Dissertations / Theses on the topic 'Turbulence Closures'
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Weatheritt, Jack. "The development of data driven approaches to further turbulence closures." Thesis, University of Southampton, 2015. https://eprints.soton.ac.uk/388092/.
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The closure of turbulence models at all levels of fidelity is addressed, using unconventional methods that rely on data. The purpose of the thesis is not to present new models of turbulence per se, but rather the main focus is to develop the methodologies that created them. The main tool, Gene Expression Programming, is a versatile evolutionary algorithm. Implementations of the algorithm allow for symbolic regression of scalar and tensor fields and the clustering of data sets. The last two applications are novel algorithms. Scalar field regression is used to construct length scale damping functions for Hybrid RANS/LES. Direct Numerical Simulation snapshots are filtered to mimic Hybrid RANS/LES flow fields and from this new damping functions are created. Two closures are constructed, one from data in a turbulent pipe and another from slices along the classic backward facing step geometry. The new closures are tested for a range of separated flow applications. Tests alongside existing closures of the same class show that both new methods adapt to the local mesh resolution and turbulence level at least as well as other hybrid closures. Tensor field regression is used to construct non-linear stress-strain relationships in a Reynolds-Averaged Navier-Stokes framework. A common two-equation model is modified by including a further term that accounts for extra anisotropy with respect to the Boussinesq approximation. This model term, regressed from time averaged Direct Numerical Simulation data, turns the linear closure into an Explicit Algebraic Stress Model. The training data is taken from the reverse flow region behind a backward facing step. When applied to the classic periodic hills case, the subclass of models generated are found to greatly improve the prediction with respect to the linear model. A subclass of models is created in order to test the ability of the evolutionary algorithm. The deviation from the periodic hills reference data is quantified and used as a metric for model performance. The key finding is that improved performance of the Gene Expression Programming framework corresponded to improved prediction of the periodic hills. The final application of Gene Expression Programming, the clustering of datasets, is used to group Reynolds stress structures into distinct types. Firstly, reference Direct Numerical Simulation data obtained in a turbulent channel is categorised into six distinct groups. These groups are then compared to structures from Hybrid RANS/LES. These groups help to show that Hybrid RANS/LES structures do not correctly capture the near-wall cycle of turbulence. Instead there is an artificial cycle that is characterised by an incorrect buffer layer, defined by tall, long and thin structures. Further, streaky structures lie on the interface between Reynolds-Averaged Navier-Stokes and Large Eddy Simulation. These structures are free to move in the vertical direction and seriously contribute to discrepancies in the second order statistics.
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Ilicak, Mehmet. "Development and Applications of Second-Order Turbulence Closures for Mixing in Overflows." Scholarly Repository, 2009. http://scholarlyrepository.miami.edu/oa_dissertations/225.
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Mixing between overflows and ambient water masses is a crucial problem of deep-water formation in the down-welling branch of the meridional overturning circulation of the ocean. In this dissertation work, performance of second-order turbulence closures in reproducing mixing of overflows is investigated within both hydrostatic and non-hydrostatic models. First, a 2D non-hydrostatic model is developed to simulate the Red Sea overflow in the northern channel. The model results are compared to the Red Sea Outflow Experiment. It is found that the experiments without sub-grid scale models cannot reproduce the basic structure of the overflow. The k-ε model yields unrealistically thick bottom layer (BL) and interfacial layer (IL). A new technique so-called very large eddy simulation (VLES) which allows the use of k-ε model in non-hydrostatic models is also employed. It is found that VLES results the most realistic reproduction of the observations. Furthermore, the non-hydrostatic model is improved by introducing laterally average terms, so the model can simulate the constrictions not only in the z-direction but also in the y-direction. Observational data from the Bosphorus Strait is employed to test the spatially average 2D non-hydrostatic model (SAM) in a realistic application. The simulations from SAM with a simple Smagorinsky type closure appear to be excessively diffusive and noisy. We show that SAM can benefit significantly from VLES turbulence closures. Second, the performance of different second-order turbulence closures is extensively tested in a hydrostatic model. Four different two-equation turbulence closures (k-&epsilon, k-&omega, Mellor-Yamada 2.5 (MY2.5) and a modified version of k- &epsilon) and K-Profile Parameterization (KPP) are selected for the comparison of 3D numerical simulations of the Red Sea overflow. All two-equation turbulence models are able to capture the vertical structure of the Red Sea overflow consisting of the BL and IL. MY2.5 with Galperin stability functions produce the largest salinity deviations from the observations along two sections across the overflow and the modified k-&epsilon exhibits the smallest deviations. The rest of the closures fall in between, showing deviations similar to one another. Four different closures (k- &epsilon, k-&omega, MY2.5KC and KPP) are also employed to simulate the Mediterranean outflow. The numerical results are compared with observational data obtained in the 1988 Gulf of Cadiz Expedition. The simulations with two-equation closures reproduce the observed properties of the overflow quite well, especially the evolution of temperature and salinity profiles. The vertically integrated turbulent salt flux displays that the overflow goes under significant mixing outside the west edge of the Strait of Gibraltar. The volume transport and water properties of the outflow are modified significantly in the first 50 km after the overflow exits the strait. The k-&epsilon and k-&omega cases show the best agreement with the observations. Finally, the interaction between the Red Sea overflow and Gulf of Aden (GOA) eddies has been investigated. It is found that the overflow is mainly transported by the undercurrent at the west side of the gulf. The transport of the overflow is episodic depending strength and location of GOA eddies. The most crucial finding is that the Red Sea overflow leaves the Gulf of Aden in patches rather than one steady current. Multiple GOA eddies induce lateral stirring, thus diapycnal mixing of the Red Sea outflow.
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Shen, Bing. "The application of second-moment turbulence closures to 2D pulverised-coal flames." Thesis, Imperial College London, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.420722.
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Pascau, Benito Antonio. "The application of second order turbulence closures to isothermal and combusting swirling flows." Thesis, Imperial College London, 1990. http://hdl.handle.net/10044/1/46491.
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Marquis, Andrew James. "The application of high order convection approximations and second order turbulence closures to recirculating flows." Thesis, Imperial College London, 1986. http://hdl.handle.net/10044/1/38095.
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Kirwin, P. J. "Investigation and development of two-equation turbulence closures with reference to mixed convection in vertical pipes." Thesis, University of Manchester, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.525619.
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Rasam, Amin. "Anisotropy-resolving subgrid-scale modelling using explicit algebraic closures for large eddy simulation." Doctoral thesis, KTH, Turbulens, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-142401.
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The present thesis deals with the development and performance analysis ofanisotropy-resolving models for the small, unresolved scales (”sub-grid scales”,SGS) in large eddy simulation (LES). The models are characterised by a descriptionof anisotropy by use of explicit algebraic models for both the subgridscale(SGS) stress tensor (EASSM) and SGS scalar flux vector (EASSFM). Extensiveanalysis of the performance of the explicit algebraic SGS stress model(EASSM) has been performed and comparisons made with the conventionalisotropic dynamic eddy viscosity model (DEVM). The studies include LES ofplane channel flow at relatively high Reynolds numbers and a wide range ofresolutions and LES of separated flow in a channel with streamwise periodichill-shaped constrictions (periodic hill flow) at coarse resolutions. The formersimulations were carried out with a pseudo-spectral Navier–Stokes solver, whilethe latter simulations were computed with a second-order, finite-volume basedsolver for unstructured grids. The LESs of channel flow demonstrate that theEASSM gives a good description of the SGS anisotropy, which in turn gives ahigh degree of resolution independence, contrary to the behaviour of LES predictionsusing the DEVM. LESs of periodic hill flow showed that the EASSMalso for this case gives significantly better flow predictions than the DEVM.In particular, the reattachment point was much better predicted with the EASSMand reasonably well predicted even at very coarse resolutions, where theDEVM is unable to predict a proper flow separation.The explicit algebraic SGS scalar flux model (EASSFM) is developed toimprove LES predictions of complex anisotropic flows with turbulent heat ormass transfer, and can be described as a nonlinear tensor eddy diffusivity model.It was tested in combination with the EASSM for the SGS stresses, and itsperformance was compared to the conventional dynamic eddy diffusivity model(DEDM) in channel flow with and without system rotation in the wall-normaldirection. EASSM and EASSFM gave predictions of high accuracy for meanvelocity and mean scalar fields, as well as stresses and scalar flux components.An extension of the EASSM and EASSFM, based on stochastic differentialequations of Langevin type, gave further improvements. In contrast to conventionalmodels, these extended models are able to describe intermittent transferof energy from the small, unresolved scales, to the resolved large ones.The present study shows that the EASSM/EASSFM gives a clear improvementof LES of wall-bounded flows in simple, as well as in complex geometriesin comparison with simpler SGS models. This is also shown to hold for a widerange of resolutions and is particularly accentuated for coarse resolution. The advantages are also demonstrated both for high-order numerical schemes andfor solvers using low-order finite volume methods. The models therefore havea clear potential for more applied computational fluid mechanics.QC 20140304
Explicit algebraic sub-grid scale modelling for large-eddy simulations
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Sotgiu, Corrado [Verfasser]. "An investigation of anisotropic RANS turbulence closures for the heat transfer prediction in ribbed cooling passages / Corrado Sotgiu." München : Verlag Dr. Hut, 2021. http://d-nb.info/123842273X/34.
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Nenja, A. "Turbulence closure models." Thesis, Вид-во СумДУ, 2005. http://essuir.sumdu.edu.ua/handle/123456789/19821.
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Zonato, Andrea. "Modeling the Urban Boundary Layer in Complex Terrain." Doctoral thesis, Università degli studi di Trento, 2021. http://hdl.handle.net/11572/323566.
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In this work, various topics regarding (urban) boundary simulations for a city situated in the alps will be discussed. First of all, we will present novel parameterization adopted to take into account the effect of mitigation strategies, such as rooftop Photovoltaic Panels and Green Roofs, on the urban environment, and their effect on average temperature and energy consumption by buildings. Secondly, a new turbulence closure, that adopts a diagnostic equation for dissipation rate, and then independent on mixing length scales, will be introduced. The new turbulence closure, implemented into the WRF model, has been coupled with multi-layer urban parameterization schemes and compared with high-resolution CFD and LES simulations.
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Pratt, Jane. "Turbulence, Magnetics, and Closure Equations." Scholarship @ Claremont, 2003. https://scholarship.claremont.edu/hmc_theses/133.
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When a ferromagnet is heated, it loses its magnetism. Stars and planets have magnetic fields, as does the Earth. But it is known that the center of the Earth is very hot. Therefore, to sustain the large magnetic field of a planet, we cannot look to simple ferromagnetism like that of a bar magnet, but we have to look at the movement of electric charges within the Earth’s molten core to generate magnetic field. This magnetic field sustainment against ohmic dissipation by turbulent flow is referred to as the turbulent dynamo effect. Theoretical research into the mechanisms that create the dynamo has been actively pursued for several decades, culminating recently in massive computer simulations of the Earth’s core. Most of these studies have employed the equations of magnetohydrodynamics (MHD), a nonlinear theory of electrically conducting fluids. The EDQNM (Eddy-Damped Quasi-Normal Markovian) closure is a statistical model designed so that the turbulence equations derived from Navier-Stokes dynamics can be closed and satisfy the realizability condition of positivity of the kinetic energy spectrum. In case of MHD turbulence, realizability requires more work. We have proved in an earlier work that equations analogous to those expected of the EDQNM closure for MHD without mean fields satisfy the appropriate realizability conditions (Turner and Pratt 1999). In this work, we discuss requirements needed to make the MHD equations realizable with mean fields, extending those of neutral fluid turbulence by Turner [1]. Finally, we discuss direct numerical simulations and the correspondence of the statistical theories with simulation results.
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Sabat, Macole. "Modèles euleriens et méthodes numériques pour la description des sprays polydisperses turbulents." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLC086.
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De nos jours, la simulation des écoulements diphasiques a de plus en plus d’importance dans les chambres de combustion aéronautiques en tant qu’un des éléments requis pour analyser et maîtriser le processus complet de combustion, afin d’améliorer la performance du moteur et de mieux prédire les émissions polluantes. Dans les applications industrielles, la modélisation du combustible liquide trouvé en aval de l’injecteur sous forme de brouillard de gouttes polydisperse, appelé spray, est de préférence faite à l’aide de méthodes Eulériennes. Ce choix s’explique par les avantages qu’offrent ces méthodes par rapport aux méthodes Lagrangiennes, notamment la convergence statistique intrinsèque, le couplage aisé avec la phase gazeuse ainsi que l’efficacité pour le calcul haute performance. Dans la présente thèse, on utilise une approche Eulérienne basée sur une fermeture au niveau cinétique de type distribution Gaussienne Anisotrope (AG). L’AG résout des moments de vitesse jusqu’au deuxième ordre et permet de capter les croisements des trajectoires (PTC) à petite échelle de manière statistique. Le système d’équations obtenu est hyperbolique, le problème est bien-posé et satisfait les conditions de réalisabilité. L’AG est comparé au modèle monocinétique (MK) d’ordre 1 en vitesse. Il est approprié pour la description des particules faiblement inertielles. Il mène à un système faiblement hyperbolique qui peut générer des singularités. Plusieurs schémas numériques, utilisés pour résoudre les systèmes hyperboliques et faible- ment hyperboliques, sont évalués. Ces schémas sont classifiés selon leur capacité à traiter les singularités naturellement présentes dans les modèles Eulériens, sans perdre l’ordre global de la méthode ni rompre les conditions de réalisabilité. L’AG est testé sur un champ turbulent 3D chargé de particules dans des simulations numériques directes. Le code ASPHODELE est utilisé pour la phase gazeuse et l’AG est implémenté dans le code MUSES3D pour le spray. Les résultats sont comparés aux de simulations Lagrangiennes de référence et aux modèle MK. L’AG est validé pour des gouttes modérément inertielles à travers des résultats qualitatifs et quantitatifs. Il s’avère prometteur pour les applications complexes comprenant des PTC à petite échelle. Finalement, l’AG est étendu à la simulation aux grandes échelles nécessaire dans les cas réels turbulents dans le domaine industriel en se basant sur un filtrage au niveau cinétique. Cette stratégie aide à garantir les conditions de réalisabilités. Des résultats préliminaires sont évalués en 2D pour tester la sensibilité des résultats LES sur les paramètres des modèles de fermetures de sous maillesIn aeronautical combustion chambers, the ability to simulate two-phase flows gains increasing importance nowadays since it is one of the elements needed for the full understanding and prediction of the combustion process. This matter is motivated by the objective of improving the engine performance and better predicting the pollutant emissions. On the industrial scale, the description of the fuel spray found downstream of the injector is preferably done through Eulerian methods. This is due to the intrinsic statistical convergence of these methods, their natural coupling to the gas phase and their efficiency in terms of High Performance Computing compared to Lagrangian methods. In this thesis, the use of Kinetic-Based Moment Method with an Anisotropic Gaussian (AG) closure is investigated. By solving all velocity moments up to second order, this model reproduces statistically the main features of small scale Particles Trajectories Crossing (PTC). The resulting hyperbolic system of equations is mathematically well-posed and satisfies the realizability properties. This model is compared to the first order model in the KBMM hierarchy, the monokinetic model MK which is suitable of low inertia particles. The latter leads to a weakly hyperbolic system that can generate δ-shocks. Several schemes are compared for the resolution of the hyperbolic and weakly hyperbolic system of equations. These methods are assessed based on their ability to handle the naturally en- countered singularities due to the moment closures, especially without globally degenerating to lower order or violating the realizability constraints. The AG is evaluated for the Direct Numerical Simulation of 3D turbulent particle-laden flows by using ASPHODELE solver for the gas phase, and MUSES3D solver for the Eulerian spray in which the new model is implemented. The results are compared to the reference Lagrangian simulation as well as the MK results. Through the qualitative and quantitative results, the AG is found to be a predictive method for the description of moderately inertial particles and is a good candidate for complex simulations in realistic configurations where small scale PTC occurs. Finally, within the framework of industrial turbulence simulations a fully kinetic Large Eddy Simulation formalism is derived based on the AG model. This strategy of directly applying the filter on the kinetic level is helpful to devise realizability conditions. Preliminary results for the AG-LES model are evaluated in 2D, in order to investigate the sensitivity of the LES result on the subgrid closures
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Sadek, Shereef Aly. "A Basic Three-Dimensional Turbulent Boundary Layer Experiment To Test Second-Moment Closure Models." Diss., Virginia Tech, 2008. http://hdl.handle.net/10919/29706.
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In this work, a three-dimensional turbulent boundary layer experiment was set up with alternating stream-wise and span-wise pressure gradients. The pressure gradients are generated as a result of the test section wavy side wall shape. Each side had six sine waves with a trough to peak magnitude to wavelength ratio of 0.25. Boundary layer control was used so that the flow over the side walls remains attached. The mean flow velocity components, static and total pressures were measured at six plane along the stream-wise direction. The alternating mean span-wise and stream-wise pressure gradients created alternating stream-wise and span-wise vorticity fluxes, respectively, along the test section. As the flow developed downstream the vorticity created at the tunnel floor and ceiling diffused away from the wall.
The vorticity components in the stream-wise and span-wise directions are strengthened due to stretching and tilting terms in the vorticity transport equations. The positive-z half of the test section contains large areas that generate positive vorticity flux in the trough region and smaller areas generating negative vorticity around the wave peak. The opposite is true for the negative-z half of the test-section. This results in a large positive stream-wise vorticity in the positive-z half and negative stream-wise vorticity in the negative-z half of the test-section. The smaller regions of opposite sign vorticity in each half tend to mix the flow such that as they diffuse away from the wall, the turbulent stresses are more uniform.
Turbulent fluctuating velocity components were measured using Laser Doppler Velocimetery. Mean velocities as well as Reynolds stresses and triple velocity component correlations were measured at thirty stations along the last wave in the test section. Profiles at the center of the test section showed three dimensionality, but exhibited high turbulence intensities in the outer layer.
Profiles off the test section centerline are highly three dimensional with multiple peaks in the normal stress profiles. The flow also reaches a state where all the normal stresses have equal magnitudes while the shear stresses are non-zero.
Flow angles, flow gradient angles and shear stress angles show very large differences between wall values and outer layer vlaues. The shear stress angle lagged the flow gradient angle indicating non-equilibrium.
A turbulent kinetic energy transport budget is performed for all profiles and the turbulence kinetic energy dissipation rate is estimated. Spectral measurements were also made and an independent estimate of the kinetic energy dissipation rate is made. These estimates agree very well with those estimates made by balancing the turbulence kinetic energy transport equation.
Multiple turbulent diffusion models are compared to measured quantities. The models varied in agreement with experimental data. However, fair agreement with turbulence kinetic energy turbulent diffusion is observed. A model for the dissipation rate tensor anisotropy is used to extract estimates of the pressure-strain tensor from the Reynolds stress transport equations. The pressure-strain estimates are compared with some of the models in the literature. The comparison showed poor agreement with estimated pressure-strain values extracted from experimental data.
A tentative model for the turbulent Reynolds shear stress angle is developed that captures the shear stress angle near wall behavior to a very good extent. The model contains one constant that is related to mean flow variables. However, the developed expression needs modification so that the prediction is improved along the entire boundary layer thickness.Ph. D.
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Eremin, Aleksandr. "Numerical implementation of the wave-turbulence closure in a rotating channel." Thesis, Lyon, 2019. http://www.theses.fr/2019LYSEC031.
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L’étude porte sur un problème de turbulence dans un canal mis en rotation rapide. Dans ce cas, la non linéarité est dominée par la rotation, ce qui suggère d’utiliser la théorie de la turbulence d’ondes.La première partie de ce travail porte sur l’étude du modèle de fermeture pour la turbulence d’ondes (WTC pour « wave-turbulence closure »). Nous exprimons le champ de vitesse comme combinaison de modes de guide d’onde inertiels. Le confinement en canal implique aussi une discrétisation de la composante du vecteur d’onde normale à la paroi. Le transfert non linéaire est dominé par les interactions résonantes entre triades de vecteurs d’ondes. La viscosité, qui se manifeste par l’amortissement des modes, est la somme de deux contributions : l’une volumique, l’autre due à la paroi. Le taux d’amortissement en volume croît comme le carré du nombre d’onde, et inhibe la cascade d’énergie en-deçà d’une certaine échelle de longueur.L’implémentation numérique du modèle utilise un schéma d’avancement en temps qui assure la propriété de réalisabilité du modèle ainsi que la prise en compte des discontinuités spectrales prédites par la théorie de turbulence d’ondes. Les résultats de notre étude numérique du modèle WTC montrent que l’évolution en temps de la turbulence se produit en deux phases. Pendant la première phase, l’amortissement dû à la paroi est dominant, mais à la suite de la cascade d’énergie vers les petites échelles, l’amortissement volumique prend le dessus pendant la seconde phase. Lorsque le coefficient d’amortissement volumique est suffisamment petit, la transition entre les deux phases se produit brusquement à un instant qui est indépendant à la fois des coefficients d’amortissement volumique et de paroi, mais qui varie significativement avec le troisième paramètre du problème qu’est la largeur spectrale initiale. L’évolution du spectre révèle le développement d’une zone inertielle dont la pente se trouve presque indépendante des paramètres du problème. Le transfert d’énergie parallèlement aux parois du canal apparaît être plus efficace que dans la direction normale.En vue de réaliser des simulations numériques directes (DNS pour « Direct Numerical Simulations »), il a fallu développer une méthode appropriée à l’initialisation d’un champ de vitesse possédant les propriétés statistiques prescrites par le modèle. La comparaison des résultats de DNS et de WTC nécessite la construction de la matrice spectrale aux temps ultérieurs. Ceci a nécessité le développement de méthodes d’analyse spectrale et leur incorporation au sein du code de DNS existant. Malgré l’utilisation d’un super-calculateur et du calcul massivement parallèle, seuls trois calculs de DNS ont été possibles. Ces calculs utilisent les mêmes paramètres physiques mais différentes périodes spatiales pour la DNS, afin de vérifier la convergence en fonction des paramètres numériques. Idéalement, de nombreuses réalisations devraient être lancées et une moyenne d’ensemble prise pour calculer la matrice spectrale. Ceci n’étant pas possible avec un seul calcul, nous avons plutôt développé une méthode s’appuyant sur l’isotropie statistique dans les directions parallèles aux parois, dans laquelle les moyennes sont faites sur des domaines annulaires de l’espace spectral. Malheureusement, nos résultats indiquent que la non linéarité n’est pas suffisamment faible au nombre de Rossby utilisé dans les DNS. Par conséquent, un abaissement supplémentaire du nombre de Rossby serait nécessaire pour atteindre le régime d’applicabilité de la théorie de turbulence d’ondes. Ceci n’est cependant pas envisageable avec la puissance de calcul à dispositionA problem of turbulence in a rapidly rotating channel is investigated. The rapid rotation means that nonlinearity is dominated by rotation suggesting application of wave-turbulence theory.The first part of the work is devoted to study of the wave-turbulence closure (WTC) model. We express the velocity field as a combination of inertial waveguide modes. In its turn, confinement implies discretization of the wall-normal component of the wave vector. The nonlinear transfer is dominated by resonant interactions of wave-vector triads. Viscosity is present via modal damping, which is the sum of two components: volumetric and wall damping. The volumetric-damping rate grows as the square of the wavenumber inhibiting the energy cascade below a certain scale.The numerical implementation of the model uses a time-marching scheme ensuring the realizability property of the model and explicit consideration of the spectral discontinuities predicted by the wave-turbulence theory. According to the results of numerical investigation of the WTC model the time evolution of the turbulence occurs in two phases. During the first phase wall damping dominates, but following an energy cascade to the small scales, volumetric damping takes over during the second phase. Provided the volumetric-damping coefficient is sufficiently small, the transition between the phases takes place abruptly at a time which is insensitive to both wall- and volumetric- damping coefficients, but varies significantly with the third parameter of the problem, which is the initial spectral width. Evolution of the spectra reveals the development of an inertial range whose exponent is found to be almost independent of the problem parameters. The transfer of energy parallel to the channel walls is found to be more efficient than in the cross-channel direction.To perform direct numerical simulations (DNS) an appropriate method for initialization of the velocity field possessing the statistical properties prescribed by the model is developed. Comparison of the DNS and WTC results requires construction of the spectral matrix at later times. This involved the development of spectral analysis methods and their incorporation into the existing DNS code. Despite running the DNS on a super-computer and using many processors in parallel, only three runs were feasible. Those runs used the same physical parameters and different DNS spatial periods to check for convergence with respect to that numerical parameter. In an ideal world, many realizations would be performed and the ensemble average taken to calculate the spectral matrix. Given one run, this is not possible, so we instead developed a method based on statistical isotropy in the directions parallel to the walls in which averages are taken over annular regions in spectral space. Unfortunately, the results indicate that nonlinearity is not small enough for the Rossby number used in the DNS. That is, further reduction of the Rossby number would be needed to reach the regime of applicability of the wave-turbulence theory. This is not achievable with the computer power available
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Woolley, Robert Michael. "Conditional moment closures for turbulent reacting flows." Thesis, University of Leeds, 2003. http://etheses.whiterose.ac.uk/3304/.
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Mathematical modelling of the turbulent combustion process is becoming increasingly applied in calculations to assist in the design and analysis of practical combustion devices for efficiency-improvement and emission reduction. The current requirement to accurately predict pollutant emissions in many applications has increased the need for linking turbulent flow calculations and finite- rate chemistry effects in a rigorous way. Several methodologies are available for modelling such interactions, including the transported probability density function (PDF) approach and the conditional moment closure (CMC) method. Although in the early stages of its development, the CMC method has been shown to be a promising technique for predicting a wide range of practical problems. These include both premixed and non-premixed combustion, relatively slow chemistry effects, and ignition and extinction phenomena. This study concerns the CMC approach, and addresses the application of a number of models to a wide range of flows displaying varied compositions and geometries, including hydrogen and methane, and rim-stable and lifted jets. The impact of the choice of chemistry mechanism is considered for all the flows, and a higherorder CMC chemistry closure is investigated for the hydrogen flames. Analysis is made as to the ability of a parabolic CMC model to predict such flows, and the performance of the sub-model interactions is also reported on. The method of coupling the turbulent mixing field and the chemical kine tics is also investigated, and the effects of Reynolds stress and k - E turbulence closures upon subsequent CMC calculations are compared in all the flows considered. Overall, the results shown and conclusions drawn are very promising with respect to the possible future development of CMC. Requirements essential for this step forward of CMC methodologies for use in modelling practical geometries are specified, and an outline for the continuation of these studies is presented.
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Vaos, Evangelos. "Second moment closures for turbulent flows with reacting scalars." Thesis, Imperial College London, 1999. http://hdl.handle.net/10044/1/8438.
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Terentiev, Leonid. "The turbulence closure model based on linear anisotropy invariant analysis." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=979794781.
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Martin, Scott Montgomery. "The conditional moment closure method for modeling lean premixed turbulent combustion /." Thesis, Connect to this title online; UW restricted, 2003. http://hdl.handle.net/1773/7088.
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Fu, Song. "Computational modelling of turbulent swirling flows with second-moment closures." Thesis, University of Manchester, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.267917.
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This work focuses on the simulation of turbulent swirling flows within the framework of second-moment closure. The main objectives are to assess the performance of currently available turbulence models in predicting such flows, and to develop new closure models which would further enhance current predictive ability, and hence, to provide a reliable turbulence model for engineering applications that would help the design process and reduce the development costs of industrial combustion systems. Attention is confined to isothermal flows, and predictions have been carried out for three major swirling cases: a weakly and a strongly swirling free jet and a confined strongly swirling flow in which an annular swirling stream is discharged together with a non-swirling central jet into a suddenly enlarging circular chamber. In the last case, mass transfer has also been examined by predicting the behaviour of an inert scalar tracer with which the central jet has been laced. The existing turbulence models examined are the standard versions of the k — e Boussinesq-viscosity model, the algebraic stress closure and the differential stress closure (BVM, ASM and DSM, respectively), as well as modified ASM and DSM variants. One outcome of this study is that neither the standard versions of the BVM, ASM and DSM nor their previously modified forms examined here predict adequately swirling-flow behaviour. An important conclusion emerging from preliminary efforts has been that the algebraic approximation of stress transport in terms of the transport of turbulence energy—which is a widely used practice—is fundamentally flawed in the presence of swirl. Specifically, the method returns a physically unrealistic behaviour of the normal stresses. It is this conclusion which eventually led to the ASM methodology being discarded and to the exclusive use of the differential methodology. Within the framework of differential closures, two new pressure-strain models have been proposed, namely the Isotropization of Production and Convection Model (IPCM) and the Cubic Quasi-Isotropic Model (CQIM). The former emerged as an extension of the standard DSM approach with the inclusion of the convection tensor into the turbulence isotropization mechanism, whereas the latter follows from a more rational and fundamental approach in which non-linear anisotropy effects have been incorporated, with the resulting model made to satisfy the limit of two-dimensional turbulence. Comparisons between predicted solutions and measurements for swirling flow show that the IPCM produces a marked improvement over all the other models considered, while it does not significantly alter the behaviour of the standard stress closure in non-swirling conditions. Only very limited improvement is achieved by the CQIM, however, despite its success in predicting nearly homogeneous shear flows. The merits and weaknesses of all the models examined are discussed in detail, and the IPCM is recommended as the best approach for predictions of swirling flows. Within the study of the confined case, considerations were extended to the modelling of scalar transport by a second-moment flux closure, and comparisons are made between eddy-diffusivity and flux-closure calculations and experimental data. Computational results show that the distribution of the scalar field is primarily governed by aero-dynamic features. There are indications, however, that the flux model is superior to the eddy-diffusivity model.
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De, Paola Giorgio. "Conditional moment closure for autoignition in turbulent flows." Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.613313.
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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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Akroyd, Jethro. "Mean reaction rate closures for nanoparticle formation in turbulent reacting flow." Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/244968.
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This thesis investigates mean reaction rate closures for turbulent reacting flow. The closures model the mean rate of reaction in the flow and are applied to simulations of nanoparticle formation. The simulations couple detailed chemical reaction, particle population dynamics and turbulent flow, and offer the potential to improve the understanding of a range of industrial processes. The numerical behaviour of a mean reaction rate closure based on the direct quadrature method of moments using the interaction by exchange with the mean micromixing model (DQMoM-IEM) is studied in detail. An analytic expression is presented for the source terms and a filter function introduced to address issues of boundedness and singularity. Analytic integrals are presented for special cases of specific terms. The implementation of the method in the Star-CD computational fluid dynamics code is described in detail and validated against a test problem. The numerical performance of DQMoM-IEM is systematically compared to the stochastic fields (SF) turbulent reaction model. The methods share many similarities and are presented in a common mathematical framework for the first time. They differ in their treatment of key terms that make DQMoM-IEM numerically challenging. A variance reduction technique using antithetic sampling is introduced to increase the efficiency of the SF method. However, DQMoM-IEM is shown to remain competitive for the test problem considered. A new methodology is presented to couple a detailed particle model to simulations of turbulent reacting flow. A projected fields (PF) method based on DQMoM-IEM is used to combine detailed chemistry and the method of moments with interpolative closure (MoMIC) population balance model in Star-CD. The method is applied to the example of the chloride process for the industrial synthesis of titania nanoparticles and includes full coupling between the flow, chemistry and particles undergoing simultaneous inception, coagulation and surface growth.
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Arduini, Gabriele. "Modelling the stable, nocturnal boundary layer: different approaches to turbulence closure problem." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2013. http://amslaurea.unibo.it/5625/.
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Il lavoro è dedicato all'analisi fisica e alla modellizzazione dello strato limite atmosferico in condizioni stabili. L'obiettivo principale è quello di migliorare i modelli di parametrizzazione della turbulenza attualmente utilizzati dai modelli meteorologici a grande scala. Questi modelli di parametrizzazione della turbolenza consistono nell' esprimere gli stress di Reynolds come funzioni dei campi medi (componenti orizzontali della velocità e temperatura potenziale) usando delle chiusure. La maggior parte delle chiusure sono state sviluppate per i casi quasi-neutrali, e la difficoltà è trattare l'effetto della stabilità in modo rigoroso. Studieremo in dettaglio due differenti modelli di chiusura della turbolenza per lo strato limite stabile basati su assunzioni diverse: uno schema TKE-l (Mellor-Yamada,1982), che è usato nel modello di previsione BOLAM (Bologna Limited Area Model), e uno schema sviluppato recentemente da Mauritsen et al. (2007). Le assunzioni delle chiusure dei due schemi sono analizzate con dati sperimentali provenienti dalla torre di Cabauw in Olanda e dal sito CIBA in Spagna. Questi schemi di parametrizzazione della turbolenza sono quindi inseriti all'interno di un modello colonnare dello strato limite atmosferico, per testare le loro predizioni senza influenze esterne. Il confronto tra i differenti schemi è effettuato su un caso ben documentato in letteratura, il "GABLS1". Per confermare la validità delle predizioni, un dataset tridimensionale è creato simulando lo stesso caso GABLS1 con una Large Eddy Simulation. ARPS (Advanced Regional Prediction System) è stato usato per questo scopo. La stratificazione stabile vincola il passo di griglia, poichè la LES deve essere ad una risoluzione abbastanza elevata affinchè le tipiche scale verticali di moto siano correttamente risolte. Il confronto di questo dataset tridimensionale con le predizioni degli schemi turbolenti permettono di proporre un insieme di nuove chiusure atte a migliorare il modello di turbolenza di BOLAM. Il lavoro è stato compiuto all' ISAC-CNR di Bologna e al LEGI di Grenoble.
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Moustafa, Mohamed Zaki. "Advanced turbulence closure models and their application to buoyant and nonbuoyant flows." W&M ScholarWorks, 1988. https://scholarworks.wm.edu/etd/1539616785.
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In recent years, different approaches for modeling Reynolds stresses have been pursued by various authors. Two different closure schemes, representing two schools of thought, have been selected. The first closure scheme is the k-&\varepsilon& model. The second closure scheme is the level two model. Each group has claimed that the proposed models are an improvement and an advancement over previous models employing the mixing length hypothesis as a closure scheme. Neither group however, has made thorough comparisons between the newly proposed schemes and the most commonly used approach the mixing length hypothesis. The main objective of this paper is to test the applicability of the standard k-&\varepsilon& and the level two models in buoyant and non-buoyant flows. In addition, comparisons have been made between these new closure schemes and an existing mixing length model. Results obtained by employing the level two model for buoyant and non-buoyant flows are considered to be an improvement over those obtained by employing the mixing length model. A single set of constants was used for all level two model applications. Unlike the mixing-length closure scheme which requires tuning of the constants employed by the model for each individual application, there was no further tuning required for the level two model. Comparison between model results has shown no substantial improvement of the k-&\varepsilon& model over the level two model.
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Hodara, Joachim. "Hybrid RANS-LES closure for separated flows in the transitional regime." Diss., Georgia Institute of Technology, 2016. http://hdl.handle.net/1853/54995.
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The aerodynamics of modern rotorcraft is highly complex and has proven to be an arduous challenge for computational fluid dynamics (CFD). Flow features such as massively separated boundary layers or transition to turbulence are common in engineering applications and need to be accurately captured in order to predict the vehicle performance. The recent advances in numerical methods and turbulence modeling have resolved each of these issues independent of the other. First, state-of-the-art hybrid RANS-LES turbulence closures have shown great promise in capturing the unsteady flow details and integrated performance quantities for stalled flows. Similarly, the correlation-based transition model of Langtry and Menter has been successfully applied to a wide range of applications involving attached or mildly separated flows. However, there still lacks a unified approach that can tackle massively separated flows in the transitional flow region. In this effort, the two approaches have been combined and expended to yield a methodology capable of accurately predicting the features in these highly complex unsteady turbulent flows at a reasonable computational cost. Comparisons are evaluated on several cases, including a transitional flat plate, circular cylinder in crossflow and NACA 63-415 wing. Cost and accuracy correlations with URANS and prior hybrid URANS-LES approaches with and without transition modeling indicate that this new method can capture both separation and transition more accurately and cost effectively.
This new turbulence approach has been applied to the study of wings in the reverse flow regime. The flight envelope of modern helicopters has increased significantly over the last few decades, with design concepts now reaching advance ratios up to μ = 1. In these extreme conditions, the freestream velocity exceeds the rotational speed of the blades, and a large region of the retreating side of the rotor disk experiences reverse flow. For a conventional airfoil with a sharp trailing edge, the reverse flow regime is generally characterized by massive boundary layer separation and bluff body vortex shedding. This complex aerodynamic environment has been utilized to evaluate the new hybrid transitional approach. The assessment has proven the efficiency of the new hybrid model, and it has provided a transformative advancement to the modeling of dynamic stall.
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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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Amzin, Shokri. "Computations of turbulent premixed flames using conditional moment closure." Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/244193.
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Lean premixed combustion is at present one of the most promising methods to reduce emissions and to maintain high efficiency in combustion systems. As the emission legislation becomes more stringent, modelling of turbulent premixed combustion has become an important tool for designing efficient and environmentally friendlier combustion systems. However, in order to predict these emissions reliable predictive models are required. One of the methods used for predicting pollutants is the conditional moment closure (CMC), which is suitable to predict pollutants with slow time scales. Despite the fact that CMC has been successfully applied to various non-premixed combustion systems, its application to premixed flames is not fully tested and validated. The main difficulty is associated with the modelling of the conditional scalar dissipation rate (CSDR) of the conditioning scalar, the progress variable. In premixed CMC, this term is an important quantity and represents the rate of mixing at small scales of relevance for combustion. The numerical accuracy of the CMC method depends on the accuracy of the CSDR model. In this study, two different models for CSDR, an algebraic model and an inverse problem model, are validated using two different DNS data sets. The algebraic model along with standard k-ε turbulence modelling is used in the computations of stoichiometric and very lean pilot stabilized Bunsen flames using the RANS-CMC method. A first order closure is used for the conditional mean reaction rate. The computed nonreacting and reacting scalars are in reasonable agreement with the experiments and are consistent with earlier computations using flamlets and transported PDF methods for the stoichiometric flames, and transported PDF methods for the very lean flames. Sensitivity to chemical kinetics mechanism is also assessed.
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Tremblay, Frédéric. "Introduction of a second-moment closure turbulence model in a finite element formulation." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp01/MQ29632.pdf.
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Tremblay, Frédéric 1970. "Introduction of a second-moment closure turbulence model in a finite element formulation." Thesis, McGill University, 1997. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=27258.
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The present thesis deals with the successful introduction of a second-moment closure turbulence model into a computer program using the Finite Element Method to solve the Navier-Stokes equations. The implementation presented has the advantage of using an equal interpolation for all the variables. It is also very economical in terms of the amount of memory required from the computer, since a fully decoupled formulation has been adopted, along with an iterative solver which permits to store in memory only the non-zero coefficients of the linear system of equations to be solved. Specialized elements are used to avoid resolving the near-wall region of the flow. The apparent viscosity concept is derived for the finite element formulation, along with a correction factor which permits a better representation of the Reynolds stresses. The RSM is compared to the older $k - epsilon$ model in two test cases where experimental data was available. The conclusion drawn from this work is that the RSM is able to reproduce more phenomenon occurring in turbulent flows than the $k - epsilon$ model. It is thought that the $k - epsilon$ model will gradually be supplanted by more complex models, as more computing power become available.
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Zhou, Ye. "Renormalization group theory technique and subgrid scale closure for fluid and plasma turbulence." W&M ScholarWorks, 1987. https://scholarworks.wm.edu/etd/1539623774.
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Renormalization group theory is applied to incompressible three-dimension Navier-Stokes turbulence so as to eliminate unresolvable small scales. The renormalized Navier-Stokes equation includes a triple nonlinearity with the eddy viscosity exhibiting a mild cusp behavior, in qualitative agreement with the test-field model results of Kraichnan. For the cusp behavior to arise, not only is the triple nonlinearity necessary but the effects of pressure must be incorporated in the triple term.;Renormalization group theory is also applied to a model Alfven wave turbulence equation. In particular, the effect of small unresolvable subgrid scales on the large scales is computed. It is found that the removal of the subgrid scales leads to a renormalized response function. (i) This response function can be calculated analytically via the difference renormalization group technique. Strong absorption can occur around the Alfven frequency for sharply peaked subgrid frequency spectra. (ii) With the {dollar}\epsilon{dollar} - expansion renormalization group approach, the Lorenzian wavenumber spectrum of Chen and Mahajan can be recovered for finite {dollar}\epsilon{dollar}, but the nonlinear coupling constant still remains small, fully justifying the neglect of higher order nonlinearities introduced by the renormalization group procedure.
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Cheok, Van Seng Joseph. "Modelling the vertical structure of flows in the shelf seas." Thesis, Bangor University, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320063.
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Wright, Yuri Martin. "Numerical investigation of turbulent spray combustion with conditional moment closure /." Zürich : ETH, 2005. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=16386.
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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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Casel, Brian(Brian Scott). "Improved turbulent lift momentum closure for multiphase computational fluid dynamics." Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/129889.
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Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, February, 2020Cataloged from student-submitted PDF of thesis.
Includes bibliographical references (pages 63-65).
More efficient boiling heat transfer systems in nuclear reactors can help lower the costs of a large, low carbon energy source. Multiphase computational fluid dynamics (M-CFD) can be utilized in the design of these systems, but requires additional modeling for interphase transfer of mass, momentum, and energy [1]. Within the momentum transfer between phases, the interfacial lift force strongly affects the lateral migration of the gas phase in bubbly flow, which strongly impacts the predictions of pressure drop and heat transfer [2]. Recent work from Sugrue has proposed an improved physical representation of the turbulent lift force utilizing a combination of direct numerical simulation (DNS) data and a numerical optimization of the lift coefficient using experimental data [3].
The resulting Sugrue lift model yielded consistent and improved predictions of lateral redistribution of the gas phase in adiabatic air-water experiments; however, application to developing, bubbly flow has shown there is potential to further improve the accuracy of the formulation [4, 5]. In this work, a systematic optimization to the turbulent lift model is performed to adjust the Sugrue model and a new turbulent lift model is proposed. Both formulations out-perform the original Sugrue model on the Hibiki [6] experiment and the new turbulent lift model marginally improves performance on the TOPFLOW [7] experiments. Additionally, machine learning methods including k-nearest neighbors, principal component analysis, linear regression, random forests, and neural networks, are used to analyze M-CFD data to highlight parameters for future modeling.
The linear regression and random forest methods both suggest that superficial liquid and gas velocities (J[subscript l] and J[subscript g]), and slip ratio (S) are the three most important variables for modeling the lift coefficient. Additional data is needed to extract more precise modeling information from the candidate machine learning models in future study.
by Brian Casel.
S.M.
S.M. Massachusetts Institute of Technology, Department of Nuclear Science and Engineering
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Ziegenhein, Thomas, Dirk Lucas, Roland Rzehak, and Eckhard Krepper. "Closure relations for CFD simulation of bubble columns." Helmholtz-Zentrum Dresden-Rossendorf, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:d120-qucosa-144231.
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This paper describes the modelling of bubbly flow in a bubble column considering non-drag forces, polydispersity and bubble induced turbulence using the Eulerian two-fluid approach. The set of used closure models describing the momentum exchange between the phases was chosen on basis of broad experiences in modelling bubbly flows at the Helmholtz-Zentrum Dresden-Rossendorf. Polydispersity is modeled using the inhomogeneous multiple size group (iMUSIG) model, which was developed by ANSYS/CFX and Helmholtz-Zentrum Dresden-Rossendorf. Through the importance of a comprehensive turbulence modeling for coalescence and break-up models, bubble induced turbulence models are investigated. A baseline has been used which was chosen on the basis of our previous work without any adjustments. Several variants taken from the literature are shown for comparison. Transient CFD simulations are compared with the experimental measurements and Large Eddy Simulations of Akbar et al. (2012).
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Smith, Nigel Stuart Allen. "Development of the conditional moment closure method for modelling turbulent combustion." Phd thesis, Department of Mechanical and Mechatronic Engineering, 1994. http://hdl.handle.net/2123/8917.
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Huang, G. P. G. "The computation of elliptic turbulent flows with second-moment-closure models." Thesis, University of Manchester, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.377632.
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Devaud, Cecile Brigitte Claudine. "Conditional moment closure applied to lifted and attached turbulent jet flames." Thesis, University of Cambridge, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.624249.
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Bos, Wouter. "Passive scalar mixing in turbulent flow." Phd thesis, Ecole Centrale de Lyon, 2005. http://tel.archives-ouvertes.fr/tel-00199364.
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Le mélange d'un scalaire passif par un écoulement turbulent est étudié. D'abord, la simulation numérique directe (DNS), la simulation des grandes échelles (LES) et des arguments dimensionnels sont employés pour étudier le spectre du flux de scalaire dans une turbulence isotrope avec un gradient moyen uniforme de scalaire. Une loi d'échelle est dérivée. Cette loi conduit à des pentes du spectre variant entre -5/3 et -7/3 en zone inertielle. De premiers résultats de LES plaident en faveur d'un comportement en K^-2. Ensuite, en utilisant une fermeture en deux points (EDQNM), nous montrons qu'aux nombres de Reynolds très élevés, le spectre de flux de scalaire dans la zone intertielle se comporte en K^-7/3. Ce résultat est en accord avec l'analyse dimensionnelle classique de Lumley (1967). Aux nombres de Reynolds correspondant aux expériences de laboratoire, la fermeture conduit à des spectres plus près de K^-2. Nous montrons ensuite que le comportement en K^-2 trouvé en LES est induit par le forçage à grande échelle. La fermeture est alors appliquée au cas des écoulements homogènes cisaillés et les spectres du flux de scalaire longitudinal et transverse sont étudiés. Le spectre du flux longitudinal est trouvé proportionnelle à K^-23/9. Ce résultat est en accord avec l'expérience mais est en désaccord avec l'analyse dimensionnelle classique. Finalement, nous montrons que le lien entre la dispersion de particules et le mélange d'un scalaire permet de formuler une fermeture en deux points et un temps qui ne nécessite l'introduction d'aucune constante dans le modèle.
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DeSena, Geoffrey. "Improvement of RANS Forest Model via Closure Coefficient Modification." Thesis, Uppsala universitet, Institutionen för geovetenskaper, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-326514.
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As wind farms continue to take up more land throughout Northern Europe, developers are looking to sparsely populated areas, particularly in northern Fennoscandia, which hosts strong winds but also mixed and patchy forests over complex terrain. The complexity makes wind resource assessments difficult, raising uncertainty and therefore cost. Computational fluid dynamics (CFD) has the potential to increase the accuracy and reliability of wind models, but the most common form of commercial CFD modeling, Reynolds averaged Navier-Stokes (RANS), makes limiting assumptions about the effect of the forest on the wind. The wind resource assessment and energy estimation tool WindSim® , developed by WindSim AS, utilizes a porous medium model of a homogeneous forest with the influence of the forest on the airflow as a drag force term in the momentum equations. This method has provided reliable wind speed results but has been less reliable in estimating turbulence characteristics. The measure we evaluate in this study is turbulence intensity (TI). In this investigation, we make two types of modifications to the model and evaluate their impact on the TI estimates by using a benchmark data set collected by Meroney [1]. The first method is a variable profile of leaf area index (LAI) to represent the physical shape of the forest more accurately, and the second is a series of modifications to the closure coefficients in the turbulence transport equations. These modifications focus on the work of Lopes et al. [2], who used a large eddy simulation (LES) model to show that the turbulence production terms originally proposed by Green [3], expanded upon by Sanz [4], and widely used in the industry are unnecessary. Our investigations found that the implementation of a variable LAI profile has a small but non-negligible effect and that the elimination of the production terms from the turbulence transport equations does lead to a significant reduction in TI immediately above the forest. Both methods have minor effects on wind speed estimates, but the modification of closure coefficients has a much more significant impact on the TI. The coefficients proposed by Lopes et al. [2] drastically reduce TI estimates, but the model is still unable to reflect the Meroney data throughout the forest. Continued modification to new closure coefficients in combination with a variable forest LAI and other modifications such as a limited length scale may lead to significant improvement in TI estimates in future models, but these modifications must be compared against real-world data to ensure their applicability.
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Genin, Franklin Marie. "Study of compressible turbulent flows in supersonic environment by large-eddy simulation." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/28085.
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Thesis (M. S.)--Aerospace Engineering, Georgia Institute of Technology, 2009.Committee Chair: Menon, Suresh; Committee Member: Ruffin, Stephen; Committee Member: Sankar, Lakshmi; Committee Member: Seitzman, Jerry; Committee Member: Stoesser, Thorsten
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Ukai, Satoshi [Verfasser], and Andreas [Akademischer Betreuer] Kronenburg. "Conditional moment closure modelling of turbulent spray flames / Satoshi Ukai. Betreuer: Andreas Kronenburg." Stuttgart : Universitätsbibliothek der Universität Stuttgart, 2015. http://d-nb.info/1074139739/34.
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Lin, Chao-An. "Three-dimensional computations of injection into swirling cross-flow using second-moment closure." Thesis, University of Manchester, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.280543.
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Neske, Stefanie [Verfasser], Yaping [Gutachter] Shao, and Hendrik [Gutachter] Elbern. "Towards an improved Turbulence Closure Scheme by analysing ICON Model Simulations / Stefanie Neske ; Gutachter: Yaping Shao, Hendrik Elbern." Köln : Universitäts- und Stadtbibliothek Köln, 2017. http://d-nb.info/113277117X/34.
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Tselepidakis, Demetrios P. "Development and application of a new second-moment closure for turbulent flows near walls." Thesis, University of Manchester, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.332657.
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Chan, Lucilla. "Turbulent boundary-layer flow separation as portrayed by a two-dimensional, second-order closure model." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/NQ66343.pdf.
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Cleary, Matthew John. "CMC Modelling of Enclosure Fires." University of Sydney. Aerospace, Mechanical and Mechatronic Engineering, 2005. http://hdl.handle.net/2123/696.
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This thesis describes the implementation of the conditional moment closure (CMC) combustion model in a numerical scheme and its application to the modelling of enclosure fires. Prediction of carbon monoxide (CO) in the upper smoke layer of enclosure fires is of primary interest because it is a common cause of death. The CO concentration cannot be easily predicted by empirical means, so a method is needed which models the chemistry of a quenched, turbulent fire plume and subsequent mixing within an enclosed space. CMC is a turbulent combustion model which has been researched for over a decade. It has provided predictions of major and minor species in jet diffusion flames. The extension to enclosure fires is a new application for which the flow is complex and temperatures are well below adiabatic conditions. Advances are made in the numerical implementation of CMC. The governing combustion equations are cast in a conserved, finite volume formulation for which boundary conditions are uniquely defined. Computational efficiency is improved through two criteria which allow the reduction in the size of the computational domain without any loss of accuracy. Modelling results are compared to experimental data for natural gas fires burning under a hood. Comparison is made in the recirculating, post-flame region of the flow where temperatures are low and reactions are quenched. Due to the spatial flux terms contained in the governing equations, CMC is able to model the situation where chemical species are produced in the high temperature fire-plume and then transported to non-reacting regions. Predictions of CO and other species are in reasonable agreement with the experimental data over a range of lean and rich hood-fire conditions. Sensitivity of results to chemistry, temperature and modelling closures is inves- tigated. Species predictions are shown to be quite different for the two detailed chemical mechanisms used. Temperature conditions within the hood effect the for- mation of species in the plume prior to quenching and subsequently species predic- tions in the post-flame region are also effected. Clipped Gaussian and ß-function probability density functions (PDFs) are used for the stochastic mixture fraction. Species predictions in the plume are sensitive to the form of the PDF but in the post-flame region, where the ß-function approaches a Gaussian form, predictions are relatively insensitive. Two models are used for the conditional scalar dissipation: a uniform model, where the conditional quantity is set equal to the unconditional scalar dissipation across all mixture fraction space; and a model which is consistent with the PDF transport equation. In the plume, predictions of minor species are sensitive to the modelling used, but in the recirculating, post-flame region species are not significantly effected.
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Gao, Yuan. "Development of closures of scalar dissipation rate for large eddy simulation of turbulent premixed combustion using direct numerical simulation data." Thesis, University of Newcastle upon Tyne, 2016. http://hdl.handle.net/10443/3223.
Full textAbstract:
In turbulent premixed combustion, the mean reaction rate can be modelled based on scalar dissipation rate (SDR) in the context of both Reynolds Averaged Navier-Stockes (RANS) and Large Eddy Simulations (LES) simulations. The SDR, which characterises the mixing rate of the unburnt reactants and hot burnt products, itself requires modelling as well. The SDR based reaction rate closure has been studied extensively in the context of RANS. However, modelling of SDR and SDR based reaction rate closure are yet to be addressed in the context of LES for turbulent premixed combustion. There are two major approaches for SDR based reaction rate modelling, which are algebraic closure and SDR transport equation based closure respectively. Several Direct Numerical Simulation (DNS) databases, part of which were generated by this study, have been explicitly filtered using a Gaussian filter for both a-priori analysis of Favre filtered SDR and filtered SDR transport equation and a-posteriori assessment of the SDR based reaction rate closure. In the a-priori DNS analysis, a three-dimensional DNS database of freely propagating statistically planar flames for a range of different heat release parameter, global Lewis number and turbulent Reynolds number has been LES filtered using a Gaussian filter. An existing SDR based reaction rate closure for RANS simulations has been extended for LES and a satisfactory performance of this LES closure is observed for a range of filter widths, covering both laboratories scale to practical scales. When the generation and destruction of the scalar gradient are at equilibrium, it is viable for an algebraic SDR model in the context of LES. A-priori DNS assessment of algebraic SDR closures based on passive scalar mixing model and a power-law has been conducted, which have been found unsuitable for the reactive turbulent flows of premixed flames. Subsequently, a new algebraic model of Favre-filtered SDR has been proposed by extending a popular algebraic model of RANS averaged SDR into the context of LES. The performances of the newly proposed algebraic closure were assessed with respect to Favre-filtered SDR directly extracted from the DNS datasets. It has been found that the newly proposed SDR model for LES predicts both local and volume-averaged behaviours of SDR satisfactorily. However, when the generation and destruction of the scalar gradient are not at equilibrium, the Favre-filtered SDR transport equation need to be modelled for both RANS and LES. The statistical behaviours of the SDR transport equation have been studied for different global Lewis number, turbulent Reynolds number and heat release parameter at different filter widths. Based on the scaling analysis of all the unclosed terms in the Favre-filtered SDR transport equation, models are proposed for those terms in the context of LES and their performances have been assessed with respect to their corresponding values obtained from explicitly filtered DNS data. These newly proposed models are found to satisfactorily predict both the qualitative and quantitative behaviours of these unclosed terms for a range of different values of filter widths, heat release parameter, global Lewis number and turbulent Reynolds number. The newly proposed algebraic closure and transport equation based closure of Favrefiltered SDR in the context of LES, which were proposed based on simple chemistry DNS database, are assessed by a v-flame detail chemistry DNS database as a-posteriori assessment. The algebraic model is found to capture both qualitative and quantitative behaviours of SDR with reaction progress variable defined based on both deficient reactants and products. The models of the unclosed terms of SDR transport equation are found to capture the behaviours of the explicitly filtered terms of the detail chemistry DNS database in an order-of-magnitude sense. Further improvement is required in order to address the effects of diffusivity gradient, the gradients of reaction rate and molecular dissipation of SDR.
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Júnior, Guilherme Barbosa Lopes. "Organização de equações estatísticas para transferência de massa em processos turbulentos." Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/18/18138/tde-03052012-155115/.
Full textAbstract:
Em mecânica dos fluidos, especificamente em processos turbulentos, o problema de fechamento representa um dos maiores desafios para qualquer pessoa interessada nesta área. Durante décadas, cientistas vêm usando abordagens estatísticas com o objetivo de \"fechar\" o problema ou, pelo menos, diminuir as dificuldades inerentes. Assim, o presente trabalho apresenta uma criteriosa análise com base em ferramentas estatísticas em que ondas quadradas aleatórias, aliadas a um número fixo de parâmetros, foram utilizadas para criar equações paramétricas para representar um fluxo turbulento unidimensional com uma abordagem a priori, diferenciando de outras abordagens aplicadas amplamente na área, que utilizam uma abordagem a posteriori. Em seguida, simulações foram realizadas, a fim de avaliar o comportamento do modelo. Nas simulações pôde-se reproduzir o comportamento observado na literatura e estipular a abrangência do método. Além disso, uma importante discussão acerca das condições de contorno foi desenvolvida.In fluid mechanics, specifically in turbulent processes, the closure problem represents one of the biggest challenges for anyone interested in this area. For decades, scientists have been using statistical approaches aiming to close the problem or, at least, decrease the inherent difficulties. So, the present project presents a judicious analyze based on statistical tools in which random square waves, allied with a fixed numbers of parameters, were used to create parametric equations to represent a turbulent flow with an a priori approach, differentiating from other approaches broadly applied in the area, which use an a posteriori approach. Then simulations were done, in order to evaluate the behavior of the model. In the simulations, the behavior of some data from the literature could be followed and the scope of the method was stipulated. Besides this, an important discussion about boundary conditions was developed.