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1
Sung, Kyung-Sub. "Turbulent dispersion in strongly stratified turbulence." Thesis, Imperial College London, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.582577.
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The first part is the derivation of one-particle vertical diffusion for stably stratified turbulence with or without rapid rotation. Nicolleau & Vassilicos (2000) have analytically calculated vertical one-particle diffusion in stably stratified turbulence without rotation. One-particle vertical diffusion for turbulence with stable stratification and with or without rapid rotation has been derived here analytically using the solutions of the linearized equations of motions. The second part is an attempt to explain the depletion of horizontal pair diffusion in strongly stratified turbulence. "Recently, Nicolleau et al. (2005) have shown that in their Kinematic Simulations (KS) of vertically stably and strongly stratified homogeneous turbulence (Froude number smaller than 1). horizontal pair diffusion is significantly depleted by comparison to unstratified isotropic and homogeneous two- and three-dimensional turbulence. We have seeked to explain this depletion of horizontal pair diffusion by vertical stratification in terms of the probability density function of the horizontal divergence of the velocity field and the statistics of stagnation points following the recent approach to Richardson pair diffusion by Davila & Vassilicos (2003), Goto & Vassilicos (2004), Goto et al. (2005) and Osborne et al. (2005). We measure the number density of stagnation points in the KS of three-dimensional strongly stratified turbulence and find that it is virtually identical to what it is in KS of three-dimensional isotropic turbulence The third part is a study of the vertical motions of small, spherical inertial particles in strongly stratified turbulence.
2
Alves, Portela Felipe. "Turbulence cascade in an inhomogeneous turbulent flow." Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/63233.
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The inhomogeneous, anisotropic turbulence downstream of a square prism is investigated by means of direct numerical simulations (DNS) and two-point statistics. As noted by Moffatt (2002) “it now seems that the intense preoccupation [...] with the problem of homogeneous isotropic turbulence was perhaps misguided” acknowledging there is now a revived interest in studying inhomogeneous turbulence. The full description of the turbulence cascade requires a two-point analysis which re- volves around the recently derived Kármán-Howarth-Monin-Hill equation (KHMH). This equation is the inhomogeneous/anisotropic analogue to the so-called Kolmogorov equation (or Kármán-Howarth equation) used in Kolmogorov’s 1941 seminal papers (K41) which are the foundation to the most successful turbulence theory to date. Particular focus is placed on the near wake region where the turbulence is anticipated to be highly inhomogeneous and anisotropic. Because DNS gives direct access to all ve- locity components and their derivatives, all terms of the KHMH can be computed directly without resorting to any simplifications. Computation of the term associated with the non-linear inter-scale transfer of energy (Π) revealed that this rate is roughly constant over a range of scales which increases (within the bounds of our database) with distance to the wake generator, provided that the orientations of the pairs of points are averaged-out on the plane of the wake. This observation appears in tandem with a near −5/3 power law in the spectra of fluctuating velocities which deteriorates as the constancy of Π improves. The constant non-linear inter-scale transfer plays a major role in K41 and is required for deriving the 2/3-law (which is real space equivalent of the −5/3). We extend our analysis to a triple decomposition where the organised motion associ- ated with the vortex shedding is disentangled from the stochastic motions which do not display a distinct time signature. The imprint of the shedding-associated motion upon the stochastic component is observed to contribute to the small-scale anisotropy of the stochastic motion. Even though the dynamics of the shedding-associated motion differs drastically from that of the stochastic one, we find that both contributions are required in order to preserve the constant inter-scale transfer of energy. We further find that the inter- scale fluxes resulting from this decomposition display local (in scale-space) combinations of direct and inverse cascades. While the inter-scale fluxes associated with the coherent motion can be explained on the basis of simple geometrical arguments, the stochastic motion shows a persistent inverse cascade at orientations normal to the centreline despite its energy appearing to be roughly isotropically distributed.
3
Ahmed, Umair. "Flame turbulence interaction in premixed turbulent combustion." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/flame-turbulence-interaction-in-premixed-turbulent-combustion(f23c7263-df3d-41fa-90ed-41735fcaa34a).html.
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4
Rind, Elad. "Turbulent wakes in turbulent streams." Thesis, University of Southampton, 2011. https://eprints.soton.ac.uk/193955/.
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Direct numerical simulation and wind tunnel experiments have been used to study the effects of free-stream turbulence on axisymmetric wakes. In both cases the wake was introduced to various turbulent streams having various levels of turbulence intensity and length scales. It was found that the presence of the free-stream turbulence changes the wake’s decay rate and does not allow self-similarity to occur (unless maybe very far downstream and way beyond the current measurements reached). Also, the free-stream turbulence was found to be causing a significant transformation in the turbulence structure inside the wake, where the latter was found to be gradually evolving towards the former. Last, the fact that the two approaches were modelling two different problems led to some differences in their results emphasising the importance of the flow structure around the wake generating body in shaping the far wake region.
5
Vosskuhle, Michel. "Particle collisions in turbulent flows." Phd thesis, Ecole normale supérieure de lyon - ENS LYON, 2013. http://tel.archives-ouvertes.fr/tel-00946618.
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Cette thèse est consacrée au mécanisme conduisant à des taux de collisions importants dans les suspensions turbulentes de particules inertielles. Le travail a été effectué en suivant numériquement des particules, par simulations directes des équations de Navier-Stokes, et également par étude de modèles simplifiés. Les applications de ce domaine sont nombreuses aussi bien dans un contexte industriel que naturel (astrophysique, géophysique). L'approximation des collisions fantômes (ACF), souvent utilisée pour déterminer les taux de collision numériquement, consiste à compter dans une simulation, le nombre de fois que la distance entre les centres de deux particules devient plus faible qu'une distance seuil. Plusieurs arguments théoriques suggéreraient que cette approximation conduit à une surestimation du taux de collision. Cette thèse fournit non seulement une estimation quantitative de cette surestimation, mais également une compréhension détaillée des mécanismes des erreurs faites par l'ACF. Nous trouvons qu'une paire de particules peut subir des collisions répétées avec une grande probabilité. Ceci est relié à l'observation que, dans un écoulement turbulent, certaines paires de particules peuvent rester proches pendant très longtemps. Une deuxième classe de résultats obtenus dans cette thèse a permis une compréhension quantitative des très forts taux de collisions souvent observés. Nous montrons que lorsque l'inertie des particules n'est pas très petite, l'effet " fronde/caustiques ", à savoir, l'éjection de particules par des tourbillons intenses, est responsable du taux de collision élevé. En comparaison, la concentration préférentielle de particules dans certaines régions de l'espace joue un rôle mineur.
6
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
7
Sanderson, V. E. "Turbulence modelling of turbulent buoyant jets and compartment fires." Thesis, Cranfield University, 2001. http://hdl.handle.net/1826/137.
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Turbulent buoyant jets are a major feature in fire hazards. The solution of the
Reynolds Averaged Navier-Stokes (RANS) equations through computational fluid
dynamic (CFD) techniques allow such flows to be simulated. The use of Reynolds
averaging requires an empirical model to close the set of equations, this is known as
the turbulence model. This thesis undertakes to investigate linear and nonlinear
approaches to turbulence modelling and to apply the knowledge gained to the
simulation of compartment fires. The principle contribution of this work is the reanalysis
of the standard k- ε turbulence model and the implementation and
application of more sophisticated models as applied to thermal plumes.
Validation in this work, of the standard k- ε model against the most recent
experimental data, counters the established view that the model is inadequate for the
simulation of buoyant flows. Examination of previous experimental data suggests
that the measurements were not taken in the self-similar region resulting in
misleading comparisons with published numerical solutions. This is a significant
conclusion that impacts of the general approach taken to modelling turbulence in
this field.
A number of methods for modelling the Reynolds stresses and the turbulent scalar
fluxes have been considered and, in some cases for the first time, are applied to nonisothermal
flows. The relative influence of each model has been assessed enabling
its performance to be gauged. The results from this have made a valuable
contribution to the knowledge in the field and have enabled the acquired experience
to be applied to the simulation of compartment fires.
The overall conclusion drawn from this thesis is that for the simulation of
compartment fires, the most appropriate approach with current computational
resources, is still the buoyancy corrected standard k- ε model. However, the
turbulence scalar flux should be modelled by the generalised gradient diffusion
hypothesis (GGDH) rather than the eddy-diffusivity assumption.
8
Khorsandi, Babak. "Effect of background turbulence on an axisymmetric turbulent jet." Thesis, McGill University, 2011. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=104661.
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The effect of background turbulence on a turbulent jet was investigated experimentally. The primary objective of this work was to study the effect of different levels of the background turbulence on the dynamics and mixing of an axisymmetric turbulent jet at different Reynolds numbers. The secondary objective, which arose during the experiments, was to improve the acoustic Doppler velocimetry measurements which were found to be inaccurate when measuring turbulence statistics. In addition to acoustic Doppler velocimetry (ADV), flying hot-film anemometry was employed in this study. To move the hot-film probe at constant speeds, a high precision traversing mechanism was designed and built. A data acquisition system and LabVIEW programs were also developed to acquire data and control the traversing mechanism. The experiments started by benchmarking the two measurement techniques in an axisymmetric turbulent jet. Comparing the results with those of the other studies validated the use of flying hot-film anemometry to estimate the mean and the root-mean square (RMS) velocities. The experiments also validated the use of ADV for measurement of the mean velocities (measured in three Cartesian directions) and the RMS velocity (measured in the z-direction only). RMS velocities measured by the ADV along the x- and y-direction of the probe were overestimated.Attempts to improve the turbulence statistics measured by the ADV using the post-processing and noise-reduction methods presented in the literature were undertaken. However, the RMS velocities remained higher than the accepted values. In addition, a noise-reduction method was presented in this study which reduced the RMS velocities down to the accepted values. It was also attempted to relate Doppler noise to current velocity, and thus improve the results by subtracting the Doppler noise from the measured RMS velocities in the jet. However, no relationship was found between the Doppler noise and the mean velocity. The effect of different levels of background turbulence on the dynamics and mixing of an axisymmetric turbulent jet at different Reynolds numbers was then investigated. The background turbulence was generated by a random jet array. To confirm that the turbulence is approximately homogeneous and isotropic and has a low mean flow, the background flow was first characterized. Velocity measurements in an axisymmetric jet issuing into two different levels of background turbulence were then conducted. Three different jet Reynolds numbers were tested (Re = UJD/ν, where UJ is the jet exit velocity, D is the exit diameter of the jet, and ν is the kinematic viscosity). The results showed that (compared to the jet in a quiescent ambient) the mean axial velocities decay faster in the presence of background turbulence, while the mean radial velocities increase, especially close to the edges of the jet. At lower Reynolds numbers, the jet structure was destroyed in the near-field of the jet. The increase in the level of the background turbulence resulted in a faster decay of the mean axial velocities. The RMS velocity of the jet issuing into the turbulent background also increased, indicating that the level of turbulence in the jet increases. In addition, the jet's width increased in the presence of the background turbulence. The mass flow rate of the jet decreased in the presence of the background turbulence from which it can be inferred that the entrainment into the jet is reduced. The effect of background turbulence on entrainment mechanisms – large-scale engulfment and small-scale nibbling – is discussed. It is concluded that in the presence of background turbulence, engulfment is expected to be the main entrainment mechanism.L'effet de la turbulence ambiante sur l'évolution d'un jet turbulent est étudié dans le cadre de cette recherche expérimentale. L'objectif primaire de ce travail est l'étude de l'effet de l'intensité de la turbulence ambiante sur l'évolution d'un jet turbulent, à trois nombres de Reynolds différents. L'objectif secondaire est l'amélioration des mesures de vélocimétrie acoustique Doppler qui se sont avérées inexactes au cours de ce travail. Un dispositif à anémométrie à fil chaud volant a aussi été développé pour effectuer des mesures dans le cadre de cette étude. A cette fin, un mécanisme de translation a été conçu pour déplacer la sonde à vitesse constante. Un système d'acquisition de données et des programmes LabVIEW ont été développés pour enregistrer les données et contrôler le mécanisme. De premières expériences (dans un jet turbulent axisymétrique en milieu tranquille) ont prouvé le bien-fondé i) des mesures de vitesses moyenne et moyenne quadratique par anémométrie à fil chaud volant, et ii) des mesures de vitesse moyenne (dans tous le sens) et de vitesse moyenne quadratique (dans le sens z) par vélocimétrie acoustique Doppler. Les mesures par vélocimétrie acoustique Doppler dans les sens x et y étaient surestimées. L'amélioration des mesures de vitesse moyenne quadratique par vélocimétrie acoustique Doppler a été tentée par moyen de techniques de réduction de bruit existantes. Néanmoins, les vitesses moyennes quadratiques restaient surestimées. Une nouvelle technique de réduction de bruit (qui avait pour résultat des vitesses moyennes quadratiques précises) a été proposée dans le cadre de cette étude. En outre, des expériences ayant pour but de quantifier le rapport entre le bruit Doppler et la vitesse de l'écoulement ont été entreprises (pour pouvoir soustraire le bruit Doppler des mesures de vitesses moyennes quadratiques). Cependant, celles-ci n'ont trouvé aucun rapport entre ces deux quantités. Par la suite, l'effet de l'intensité de la turbulence ambiante sur l'évolution d'un jet turbulent axisymétrique, à trois nombres de Reynolds différents, a été étudié. La turbulence ambiante a été produite par moyen d'une maille de jets aléatoires. La turbulence ambiante s'est avérée, par moyen de mesures d'anémométrie à fil chaud volant et de vélocimétrie acoustique Doppler, homogène est isotrope. L'évolution d'un jet turbulent (à trois nombres de Reynolds) émis en milieux turbulents (de deux intensités différentes) a ensuite été étudiée. Les mesures ont démontré que la turbulence ambiante i) réduisait la vitesse axiale moyenne du jet (en augmentant le taux de décroissance), et ii) augmentait la vitesse radiale moyenne du jet (surtout prés du bord du jet). Pour les jets à nombre de Reynolds bas, la structure du jet a été détruite dans le champ proche du jet. Les vitesses moyennes quadratiques du jet émis en milieu turbulent étaient plus grandes, indiquant une croissance du niveau de turbulence dans le jet. En outre, la demi-largeur du jet augmentait en milieu turbulent. Par contre, en environnement turbulent, le débit massique du jet émis a diminué, ce qui implique que le taux d'entraînement du jet est aussi réduit. L'effet de la turbulence ambiante sur les mécanismes de l'entraînement (par engloutissement à grande échelle ou par grignotage) est examiné. Il est conclu que, en environnement turbulent, l'engloutissement est le mécanisme d'entraînement principal.
9
Irvine, Mark Rankin. "Turbulence and turbulent transport above and within coniferous forests." Thesis, University of Liverpool, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240324.
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Mergheni, Mohamed Ali. "Interactions particules - turbulence dans un jet axisymétrique diphasique turbulent." Rouen, 2008. http://www.theses.fr/2008ROUES067.
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Ce travail de thèse s'inscrit dans le cadre des études sur les écoulements turbulents gaz-solide et porte sur une étude numérique et une étude expérimentale de jets ronds coaxiaux diphasiques où le rapport des vitesses entre les jets externe et interne est supérieur et inférieur à un. Le but est de contribuer à la caractérisation des interactions entre la phase porteuse gazeuse et la phase dispersée et leur effet sur la modification de l'écoulement porteur. Le premier travail s'appuie sur une simulation de type Eulérienne / Lagrangienne qui résout les équations moyennées de Navier Stokes par la méthode des volumes finis. La turbulence du fluide est traitée par le modèle k-E standard. Le traitement de la phase dispersée consiste à un suivi Lagrangien de particules au sein de l'écoulement d'air. Le chargement en particules est suffisamment important pour que les particules influent sur la phase gazeuse (couplage) mais suffisamment faible pour pouvoir négliger les collisions interparticulaires. Le second travail consiste à réaliser un dispositif expérimental de jet gazeux ensemencé de particules solides (dp=100-212γm) issu d'un injecteur coaxial. L'écoulement diphasique est obtenu en utilisant un système d'ensemencement de particules assurant une injection régulière et homogène des particules dans le jet central. L'originalité de l'expérience consiste à mesurer simultanément les vitesses des particules et du fluide par une méthode optique non intrusive afin d'analyser le couplage entre deux phases. Ces résultats ont été obtenus à l'aide d'une chaîne de mesures optique PDA (Phase Doppler Anémométrie). L'analyse des caractéristiques dynamiques du fluide diphasique dans la zone proche de l'injecteur coaxial met en évidence que la vitesse de l'écoulement chargé est inférieure à la vitesse du fluide sans particules et que la présence des particules amplifie la turbulence du fluide lorsque la vitesse du jet centrale est supérieure à la vitesse du jet annulaire (ru>1). Ainsi, on note un décalage du pic de turbulence vers l'intérieur du jet central. Plus loin la vitesse moyenne du fluide en présence de particules devient supérieure à celle du jet monophasique à cause des transferts de quantité de mouvement des particules vers le fluide et on remarque une atténuation de la turbulence. Par contre, lorsque la vitesse du jet annulaire est supérieure à la vitesse du jet central (ru<1) on remarque une atténuation de la turbulence par la présence des particules et un décalage du pic de turbulence vers l'extérieur du jet central. On peut dire que la présence de particules solides permet à la turbulence de s'installer plus rapidement au sein du fluide pour ru>1. Lorsque ru<1, les particules ont tendance à calmer l'écoulement. Pour examiner l'approche numérique, les comparaisons avec mes travaux expérimentaux ont été réalisés. Les effets observés dans la partie expérimentale ont été reproduits dans deux cas différents (ru>1 et ru<1).
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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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Louch, Derek Stanley. "Vorticity and turbulent transport in premixed turbulent combustion." Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.625005.
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Donzis, Diego Aaron. "Scaling of turbulence and turbulent mixing using Terascale numerical simulations." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/19794.
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Fundamental aspects of turbulence and turbulent mixing are investigated using direct numerical simulations (DNS) of stationary isotropic turbulence, with Taylor-scale Reynolds numbers ranging from 8 to 650 and Schmidt numbers from 1/8 to 1024. The primary emphasis is on important scaling issues that arise in the study of intermittency, mixing and turbulence under solid-body rotation.
Simulations up to 2048^3 in size have been performed using large resource allocations on Terascale computers
at leading supercomputing centers.
Substantial efforts in algorithmic development have also been undertaken
and resulted in
a new code based on a two-dimensional domain decomposition
which allows
the use of very large number of processors.Benchmark tests indicate
very good parallel performance
for resolutions up to 4096^3 on up to 32768 processors.
Investigation of intermittency through the statistics of
dissipation and enstrophy in a series
of simulations at the same Reynolds number but different
resolution indicate that accurate
results in high-order moments require a higher degree
of fine-scale resolution than commonly practiced.
At the highest Reynolds number in our simulations (400 and 650)
dissipation and enstrophy exhibit
extreme fluctuations of O(1000) the mean
which have not been studied in
the literature before and suggest a universal scaling
of small scales.
Simulations at Reynolds number of 650 on 2048^3 grids
with scalars at Sc=1/8 and 1
have allowed us to obtain the clearest evidence of attainment of
inertial-convective scaling in the scalar spectrum
in numerical simulations to date whereas
results at high Sc support k^{-1} viscous-convective scaling.
Intermittency for scalars as measured by the tail of the PDF of scalar dissipation
and moments of scalar gradient fluctuations is found to saturate at high Sc.
Persistent departures from isotropy are observed as the Reynolds number increases.
However, results suggest a return to isotropy
at high Schmidt numbers, a tendency that appears to be stronger
at high Reynolds numbers.
The effects of the Coriolis force on
turbulence under solid-body rotation are investigated using
simulations on enlarged solution domains which
reduce the effects of
periodic boundary conditions.
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Keshava, Iyer Kartik P. "Studies of turbulence structure and turbulent mixing using petascale computing." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/52260.
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A large direct numerical simulation database spanning a wide range of Reynolds and Schmidt number is used to examine fundamental laws governing passive scalar mixing and turbulence structure. Efficient parallel algorithms have been developed to calculate quantities useful in examining the Kolmogorov small-scale phenomenology. These new algorithms are used to analyze data sets with Taylor scale Reynolds numbers as high as 650 with grid-spacing as small as the Kolmogrov length scale. Direct numerical simulation codes using pseudo-spectral methods typically use transpose based three-dimensional (3D) Fast Fourier Transforms (FFT). The ALLTOALL type routines to perform global transposes have a quadratic dependence on message size and typically show limited scaling at very large problem sizes. A hybrid MPI/OpenMP 3D FFT kernel has been developed that divides the work among the threads and schedules them in a pipelined fashion. All threads perform the communication, although not concurrently, with the aim of minimizing thread-idling time and increasing the overlap between communication and computation. The new algorithm is seen to reduce the communication time by as much as 30% at large core-counts, as compared to pure-MPI communication. Turbulent mixing is important in a wide range of fields ranging from combustion to cosmology. Schmidt numbers range from O(1) to O(0.01) in these applications. The Schmidt number dependence of the second-order scalar structure function and the applicability of the so-called Yaglomﾒs relation is examined in isotropic turbulence with a uniform mean scalar gradient. At the moderate Reynolds numbers currently achievable, the dynamics of strongly diffusive scalars is inherently different from moderately diffusive Schmidt numbers. Results at Schmidt number as low as 1/2048 show that the range of scales in the scalar field become quite narrow with the distribution of the small-scales approaching a Gaussian shape. A much weaker alignment between velocity gradients and principal strain rates and a strong departure from Yaglomﾒs relation have also been observed. Evaluation of different terms in the scalar structure function budget equation assuming statistical stationarity in time shows that with decreasing Schmidt number, the production and diffusion terms dominate at the intermediate scales possibly leading to non-universal behavior for the low-to-moderate Peclet number regime considered in this study. One of the few exact, non-trivial results in hydrodynamic theory is the so-called Kolmogorov 4/5th law. Agreement for the third order longitudinal structure function with the 4/5 plateau is used to measure the extent of the inertial range, both in experiments and simulations. Direct numerical simulation techniques to obtain the third order structure structure functions typically use component averaging, combined with time averaging over multiple eddy-turnover times. However, anisotropic large scale effects tend to limit the inertial range with significant variance in the components of the structure functions in the intermediate scale ranges along the Cartesian directions. The net result is that the asymptotic 4/5 plateau is not attained. Motivated by recent theoretical developments we present an efficient parallel algorithm to compute spherical averages in a periodic domain. The spherically averaged third-order structure function is shown to attain the K41 plateau in time-local fashion, which decreases the need for running direct numerical simulations for multiple eddy-turnover times. It is well known that the intermittent character of the energy dissipation rate leads to discrepancies between experiments and theory in calculating higher order moments of velocity increments. As a correction, the use of three-dimensional local averages has been proposed in the literature. Kolmogorov used the local 3D averaged dissipation rate to propose a refined similarity theory. An algorithm to calculate 3D local averages has been developed which is shown to scale well up to 32k cores. The algorithm, computes local averages over overlapping regions in space for a range of separation distances, resulting in N^3 samples of the locally averaged dissipation for each averaging length. In light of this new calculation, the refined similarity theory of Kolmogorov is examined using the 3D local averages at high Reynolds number and/or high resolution.
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Islam, Asiful. "An Implicit Hybrid Turbulence Model for Wall-Bounded Turbulent Aerodynamics." Thesis, The University of Sydney, 2019. https://hdl.handle.net/2123/21285.
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Resolving the small-scale streaks in viscous boundary layers require excessively refined, isotropic grids beyond the capability of all but the most powerful supercomputing facilities. This thesis contributes to the widespread effort in developing hybrid turbulence closures, which aim to reduce near-wall resolution requirements yet maintain turbulence-resolving capability in free-shear flow regions. This thesis evaluated the performance of an Implicit Large-Eddy Simulation (ILES) approach implemented within a very high-order accurate framework for a structured finite-volume, compressible solver. A new hybrid RANS-ILES model was proposed and systematically developed using hybrid length-scale modifications, a blending function, a boundary layer detection variable and a smoothing function. Different formulations for each of these mechanisms were numerically assessed, classified and eliminated to form the most optimal algorithm. This avoids some limitations of eddy-viscosity based subgrid-scale models used in conventional Large-Eddy Simulation and reduces case-specific calibration. The RANS-ILES hybrid model was used to simulate turbulent boundary layers highlighting its robust performance even on grids designed to induce Modelled-Stress Depletion. Under-resolved ILES results were degraded compared to URANS for such grids. Two Reynolds number regimes were used for flow around a cylinder and force coefficients, separation angles, wake profiles and shedding frequencies agreed well with past hybrid models, LES and experiments. The auxiliary transport equation successfully identified turbulent and non-turbulent regions in two automotive test cases. Surface pressures agreed well with experiments and a commercial LBM solver, despite some modelling simplifications. These simulations also leveraged high-order numerics and accurately predicted dominant spectral modes and coherent structures.
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Desjonqueres, Philippe. "Modélisation lagrangienne du comportement de particules discrètes en écoulement turbulent." Rouen, 1987. http://www.theses.fr/1987ROUES004.
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Sciacovelli, Luca. "Simulation numérique d'écoulements turbulents de gaz dense." Thesis, Paris, ENSAM, 2016. http://www.theses.fr/2016ENAM0061/document.
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Les écoulements turbulents de gaz denses, qui sont d’un grand intérêt pour un large éventail d'applications, sont le siège de phénomènes physiques encore peu connus et difficiles à étudier par des approches expérimentale. Dans ce travail, nous étudions pour la première fois l’influence des effets de gaz denses sur la structure de la turbulence compressible à l’aide de simulations numériques. Le fluide considéré est le PP11, un fluorocarbure lourd, dont le comportement thermodynamique a été représenté à l’aide de différentes lois d’état, afin de quantifier la sensibilité des solutions aux choix de modélisation. Nous avons considéré d’abord la décroissance d’une turbulence homogène isotrope compressible. Les fluctuations de température sont négligeables, alors que celles de la vitesse du son sont importantes à cause de leur forte dépendance de la densité. Le comportement particulier de la vitesse du son modifie de manière significative la structure de la turbulence, conduisant à la formation de shocklets de détente. L’analyse de la contribution des différentes structures à la dissipation d’énergie et à la génération d’enstrophie montre que, pour un gaz dense, les régions de forte dilatation jouent un rôle similaire à celles de forte compression, contrairement aux gaz parfaits, dans lesquels le comportement est fortement dissymétrique. Ensuite, nous avons mené des simulations numériques pour une configuration de canal plan en régime supersonique, pour plusieurs valeurs des nombres de Mach et de Reynolds. Les résultats confirment la validité de l’hypothèse de Morkovin. L’introduction d’une loi d’échelle semi-locale prenant en compte le variations de densité et viscosité, permet de comparer les profils des grandeurs turbulentes (contraintes de Reynolds, anisotropie, budgets d’énergie) avec ces observés en gaz parfait. Les variables thermodynamiques, quant à elles, présentent une évolution très différente pour un gaz parfait et pour un gaz dense, la chaleur spécifique élevée de ce dernier conduisant à un découplage des effets dynamiques et thermiques et à un comportement proche de celui d’un fluide incompressible avec des propriétés variablesDense gas turbulent flows, of great interest for a wide range of engineering applications, exhibit physical phenomena that are still poorly understood and difficult to reproduce experimentally. In this work, we study for the first time the influence of dense gas effects on the structure of compressible turbulence by means of numerical simulations. The fluid considered is PP11, a heavy fluorocarbon, whose thermodynamic behavior is described by means of different equations of state to quantify the sensitivity of solutions to modelling choices. First, we considered the decay of compressible homogeneous isotropic turbulence. Temperature fluctuations are found to be negligible, whereas those of the speed of sound are large because of the strong dependence on density. The peculiar behavior of the speed of sound significantly modifies the structure of the turbulence, leading to the occurrence of expansion shocklets. The analysis of the contribution of the different structures to energy dissipation and enstrophy generation shows that, for a dense gas, high expansion regions play a role similar to high compression ones, unlike perfect gases, in which the observed behaviour is highly asymmetric. Then, we carried out numerical simulations of a supersonic turbulent channel flow for several values of Mach and Reynolds numbers. The results confirm the validity of the Morkovin' hypothesis. The introduction of a semi-local scaling, taking into account density and viscosity variations across the channel, allow to compare the wall-normal profiles of turbulent quantities (Reynolds stresses, anisotropy, energy budgets) with those observed in ideal gases. Nevertheless, the thermodynamic variables exhibit a different evolution between perfect and dense gases, since the high specific heats of the latter lead to a decoupling of dynamic and thermal effects, and to a behavior close to that of variable property incompressible fluids
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Bernard, Donald Edward. "Optimization of Turbulent Prandtl Number in Turbulent, Wall Bounded Flows." ScholarWorks @ UVM, 2018. https://scholarworks.uvm.edu/graddis/824.
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After nearly 50 years of development, Computational Fluid Dynamics (CFD) has become an indispensable component of research, forecasting, design, prototyping and testing for a very broad spectrum of fields including geophysics, and most engineering fields (mechanical, aerospace, biomedical, chemical and civil engineering). The fastest and most affordable CFD approach, called Reynolds-Average-Navier-Stokes (RANS) can predict the drag around a car in just a few minutes of simulation. This feat is possible thanks to simplifying assumptions, semi-empirical models and empirical models that render the flow governing equations solvable at low computational costs. The fidelity of RANS model is good to excellent for the prediction of flow rate in pipes or ducts, drag, and lift of solid objects in Newtonian flows (e.g. air, water). RANS solutions for the prediction of scalar (e.g. temperature, pollutants, combustable chemical species) transport do not generally achieve the same level of fidelity. The main culprit is an assumption, called Reynolds analogy, which assumes analogy between the transport of momentum and scalar. This assumption is found to be somewhat valid in simple flows but fails for flows in complex geometries and/or in complex fluids.
This research explores optimization methods to improve upon existing RANS models for scalar transport. Using high fidelity direct numerical simulations (numerical solutions in time and space of the exact transport equations), the most common RANS model is a-priori tested and investigated for the transport of temperature (as a passive scalar) in a turbulent channel flow. This one constant model is then modified to improve the prediction of the temperature distribution profile and the wall heat flux. The resulting modifications provide insights in the model’s missing physics and opens new areas of investigation for the improvement of the modeling of turbulent scalar transport.
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Seshasayanan, Kannabiran. "Rotating turbulent dynamos." Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066158/document.
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Dans cette thèse, nous étudions l’effet de la turbulence en rotation sur l’instabilité dynamo. Nous étudions les différentes limites de la turbulence en rotation numériquement et théoriquement. D’abord, nous avons considéré l’effet dynamo engendré par les écoulements quasi-bidimensionnel (un écoulement avec trois composantes de vitesse qui dépendent de deux directions), qui modélise la limite de rotation très rapide. Nous avons étudié l’amplitude de saturation du champ magnétique en fonction du nombre de Prandt magnétique pour ce type d’écoulement. Un modèle théorique est développé et comparé avec les résultats numériques. Nous avons aussi regardé l’effet d’une vitesse bruitée sur le taux de croissance des différents moments du champ magnétique. Nous avons étudié l’écoulement 3D en rotation globale pour différents régimes du paramètre de contrôle. Pour l’écoulement hydrodynamique, nous avons étudié la transition vers une cascade inverse et les différents types de saturation de la cascade inverse. Nous avons regardé l’instabilité dynamo de ces écoulements. Nous avons montré que la rotation modifie le mode le plus instable et dans certains cas peut réduire le seuil de l’instabilité dynamoIn this thesis, we study the effect of rotating turbulent flows on the dynamo instability. We study the different limits of rotating turbulence using numerical simulations and theoretical tools. We first look at the dynamo instability driven by quasi-twodimensional flows (flows with three components varying along two directions), which models the limit of very fast rotation. We look at the saturation amplitude of the magnetic field as a function of the magnetic Prandtl number for such flows. A theoretical model for the dynamo instability is later developed and compared with the numerical results. We also study the effect of a fluctuating velocity field on the growth rate of different moments of the magnetic field. The three dimensional rotating flow is then studied for different range of parameters. For the hydrodynamic problem, we study the transition to an inverse cascade and the different saturation mechanism of the inverse cascade. Later the dynamoinstability driven by such flows is investigated. We show that the effect of rotation modifies the most unstable mode and in some cases can reduce the dynamo threshold
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McIlhenny, Julia F. "Artificial turbulent bursts." Link to electronic thesis, 2002. http://www.wpi.edu/Pubs/ETD/Available/etd-0110102-201212.
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Ramesh, Chandra D. S. "Turbulent Mixed Convection." Thesis, Indian Institute of Science, 2000. https://etd.iisc.ac.in/handle/2005/236.
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Turbulent mixed convection is a complicated flow where the buoyancy and shear forces compete with each other in affecting the flow dynamics. This thesis deals with the near wall dynamics in a turbulent mixed convection flow over an isothermal horizontal heated plate. We distinguish between two types of mixed convection ; low-speed mixed convection (LSM) and high-speed mixed convection (HSM). In LSM the entire boundary layer, including the near-wall region, is dominated by buoyancy; in HSM the near-wall region, is dominated by shear and the outer region by buoyancy. We show that the value of the parameter (* = ^ determines whether the flow is LSM or HSM. Here yr is the friction length scale and L is the Monin-Obukhov length scale.
In the present thesis we proposed a model for the near-wall dynamics in LSM. We assume the coherent structure near-wall for low-speed mixed convection to be streamwise aligned periodic array of laminar plumes and give a 2d model for the near wall dynamics, Here the equation to solve for the streamwise velocity is linear with the vertical and spanwise velocities given by the free convection model of Theerthan and Arakeri [1]. We determine the profiles of streamwise velocity, Reynolds shear stress and RMS of the fluctuations of the three components of velocity. From the model we obtain the scaling for wall shear stress rw as rw oc (UooAT*), where Uoo is the free-stream velocity and AT is the temperature difference between the free-stream and the horizontal surface.A similar scaling for rw was obtained in the experiments of Ingersoll [5] and by Narasimha et al [11] in the atmospheric boundary layer under low wind speed conditions. We also derive a formula for boundary layer thickness 5(x) which predicts the boundary layer growth for the combination free-stream velocity Uoo and AT in the low-speed mixed convection regime.
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Ramesh, Chandra D. S. "Turbulent Mixed Convection." Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/236.
Full textAbstract:
Turbulent mixed convection is a complicated flow where the buoyancy and shear forces compete with each other in affecting the flow dynamics. This thesis deals with the near wall dynamics in a turbulent mixed convection flow over an isothermal horizontal heated plate. We distinguish between two types of mixed convection ; low-speed mixed convection (LSM) and high-speed mixed convection (HSM). In LSM the entire boundary layer, including the near-wall region, is dominated by buoyancy; in HSM the near-wall region, is dominated by shear and the outer region by buoyancy. We show that the value of the parameter (* = ^ determines whether the flow is LSM or HSM. Here yr is the friction length scale and L is the Monin-Obukhov length scale.
In the present thesis we proposed a model for the near-wall dynamics in LSM. We assume the coherent structure near-wall for low-speed mixed convection to be streamwise aligned periodic array of laminar plumes and give a 2d model for the near wall dynamics, Here the equation to solve for the streamwise velocity is linear with the vertical and spanwise velocities given by the free convection model of Theerthan and Arakeri [1]. We determine the profiles of streamwise velocity, Reynolds shear stress and RMS of the fluctuations of the three components of velocity. From the model we obtain the scaling for wall shear stress rw as rw oc (UooAT*), where Uoo is the free-stream velocity and AT is the temperature difference between the free-stream and the horizontal surface.A similar scaling for rw was obtained in the experiments of Ingersoll [5] and by Narasimha et al [11] in the atmospheric boundary layer under low wind speed conditions. We also derive a formula for boundary layer thickness 5(x) which predicts the boundary layer growth for the combination free-stream velocity Uoo and AT in the low-speed mixed convection regime.
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CABRIT, BERTRAND. "Diffusion collective de la lumiere par un gaz turbulent : dispersion moleculaire et turbulente." Paris 6, 1992. http://www.theses.fr/1992PA066427.
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La lumiere diffusee par un fluide est mesuree a l'aide d'un spectrometre a melange optique de tres bonne resolution spectrale. Les conditions experimentales sont telles que les longueurs d'onde analysees sont tres grandes devant le libre parcours moyen des molecules. Dans ce cas, le spectre frequentiel du champ electrique de l'onde optique diffusee est caracteristique des fluctuations de la densite du fluide (modes hydrodynamiques). Les milieux observes sont des gaz turbulents (jet d'air, couche de melange supersonique, plasma). Ces ecoulements diffusent fortement la lumiere. La section efficace de diffusion est plusieurs ordres de grandeurs superieurs a la section efficace pour un gaz a l'equilibre thermodynamique. Les fluctuations de densite qui diffusent la lumiere verifient une equation de contaminant passif. La forme du spectre frequentiel est analysee en terme de mouvements turbulents lagrangiens et browniens. Trois situations sont successivement etudiees: le spectre reproduit la densite de probabilite de la vitesse du fluide (ddp); le spectre est elargi par la diffusion moleculaire; le spectre est retreci par la diffusion lagrangienne. Dans le premier cas nous montrons que le dispositif experimental permet une mesure precise et non-perturbative de la ddp de la vitesse de l'ecoulement. Dans le second cas nous mesurons le coefficient de diffusion thermique moleculaire. Dans le dernier cas la largeur du spectre fournit une mesure du coefficient de diffusion turbulente
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MacAulay, Phillip N. "An investigation of structure in a turbulent boundary layer developing on a smooth wall." Thesis, University of British Columbia, 1990. http://hdl.handle.net/2429/30002.
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The structure of a stable smooth wall zero pressure gradient turbulent boundary layer is investigated experimentally in order to determine the dominant outer region structure and to develop a hypothetical generalized boundary layer flow model. Three hot wire configurations, two vertically separated X-wires and a leading straight wire, a horizontal rake of 5 straight wires, and a vertical rake of 5 straight wires were used in the experiments,
conducted at Reɵ = 8200. The basis for data reduction procedures came from crosscorrelations and the Variable Interval Time Average (VITA) technique.
Three structure types are reported in the literature to be important: streaks and counter rotating streamwise vorticity, wall scaled hairpins or ring vortices, and large scale (0(ઠ)) bulges. A simple pictorial model consisting of three Reɵ dependent interdeveloping stages, which integrate all three structure types, is presented and discussed in relation to the literature and experiments performed. The rake data indicate that the positive ([formula omitted]u/[formula omitted]t) VITA detected velocity front has a scale much larger than that of the wall scaled eddies which typically have a scale of 100-300 y[formula omitted], and that this velocity front exhibits characteristics that are consistent with the trailing velocity front described in the model. The general convection velocity from basic crosscorrelations and the convection velocity of the positive VITA detected velocity front both had values 90-100% of the local mean velocity over most of the boundary layer. Evidence of small scale structure concentration on the downstream edge of the trailing velocity front is presented. A new method used to determine the average structure inclination angle associated with the trailing velocity front is presented and demonstrates that the generalized structure inclination angle, calculated from basic crosscorrelations between vertically separated
sensors, does not indicate structure shape, but is associated with the bulk flow associated with the structure. The new method appears to give results that are consistent with flow visualization and more accurately estimates the inclination angle associated with the most dominant feature of the outer flow, the positive VITA velocity front. Although the model presented is somewhat crude and further development and refinement are required, the model appears to agree with most data in the literature, as well as the present experimental results.Applied Science, Faculty of
Mechanical Engineering, Department of
Graduate
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Wu, Jiunn-Chi. "A study of unsteady turbulent flow past airfoils." Diss., Georgia Institute of Technology, 1988. http://hdl.handle.net/1853/13091.
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Onokpe, Oghenekevwe Owin. "Numerical investigation of turbulent hydrogen/air diffusion flames and turbulence radiation interactions." Thesis, Heriot-Watt University, 2011. http://hdl.handle.net/10399/2448.
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An investigation of the flame structure and radiation properties of turbulent hydrogen/air diffusion flames is reported. The laminar flamelet-conserved scalar probability density function approach is used to predict the scalar distributions throughout a laboratory-scale axisymmetric buoyant hydrogen diffusion flame. Predictions are compared with published measurements of mean and root mean square (RMS) temperatures and species concentrations based on the laminar flamelet concept. Predictions of spectral intensity and received heat flux are made with a narrow-band radiation model using mean properties, stochastic and mean emission methods to evaluate the effects of turbulence radiation interactions (TRI) and modelling TRI to predict the received radiant heat flux was very important. The predictions were, on the whole in good agreement with published measured data available in the open literature. Present study centres on the development of novel numerical models to predict TRI in turbulent hydrogen flames, implemented in a sophisticated way using enthalpy perturbation equation to account for radiative heat loss. This thesis highlights novel accomplishments in areas such as modelling lifted hydrogen jet flames, flame structures and external radiation fields where significant findings are reported. Firstly, successful extension of the lift-off model to hydrogen jet flames using strain rate as stretch parameter to accurately predict the lift-off height and affirming the smallscale strain rate model is better than the large-scale strain rate model which is different to methane lifted jet flames. Secondly, different jet flames were investigated using two different probability density functions (PDFs) and two transport equations taking into account fluctuations of temperature T ¢2 and water vapours 2 2 H O X¢ . The new Truncated Gaussian PDF was confirmed to give better predictions than other methods. Lastly, of the three approaches considered in modelling TRI the stochastic method proved the most accurate to predict the spectral intensity distribution and radiative heat flux distribution.
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Dallas, Vassilios. "Multiscale structure of turbulent channel flow and polymer, dynamics in viscoelastic turbulence." Thesis, Imperial College London, 2010. http://hdl.handle.net/10044/1/5855.
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This thesis focuses on two important issues in turbulence theory of wall-bounded flows. One is the recent debate on the form of the mean velocity profile (is it a log-law or a power-law with very weak power exponent?) and on its scalings with Reynolds number. In particular, this study relates the mean flow profile of the turbulent channel flow with the underlying topological structure of the fluctuating velocity field through the concept of critical points, a dynamical systems concept that is a natural way to quantify the multiscale structure of turbulence. This connection gives a new phenomenological picture of wall-bounded turbulence in terms of the topology of the flow. This theory validated against existing data, indicates that the issue on the form of the mean velocity profile at the asymptotic limit of infinite Reynolds number could be resolved by understanding the scaling of turbulent kinetic energy with Reynolds number. The other major issue addressed here is on the fundamental mechanism(s) of viscoelastic turbulence that lead to the polymer-induced turbulent drag reduction phenomenon and its dynamical aspects. A great challenge in this problem is the computation of viscoelastic turbulent flows, since the understanding of polymer physics is restricted to mechanical models. An effective numerical method to solve the governing equation for polymers modelled as nonlinear springs, without using any artificial assumptions as usual, was implemented here for the first time on a three-dimensional channel flow geometry. The superiority of this algorithm is depicted on the results, which are much closer to experimental observations. This allowed a more detailed study of the polymer-turbulence dynamical interactions, which yields a clearer picture on a mechanism that is governed by the polymer-turbulence energy transfers.
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Laenen, François. "Modulation de mélange, transport et turbulence dans des suspensions solides : étude et modélisation." Thesis, Université Côte d'Azur (ComUE), 2017. http://www.theses.fr/2017AZUR4010/document.
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Le transport de particules par des écoulements turbulents est un phénomène présent dans de nombreux écoulements naturels et industriels, tels que la dispersion de polluants dans l'atmosphère ou du phytoplancton et plastiques dans et à la surface des océans. Les modèles prédictifs classiques ne peuvent prévoir avec précision la formation de larges fluctuations de concentrations. La première partie de cette thèse concerne une étude de la dispersion turbulente de traceurs émis à partir d'une source ponctuelle et continue. Les fluctuations spatiales de masse sont déterminées en fonction de la distance à la source et à l'échelle d'observation. La combinaison de plusieurs phénomènes physiques à l'origine du mélange limite la validité d'une caractérisation de géométrie fractale. Une approche alternative est proposée, permettant d'interpréter les fluctuations massiques en terme des différents régimes de séparation de pair dans des écoulements turbulents. La seconde partie concerne des particules ayant une inertie finie, dont la dispersion dans l'espace des vitesses requiert de développer des techniques de modélisation adaptées. Une méthode numérique originale est proposée pour exprimer la distribution des particules dans l'espace position-vitesse. Cette méthode est ensuite utilisée pour décrire la modulation de la turbulence bi- dimensionnelle par des particules inertielles. A grand nombres de Stokes, l'effet montré est analogue à celui d'une friction effective à grande échelle. Aux petits Stokes, le spectre de l'énergie cinétique du fluide et les transferts non-linéaires sont modifiées d'une manière non trivialeThe transport of particles by turbulent flows is ubiquitous in nature and industry. It occurs in planet formation, plankton dynamics and combustion in engines. For the dispersion of atmospheric pollutants, traditional predictive models based on eddy diffusivity cannot accurately reproduce high concentration fluctuations, which are of primal importance for ecological and health issues. The first part of this thesis relates to the dispersion by turbulence of tracers continuously emitted from a point source. Mass fluctuations are characterized as a function of the distance from the source and of the observation scale. The combination of various physical mixing processes limits the use of fractal geometric tools. An alternative approach is proposed, allowing to interpret mass fluctuations in terms of the various regimes of pair separation in turbulent flows. The second part concerns particles with a finite and possibly large inertia, whose dispersion in velocity requires developing efficient modelling techniques. A novel numerical method is proposed to express inertial particles distribution in the position-velocity phase space. Its convergence is validated by comparison to Lagrangian measurements. This method is then used to describe the modulation of two-dimensional turbulence by large-Stokes-number heavy particles. At high inertia, the effect is found to be analogous to an effective large-scale friction. At small Stokes numbers, kinetic energy spectrum and nonlinear transfers are shown to be modified in a non-trivial way which relates to the development of instabilities at vortices boundaries
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COHEN, JACOB. "INSTABILITIES IN TURBULENT FREE SHEAR FLOWS." Diss., The University of Arizona, 1986. http://hdl.handle.net/10150/188143.
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The evolution of the large scale structures and the mean field were investigated in axisymmetric and plane mixing layers. Some aspects of the linear instability of an axisymmetric jet have been demonstrated. The axisymmetric geometry admits two additional length scales with relation to the two-dimensional shear layer: the radius of the jet column and the azimuthal wavelength. The importance of these two length scales in governing the instability of an axisymmetric jet was explored. The special case of a thin axisymmetric shear layer was analyzed and the results stressing the evolution of different azimuthal modes were compared with some phase-locked data which was produced by subjecting the jet to axisymmetric and helical excitation. The importance of the initial spectral distribution in a natural jet was demonstrated when it is used as an input to the amplification curve obtained from linear stability theory to predict a measured spectral distribution at a further downstream location. The inclusion of the nonlinear terms in the stability analysis reveals two main interactions: mean flow-wave interaction and wave-wave interaction. The modification of the mean flow of an axisymmetric jet was examined by exciting two azimuthal modes simultaneously. The interaction resulted in an azimuthal modulation of the mean velocity profile having a cosine shape. Effectively, the geometry of the jet was modified without changing the geometry of the nozzle. The coupling between an excited periodic disturbance and the mean flow was analyzed and the spatial evolution of both were compared with experimental results obtained in a plane mixing layer. The behavior of the concommittant Reynolds stresses is discussed in detail. The conditions under which one disturbance will transfer energy to another were derived and demonstrated in an axisymmetric jet. The interaction between a large amplitude plane wave with a weak subharmonic component was shown to enhance the amplification rate of the subharmonic. It was further shown that the nonlinear interaction between two azimuthal modes can produce a third azimuthal mode which was not initially present in the flow. The coupling between a fundamental wave and its subharmonic in a parallel plane mixing layer was demonstrated numerically.
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Ünsal, Bülent. "Time-dependent laminar, transitional and turbulent pipe flows = Zeitabhängige laminare, transitionale und turbulente Rohrströmungen." kostenfrei, 2008. http://d-nb.info/990023168/34.
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Padovani, Lorenzo. "Enstrophy Analysis of a Turbulent Temporal Plume." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2021.
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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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Gete, Zenebe. "et-enhanced turbulent combustion." Thesis, University of British Columbia, 1991. http://hdl.handle.net/2429/29969.
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A study of the squish-jet design concept in spark ignition engines, with central ignition, was conducted in a constant volume chamber. The effects of jet size, jet number and jet orientation in generating turbulence and jet enhanced turbulent combustion were investigated. Three sets of configurations with three port sizes were used in this study. The research was carried out in three stages:
1.Qualitative information was obtained from flow visualization experiments via schlieren photography at 1000 frames per second. The flow medium was air. A sequence of frames at specific time intervals were selected to study the results from the respective configurations
and jet sizes. The swirling nature of the flow is vivid in the offset arrangement.
2.Pre-ignition pressure and combustion pressure traces were measured with a piezoelectric
pressure transducer from which characterising parameters such as maximum pressure,
ignition advance and mass burn rate were analysed. Mass fraction curves were calculated
using the simple model of fractional pressure rise. A maximum pressure increase of 66% over the reference quiescent combustion case, and combustion duration reduction of 77% were obtained for the offset arrangement with 2 mm diameter port. Comparisons of the times required for 10%, 50% and 90% mass burned are identified and confirmed that it took the 2 mm jet the shortest time to burn 90% of the mixture in the chamber.
3.Two-component velocity measurements were made using an LDV system. Measurements
were taken in the central vertical plane of the chamber at specified locations. The data collected were window ensemble- averaged for the mean and fluctuating velocities over a number of cycles. Data intermittency and low data rate precluded, however, cycle-by-cycle analysis. Mean tangential velocities were calculated for each case and the data
were used to construct a movie of the tangential velocity as a function of time, suitable for quantitative flow visualization. The vortical nature of the flow was recorded, the distribution being neither solid body rotation nor free vortex, but some complex fluid motion.
The jet scale and orientation influence the in generation of turbulence flow field in the chamber, affecting the rate of combustion and the ensuing maximum pressure rise. The offset jet arrangement gives the best results, whereas radially opposed jets have a reduced effect. Increasing the number of jets in opposed arrangement does not enhance turbulent flow. Turbulent flow in the spark region during the onset of ignition was found to be important.Applied Science, Faculty of
Mechanical Engineering, Department of
Graduate
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Adams, Thomas M. "Turbulent convection in microchannels." Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/19421.
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Kaye, Nigel Gregory. "Interaction of turbulent plumes." Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.323741.
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Asgyer, Abulkasem A. "Turbulent premixed impinging flames." Thesis, University of Manchester, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488202.
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Abdullatif, Tawfik A. "Turbulent diffusion impinging flames." Thesis, University of Manchester, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488402.
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Nokes, Roger Ian. "Problems in turbulent dispersion." Thesis, University of Canterbury. Civil Engineering, 1985. http://hdl.handle.net/10092/7711.
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Solutions to a number of turbulent dispersion problems, involving a contaminant issuing from a steady source into uniform, steady open channel flow, are presented. These problems include the two and three-dimensional dispersion of a neutrally buoyant contaminant and the two-dimensional dispersion of buoyant particles and are modelled with the diffusion equation incorporating turbulent diffusion coefficients. In order to ensure that the solutions simulate the physical processes as accurately as possible the experimentally determined logarithmic velocity profile and the theoretically deduced parabolic diffusivity are used.
The solutions take the form of one or two eigenfunction expansions, the eigenfunctions and eigenvalues of which are governed by Sturm-Liouville theory. Generally the power series method for solving ordinary differential equations is employed to derive the eigenfunctions and eigenvalues and in nearly all cases this method is found to be accurate, straightforward in its use and efficient with computing resources.
Much useful information is deduced from the eigenfunctions and eigenvalues. The rate at which equilibrium conditions are approached and the ideal source position, that from which the contaminant is most rapidly mixed, come naturally from these quantities.
An experimental programme, with the aims of verifying the theoretical solution for vertical dispersion of a neutrally buoyant contaminant and measuring the lateral turbulent diffusion coefficient, is described. The two-dimensional results for vertical mixing strongly support the theoretical predictions, using the measured logarithmic velocity profile and the deduced parabolic diffusivity in the turbulent diffusion equation, and indeed confirm the location of the ideal source derived from theory.
The values of the depth-averaged lateral diffusivity obtained from the experiments lie at the lower end of the range of values obtained by other experimentalists. A reanalysis of these previously published results demonstrates that, provided the natural turbulence of a wide channel is the only mixing mechanism present, the depth-averaged lateral diffusivity, non-dimensionalised by the flow depth and shear velocity, is in fact independent of all flow parameters, except when the friction factor is small.
The dependence of the rate of lateral spreading on height in the flow and the location of the source demonstrates, at least qualitatively, that the vertical dependence of the lateral diffusivity is in essence the same as the velocity distribution.
Verification of the theoretical solution for dispersion of buoyant particles is achieved with the experimental results of Jobson and Sayre (1970) which indicate that the theoretical model is valid for fine particles dispersing in strongly turbulent flow.
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Goh, Eng Yew. "Turbulent slender flow calculations." Thesis, Imperial College London, 1990. http://hdl.handle.net/10044/1/46316.
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Markides, Christos Nicolaos. "Autoignition in turbulent flows." Thesis, University of Cambridge, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.613893.
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Rigas, Georgios. "Modelling of turbulent wakes." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/26590.
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The dynamics of the turbulent three-dimensional wake generated by an axisymmetric bluff body with blunt trailing edge are experimentally and theoretically investigated at a diameter based Reynolds number of 188,000. A detailed analysis of the base pressure measurements shows that the large scale structures of the turbulent three-dimensional wake retain the structure of the laminar instabilities observed in the transitional regimes, in a statistical sense. These persisting instabilities at the turbulent regime, are associated with spatial and temporal symmetry breaking, giving rise to spatial reflectional symmetry and quasi-periodic vortex shedding. The influence of turbulence recovers the lost temporal and spatial symmetries in the long-time average. It is shown that the turbulent spatial dynamics are reproduced by a simple stochastic model the deterministic part of which accounts for the spatial symmetry breaking and gives rise to steady large scale structures through a supercritical pitchfork bifurcation, and the stochastic part modelling in a phenomenological sense the turbulent fluctuations acting on the large scale structures. The axisymmetric body wake is further investigated when axisymmetric slot-jet zero-net-mass-flux forcing is applied on the rear base. Landau-like models that capture the weakly nonlinear interaction between the global vortex shedding mode and axisymmetric forcing are derived from the phase-averaged Navier-Stokes equations. The Landau-like models describe accurately the forced response by means of measured base pressure of the global vortex shedding mode. With the present analysis it is demonstrated that the concept of weakly nonlinear global modes can be extended to a fully turbulent flow, far from the critical bifurcation Reynolds number, and a general framework for the description of systems with broken symmetries---giving rise to global dynamics---and turbulent dynamics is provided. The novel results presented here advance the understanding of the dynamics of three-dimensional turbulent wakes and pave the way for turbulence prediction and control.
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Brown, A. R. "Modelling turbulent sheared convection." Thesis, University of Surrey, 1995. http://epubs.surrey.ac.uk/843794/.
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Large-eddy simulations of the atmospheric boundary layer have been performed over a range of stabilities between neutral and free convective conditions. The variation of various non-dimensionalized turbulence statistics over this stability range is presented and the results are compared with observations where possible. The robustness of the model results is also assessed by comparing those from high and low resolution simulations, and by reference to a number of additional sensitivity tests. The simulation results for the variation with stability of the mean wind and temperature profiles and various similarity coefficients are presented. The large-eddy model datasets are then used to evaluate critically the performance of a number of simple closure schemes suitable for use in boundary layer parametrizations in large-scale weather forecasting and climate prediction models. The potential significance of the shortcomings of the simplest mixing length schemes is discussed, and an assessment is made of the types of closure most likely to give a significant improvement in performance without an excessive computational overhead. Results are also presented from large-eddy simulations of the baroclinic boundary layer. The effects of the shear in the geostrophic wind on scaled turbulence statistics and the mean wind profiles are discussed. It is shown that this shear does not lead to significant degradation of the performance of two simple closure models, in either neutral or convective conditions. Finally simulation results for the entrainment flux at the top of the boundary layer are presented. A parametrization of this flux is developed, based on the boundary layer root mean square vertical velocity.
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Prabhudesai, Gaurav. "Fluctuations in turbulent flows." Thesis, Université Paris sciences et lettres, 2021. http://www.theses.fr/2021UPSLE001.
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Dans cette thèse, nous étudions les fluctuations de vitesse et de température dans un écoulement turbulent, et leurs implications sur la propagation d’ondes acoustiques en turbulence. La première partie est consacrée à l’étude de la génération spontanée de fluctuations de température par un écoulement turbulent. Nous démontrons que ces fluctuations de température proviennent de deux types de structures intermittentes, les filaments de vorticité et les structures dissipatives. Dans la seconde partie, nous étudions les fluctuations spatio-temporelles de la vitesse, en utilisant la fonction de cohérence. Nous démontrons que la fonction de cohérence résulte d’un balayage des fluctuations de vitesse dans la gamme inertielle par les fluctuations plus lentes de l’échelle intégrale. L’effet du balayage est particulièrement intéressant pour l’étude des écoulements turbulents, car il ne rentre pas dans le cadre de la théorie K41 de Kolmogorov. Enfin, nous étudions la propagation d’ondes acoustiques à travers un écoulement turbulent, et en particulier les fluctuations de phase et d’amplitude. Nous montrons que les fluctuations de phase peuvent être déduites de la fonction de cohérence des fluctuations de vitesse. Ces fluctuations résultent ainsi d’un effet de balayage de l’onde acoustique par l’écoulement turbulent. Nous mesurons également la variation de la vitesse du son induite par un effet de diffusion multiple par l’écoulement turbulentIn this thesis, we study the velocity and temperature fluctuations in a turbulent flow and their implications on the propagation of acoustic waves in turbulence. The first part is devoted to the study of the spontaneous generation of temperature fluctuations by a turbulent flow. We demonstrate that these temperature fluctuations originate from two types of intermittent structures, vorticity filaments and dissipative structures. In the second part, we study the spatio-temporal fluctuations of velocity, using the coherence function. We demonstrate that the coherence function results from the sweeping of the velocity fluctuations in the inertial range by the slower fluctuations of the integral scale. The sweeping effect is particularly interesting for the study of turbulent flows, because it does not come within the scope of Kolmogorov’s K41 theory. Finally, we study the propagation of acoustic waves through a turbulent flow and in particular phase and amplitude fluctuations. We show that phase fluctuations can be deduced from the coherence function of turbulent speed fluctuations. These fluctuations thus result from a sweeping effect of the acoustic wave by the turbulent flow. We also measure the variation in the speed of sound induced by a multiple scattering effect of the turbulent flow
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Sutton, Phil J. "Saturn's turbulent F ring." Thesis, Loughborough University, 2015. https://dspace.lboro.ac.uk/2134/18475.
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As our abilities to utilise high performance computing to theoretically probe many astrophysical systems increases, a genuine need to relate to real systems becomes ever more important. Here, Saturn s rings can be used as a nearby laboratory to investigate in real time many astrophysical processes. One such system is the narrow F ring and its interaction with its inner shepherd moon Prometheus. Through numerical modelling and direct observations of the in-situ spacecraft Cassini we find new and exciting dynamics. These might help explain some of the asymmetries witnessed in the distribution of embedded moonlets and azimuthal ring brightness known to exist within the F ring. Spatially we find asymmetry in the Prometheus induced channel edges with regards to density, velocity and acceleration variations of ring particles. Channel edges that show fans (embedded moonlets) are also the locations of highly localised increases in densities, velocity and acceleration changes where opposing edges are considerably less localised in their distribution. As a result of the highly localised nature of the velocity and acceleration changes chaotic fluctuations in density were witnessed. However, this could seek to work in favour of creating coherent objects at this channel edge as density increases were significantly large. Thus, density here had a greater chance of being enhanced beyond the local Roche density. Accompanied with these dynamics was the discovery of a non-zero component to vorticity in the perturbed area of the F ring post encounter. By removal of the background Keplerian flow we find that encounters typically created a large scale rotation of ~10,000 km^2. Within this area a much more rich distribution of local rotations is also seen located in and around the channel edges. Although the real F ring and our models are non-hydrodynamical in nature the existence of a curl in the velocity vector field in the perturbed region could offer some interesting implications for those systems that are gas rich.
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MARCHISIO, DANIELE. "Precipitation in turbulent fluids." Doctoral thesis, Politecnico di Torino, 2002. http://hdl.handle.net/11583/2550946.
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Gerbi, Gregory Peter. "Observations of turbulent fluxes and turbulence dynamics in the ocean surface boundary layer." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45778.
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Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2008.Includes bibliographical references (p. 110-119).
This study presents observations of turbulence dynamics made during the low winds portion of the Coupled Boundary Layers and Air-Sea Transfer experiment (CBLAST-Low). Observations were made of turbulent fluxes, turbulent kinetic energy, and the length scales of flux-carrying and energy-containing eddies in the ocean surface boundary layer. A new technique was developed to separate wave and turbulent motions spectrally, using ideas for turbulence spectra that were developed in the study of the bottom boundary layer of the atmosphere. The observations of turbulent fluxes allowed the closing of heat and momentum budgets across the air-sea interface. The observations also show that flux-carrying eddies are similar in size to those expected in rigid-boundary turbulence, but that energy-containing eddies are smaller than those in rigid-boundary turbulence. This suggests that the relationship between turbulent kinetic energy, depth, and turbulent diffusivity are different in the ocean surface boundary layer than in rigid-boundary turbulence. The observations confirm previous speculation that surface wave breaking provides a surface source of turbulent kinetic energy that is transported to depth where it dissipates. A model that includes the effects of shear production, wave breaking and dissipation is able to reproduce the enhancement of turbulent kinetic energy near the wavy ocean surface. However, because of the different length scale relations in the ocean surface boundary layer, the empirical constants in the energy model are different from the values that are used to model rigid-boundary turbulence. The ocean surface boundary layer is observed to have small but finite temperature gradients that are related to the boundary fluxes of heat and momentum, as assumed by closure models. However, the turbulent diffusivity of heat in the surface boundary layer is larger than predicted by rigid-boundary closure models. Including the combined effects of wave breaking, stress, and buoyancy forcing allows a closure model to predict the turbulent diffusivity for heat in the ocean surface boundary layer.
by Gregory Peter Gerbi.
Ph.D.
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Kishi, Tatsuro. "Scaling laws for turbulent relative dispersion in two-dimensional energy inverse-cascade turbulence." Doctoral thesis, Kyoto University, 2021. http://hdl.handle.net/2433/263445.
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Cisse, Mamadou. "Suspensions turbulentes de particules de tailles finies : dynamique, modification collective de l'écoulement turbulent." Thesis, Nice, 2015. http://www.theses.fr/2015NICE4014/document.
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Les travaux numériques et expérimentaux de cette thèse contribuent à une meilleure compréhension de la dynamique de grosses particules dans un écoulement turbulent. Un premier volet m’a permis de quantifier leur mouvement relatif au fluide, ainsi que leur influence locale sur l’écoulement turbulent. Dans un second volet, j'ai trouvé que l'effet collectif des particules est d'atténuer l’amplitude des fluctuations turbulentes. En revanche, celles-ci n’ont pas d’influence sur les propriétés statistiques fines de l’écoulement. Aussi, ces mesures suggèrent l’existence d’une transition de phase dans les grandes échelles de l’écoulement au-delà d’un seuil critique du nombre de particulesThe numerical and experimental work of this thesis contribute to a better understanding of the dynamics of large particles in a turbulent flow. The first part allowed me to quantify their relative motion to the flow and their local influence on the surrounding flow. In a second part, I found that the collective effect of particles is to reduce the amplitude of turbulent fluctuations. In revanche, they have no influence on the fine statistical properties of the flow. Also, these measurements suggest the existence of a phase transition in the larger scales of the flow beyond a critical threshold of the number of particles
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黎敦楠 and Tun-nam Lai. "Turbulent transport of airborne pollutant near a low hill." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2002. http://hub.hku.hk/bib/B31227491.
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Nguyen, Dinh Duong. "Some results on turbulent models." Thesis, Rennes 1, 2020. https://ged.univ-rennes1.fr/nuxeo/site/esupversions/c3bf8d92-25f2-4242-af95-a9625f7ab4a0.
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L'objectif de la thèse est double : d'une part la thèse propose de nouveaux modèles turbulents et leur analyse également. Plus précisément, sur la base d'une modélisation de turbulence de base, de nouvelles formes d'hypothèse de Boussinesq - qui prennent en compte la rétrodiffusion d'énergie - sont obtenues. Ensuite, des outils d'analyse fonctionnelle sont appliqués pour prouver l'existence et l'unicité de solutions faibles aux modèles proposés. D'autre part, le manuscrit donne le taux de convergence des modèles de $\alpha $-régularisation aux équations de Navier-Stokes. Plus précisément, l'erreur de modélisation est étudiée dans le cas d'un réglage périodique bidimensionnel de l'espaceThe aim of the dissertation is twofold: On one hand the thesis provides new turbulent models and their analysis as well. More precisely, based on basic turbulence modeling new forms of Boussinesq assumption --which take into account of back-scatter of energy-- are obtained. Then functional analysis tools are applied to prove the existence and uniqueness of weak solutions to the proposed models. On the other hand the manuscript gives the rate of convergence of $\alpha$-regularization models to the Navier-Stokes equations. More exactly, the modeling error is investigated in the case of two-dimensional space-periodic setting
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Schönfeldt, Hans-Jürgen. "Zur Bestimmung turbulenter Transporte." Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-215504.
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Die Zerlegung von Beobachtungsgrößen in sogenannte Mittelwerte und Fluktuationen führt zur Parametrisierung des turbulenten Flusses aber auch zu Problemen. Der Erwartungswert der turbulenten Größe ψ ist das Ensemble Mittel über eine große Zahl von Realisierungen, falls ψ normalverteilt ist. Geophysikalische Daten bestehen jedoch aus Zeitreihen und/oder räumlichen Daten. Daher muß jeder vernünftige Mittelungsprozeß von ψ in der Zeit und/oder im Raum durchgeführt werden. Um die Fluktuationen ψ' von ψ zu trennen, müssen wir den Erwartungswert von ψ bestimmen, d.h. die Fluktuationen in den langen Zeit- und/oder Raumskalen. Für dieses Problem ist der Mittelwert über das Meßintervall eine schlechte Approximation, das gleitende Mittel eine bessere und der numerisch tiefpassgefilterte Wert die bestmögliche Approximation. Eine Fluktuationsmessung (surface flux) im Bereich niedriger Flüsse wurde ausgewertet 1) nach der gewöhnlichen Methode und 2) mit einem numerischen Tiefpass Lanczos-Filter. Mit 2) erhielten wir bessere ErgebnisseDecomposition of some observables into so-called mean parts and fluctuations leads to parameterisation of turbulent flow but is also the cause of different problems. The expectation of the turbulent field ψ is, the ensemble mean over a large number of realizations if ψ follows a normal distribution. Geophysical data, however, consist of time- and/or space series. Thus every reasonable averaging process of ψ must be over time and/or space. To separate fluctuations ψ' from ψ we must estimate the expectation value of ψ, i.e. fluctuations on long time and/or space scales. For this problem the mean over the measuring interval is an inexact approximation, the moving mean is better but the numerically low-pass filtered value probably the best possible approximation. A surface flux measurement in low flux regime is evaluated with 1) a usual procedure and 2) with a numerical low-pass Lanczos-filter. With 2) we obtain better results