a thorough assessment of turbulent non-premixed combustion modelling techniques, emphasising the fundamental issue of turbulence-chemistry interaction. The combustion models studied are the flame-sheet, equilibrium, eddy breakup and laminar flamelet models. An in-house CFD code is developed and all the combustion models are implemented. Fundamental numerical issues involving the discretisation schemes are addressed by employing three discretisation schemes namely, hybrid, power law and TVD. The combustion models are evaluated for a number of fuels ranging from simple H2/CO and CO/H2/N2 to more complex Cl4/H2 burning in bluff body stabilised burners at different inlet fuel velocities. The bluff body burner with its complex recirculation zone provides a suitable model problem for industrial flows. The initial and boundary conditions are simple and well-defined. The bluff body burner also provides a controlled environment for the study of turbulence-chemistry interaction at the neck zone. The high quality experimental database available from the University of Sydney and other reported measurements are used for the validation and evaluation of combustion models. The present calculations show that all the combustion models provide good predictions for near equilibrium flames for temperature and major species. Although the equilibrium chemistry model is capable of predicting minor species, the predictive accuracy is found to be inadequate when compared to the experimental data. The laminae flamelet model is the only model which has yielded good predictions for the minor species. For flames at higher velocities. the laminar flamelet model again has provided better predictions compared to predictions of other models considered. With different fuels, the laminar flamelet model predictions for CO/H2/N2 fuel are better than those for CH4/H2 fuel. The reasons for this discrepancy are discussed in detail. The effects of differential diffusion are studied in the laminar flamelet modelling strategy. The flamelet with unity Lewis number is found to give a better representation of the transport of species. The laminar flamelet model has yielded reasonably good predictions for NO mass fraction. The predictions of NO mass fraction are found to be very sensitive to differential diffusion effects. This study has also considered the issue of inclusion of radiative heat transfer in the laminar flamelet model. The radiation effects are found to be important only where the temperature is very high. The study undertaken and reported in this thesis shows that the presently available laminar flamelet modelling concepts are capable of predicting species concentrations and temperature fields with an adequate degree of accuracy. The flamelet model is also well suited for the prediction of NO emissions. The inclusion of radiation heat transfer has enhanced the predictive capability of the laminar flamelet model.
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Ahmed, S. F. A. F. S. "Spark ignition of turbulent non-premixed flames." Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.595391.
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This thesis investigates the spark ignition of various turbulent non-premixed flames namely, jet, counter-flow and bluff-body flames. This detailed fundamental study of spark ignition aims to provide useful information for solving the high-altitude relight problem in the aviation gas turbine. A specially designed ignition unit has been built. Different spark parameters and flow conditions have been examined to study their effects on the ignition probability defined as successful flame establishment. The ignition probability results have been correlated with the measured or estimated flow velocity and mixture fraction. The whole ignition and flame propagation events have been visualized by a high-speed camera and OH-PLIF. In the jet flames, it was found that after an initially spherical shape, the flame took a cylindrical shape with a propagating edge upstream. The probability of successful ignition Pign increases with high spark energy, thin electrode diameter and wide gap, but decreases with increasing dilution of the jet with air. The flame kernel growth rate is high when the ignition probability is high for all parameters, except for jet velocity. Increasing the jet velocity decreases the ignition probability at all locations. The estimated net propagation speed relative to the incoming flow was about 3 to 6 laminar burning velocities of a stoichiometric mixture SL. In the counter-flow flames, it was found that the flame spread as an edge flame with a large scatter in its radial position. Pign decreased with bulk velocity, which suggests that the local strain rate can be detrimental to ignition so that, even with the strongest spark tested, ignition could not be achieved at a bulk velocity about 90% of the extinction velocity. Pign was greater than zero even in regions well into the fuel and air streams where the mixture fraction fluctuations were virtually zero, giving zero probability of finding flammable mixture at the spark location. The estimated edge flame speed relative to the radial flow is higher than SL for the premixed flame and is less than SL for the non-premixed flames.
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Smith, Sarah Elizabeth. "Turbulent duct flow of non-Newtonian liquids." Thesis, University of Liverpool, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399184.
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The turbulent flow of non-Newtonian fluids in straight ducts has been investigated. Specifically, the fully developed circular pipe flow, axisymmetric sudden expansion flow and fully developed square duct flow were studied. The pipe flow study analysed previous measurements of the mean velocity profiles and friction factor-Reynolds number characteristics of different non-Newtonian fluids in pipe fully developed pipe flows. An investigation of different nondimensional parameters permitted initial progress on developing a correlation between drag reduction and fluid rheology to be made. Comparison of the ranking orders of drag reduction, fluid extensional viscosity and fluid elasticity revealed that these fluid properties are most strongly correlated with drag reduction at low shear/strain rates (that is, in the buffer and outer regions of the boundary layer). The sudden expansion geometry was investigated for flows of aqueous Xanthan gum solution and two reference Newtonian fluids. A smooth contraction was placed at the inlet to the sudden expansion. Few significant differences were observed between the mean flow behaviours of the test fluids for the turbulent Reynolds numbers tested (26,000 and 80,000). These results may reflect the manner in which the rigid, rod-like molecules found in Xanthan gum influence the flow behaviour. Turbulence measurements indicated that all three turbulence components were suppressed for the polymer solution flow within the free shear layer downstream of the expansion. The turbulent flow of two non-Newtonian fluids (a blend ofXanthan gum and Carboxymethylcellulose in water and an aqueous solution of polyacrylamide) in a square duct were compared with a turbulent Newtonian square duct flow. Although suppression of the transverse turbulence components was noted, the polymer solutions also strongly affected the behaviour of the secondary flows found in turbulent non-circular duct flows of Newtonian fluids. Specifically, the secondary flows appeared to be weakened in the polymer blend flow and completely suppressed in the polyacrylamide solution flow. It is anticipated that fluid elasticity is influential in this suppression
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Richardson, Edward S. "Ignition modelling for turbulent non-premixed flows." Thesis, University of Cambridge, 2007. https://eprints.soton.ac.uk/203167/.
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De, Bruyn Kops Stephen M. "Numerical simulation of non-premixed turbulent combustion /." Thesis, Connect to this title online; UW restricted, 1999. http://hdl.handle.net/1773/7140.
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Branley, Niall Thomas. "Large eddy simulation of non-premixed turbulent flames." Thesis, Imperial College London, 2000. http://hdl.handle.net/10044/1/8584.
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Odedra, Anand. "Unsteady flamelet modelling of turbulent non-premixed combustion." Thesis, Loughborough University, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.497217.
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In this study, RANS based axisymmetric simulations of the jet flames, bluff-body flames and swirling flames have been attempted by employing steady and unsteady flamelet models. The jet flames have been studied for pure hydrogen and diluted hydrogen (CO/H2/N2) fuels. The bluff-body flames have been studied for three different fuels CH4/H2, H2/CO and CH3OH. The swirling flame has been investigated for CH4/H2 fuel. The importance of unsteady effects is thoroughly assessed for combustion predictions. The transient effects are considered in a post-processing manner employing the Lagrangian Flamelet Model (LFM) for jet flames and the Eulerian Particle Flamelet Model (EPFM) for recirculating bluff-body and swirling flames.
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Kim, Ik Soo. "Conditional moment closure for non-premixed turbulent combustion." Thesis, University of Cambridge, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.614939.
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Niall, Branley. "Large eddy simulation of non-premixed turbulent flames." Boston Spa, U.K. : British Library Document Supply Centre, 1999. http://ethos.bl.uk/OrderDetails.do?did=1&uin=uk.bl.ethos.314128.
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Ouchene, Rafik. "Dispersion de particules non-sphériques en écoulement turbulent." Thesis, Université de Lorraine, 2015. http://www.theses.fr/2015LORR0275/document.
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Ce travail de thèse fait partie intégrante de l’ANR PLAYER (début janvier 2012), projet visant à étendre les simulations d'écoulements gaz-particules à des particules non-sphériques ayant une inertie couvrant une large gamme. Les avancées de cette ANR portent notamment sur la détermination des forces et couples élémentaires sur de tels objets avec la question du nombre de degrés de liberté supplémentaires à prendre en compte, l'impact de la forme et de l’effet d'inertie ainsi que l’influence d’une force extérieure telle que la gravité sur les interactions particule-turbulence. Dans ce cadre, l’objectif de ce travail de thèse est d'étudier finement la dispersion de particules non-sphériques rigides dans un écoulement turbulent à l’échelle mésocospique (il est supposé que les particules sont des points matériels). Pour ce faire, un suivi lagrangien de particules ellipsoïdales couplé à un code de simulation numérique directe d’un écoulement turbulent de canal a été utilisé. Cette méthode nécessite alors une bonne estimation des forces et couples hydrodynamiques agissant sur ce type de particules, ainsi qu’un couplage des équations du mouvement de translation et de rotation. En se basant sur les résultats obtenus par une simulation numérique directe résolue à l’échelle de la particule (Ansys Fluent, body-fitted method), nous avons établi, dans un premier temps, des corrélations pour les coefficients hydrodynamiques (traînée, portance, couple de tangage) dépendant du nombre de Reynolds particulaire, de la forme, et de l'orientation des particules. L’originalité de ce travail réside en la validité de ces corrélations pour des gammes étendues de facteurs de forme (rapport entre la longueur et la largeur de la particule w ∈ [0,2-32] et de Reynolds particulaires Rep ∈ [1-240]. Ces corrélations ainsi que les équations du mouvement de rotation ont été ensuite intégrées dans le code « maison » de simulation numérique directe d’un écoulement turbulent gaz-solide à l’échelle mésocospique. Après avoir validé ce code à travers différents cas tests, nous avons étudié la dispersion de différentes particules ellipsoïdales dans un écoulement de canal turbulent pour un nombre de Reynolds modéré. Trois principaux effets sont à l’étude : l’effet de forme, l'effet d'inertie et l'effet du croisement de trajectoiresThe present work is a part of a program research ANR PLAYER (started from January 2012), the aim of the project is to extend the simulations of gaz-particles flow to the non-spherical particles with a large range of inertia. The main objectives of this project consist, firstly, on the founding of hydrodynamic forces and torques occurring on these non-spherical particles. As results, we focus on the additional degrees of freedom that must be considered, shape effects and effects of inertia. Secondly, we are interested on the study of particle-turbulence interaction and particle-particle interaction. The aim of this Phd thesis consists on the studying of the dispersion of solide non-spherical particles in turbulent channel flow at mesoscopic scale. In order to achieve this work, we considered a one way coupling and we used a technique of Particles Lagrangian Tracking coupled with a Direct Numerical Simulation of the turbulent channel flow (DNS/PLT). This technique requires a well prediction of hydrodynamic forces and torques occurring on each particle. In addition, this technique requires a coupling of translational and rotational motions. Firstly, a Direct Numerical Simulation is used with a body-fitted method in CFD code Ansys-Fluent to simulate flow around ellipsoids. Based on the obtained results, models of correlation for hydrodynamic coeffients (drag, lift and torque) are proposed. The major results of this part is the accuracy models for a large ranges of particles Reynolds number, aspect ratio and orientations. Indeed these models take the particle Reynolds number Rep ∈ [1-240], the shape (aspect ratio w ∈ [0.2-32]) and the orientation of the particle into account. Secondly, these models of correlation as well as translational and rotational motions are implemented in the in-house DNS code. After a rigorous validation of the code using a different test cases, simulations of dispersion of ellipsoidals particles in a tubulent channel flow is performed for a moderate Reynolds number. Three main effects are investigated in this study: shape effect, inertial effect and the “effect of crossing trajectories”
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Bodart, Julien. "Effet de blocage dans un écoulement turbulent non cisaillé." Phd thesis, Institut National Polytechnique de Toulouse - INPT, 2009. http://tel.archives-ouvertes.fr/tel-00449404.
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Un code de résolution des équations de Navier-Stokes pour un fluide incompressible a été développé en utilisant une approche mixte spectral/différences finies, compatible avec une mise en \oe uvre dans un environnement massivement parallèle. On procède, grâce à ce nouvel outil, à des simulations directes de la turbulence dans une configuration où l'agitation est synthétisée à l'aide d'un forçage aléatoire. La production de turbulence est confinée dans une couche centrale du domaine et s'auto-diffuse en direction d'une surface libre ou d'une paroi adhérente. Dans cette configuration on obtient un état statistiquement stationnaire où le cisaillement moyen, généralement à l'origine de la production de la turbulence, est nul. Ces conditions permettent de mieux comprendre l'origine du transfert intercomposantes, caractéristique de la partie lente du terme de corrélation pression-déformation dans les équations-bilan des tensions de Reynolds. L'accent est mis sur l'analyse de ce transfert lorsqu'il s'effectue sous l'influence de l'effet de blocage au voisinage d'une surface. Les résultats obtenus permettront de mieux appréhender la modélisation des termes de corrélation pression-déformation au voisinage d'une paroi dans les modèles de fermeture au second ordre.
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Ekoto, Isaac Wesley. "Supersonic turbulent boundary layers with periodic mechanical non-equilibrium." Texas A&M University, 2006. http://hdl.handle.net/1969.1/4709.
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Previous studies have shown that favorable pressure gradients reduce the
turbulence levels and length scales in supersonic flow. Wall roughness has been shown to
reduce the large-scales in wall bounded flow. Based on these previous observations new
questions have been raised. The fundamental questions this dissertation addressed are: (1)
What are the effects of wall topology with sharp versus blunt leading edges? and (2) Is it
possible that a further reduction of turbulent scales can occur if surface roughness and
favorable pressure gradients are combined? To answer these questions and to enhance the
current experimental database, an experimental analysis was performed to provide high
fidelity documentation of the mean and turbulent flow properties along with surface and
flow visualizations of a high-speed ( 2.86 M = ), high Reynolds number (Re 60,000 q û )
supersonic turbulent boundary layer distorted by curvature-induced favorable pressure
gradients and large-scale ( 300 s k + û ) uniform surface roughness. Nine models were tested
at three separate locations. Three pressure gradient models strengths (a nominally zero, a
weak, and a strong favorable pressure gradient) and three roughness topologies
(aerodynamically smooth, square, and diamond shaped roughness elements) were used.
Highly resolved planar measurements of mean and fluctuating velocity components were
accomplished using particle image velocimetry. Stagnation pressure profiles were acquired with a traversing Pitot probe. Surface pressure distributions were characterized
using pressure sensitive paint. Finally flow visualization was accomplished using
schlieren photographs.
Roughness topology had a significant effect on the boundary layer mean and
turbulent properties due to shock boundary layer interactions. Favorable pressure
gradients had the expected stabilizing effect on turbulent properties, but the
improvements were less significant for models with surface roughness near the wall due
to increased tendency towards flow separation. It was documented that proper roughness
selection coupled with a sufficiently strong favorable pressure gradient produced regions
of âÂÂnegativeâ production in the transport of turbulent stress. This led to localized areas of
significant turbulence stress reduction. With proper roughness selection and sufficient
favorable pressure gradient strength, it is believed that localized relaminarization of the
boundary layer is possible.
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Hockey, Randal Myles. "Turbulent Newtonian and non-Newtonian flows in a stirred reactor." Thesis, Imperial College London, 1990. http://hdl.handle.net/10044/1/46341.
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Chai, Michael I. B. "Soot modeling of a turbulent non-premixed methane/air flame." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/MQ63115.pdf.
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Di, Mare F. "Large eddy simulation of reacting and non-reacting turbulent flows." Thesis, Imperial College London, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.275335.
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Sadasivuni, S. K. "LES modelling of non-premixed and partially premixed turbulent flames." Thesis, Loughborough University, 2009. https://dspace.lboro.ac.uk/2134/5804.
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A large eddy simulation (LES) model has been developed and validated for turbulent non-premixed and partially premixed combustion systems. LES based combustion modelling strategy has the ability to capture the detailed structure of turbulent flames and account for the effects of radiation heat loss. Effects of radiation heat loss is modelled by employing an enthalpy-defect based non-adiabatic flamelet model (NAFM) in conjunction with a steady non-adiabatic flamelet approach. The steady laminar flamelet model (SLFM) is used with multiple flamelet solutions through the development of pre-integrated look up tables. The performance of the non-adiabatic model is assessed against experimental measurements of turbulent CH4/H2 bluff-body stabilized and swirl stabilized jet flames carried out by the University of Sydney combustion group. Significant enhancements in the predictions of mean thermal structure have been observed with both bluff body and swirl stabilized flames by the consideration of radiation heat loss through the non-adiabatic flamelet model. In particular, mass fractions of product species like CO2 and H2O have been improved with the consideration of radiation heat loss. From the Sydney University data the HM3e flame was also investigated with SLFM using multiple flamelet strategy and reasonably fair amount of success has been achieved. In this work, unsteady flamelet/progress variable (UFPV) approach based combustion model which has the potential to describe both non-premixed and partially premixed combustion, has been developed and incorporated in an in-house LES code. The probability density function (PDF) for reaction progress variable and scalar dissipation rate is assumed to follow a delta distribution while mixture fraction takes the shape of a beta PDF. The performance of the developed model in predicting the thermal structure of a partially premixed lifted turbulent jet flame in vitiated co-flow has been evaluated. The UFPV model has been found to successfully predict the flame lift-off, in contrast SLFM results in a false attached flame. The mean lift-off height is however over-predicted by UFPV-δ function model by ~20% for methane based flame and under-predicted by ~50% for hydrogen based flame. The form of the PDF for the reaction progress variable and inclusion of a scalar dissipation rate thus seems to have a strong influence on the predictions of gross characteristics of the flame. Inclusion of scalar dissipation rate in the calculations appears to be successful in predicting the flame extinction and re-ignition phenomena. The beta PDF distribution for the reaction progress variable would be a true prospect for extending the current simulation to predict the flame characteristics to a higher degree.
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Yunardi, Y. "Conditional moment closure modelling of sooting, turbulent non-premixed flames." Thesis, University of Leeds, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.445391.
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Ali, Shaukat. "Direct quadrature conditional moment closure for turbulent non-premixed combustion." Thesis, Queen Mary, University of London, 2014. http://qmro.qmul.ac.uk/xmlui/handle/123456789/7868.
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The accurate description of the turbulence chemistry interactions that can determine chemical conversion rates and flame stability in turbulent combustion modelling is a challenging research area. This thesis presents the development and implementation of a model for the treatment of fluctuations around the conditional mean (i.e., the auto-ignition and extinction phenomenon) of realistic turbulence-chemistry interactions in computational fluid dynamics (CFD) software. The wider objective is to apply the model to advanced combustion modelling and extend the present analysis to larger hydrocarbon fuels and particularly focus on the ability of the model to capture the effects of particulate formation such as soot. A comprehensive approach for modelling of turbulent combustion is developed in this work. A direct quadrature conditional moment closure (DQCMC) method for the treatment of realistic turbulence-chemistry interactions in computational fluid dynamics (CFD) software is described. The method which is based on the direct quadrature method of moments (DQMOM) coupled with the Conditional Moment Closure (CMC) equations is in simplified form and easily implementable in existing CMC formulation for CFD code. The observed fluctuations of scalar dissipation around the conditional mean values are captured by the treatment of a set of mixing environments, each with its pre-defined weight. In the DQCMC method the resulting equations are similar to that of the first-order CMC, and the “diffusion in the mixture fraction space” term is strictly positive and no correction factors are used. Results have been presented for two mixing environments, where the resulting matrices of the DQCMC can be inverted analytically. Initially the DQCMC is tested for a simple hydrogen flame using a multi species chemical scheme containing nine species. The effects of the fluctuations around the conditional means are captured qualitatively and the predicted results are in very good agreement with observed trends from direct numerical simulations (DNS). To extend the analysis further and validate the model for larger hydrocarbon fuel, the simulations have been performed for n-heptane flame using detailed multi species chemical scheme containing 67 species. The hydrocarbon fuel showed improved results in comparison to the simple hydrogen flame. It suggests that higher hydrocarbons are more sensitive to local scalar dissipation rate and the fluctuations around the conditional means than the hydrogen. Finally, the DQCMC is coupled with a semi-empirical soot model to study the effects of particulate formation such as soot. The modelling results show to predict qualitatively the trends from DNS and are in very good agreement with available experimental data from a shock tube concerning ignition delays time. Furthermore, the findings suggest that the DQCMC approach is a promising framework for soot modelling.
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HUAPAYA, LUIS ENRIQUE ALVA. "NUMERICAL AND EXPERIMENTAL CHARACTERIZATION OF A NON-PREMIXED TURBULENT FLAME." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2008. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=11881@1.
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COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIORCENTRO DE PESQUISAS LEOPOLDO AMÉRICO MIGUEZ DE MELLO
Neste trabalho se apresenta um estudo experimental e numérico de escoamentos turbulentos quimicamente reativos em um queimador tipo obstáculo. O objetivo principal é estudar uma chama turbulenta não pré-misturada de configuração geométrica simples. Esta chama, que queima gás natural e ar, é estabilizada por um queimador tipo obstáculo. Inicialmente um estudo bibliográfico do estado da arte de experimentos e da comparação entre experimentos e modelagem neste tipo de queimador é apresentado. Na sequência, a formulação matemática, clássica, de dinâmica dos fluidos computacional é exposta, seguida da apresentação da técnica de medição empregada nos experimentos, a fluorescência induzida por plano laser (PLIF). A discussão dos resultados obtidos neste trabalho é divida em três etapas. Na primeira, comparam-se os resultados de modelagem computacional usando quatro modelos de turbulência e dois modelos de combustão com dados experimentais encontrados na literatura. Esta comparação coloca em evidência o conjunto de modelos que possui melhor capacidade preditiva no que diz respeito a este tipo de configuração. A segunda etapa consiste na apresentação dos resultados experimentais obtidos, os quais permitem caracterizar, em três regimes de combustão distintos, a presença de uma espécie química existente durante o processo de combustão, no caso, o radical hidroxila (OH). Esta caracterização é realizada pelo exame tanto da estrutura instantânea da chama turbulenta quanto da média. Por fim, comparam-se os resultados da modelagem com aqueles obtidos no presente aparato experimental. Esta comparação coloca em evidência as deficiências dos modelos clássicos de combustão empregados e indica a necessidade de serem realizadas medidas simultâneas de velocidade e de concentração de espécies químicas que possibilitem o desenvolvimento de novos modelos de combustão em escoamento turbulento.
This work presents an experimental and numerical study of turbulent chemically reactive flows in a bluff body type burner. The main objective of the present work is to study a non-premixed turbulent flame on a simple geometric configuration. This flame, which burns natural gas and air, is stabilized downstream to a bluff-body. Initially, literature is reviewed on the previous experimental and modeling studies which have been performed on this kind of burner. Then, the mathematical formulation of computational fluid dynamic problem is presented. This is followed by the introduction of the experimental measurements techniques which involve planar laser induced fluorescence (PLIF). The discussion of results obtained in this work is divided in three sections. First, a comparison is made between numerical simulations, using four different turbulent models and two different combustion models, and experimental data found in the literature. Is allows to assess capacity of the different models to predict the reactive flow configuration studied. The second section presents the experimental results obtained for three combustion regimes, which are characterized by laser induced fluorescence emission of the hydroxil radical species (OH). This characterization involves the analysis the instantaneous and the average structure of the turbulent flame. Finally, the modeling results are compared to the experimental data obtained. This comparison in evidences the necessity to perform the simultaneous measurement of velocity and chemical species concentration in order to allow for the development of new models of combustion in turbulent flows.
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Van, Sittert Fritz Peter. "The effect of pipe roughness on non-Newtonian turbulent flow." Thesis, Cape Technikon, 1999. http://hdl.handle.net/20.500.11838/1035.
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Thesis (MTech (Civil Engineering))--Cape Technikon, Cape Town, 1999Pipe roughness is known to greatly increase the turbulent flow friction factor for Newtonian fluids. The well-known Moody diagram shows that an order of magnitude increase in the friction is possible due to the effect of pipe roughness. However, since the classical work of Nikuradse (1926 -1933), very little has been done in this area. In particular, the effects that pipe roughness might have on non-Newtonian turbulent flow head loss, has been all but totally ignored. This thesis is directed at helping to alleviate this problem. An experimental investigation has been implemented in order to quantify the effect that pipe roughness has on non-Newtonian turbulent flow head loss predictions. The Balanced Beam Tube Viscometer (BBTV), developed at the University of Cape Town, has been rebuilt and refined at the Cape Technikon and is being used for research in this field. The BBTV has been fitted with pipes of varying roughness. The roughness of smooth P\'C pipes was artificially altered using methods similar to those of Nikuradse. This has enabled the accumulation of flow data in laminar and turbulent flow in pipes that are both hydraulically smooth and rough Newtonian and non-Newtonian fluids have been used for the tests. The data have been subjected to analysis using various theories and scaling laws. The strengths and problems associated with each approach are discussed and It is concluded that roughness does have a significant effect on Newtonian as well as non-Newtonlan flow.
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Slatter, Paul Thomas. "Transitional and turbulent flow of non-Newtonian slurries in pipes." Doctoral thesis, University of Cape Town, 1995. http://hdl.handle.net/11427/21487.
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The only reliable approach open to designers of pipeline systems conveying non-Newtonian slurries in the turbulent flow regime has been large scale-pipe-tests. This thesis addresses this design problem, with particular emphasis on the theoretical modelling of the laminar/turbulent transition and turbulent flow behaviour of these slurries in pipes. The literature and theory pertinent to-the flow of slurries in pipes-is examined. The development of non-Newtonian Reynolds numbers and laminar/turbulent transition criteria are presented and existing theoretical models for predicting turbulent flow are reviewed. Three test facilities were built for the establishment of a data base of non-Newtonian slurry behaviour - a tube viscometer and two pumped recirculating pipe test rigs.
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Tyves, Natalie. "Numerical simulations of turbulent non-premixed combustion in a regenerative furnace." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/mq52670.pdf.
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Tansley, G. D. "Computational investigation of turbulent, non-Newtonian flow in heart valve conduits." Thesis, Nottingham Trent University, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380266.
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Dimitriadis, Konstantinos Panagiotou. "Computation of three-dimensional turbulent flow in non-orthogonal duct junctions." Thesis, University of Manchester, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.314921.
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Presti, Felice. "Investigation of transitional and turbulent pipe flow of non-Newtonian fluids." Thesis, University of Liverpool, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399138.
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Poole, Robert John. "Turbulent flow of Newtonian and non-Newtonian liquids through sudden expansions." Thesis, University of Liverpool, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399176.
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Saito, Marcelo Batista. "Analysis of thermal non-equilibrium for turbulent transport in porous media." Instituto Tecnológico de Aeronáutica, 2006. http://www.bd.bibl.ita.br/tde_busca/arquivo.php?codArquivo=302.
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The literature has documented proposals for macroscopic energy equation modeling for porous media considering the local thermal equilibrium hypothesis and laminar flow. In addition, a two-energy equation model has been proposed for conduction and laminar convection in packed beds. With the aim of contributing to new developments, this work treats turbulent heat transport modeling in porous media under the local thermal non-equilibrium assumption. Macroscopic time-average equations for continuity, momentum and energy are presented based on the recently established double decomposition concept (spatial deviations and temporal fluctuations of flow properties). Interfacial heat transfer coefficients are numerically determined for an infinite medium over which the fully developed flow condition prevails. The numerical technique employed for discretizing the governing equations is the control volume method. Laminar and turbulent flow results for the macroscopic heat transfer coefficient, between the fluid and solid phase in a periodic cell, are presented. Furthermore, fully developed forced convection in a porous channel bounded by parallel plates is considered based on a two-energy equation model. In conclusion, solutions for temperature profile and Nusselt number are obtained and presented for laminar and turbulent flows.
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Hocine, Mohamed. "Comportement des paliers fluides en écoulement turbulent de fluides non newtoniens." Toulouse 3, 1990. http://www.theses.fr/1990TOU30050.
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Dans ce memoire, nous avons developpe l'etude des comportements des fluides, a grandes vitesses de rotation; car la theorie de reynolds laminaire n'est plus exploitable. L'elaboration d'une theorie qui permet l'approche des caracteristiques des paliers lubrifies par des fluides non newtoniens, doit reposer sur la loi fondamentale du comportement du fluide correspondant; or, pour les fluides non newtoniens il n'existe pas de loi de comportement universelle. Compte tenu que la loi choisie doit interagir avec le phenomene de turbulence; il est indispensable que cette loi presente une expression mathematique relativement simple. Nous avons donc selectionne la loi en puissance (law of power). Dans une seconde etape nous avons aborde l'etude de la turbulence, or ce phenomene complexe est decrit par plusieurs modeles, dont on a expose principalement ceux ayant ete appliques aux paliers. Il nous semble que le modele k- et le modele energie cinetique sont les plus satisfaisants; et permettent l'integration facile de la loi de comportement du fluide choisi. La resolution numerique de la turbulence a ete appliquee a un ecoulement plan. Cette premiere simulation debouche sur une formulation generalisee des coefficients gx#1 et gx#3. Ensuite ces formulations sont introduites dans l'equation de reynolds generalisee, laquelle est resolue par la technique des differences finies; et qui fournit les caracteristiques d'un palier (w, s, , cc,. . . ). Ces resultats apres avoir ete analyses et interpretes sont tout d'abord valides pour le cas newtonien et compares avec des travaux anterieurs
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Baalbaki, Daoud. "Simulation and modeling of turbulent non isothermal vapor-droplet dispersed flow." Perpignan, 2011. http://www.theses.fr/2011PERP1085.
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Le sujet traite de l’étude et de la modélisation, à l’échelle locale, des écoulements turbulents et diphasiques vapeur-gouttes dans un cœur de réacteur nucléaire lors d’un accident de perte de réfrigérant. On considère une modélisation moyennée Euler/Euler de l’écoulement diphasique. Ce travail aborde plus précisément la modélisation des termes de transfert de quantité de mouvement entre les phases et les termes de turbulence. Ainsi, nos travaux ont d’abord permis d’évaluer les limites de certains modèles utilisés dans le code de calcul NEPTUNE-CFD pour mener des études de thermo-hydraulique accidentelle au niveau local. Des solutions ont ensuite été proposées et mises en œuvre pour améliorer plus particulièrement la modélisation de l’hydrodynamique des particules et celle de leur dispersion turbulente. Cette thèse s’inscrit dans le cadre d’une collaboration entre l’IRSN et le laboratoire PROMES à PerpignanThis thesis deals with the simulation and the modeling of a turbulent vapor-droplets two-phase flow at the local scale in the core of a PWR (Pressured Water Reactor) nuclear reactor during LOCA (Loss Of Coolant Accident). We consider a Euler / Euler two-phase flow model. This work specifically treats the modeling of the terms of transfer of momentum between the phases and the terms of turbulence. Thus, first we studied the limitations of some models used in the computer code NEPTUNE-CFD for this type of flows. Solutions were then proposed and implemented to improve the modeling of the hydrodynamics of the droplets and especially that of their turbulent dispersion. This thesis is part of a collaboration between IRSN and the laboratory PROMES in Perpignan
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López, Carranza Santiago Nicolás. "Transition laminaire-turbulent en conduite cylindrique pour un fluide non Newtonien." Thesis, Université de Lorraine, 2012. http://www.theses.fr/2012LORR0118/document.
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L'objectif de cette thèse est de fournir une analyse de la transition vers la turbulence d'un fluide rhéofluidifiant (fluide de Carreau) dans une conduite cylindrique. Pour cela, un code pseudo-spectral de type Petrov-Galerkin a été développé. Une analyse linéaire de stabilité de l'écoulement laminaire est effectuée, montrant que cet écoulement est linéairement stable. Ensuite, des perturbations sous la forme des rouleaux longitudinaux contra-rotatifs sont utilisées comme condition initiale. Les termes non linéaires d'inertie et visqueux créent un écoulement secondaire avec des points d'inflexion, linéairement instable vis-à-vis de perturbations 3D. Une analyse linéaire de stabilité de ce nouvel écoulement de base bidimensionnelle est réalisée. La forme des vecteurs propres critiques est analysé. Enfin, une analyse non linéaire de stabilité de rouleaux vis-à-vis des perturbations tridimensionnelles de faible amplitude est effectuée, obtenant un retard pour la transition vers la turbulence des fluides rhéofluidifiants par rapport au cas Newtonien et une tendance à l'asymétrie du profil de vitesse axialeThe main objective of this thesis is to provide a description of the transition to turbulence of a shear thinning fluid in pipe flow. A linear stability analysis of the base flow is done. Results show that the flow is linearly stable and the optimal perturbation is given by a pair of counter rotating vortex. This kind of perturbation is used as an initial condition of a computational code which integrates the governing equations. Inertial and viscous non linear terms generate a secondary base flow with inflection points, which is linearly unstable to 3D perturbations. A secondary instability analysis is done, regarding the shape of unstable eigenvectors. Depending the rheological parameters and the size of the primary perturbation, the unstable mode might be near the wall or the center of the pipe. Finally, a non linear stability analysis of the streaks to 3D perturbations of weak amplitude, obtaining a delay in the transition to turbulence due to shear thinning
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Ravikanti, Veera Venkata Satyanarayana M. "Advanced flamelet modelling of turbulent non-premixed and partially premixed combustion." Thesis, Loughborough University, 2008. https://dspace.lboro.ac.uk/2134/34739.
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Current work focuses on the development and performance evaluation of advanced flamelet models for turbulent non-premixed and partially premixed combustion in RANS and large eddy simulation (LES) based modelling. A RANS-based combustion modelling strategy which has the ability to capture the detailed structure of turbulent non-premixed flames, including the pollutant NO, and account for the effects of radiation heat loss and transient evolution of NO, has been developed and incorporated into the in-house RANS code. The strategy employs an 'enthalpy defect'-based non-adiabatic flamelet model in conjunction with steady or unsteady nonadiabatic flamelets based NO submodels.
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Mercier, Renaud. "Turbulent combustion modeling for Large Eddy Simulation of non-adiabatic stratified flames." Thesis, Châtenay-Malabry, Ecole centrale de Paris, 2015. http://www.theses.fr/2015ECAP0042/document.
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La conception des chambres de combustion industrielles (chambres de combustion aéronautiques, fours industriels, etc.) requiert une prédiction fine des phénomènes physiques dominants. En particulier, l’interaction flamme turbulence aux échelles résolues et non-résolues, l’impact de la composition et du mélange des réactifs, l’impact des pertes thermiques et de la diffusion différentielle doivent être capturés fidèlement. C’est dans ce contexte que le modèle de combustion turbulente F-TACLES (Filtered TAbulated Chemistryfor Large Eddy Simulation) a été développé afin de coupler une méthode de chimie tabulée (FPI) avec le formalisme de la simulation aux grandes échelles(LES).Dans cette thèse, le modèle F-TACLES, initialement développé pour des écoulements adiabatiques, est étendu à la prise en compte des pertes thermiques. Un formalisme adapté à l’utilisation de bases de chimie tabulée calculées avec la diffusion différentielle est aussi proposé. Ces développements sont validés sur deux configurations : le brûleur TSF et le brûleur SWB.La modélisation de l’interaction flamme-turbulence est ensuite étudiée. Une étude de sensibilité du modèle de plissement de sous-maille de Charlette et al. (2002) à ses paramètres et sous-modèles est réalisée sur le brûleur SWB.En particulier, une méthode d’estimation dynamique des paramètres est aussi évaluée et montre d’excellents résultats. Une généralisation du formalisme de la LES pour les écoulements réactifs est ensuite proposée afin de prendre en compte explicitement les deux filtres mis en jeu dans les simulations : le filtre associé à l’écoulement et le filtre associé à la flamme. Deux stratégies de fermetures sont proposées en se basant sur des modèles existants (F-TACLES et TFLES). Le modèle obtenu, appelé modèle F2-TACLES, est ensuite validé et comparé avec F-TACLES sur la configuration semi-industrielle PRECCINSTA.Pour terminer, la capacité du modèle F-TACLES à capturer l’impact des pertes thermiques et de la composition des gaz frais sur la topologie de flammes est évaluée. Cette étude est réalisée sur une série de flammes CH4-H2-Air turbulentes en giration et prenant des formes différentes en fonction du niveau de pertes thermiques et de la composition des réactifsThe design of industrial combustion chambers (aeronautical engines, industrial furnaces, etc.) require a fine prediction of the different governing phenomena. Flame-turbulence interaction at resolved and unresolved scales, impact of reactants composition and mixing process, impact of heat losses and differential diffusion have to be correctly captured in such configurations. For that purpose,the turbulent combustion model F-TACLES (Filtered Tabulated Chemistry forLarge Eddy Simulation) has been developed to couple tabulated chemistry with large eddy simulation (LES) formalism.In this thesis, the F-TACLES model, initially developed for unity Lewis number and adiabatic flows, is extended to account for heat losses. A formalism allowing the use of chemical databases (1-D premixed flames) computed with differential diffusion is also proposed. The extended model is validated on two different configurations: the TSF burner and the SWB burner. Modeling of flame-turbulence interaction is then studied. For unresolved flame turbulence interactions, a sensitivity analysis of the Charlette et al. (2002) sub-grid scale wrinkling model to its own parameters and sub-models is performed on the SWB burner. A dynamic estimation of the model parameter is also assessed and showed very promising results. For resolved flame-turbulence interactions, a generalized formalism of the LES of reactive flows is proposed in order to account explicitly for both flame and flow filters. Two closure strategies are proposed based on the F-TACLES and TFLES models. The F2-TACLESmodel is then validated and compared to the original formulation of the FTACLES model. This study is performed on the lean premixed semi-industrial PRECCINSTA burner.The ability of the extended F-TACLES model to capture the impact of both heat losses and fresh gas composition on the flame topology is assessed. This study is conducted on a CH4-H2-Air turbulent and swirling flame series. These flames exhibit very different shapes depending on the level of heat losses and fuel composition
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Trinh, Khanh Tuoc. "Turbulent transport near the wall in newtonian and non-newtonian pipe flow." Thesis, University of Canterbury. Chemical Engineering, 1992. http://hdl.handle.net/10092/7693.
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Turbulence is present everywhere in our environment yet its complexities remain one of the great challenges in scientific and engineering research. This work began out of personal interest. As such, it involved all aspects of my background, not just my engineering training.
It is disconcerting to some westerners to think of scientists as philosophers. In the traditional Vietnamese culture, it is not possible to think of scientists as not being philosophers. In the old days we taught philosophy before we gave more practical training because all that we do IS influenced by our beliefs.
I was educated in French schools and New Zealand universities while being raised in a strongly nationalistic culture. This bicultural upbringing led at some stages of my youth to a painful crisis of identity but it greatly enriched my life and coloured my approach to science. The reader will find the influences of this background in my ideas about turbulence.
At one time, the West used to think of Asian beliefs as vague. I was first taught by an extraordinary Vietnamese electrical engineer that our description of the universe was in fact a binary progression. This is found in the Kinh Dich (in Chinese Yi-King, the Book of Change). This concept is not specific to the Vietnamese or Chinese cultures. It can be found in Indian thought, in Islamic Kybalion (their secret book), in Egyptian and South American cultures. Since that day, I have learned much more about traditional Vietnamese culture which is unconsciously reflected in some of the ideas I have developed in this research.
We believe that all phenomena are periodic. The manifestations that look continuous may in fact be thought of as an endless repetition of events that regenerate constantly. They are not static. I was not unduly surprised to learn of Fourier analysis which decomposes a continuous function into a series of periodic elements. I was thus receptive to the idea that one can define a timescale even for laminar flow, when I found it. It became eventually a transformation between the penetration and boundary layer theories of heat and mass transfer (Appendix A4).
We also believe, for example, that all things in Creation are governed by the same fundamental laws. In oriental medicine, it allowed us to understand and cure the human body by observing the laws that govern the larger universe. In essence, the organisation of the solar system is paralleled by that of the human body: there is a correspondence between the macrocosm and the microcosm. The reader will find a reflection of that thought in Chapter 9 where I endeavour to compare the information contained in time-averaged measurements to the those obtained from transient contributions.
We believe that, like us, the Earth breathes. In the teaching of Khi Cong (the equivalent of Yoga) which I studied briefly, it is important to take note of and keep in harmony with this natural cycle since we derive so many of our resources from the Earth where we live. I have often wondered whether our environmental engineering would not have benefited if those ideas had permeated into Western science.
To me, thoughts are not abstractions. They are concrete entities. We can now detect them through encephalograms. Thus thoughts that are conceived by a person are broad casted into the environment. If one is receptive, one can gather the wisdom of others around us. This is why Asian philosophy puts such an emphasis on emptying ourself of our own stray thoughts. This ultimate state of dispossession, mental as well as emotional and physical, is not easy to achieve because we tend to cling so hard to our own pre-conceived ideas. Vietnam can truly boast of having the most isolated scientific community in the world. The isolation imposed on the Vietnamese people by the Communists has forced many of us to face ourselves. In some cases it has been useful because this intellectual silence has made us much more receptive to the thoughts around us. In that sense, I have always felt that my work is simply the natural outcome of ideas evolved by my elders and colleagues in this field. In Phong Thuy (in Chinese Feng Shui, the art of shaping landscaping) the position where one buries one's ancestors is important because the configuration of the land filters the waves that are picked up by the remains of the deceased and transmitted to their descendants who are on the same wavelength. This is how innate talents are formed.
As a people, the Vietnamese have had a wonderful ability to assimilate and adapt the culture of the peoples we meet, even those who tried to conquer us. The tragic history of our country during the last hundred years has unfortunately eroded the confidence of many among us. The unfortunate policies of the Communist government during the last seventeen years have hit particularly hard and we risk losing much of our fine heritage. It is particularly painful for me to see how many of my friends, some certainly brighter than I am, have given up on research because the facilities, if they exist at all, are so much poorer than anywhere else. In the later years, the driving force behind the present work was no longer pure interest but the need to show that, whatever the circumstances, our achievements are only restricted by our perseverance with the task at hand. The work became a statement of philosophy as much as one of engineering research. My hope is that my work will make a contribution to the field, that it will be recognised as such and in a small way help the Vietnamese nation find itself again.
Trinh Khanh Tuoc
2 December 1992
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Roches, Pascal. "Étude expérimentale d'un écoulement turbulent non-cisaillé soumis à un gradient thermique." Toulouse, INPT, 2000. http://www.theses.fr/2000INPT050H.
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Le pré-traitement statistique utilisé pour simplifier le modèle mathématique complet des écoulements turbulents à masse volumique variable peut être effectué de différentes manières. Des études antérieures, portant sur ce sujet, ont mis en évidence le rôle central des flux turbulents de masse au niveau des différences qui résultent de l'utilisation de la moyenne de Favre ou de la moyenne centrée. Le but de ce travail est donc d'améliorer, par l'observation, la compréhension des mécanismes régissant les écoulements turbulents à masse volumique variable. Nous avons donc défini une configuration expérimentale amplifiant les effets de densité au détriment de ceux de la dynamique. La mesure des flux turbulents de masse nécessite l'acquisition simultanée de la vitesse et de la température à une fréquence élevée. Un examen de la métrologie actuelle nous a aidé à choisir le système de mesure adéquat pour atteindre nos objectifs. Le dépouillement de nos résultats nous a permis de mettre en évidence les mécanismes liant les effets de densité à ceux de la dynamique de l'écoulement. Plus particulièrement, nous avons montré que l'influence des flux turbulents de masse sur le comportement des écoulements compressibles est négligeable.
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Shen, Han. "Characterization of the Structure of Turbulent Non-premixed Dimethyl Ether Jet Flames." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1431017181.
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Niazi, Ardekani Mehdi. "Numerical study of non-spherical/spherical particles in laminar and turbulent flows." Licentiate thesis, KTH, Mekanik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-204421.
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The presence of solid rigid particles alters the global transport and rheological properties of the mixture in complex (and often unpredictable) ways. In recent years a few studies have been devoted to investigating the behavior of dense suspensions in the turbulent/inertial regime with the majority of theses analyses limited to mono-disperse rigid neutrally-buoyant spheres. However, one interesting parameter that is rarely studied for particles with high inertia is the particle shape. Spheroidal particles introduce an anisotropy, e.g. a tendency to orient in a certain direction, which can affect the bulk behavior of a suspension in an unexpected ways. The main focus of this study is therefore to investigate the behavior of spheroidal particles and their effect on turbulent/inertial flows. We perform fully resolved simulations of particulate flows with spherical/spheroidal particles, using an efficient/accurate numerical approach that enables us to simulate thousands of particles with high resolutions in order to capture all the fluid-solid interactions. Several conclusions are drawn from this study that reveal the importance of particle's shape effect on the behaviour of a suspension e.g. spheroidal particles tend to cluster while sedimenting. This phenomenon is observed in this work for both particles with high inertia, sedimenting in a quiescent fluid and inertialess particles (point-like tracer prolates) settling in homogenous isotropic turbulence. The mechanisms for clustering is indeed different between these two situations, however, it is the shape of particles that governs these mechanisms, as clustering is not observed for spherical particles. Another striking finding of this work is drag reduction in particulate turbulent channel flow with rigid oblate particles. Again this drag reduction is absent for spherical particles, which instead increase the drag with respect to single-phase turbulence.QC 20170328
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Kanj, Mohammed. "An Experimental Investigation of Turbulent, Near limit Non premixed and Premixed Flames." Thesis, University of Sydney, 2020. https://hdl.handle.net/2123/24366.
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This thesis presents an experimental study of turbulent near-limit flames using two cases of relevance to non-premixed and premixed conditions. For non-premixed flames turbulence-chemistry interaction is introduced by injecting a pulse of fuel to a steady flow in order to bring the flame close to blow-off and then again to steady state condition. The injection period is 10 ms. During this time, the flame transitions through conditions that have been studied earlier and these are known as piloted flames L, B, and M, which are baseline cases for the piloted burner employed here. Laser Doppler Velocimetry and planar high-speed imaging of OH-laser induced fluorescence (OH-PLIF) are employed to measure the velocity and OH fields. By moving downstream away from the burner exit plane, the pulse peak velocity decays rapidly and the pulse width shortens slightly. It is shown using the sequence of the OH-PLIF images of the transient flame that the frequency of local extinction increases and the details of fuel increase are provided within. The second near-limit condition is a highly sheared turbulent premixed flame to enable possible broadening in preheat and reaction zones and extreme finite-rate chemistry. The burner consists of two concentric tubes for the main jet surrounded by an annular pilot. Turbulent shear is generated by passing different fractions of the fuel-air mixture through the two concentric fuel tubes. The percentage of shear is changed by either changing the velocity difference between the two fuel streams or by changing the recess distance between the concentric fuel tubes. The Bunsen-like flames length shortens, and the flame tip becomes wider downstream with increasing shear rates between the two main jet streams. PLIF imaging of OH and CH show that the flame fragmentation rate increases with the increase in the shear rate, which explains the appearance of local extinction either in the flame sections near the pilot flame. The number of flame segments or number of breaks grows with the rise in the shear rate before settling at specific maximum value. In agreement with many previous studies of turbulent flames over much geometry, an increase in the local extinction near blow-off is observed.
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Padovani, Lorenzo. "Enstrophy Analysis of a Turbulent Temporal Plume." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2021.
Find full textAbstract:
The aim of the present thesis work is to analyse the enstrophy behaviour of a temporal turbulent plume. Several previous works have focused their attention on the role of vorticity or enstrophy in free shear flows, but they mainly concentrate on jets, wakes or mixing layers.
The analysis is performed on a temporal turbulent plume at time t = 40 which shows a Reλ= 89. The analyses performed start from a flow general features assessment. It is retrieved that the coherent vorticity structures inside a plume can be divided in Large Vorticity Structures (LVSs) and Intense Vorticity Structures (IVSs) and that the LVSs are responsible for the Turbulent/Non-Turbulent (T/NT) interface geometrical shape. In addition, the sensitivity to the enstrophy detection threshold is tested and verified retrieving a good interface robustness. The characteristics of the T/NT interface are analysed exploiting the traditional mean enstrophy budget equation and the conditional mean enstrophy budget equation.
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Kizildag, Deniz. "Numerical study of the non-Oberbeck-Boussinesq effects in turbulent water-filled cavities." Doctoral thesis, Universitat Politècnica de Catalunya, 2016. http://hdl.handle.net/10803/387437.
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The work carried out in the framework of the present thesis aims at shedding light into the complex phenomena involved in turbulent water-filled cavities, questioning the validity of the well-established Oberbeck-Boussinesq effects, and determining the influence of these on the flow structure and heat transfer.
First, the relevance of the variable thermophyical properties have been submitted to investigation by means of direct numerical simulations of a differentially heated cavity flow using the aspect ratio of a particular prototype. The simulations consider the so-called non-Oberbeck-Boussinesq effects, and study the temperature range for which these effects could be considered relevant. The work has been conducted employing a 2D flow assumption, estimating that this methodology -promoted by the necessity of a compromise between the accuracy and the cost of the simulations- would be valid to detect the non-Oberbeck-Boussinesq effects without the loss of generality, even though the actual flow structures of the flow are inherently 3D. The numerical results have revealed that up to the temperature difference of 30 ºC, Oberbeck-Boussinesq solution can estimate the heat transfer within 1 % error, although the loss of symmetry is certified even for a temperature difference of 10 ºC. Moreover, it has been observed that the boundary layers at hot and cold isothermal confining walls behave differently, such that the boundary layer instabilities and transition to turbulence location move downstream along the hot wall and upstream along the cold wall. As a consequence, the stratification region shifts upwards, giving rise to higher stratification numbers. Later, the non-Oberbeck-Boussinesq effects have been studied considering three-dimensional domain by means of direct numerical simulations, in the quest of analyzing their impact on the three-dimensional flow structure. The results have revealed delayed transition in the hot wall and earlier triggered transition in the cold wall boundary layers. This has been shown to be a consequence of the initial heating of the cavity due to favorable heat transfer properties in the hot wall boundary layer, which results in warmer upper cavity. As time advances, due to the influence of the stratified flow feeding the hot and cold boundary layers, the strength of the natural convection gradually decreases and increases in the hot and cold boundary layers, respectively. When a balance is attained between these two boundary layers, the cold wall boundary is found at a higher equivalent Rayleigh number, justifying its premature transition. Accordingly, the early transitioning cold wall boundary layer is thicker. This boundary layer interacts actively with the hot wall boundary layer, causing vertical oscillations in the transition to turbulent locations on both boundary layers. This interaction is also responsible for the degradation of the already shifted stratification zone. Besides the qualitative agreement in some aspects, this important effect is not captured by means of 2D simulations, which invalidates 2D flow hypothesis when it comes to describing the flow characteristics with non-Oberbeck-Boussinesq effects. As for the heat transfer, the Non-Oberbeck-Boussinesq effects do not necessarily enhance the heat transfer, as Oberbeck-Boussinesq solution is observed to overestimate the Nusselt number by about 3 %.
Last but not the least, considering the huge computational resources required for simulating these turbulent natural convection flows with water, and bearing in mind the importance of an appropriate modeling of the present phenomena, different subgrid-scale models have been analyzed in order to predict the thermal and fluid dynamics of the flow within a turbulent water-filled cavity. It has been shown that the performance of the models is directly linked to the accurate prediction of the transition to turbulence, which is the main challenge in the proper modeling of this flow.El treball realitzat en el marc de la present tesi té com a objectiu analitzar els fenòmens complexos involucrats en la convecció natural en cavitats amb aigua en règim turbulent, qüestionant la validesa de la ben establerta hipòtesi d'Oberbeck-Boussinesq. S'ha estudiat la influència dels efectes Oberbeck-Boussinesq sobre l'estructura de flux i la transferència de calor. En primer lloc, l'efecte de la dependència en la temperatura de les propietats termofísiques variables s'ha estudiat mitjançant simulacions numèriques directes del flux en una cavitat amb aigua escalfada diferencialment, emprant l'hipotèsi de flux 2D. Els resultats numèrics han revelat que per diferències de temperatura fins a 30 ºC, la solució Oberbeck-Boussinesq pot estimar la transferència de calor amb un error màxim d'1%, tot i que la pèrdua de simetria està certificada fins i tot per una diferència de temperatura de 10 ºC. D'altra banda, s'ha observat que les capes límit en les dues parets es comporten de manera diferent, de tal manera que les inestabilitats de la capa límit i el punt de transició es mouen aigües avall en la paret calenta i aigües amunt en la freda. Com a conseqüència d'això, la regió d'estratificació es desplaça cap amunt, donant lloc a un nombre d'estratificació més elevat. Tot seguit, els efectes Oberbeck-Boussinesq s'han estudiat tenint en compte el flux 3D per mitjà de simulacions numèriques directes, en la recerca d'analitzar el seu impacte en l'estructura del flux tridimensional. Els resultats han confirmat la transició retardada a la paret calenta i la transició provocada aigües amunt a la paret freda. S'ha demostrat que aquest fet és una conseqüència de l'escalfament inicial de la cavitat a causa de les propietats de transferència de calor favorables a la capa límit de la paret calenta, el que resulta en un escalfament de la part superior de la cavitat. A mesura que avança el temps, a causa de la influència del flux estratificat que alimenta les dues capes límit, la força de la convecció natural disminueix i augmenta gradualment en la capa límit de les parets calenta i freda, respectivament. Quan s'arriba a un equilibri entre aquestes dues capes límit, la capa límit de la paret freda es troba a un nombre de Rayleigh equivalent superior, justificant la seva transició prematura. En conseqüència, la capa límit de la paret freda és més gruixuda. Aquesta capa límit interactua activament amb la capa de la paret calenta, causant oscil·lacions verticals en el punt de transició en les dues capes límit. Aquesta interacció també és responsable de la degradació de la zona d'estratificació. Aquest important efecte no és capturat per mitjà de simulacions 2D, el que invalida la hipòtesi de flux 2D quan es tracta de descriure les característiques de flux amb efectes no-Oberbeck-Boussinesq. Pel que fa a la transferència de calor, els efectes no Oberbeck-Boussinesq no milloren necessàriament la transferència de calor, tal com s'observa a la solució Oberbeck-Boussinesq al sobreestimar el nombre de Nusselt en un 3%. Finalment, tenint en compte els enormes recursos computacionals necessaris per a la simulació d'aquestes cavitats en règim turbulent amb aigua, i tenint en compte la importància d'una modelització adequat dels fenòmens que s'hi troben, s'han analitzat els diferents models LES utilitzant un cas semblant, però sense efectes Boussinesq. S'ha demostrat que el rendiment dels models està directament relacionat amb la predicció precisa del punt de transició, que és el principal repte en la modelització adequada d'aquest flux.
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Düring, H. Gustavo. "Non-equilibrium dynamics of nonlinear wave systems : Turbulent regime, breakdown and wave condensation." Paris 6, 2010. http://www.theses.fr/2010PA066278.
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Dans cette thèse des problèmes relatifs à deux sujets différents ont été étudié. Tous les deux liés à la description statistique des systèmes avec nombreux degrés de liberté, mais d'une nature très différente. La première partie de ce travail est consacrée à l'étude des systèmes d'ondes faiblement non linéaires. La théorie bien établie de la turbulence des ondes est appliquée à différents systèmes, en étudiant la validité et les limites de celle-ci. Les propriétés statistiques à long terme pour les ondes élastiques dans les plaques et coques, et les ondes capillaires dans un fluide ont été considérées en fonction d'une équation cinétique pour la répartition des densités spectrales. Parmi d'autres situations, on analyse le régime turbulent, l'effet de la dissipation et la limite non dispersive. D'autre part, la description de la dynamique d'un condensat d'ondes a été mis en place, cela élargit la théorie de la turbulence d'ondes en incluant cette structure cohérente simple. Dans la deuxième partie on a étudié le problème du signe, qui représente une grosse limitation dans de nombreuses simulations numériques. Une bonne compréhension de la mécanique statistique des échantillonnages de Monte Carlo avec le problème du signe, est réalisée par analogie avec les systèmes vitreux. Une nouvelle approche est également proposée avec l'utilisation du recuit quantique, qui, loin de résoudre le problème du signe donne une compréhension plus approfondie de la structure de celui-ci.
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Al-Sharif, Sharaf. "Computation of unsteady and non-equilibrium turbulent flows using Reynolds stress transport models." Thesis, University of Manchester, 2010. https://www.research.manchester.ac.uk/portal/en/theses/computation-of-unsteady-and-nonequilibrium-turbulent-flows-using-reynolds-stress-transport-models(935dbd20-b049-4b62-9e1c-eebb261675e5).html.
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In this work the predictive capability of a number of Reynolds stress transport(RST) models was first tested in a range of non-equilibrium homogeneous flows, comparisons being drawn with existing direct numerical simulation (DNS) results and physical measurements. The cases considered include both shear and normally strained flows, in some cases with a constant applied strain rate, and in others where this varied with time. Models were generally found to perform well in homogeneous shear at low shear rates, but their performance increasingly deteriorated at higher shear rates. This was attributed mainly to weaknesses in the pressure-strain rate models, leading to over-prediction of the shear stress component of the stress anisotropy tensor at high shear rates. Performance in irrotational homogeneous strains was generally good, and was more consistent over a much wider range of strain rates. In the experimental plane strain and axisymmetric contraction cases, with time-varying strain rates, there was evidence of an accelerated dissipation rate generation. Significant improvement was achieved through the use of an alternative dissipation rate generation term, Pε , in these cases, suggesting a possible route for future modelling investigation. Subsequently, the models were also tested in the inhomogeneous case of pulsating channel flow over a wide range of frequencies, the reference for these cases being the LES of Scotti and Piomelli (2001). A particularly challenging feature in this problem set was the partial laminarisation and re-transition that occurred cyclically at low and, to a lesser extent, intermediate frequencies. None of the models tested were able to reproduce correctly all of the observed flow features, and none returned consistently superior results in all the cases examined. Finally, models were tested in the case of a plane jet interacting with a rectangular dead-end enclosure. Two geometric configurations are examined, corresponding a steady regime, and an intrinsically unsteady regime in which periodic flow oscillations are experimentally observed (Mataoui et al., 2003). In the steady case generally similar flow patterns were returned by the models tested, with some differences arising in the degree of downward deflection of the impinging jet, which in turn affected the level of turbulence energy developing in the lower part of the cavity. In the unsteady case, only two of the models tested, a two-equation k-ε model and an advanced RST model, correctly returned purely periodic solutions. The other two RST models, based on linear pressure-strain rate terms, returned unsteady flow patterns that exhibited complex oscillations with significant cycle-to-cycle variations. Unfortunately, the limited availability of reliable experimental data did not allow a detailed quantitative examination of model performance.
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Pouransari, Zeinab. "Fundamental studies of non-premixed combustion in turbulent wall jets using direct numerical simulation." Licentiate thesis, KTH, Turbulens, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-39028.
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The present thesis deals with the fundamental aspects of turbulent mixingand non-premixed combustion in wall-jet flows. Direct numerical simulations(DNS) of compressible turbulent flows are performed in a wall-jet configura-tion, which has a close resemblance to many industrial combustion applica-tions. The triple ”turbulence-chemistry-wall” interactions are also present inthis flow set-up. These interactions have been addressed by first focusing onturbulent flow effects on the isothermal reaction, including the near-wall issues.Then, by adding heat-release to the simulations, it has been concentrated onheat-release effects on various phenomena that occur in the reacting turbulentwall-jet flow. In the computational domain, fuel and oxidizer enter separatelyin a non-premixed manner and the flow is fully turbulent and subsonic in allsimulations. In the first phase of this study, the case of a turbulent wall-jetincluding an isothermal reaction without heat release is addressed in order toisolate the near-wall effects and the mixing characteristics of the flow and thekey statistics for combustion are studied in the absence of thermal effects. Adeeper insight into three-dimensional mixing and reaction characteristics in aturbulent wall-jet has been gained through investigation of the probability den-sity functions, higher order moments of velocities and reacting scalars and thescalar dissipation rates of different species. In the second phase, DNS of turbu-lent reacting wall-jets including heat release is performed, where a single-stepglobal exothermic reaction with an Arrhenius-type reaction rate is considered.The main target was to identify the heat-release effects on different mixingscales of turbulent wall-jet flow. The scalar dissipation rates, time scale ratios,two-point correlations, one and two-dimensional premultiplied spectra are usedto illustrate the heat release induced modifications. It is observed that heatrelease effects delay the transition process in the chemically reacting cases andenlarge the fluctuation intensities of density and pressure, but have a dampingeffect on all velocity fluctuation intensities. Finer small mixing scales were ob-served in the isothermal simulations and larger vortical structures formed afteradding significant amounts of heat-release. Simulations with different Damk ̈h- oler numbers, but comparable temperature-rise are performed and the expectedbehavior, a thinner flame with increasing Damk ̈hler number, is observed. Finally, some heat transfer related quantities are examined. The wall heat fluxand the corresponding Nusselt numbers are addressed. The near-wall reactioneffects on the skin friction coefficient are studied and further the reaction char-acteristics are investigated throughout the domain.QC 20110908
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Adams, Luke Wayne. "Experimental and computational study of non-turbulent flow regimes and cavern formation of non-Newtonian fluids in a stirred tank." Thesis, University of Birmingham, 2009. http://etheses.bham.ac.uk//id/eprint/394/.
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When non-Newtonian fluids are mixed in a stirred tank at low Reynolds numbers caverns can be formed around the impeller. If the fluid contains a yield stress the cavern has a fixed boundary where no flow occurs outside of it. When the fluid does not contain a yield stress a pseudo-cavern is formed, the cavern boundary is not fixed since flow can occur outside of it, but the majority of the flow is present in a region around the impeller. Mixing and cavern formation of a variety of non-Newtonian fluids are studied using experimental techniques and computational fluid dynamics (CFD). Cavern data extracted from both methods are compared with mechanistic cavern prediction models. An adapted planar laser induced fluorescence technique showed that mixing inside of a shear thinning Herschel-Bulkley fluid is very slow. Positron emission particle tracking obtained flow patterns and cavern sizes of three rheologically complex opaque fluids. CFD was able to predict the data obtained from both experimental techniques fairly well at low Reynolds numbers. A toroidal cavern model provided the best fit for single phase fluids but for the opaque fluids all models drastically over predicted the cavern size, with the cylindrical model only predicting cavern heights at high Reynolds numbers.
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Luo, Haining. "Simulation numérique directe pour un écoulement turbulent dans un T-jonction d'un fluide non-Newtonien." Thesis, Lyon, 2019. http://www.theses.fr/2019LYSEC026/document.
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Une configuration en T-jonction a été étudiée pour sa simplicité en géométrie en comparant avec d’autres mélangeurs en industrie. Plus particulièrement, j’ai effectué des simulations numériques directes avec OpenFOAM des T-jonction convergent à section circulaire et rectangulaire. Les fluides Newtonien et non-Newtonien (modèle Bird-Carreau) ont été pris en compte. Dans un premier temps, j’ai comparé mes données avec le travail expérimental de Nguyen [1] sur le T-jonction circulaire en régime deflecting. J’arrive à valider la DNS avec les données expérimentales. L’organisation des structures cohérentes sont illustrées en régime laminaire et turbulent en Newtonien et en non-Newtonien. Dans un deuxième temps, j’ai simulé deux régimes (deflecting et impinging) dans un T-jonction rectangulaire en Newtonien et en non-Newtonien. J’ai montré l’existence de structures cohérentes (par example kidney vortex) qui servent de moteur au mélange du scalaire passif propre au non-Newtonien. L’efficacité de mélange est augmentée en régime impinging par rapport au régime deflecting. Le shifting du pic de turbulence est observé uniquement en régime impingingFor the simplicity in geometry by comparing it with other mixers in the industry, flows in T-junction configuration have been studied. More specifically, Direct Numerical Simulations is carried out using OpenFOAM on a convergent T-junction configuration with circular and rectangular cross-section. Both Newtonian and non-Newtonian fluids (Bird-Carreau model) are taken into account. Firstly, DNS data is compared to Nguyen’s experimental work on the circular T-junction at regime deflecting [1]. Good agreement between simlation and experiment is achieved. The organization of coherent structures is illustrated in laminar and turbulent for both Newtonian and non-Newtonian cases. Secondly, two flow regimes (deflecting and impinging) are simulated in a rectangular T-junction for the same Newtonian and non-Newtonian fluids. The existence of non-Newtonian coherent structures (e.g. kidney vortex ) is shown. These structures are regarded as essential mixing mechanism of passive scalar mixing. The mixing efficiency is increased in regime impinging compared to regime deflecting. The shifting of the turbulence peak is only observed in regime impinging
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Claramunt, Altimira Kilian. "Numerical Simulation of Non-premixed Laminar and Turbulent Flames by means of Flamelet Modelling Approaches." Doctoral thesis, Universitat Politècnica de Catalunya, 2005. http://hdl.handle.net/10803/6680.
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Deep knowledge of combustion phenomena is of great scientific and technological interest. In fact, better design of combustion equipments (furnaces, boilers, engines, etc) can contribute both in the energy efficiency and in the reduction of pollutant formation. One of the limitations to design combustion equipments, or even predict simple flames, is the resolution of the mathematical formulation. Analytical solutions are not feasible, and recently numerical techniques have received enormous interest. Even though the ever-increasing computational capacity, the numerical resolution requires large computational resources due to the inherent complexity of the phenomenon (viz. multidimensional flames, finite rate kinetics, radiation in participating media, turbulence, etc). Thus, development of capable mathematical models reducing the complexity and the stiffness as well as efficient numerical techniques are of great interest.
The main contribution of the thesis is the analysis and application of the laminar flamelet concept to the numerical simulation of both laminar and turbulent non-premixed flames. Assuming a one-dimensional behavior of combustion phenomena in the normal direction to the flame front, and considering an appropriate coordinates transformation, flamelet approaches reduce the complexity of the problem.
The numerical methodology employed is based on the finite volume technique and a parallel multiblock algorithm is used obtaining an excellent parallel efficiency. A post-processing verification tool is applied to assess the quality of the numerical solutions.
Before dealing with flamelet approaches, a co-flow partially premixed methane/air laminar flame is studied for different levels of partial premixing. A comprehensive study is performed considering different mathematical formulations based on the full resolution of the governing equations and their validation against experimental data from the literature. Special attention is paid to the prediction of pollutant formation.
After the full resolution of the governing equations, the mathematical formulation of the flamelet equations and a deep study of the hypothesis assumed are presented. The non-premixed methane/air laminar flame is considered to apply the flamelet modelling approach, comparing the results with the simulations obtained with the full resolution of the governing equations. Steady flamelets show a proper performance to predict the main flame features when differential diffusion and radiation are neglected, while unsteady flamelets are more suitable to account for these effects as well as pollutant formation. Assumptions of the flamelet equations, the scalar dissipation rate modelling, and the evaluation of the Lagrangian flamelet time for unsteady flamelets are specially analysed.
For the numerical simulation of turbulent flames, the mathematical formulation based on mass-weighted time-averaging techniques, using RANS EVM two-equation models is considered. The laminar flamelet concept with a presumed PDF is taken into account. An extended Eddy Dissipation Concept model is also applied for comparison purposes. A piloted non-premixed methane/air turbulent flame is studied comparing the numerical results with experimental data from the literature. A clear improvement in the prediction of slow processes is shown when the transient term in the flamelet equations is retained. Radiation is a key aspect to properly define the thermal field and, consequently, species such as nitrogen oxides. Finally, the consideration of the round-jet anomaly is of significant importance to estimate the flame front position.
In conclusion, flamelet modelling simulations are revealed to be an accurate approach for the numerical simulation of laminar and turbulent non-premixed flames. Detailed chemistry can be taken into account and the stiffness of the chemistry term is solved in a pre-processing task. Pollutant formation can be predicted considering unsteady flamelets.
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Karaeren, Cenker. "Numerical Simulation Of Non-reacting Turbulent Flows Over A Constant Temperature Solid Surface In Regression." Master's thesis, METU, 2008. http://etd.lib.metu.edu.tr/upload/12609111/index.pdf.
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In this study, an attempt is made to obtain convergent and stable solutions of the K-E turbulence model equations for non-reacting turbulent flows over an isothermal solid surface in regression. A physics based mathematical model is used to describe the flow and temperature field over the moving surface. The flow is assumed to be two-dimensional, unsteady, incompressible with boundary layer approximations. Parabolized form of the standard K-E equations is adopted to simulate turbulence in the flow.
Regression of the solid surface causes the bounds of the solution domain to change with time, therefore a coordinate transformation is used in the vertical direction. The computational domain with fixed boundaries is discretized using an orthogonal grid system where a coordinate stretching is used in the vertical direction. A second order accurate, explicit finite difference technique is used for discretization of the governing equations. The final set of discretized equations is then solved using a solution algorithm specifically developed for this study. The verification of the solution algorithm includes a grid independence study, time increment study, and a comparison of the steady state results for the laminar and the turbulent flow cases. Finally, a parametric study is conducted using the proposed solution algorithm to test the stability of the numerical results for different Reynolds numbers, regression rates, and surface temperatures. It is concluded that the proposed numerical solution algorithm is capable of providing convergent and stable solutions of the two-equation turbulence model.