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1
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.
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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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Wang, Yueping. "Flow-dependent corrosion in turbulent pipe flow." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/nq23972.pdf.
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Newley, Trevor Michael Jeremy. "Turbulent air flow over hills." Thesis, University of Cambridge, 1986. https://www.repository.cam.ac.uk/handle/1810/250880.
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Shahmardi, Armin. "Turbulent Duct Flow with Polymers." Thesis, KTH, Mekanik, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-226157.
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Direct numerical simulation of the turbulent flow in a square duct with polymers has been performed and the results have been compared with the laboratory experiments done at KTH Mechanical engineering department. The polymer suspension is simulated with the FENE-P model, and the numerical results are used to elucidate the mechanism that provides drag reduction and the effect of polymers on the secondary motion which is typical of the turbulent flow in ducts. Experiments are used to support and validate the numerical data, and to discuss the Reynolds number dependency of the obtained drag reduction. The study shows that the Prandtl's secondary flow is modified by the polymers: the classical 8 regions, in the cross section with high vorticity, are bigger in the polymer flow than those in the Newtonian case, and their centers are displaced towards the center of the duct away from the wall. In plane fluctuations are reduced and streamwise coherence of the flow enhanced in the presence of polymers.Direkt numerisk simulering av det turbulenta flödet i en kvadratisk kanal med polymerer har utförts och resultaten har jämförts med de laboratorieexperiment som gjorts vid KTH: s maskintekniska avdelning. Polymersuspensionen simuleras med FENE-P-modellen och de numeriska resultaten används för att belysa mekanismen som ger dragreduktion och effekten av polymerer på sekundärrörelsen som är typisk för det turbulenta flödet i kanaler. Experiment används för att stödja och validera de numeriska data och för att diskutera Reynolds beroendet av den erhållna dragreduceringen. Studien visar att Prandtls sekundära flöde modifieras av polymererna: de klassiska 8 regionerna i tvärsnittet med hög vorticitet är större i polymerflödet än de i det newtonska fallet och deras centra är förskjutna mot centrum av kanalen bort från väggen. I planfluktuationer reduceras och strömningsförstärkt sammanhängning av flödet förbättras i närvaro av polymerer.
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DeGiuli, Eric. "Turbulent flow in geophysical channels." Thesis, University of British Columbia, 2009. http://hdl.handle.net/2429/12802.
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The problem of turbulent
ow in a rough pipe of arbitrary shape is considered.
The classical Izakson-Millikan argument for a logarithmic velocity
profile is presented, and matched asymptotic expansions are introduced.
Scaled, dimensionless equations are produced and simplified. A simple mixing
length turbulence model is presented, which closes the problem. To calibrate
the model, the mechanical problem is solved in the case of a circular
pipe. Excellent agreement with engineering relations is obtained. The mechanical
problem for a non-circular pipe is posed, and the boundary layer
problem is solved. This leaves unknown the wall stress, which is sought
through approximate methods of solution in the outer region. These are
presented and the approximate solutions thus obtained are compared to full
numerical solutions and data for a square, elliptical, and semi-elliptical pipe.
The approximations are vindicated, but agreement between the numerical
solutions and data is only moderate. Discrepancies are explained in terms
of the neglected secondary
ow.
The thermal problem is posed, with scalings taken for intended application
in glaciology. The problem is solved for a circular pipe. Heat transfer
results are presented and compared with empirical relations. The general
problem for a non-circular pipe is posed, and approximate methods of solution
are motivated, in analogy to those used for the mechanical problem.
These are used to obtain approximate solutions, which are compared with
numerical solutions, to good agreement. Possible applications of these solutions
are discussed.
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Ratnanather, John Tilak. "Numerical analysis of turbulent flow." Thesis, University of Oxford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.236094.
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ALBAGLI, RAFAEL CAMEL. "WAX DEPOSITION IN TURBULENT FLOW." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2017. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=29917@1.
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A deposição de parafina é um fenômeno presente nos sistemas de produção de petróleo (principalmente em águas profundas devido às baixas temperaturas), consistindo na aderência de frações sólidas de hidrocarbonetos nas colunas e linhas, conduzindo à redução da área aberta ao fluxo até o eventual bloqueio. A compreensão dos mecanismos que influenciam na deposição ainda não foi totalmente alcançada. Dada a relevância deste tipo de sistema para o desenvolvimento de novos campos e a ausência de uma teoria consolidada que seja capaz de explicar a evolução e as características do depósito, a limitação de produção por este fenômeno é um dos principais problemas de garantia de escoamento. Visando a aumentar o conhecimento acerca dos fenômenos existentes no processo de deposição, e identificar os mecanismos dominantes, diferentes modelos matemáticos podem ser confrontados com dados experimentais. Geralmente, os escoamentos encontrados ao longo das linhas de produção encontram-se no regime turbulento. Dessa forma, no presente trabalho, desenvolveu-se um modelo de turbulência de duas equações k–omega, acoplado com o modelo entalpia-porosidade, no qual o depósito é considerado um meio poroso. A partir de um equilíbrio termodinâmico determinam-se as espécies que saem de solução e a sua distribuição é determinada pela equação de conservação molar. As equações de conservação foram resolvidas pelo método de volumes finitos, utilizando o esquema Power-law e Euler implícito para as discretizações espacial e temporal. Comparações com dados experimentais em um duto anular foram realizadas, apresentando boa concordância para o regime permanente, mas superestimando a espessura do depósito durante o regime transiente. Constatou-se redução de espessura do depósito com o aumento do número de Reynolds.Wax deposition is a phenomenon present in oil production systems (mainly in deep water due to the low temperatures), which consists in the adhesion of solids fractions of hydrocarbon to tubing and lines, reducing the area opened to flow until be completely blocked. The comprehension of the mechanisms that influences in the deposition has not yet been fully achieved. Given the relevance of this kind of system in new fields development and the absence of a theory able to explain the deposit s evolution and characteristics, the production limitation caused by this phenomenon is one of the main issues in flow assurance. Aiming to expand the knowledge about the phenomena that exist in deposition process and identify dominant mechanisms, different mathematical models can be compared with experimental data. The flow regime in production lines is usually turbulent. Thus, in this work, a two equation k-omega turbulence model coupled to the enthalpy-porosity model, where the deposit is a porous media, was developed. From a thermodynamic equilibrium, the species that comes out of solution are determined while their distribution are determined by each molar conservation equation. The conservations equations were solved with the finite volume method, employing the Power-law and implicit Euler schemes to handle the spatial and temporal discretization. Comparisons with experimental data in an annular duct were realized, showing good agreement in the steady state. The deposit thickness, howeve, was overestimated during the transient. The deposit thickness reduction with the Reynold number increase was verified.
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Tapia, Siles Silvia Cecilia. "Robotic locomotion in turbulent flow." Paris 6, 2011. http://www.theses.fr/2011PA066414.
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Certains poissons utilisent les turbulences de leur milieu pour réduire les coûts énergétiques liés à la nage. Par exemple, les truites ont la capacité de synchroniser leur allures par rapport à la succession stéréotypée de vortex caractérisant une allée de Karman (Karman vortex street). Les truites peuvent ainsi garder une position stationnaire à contre-courant en consommant très peu d'énergie ou réduire, de 4 à 6 fois, la force nécessaire pour nager à l'intérieur d'un banc, en exploitant les allées de Karman induites par les poissons les devançant. En s'inspirant du comportement des poissons, notre travail a porté sur les méthodes de contrôle d'une telle locomotion pour des robots poissons. Dans ce cadre, nos principales les contributions sont les suivantes : Un modèle cinématique simplifié d'allée de Karman. Ce modèle donne les repères cinématiques pour modéliser les contrôleurs. L'approche présentée est basée sur des concepts de stabilité de l'allée de Karman. Le modèle proposé est une segmentation cinématique d'une allée de Karman stable. La génération et le contrôle biomimétiques de mouvements rythmiques de nage semi-passive. Trois contrôleurs sont proposés afin de fusionner le système Environnement-Corps-Control avec des approches différentes de contrôle : Approche externe. On essaye d’imiter le mouvement du poisson en ajustant les articulations pour suivre l’ondulation désirée. Approche bio inspiré. On utilise le modèle d'un Central Pattern Generator pour générer les mouvements. Approche conceptuelle. On utilise des oscillateurs Adaptatifs en Fréquence pour apprendre la fréquence du KVS
10
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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Strömgren, Tobias. "Modelling of turbulent gas-particle flow." Licentiate thesis, KTH, Mechanics, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4639.
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An Eulerian-Eulerian model for dilute gas-particle turbulent flows is
developed for engineering applications. The aim is to understand the effect of particles on turbulent flows. The model is implemented in a finite element code which is used to perform numerical simulations. The feedback from the particles on the turbulence and the mean flow of the gas in a vertical channel flow is studied. In particular, the influence of the particle response time and particle volume fraction on the preferential concentration of the particles near the walls, caused by the turbophoretic effect is explored. The study shows that the particle feedback decreases the accumulation of particles on the walls. It is also found that even a low particle volume fraction can have a significant impact on the turbulence and the mean flow of the gas. A model for the particle fluctuating velocity in turbulent gas-particle flow is derived using a set of stochastic differential
equations. Particle-particle collisions were taken into account. The model shows that the particle fluctuating velocity increases with increasing particle-particle collisions and that increasing particle response times decrease the fluctuating velocity.
12
Bos, Wouter. "Passive scalar mixing in turbulent flow." Phd thesis, Ecole Centrale de Lyon, 2005. http://tel.archives-ouvertes.fr/tel-00199364.
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Le mélange d'un scalaire passif par un écoulement turbulent est étudié. D'abord, la simulation numérique directe (DNS), la simulation des grandes échelles (LES) et des arguments dimensionnels sont employés pour étudier le spectre du flux de scalaire dans une turbulence isotrope avec un gradient moyen uniforme de scalaire. Une loi d'échelle est dérivée. Cette loi conduit à des pentes du spectre variant entre -5/3 et -7/3 en zone inertielle. De premiers résultats de LES plaident en faveur d'un comportement en K^-2. Ensuite, en utilisant une fermeture en deux points (EDQNM), nous montrons qu'aux nombres de Reynolds très élevés, le spectre de flux de scalaire dans la zone intertielle se comporte en K^-7/3. Ce résultat est en accord avec l'analyse dimensionnelle classique de Lumley (1967). Aux nombres de Reynolds correspondant aux expériences de laboratoire, la fermeture conduit à des spectres plus près de K^-2. Nous montrons ensuite que le comportement en K^-2 trouvé en LES est induit par le forçage à grande échelle. La fermeture est alors appliquée au cas des écoulements homogènes cisaillés et les spectres du flux de scalaire longitudinal et transverse sont étudiés. Le spectre du flux longitudinal est trouvé proportionnelle à K^-23/9. Ce résultat est en accord avec l'expérience mais est en désaccord avec l'analyse dimensionnelle classique. Finalement, nous montrons que le lien entre la dispersion de particules et le mélange d'un scalaire permet de formuler une fermeture en deux points et un temps qui ne nécessite l'introduction d'aucune constante dans le modèle.
13
Ferro, Marco. "Experimental study on turbulent pipe flow." Thesis, KTH, Mekanik, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-103029.
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Fully developed turbulent pipe ows have been studied experimentally for more than a century and for more than two decades by means of Direct Numerical Simulations, nonetheless there are still unresolved and of fundamental nature issues. Among those are the scaling of the mean velocity pro le or the question whether the near-wall peak in the variance pro le is Reynolds number invariant. In this thesis new experimental results on high Reynolds number turbulent pipe ows, obtained by means of hot-wire anemometry, are carefully document and results are presented, thereby extending the Reynolds number range of an available in-house experimental database (Sattarzadeh 2011). The main threads of this thesis are the spatial resolution eects and the Reynolds number scaling of wall-bounded ows and were investigated acquiring the measurements with probes of four dierent wire-lengths at dierent Reynolds numbers covering the friction Reynolds number range of 550 <R+< 2 500. The small viscous length-scales encountered required a high accuracy in the wall-position. Therefore, a vibration analysis of the probe exposed to the ow was performed on two dierent traversing systems and on several probeholder/ probe con gurations, proving that the vibrations of the probe can be large and should be taken into account when choosing the traverse system and probe-holder geometry. Results of the hot-wire velocity measurements showed that when accounting for spatial resolution eects, a clear Reynolds number eect on the statistical and spectral quantities can be observed. The peak of velocity variance, for instance, appeared to increase with the Reynolds number and the growth seems to be justi ed from the increase of the low frequency modes. This result together with the appearance of an outer peak located in the low frequency range at higher Reynolds numbers suggests that the increase of the peak of the velocity variance is due to the inuence that the large-scale motions have on the near-wall cycle of the velocity uctuations. As a side results of the velocity measurements, temperature, i.e. passive scalar, mean and variance pro le were obtained by means of cold-wire anemometry. Also here, clear spatial resolution eect on the temperature variance pro le could be documented.
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Vanderwel, Christina M. "Turbulent Diffusion in Uniformly Sheared Flow." Thesis, Université d'Ottawa / University of Ottawa, 2014. http://hdl.handle.net/10393/31806.
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The objective of this thesis research is to further the understanding of turbulent diffusion by experimentally studying the turbulent diffusion of a plume of dye released in uniformly sheared flow generated in a water tunnel. The flow studied was nearly homogeneous but strongly anisotropic and had a turbulent Reynolds number of 150. Maps of the turbulent velocity and dye concentration were measured simultaneously using stereoscopic particle image velocimetry and planar laser-induced fluorescence. A thorough analysis of the planar laser-induced fluorescence technique was performed; several previously unconsidered sources of error were identified and corrections were proposed. The measured evolutions of the mixed velocity-concentration statistics of the plume were compared with previous studies. The turbulent scalar flux vector was related to the mean concentration gradient through a first-order gradient transport model and, for the first time in an experimental flow, all components of the turbulent diffusivity tensor were measured directly. The turbulent diffusivity tensor was found to be highly anisotropic and its streamwise component appeared to be counter-gradient. The relative diffusion of the plume was also investigated and the evolution of the mean square particle separation was found to be consistent with Richardson-Obukhov scaling, with a value of Richardson's constant equal to 0.35. The fine structure of the concentration field and the mixed velocity-concentration statistics were also documented. Because of the high level of intermittency of the present plume, the scalar probability density function was strongly non-Gaussian and the conditional expectations of the velocity components and the scalar dissipation, conditioned upon the scalar value, were distinctly non-linear. Lastly, the role of coherent structures on scalar diffusion was investigated and a conditional eddy analysis demonstrated that hairpin vortices were associated closely with large scalar flux events.
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Strömgren, Tobias. "Modelling of turbulent gas-particle flow /." Stockholm : Mekanik, Kungliga Tekniska högskolan, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4639.
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Lavertu, Robert A. "Scalar dispersion in turbulent channel flow." Thesis, McGill University, 2002. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=79243.
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Experimental results of scalar dispersion from a concentrated source in an inhomogeneous turbulent flow field are presented in this thesis.The flow field is fully developed channel flow---one of the simplest inhomogeneous turbulent flows. A fine line source is used to inject the scalar (temperature). The temperature injection does not affect the flow field, hence it is passive. The line source is in the spanwise (z) direction of the channel, and is located at transverse (i.e., wall-normal) locations: y/h = 0.067, 0.17, and 1.0 (where h is the channel half-width). Measurements of the resultant thermal plume for each source location are taken for two Reynolds numbers: 10400 and 22800 (where Re = Uch/nu: U c is the centerline velocity and nu is the kinematic viscosity). Hot-wire anemometry and cold-wire thermometry are used to acquire velocity and temperature data, respectively.
It is shown that the downstream decay of the mean temperature rise is less than that of isotropic grid-generated turbulence and homogenous turbulent shear flow. The peak RMS temperature fluctuation also decays at a slower rate. For the near-wall source locations, the peak of the transverse RMS temperature profile drifts toward the channel centerline with increasing downstream distance from the source. Also, for the near-wall source locations, the scalar PDF is quasi-Gaussian, indicating improved mixing in that region. As the plume is traversed toward the centerline, the PDF evolves into a very positively skewed shape.
An extensive database of the thermal plume is compiled, which consists of the mean, RMS, and skewness profiles of the temperature field, PDFs of the temperature field, and temperature-velocity correlations. This database can be used to test numerical and/or theoretical models, highlighting their strengths and weaknesses.
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Lanerolle, Lyon Werner John. "Numerical modelling of turbulent compressible flow." Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.362004.
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Wood, Nigel. "Turbulent flow over three-dimensional hills." Thesis, University of Reading, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.306165.
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Van, Herpe Francois. "Computational aeroacoustics for turbulent duct flow." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.336488.
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Zhang, Zhentong. "Micro-bubble dynamics in turbulent flow." Thesis, Toulouse, INPT, 2019. http://www.theses.fr/2019INPT0107.
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Cette thèse est consacrée à l'étude du mouvement de petites bulles dans des écoulements turbulents homogènes isotropes. Le travail aborde différentes questions liées à la description statistique des forces hydrodynamiques exercées sur une bulle ainsi qu'à leur modélisation stochastique tenant compte des effets d'intermittence. Nous proposons tout d'abord un modèle pour l'accélération de bulles de taille inférieures à l'échelle dissipative de l'écoulement soumises à la traînée et aux forces d'inertie du fluide. Ce modèle, qui dépend du nombre de Stokes, du nombre de Reynolds et du rapport de densité, reproduit l'évolution de la variance d'accélération ainsi que l'importance relative et l'alignement des deux forces observées à partir de simulations numériques directes (DNS). Deuxièmement, sur la base de l’observation selon laquelle les statistiques d’accélération conditionnelles au taux de dissipation de l’énergie cinétique locale sont invariantes avec le nombre de Stokes et le taux de dissipation, nous proposons un modèle stochastique du vecteur d’accélération instantanée de la bulle, qui tient compte de l’intermittence à petite échelle de la turbulence. La norme de l'accélération de la bulle est obtenue en modélisant le taux de dissipation le long de la trajectoire de la bulle à partir d'un processus stochastique lognormal, tandis que son orientation est donnée par deux marches aléatoires couplées sur une même sphère afin de modéliser l'évolution de l'orientation conjointe la traînée et les forces d'inertie agissant sur la bulle. Le modèle stochastique proposé pour l'accélération des bulles permet d'améliorer les simulations de grandes turbulences (LES) d'écoulements turbulents transportant de petites bulles. Il peut reproduire efficacement l’effet des échelles turbulentes inférieures à la résolution du maillage en ajoutant une contribution aléatoire en fonction du taux de dissipation moyen local. Les comparaisons avec le DNS et les LES standard montrent que le modèle proposé améliore considérablement les statistiques de la phase de formation de bulles. Troisièmement, nous étendons les résultats précédents dans le cas de bulles à plus grand nombre de Reynolds en prenant en compte les lois de traînée non-linéaires. Nous définissons un temps de relaxation effectif basé sur le coefficient de traînée pour caractériser le mouvement de la bulle (accélération, vitesse). Finalement, nous étudions l’effet de la flottabilité et de la force de portance sur la dynamique des bulles et analysons la réduction de la vitesse moyenne ascensionnelle dans les écoulements turbulents par rapport aux écoulements au repos. On observe que la bulle explore de préférence une région ayant une accélération de fluide vers le bas qui contribue, par le biais de la force d’inertie, à réduire la vitesse de montée. De plus, comme déjà observée, la force de portance amène de préférence les bulles dans un mouvement de fluide en aval qui réduit également leur vitesse de montéeThis thesis is devoted to the study of the motion of small bubbles in homogeneous isotropic turbulent flows. The work addresses several questions related to the statistical description of the hydrodynamic forces exerted on a bubble as well as the stochastic modeling of their high frequency fluctuations. First, we propose a model for the acceleration of micro-bubbles (smaller than the dissipative scale of the flow) subjected to the drag and the fluid inertia forces. This model, that depends on the Stokes number, the Reynolds number and the density ratio, reproduces the evolution of the acceleration variance as well as the relative importance and alignment of the two forces as observed from Direct Numerical Simulations (DNS). Second, based on the observation that acceleration statistics conditional to the local kinetic energy dissipation rate are invariant with the Stokes number and the dissipation rate, we propose a stochastic model for the instantaneous bubble acceleration vector accounting for the small-scale intermittency of the turbulent flows. The norm of the bubble acceleration is obtained by modeling the dissipation rate along the bubble trajectory from a log-normal stochastic process, whereas its orientation is given by two coupled random walk on a unit sphere in order to model the evolution of the joint orientation of the drag and inertia forces acting on the bubble. Furthermore, the proposed stochastic model for the bubble acceleration is used in the context of large eddy simulations (LES) of turbulent flows laden with small bubbles. It can effectively reproduce effect of turbulent motion at scales smaller than the mesh resolution by adding a random contribution depending on local average dissipation rate. Comparisons with DNS and standard LES, show that the proposed model improves significantly the statistics of the bubbly phase. Third, we extend the previous results in the case of bubbles with large Reynolds number by considering non-linear drag laws. We define an effective relaxation time based on the drag coefficient to characterize bubble motion (acceleration,velocity). Eventually we study the effect of buoyancy and lift force on the bubble dynamics, and analyze the reduction of the average rising velocity in turbulent flow compared to quiescent flows. It is observed that bubbles preferentially explore region having downward fluid acceleration which contributes through the inertia force to reduction of the rising velocity. In addition, as already observed, the lift force brings preferably bubbles into downstream fluid motion which also reduce their rising velocity
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Weber, Francis J. "Ultrasonic beam propagation in turbulent flow." Link to electronic thesis, 2004. http://www.wpi.edu/Pubs/ETD/Available/etd-0419104-173917.
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Gundersen, Ted Ørjan Kjellevik. "Modelling of Rotating Turbulent Flows : Computer simulation of turbulent backward-facing step flow with system rotation." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for energi- og prosessteknikk, 2011. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-13925.
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An investigation of how different levels of turbulence modelling tackle the effects of system rotation has been performed. Ranging from simple one-equation models to large-eddy simulations, different approaches have been considered by means of a literature study and numerical calculations of turbulent flow over a backward-facing step subjected to spanwise rotation. The computed results were compared with results from direct numerical simulations.The literature study revealed that simple linear eddy-viscosity turbulence models are unable to predict any effects on the turbulence field due to system rotation. Eddy-viscosity models may be sensitised to rotation, but this has been done with a varying degree of success. The Reynolds stress equation models inherently respond well to system rotation, but a more costly eddy simulation will yield the most accurate result.Numerical calculations confirmed what was found in the literature. A linear eddy-viscosity model was unaffected by system rotation, while the sensitised model exhibited some effects on the mean flow field. The Reynolds stress model managed to predict all essential effects related to system rotation, although one separation bubble was oversized. This defect was attributed to a flaw in the modelling of the Reynolds stress redistribution process.
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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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Catlett, Matthew Ryan. "Flow Induced Noise from Turbulent Flow over Steps and Gaps." Thesis, Virginia Tech, 2010. http://hdl.handle.net/10919/32926.
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The existence of small surface discontinuities on a flow surface generate significant pressure fluctuations which can manifest as radiated far field sound and affect the fluctuating near wall pressure field exerted on the flow surface. A significant amount
of research has been performed on various step and gap flows; however few have dealt with step heights that are small relative to the incoming boundary layer. Fewer still have been concerned with measuring the effect on the fluctuating wall pressure field or the radiated far field sound from these small surface discontinuities. This study presents the work aimed at scaling the radiated sound from small forward and backward steps, detailing the surface pressure field as a result of these steps, and detailing the far field sound radiated from gap configurations of similar dimension. These measurements were
performed in the Virginia Tech Anechoic Wall Jet facility for step heights that ranged from approximately 10% to 100% of the incoming boundary layer height. The results show the influence of step height and boundary layer velocity on the far field sound from forward and backward steps. Very little directivity is seen for either source and the larger step heights considered in this study are shown to not be acoustically compact. A new
mixed scaling normalization is proposed for the far field spectra from both types of step, which is shown to reliably collapse the data. Backward steps are shown to be much weaker producers of far field sound than a similarly sized forward step. The implications of this behavior are discussed with respect to the far field sound measured from various gap flows. The fluctuating wall pressure field was measured upstream and downstream of both step configurations. The data shows a slow recovery of the wall pressure field with lasting disturbances up to 100 step heights downstream of the step feature.Master of Science
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Thompson, Andrew S. "Experimental charactarization of flow induced vibration in turbulent pipe flow /." Diss., CLICK HERE for online access, 2009. http://contentdm.lib.byu.edu/ETD/image/etd3162.pdf.
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Thompson, Andrew S. "Experimental Characterization of Flow Induced Vibration in Turbulent Pipe Flow." BYU ScholarsArchive, 2009. https://scholarsarchive.byu.edu/etd/1906.
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This thesis presents results of an experimental investigation that characterizes the wall vibration of a pipe with turbulent flow passing through it. Specifically, experiments were conducted using a water flow loop to address three general phenomena. The topics of investigation were: 1) How does the pipe wall vibration depend on the average flow speed, pipe diameter, and pipe thickness for an unsupported pipe? 2) How does the behavior change if the pipe is clamp supported at various clamping lengths? 3) What influence does turbulence generation caused by holed baffle plates exert on the pipe response? A single pipe material (PVC) was used with a range of internal diameters from 5.08 cm to 10.16 cm and diameter to thickness ratios ranging from 8.90 to 16.94. The average flow speed that the experiments were conducted at ranged from 0 to 11.5 m/s. Pipe vibrations were characterized by accelerometers mounted on the pipe wall at several locations along the pipe length. Rms values of the pipe wall acceleration and velocity time series were measured at various flow speeds. Power spectral densities of the accelerometer data were computed and analyzed. Concurrent wall pressure fluctuation measurements were also obtained. The results show that for a fully developed turbulent flow, the rms of the wall pressure fluctuations is proportional to the rms of the wall acceleration and each scale nominally as the square of the average fluid velocity. Also, the rms of the pipe wall acceleration increases with decreasing pipe wall thickness. When changes were made in the pipe support length, it was observed that, in general, pipe support length exercises little influence on the pipe wall acceleration. The influence of pipe support length on the pipe wall velocity is much more pronounced. A non-dimensional parameter describing the pipe wall acceleration is defined and its dependence on relevant independent non-dimensional parameters is presented. Turbulence was induced using baffle plates with various sizes (2.54 cm to 0.159 cm) and numbers of holes drilled through them to provide a constant through area of 35.48 cm2 for each plate. Cavitation exists at high speeds for the largest holed baffle plates and this significantly increases the rms of the pipe wall acceleration. As the baffle plate hole size decreases, vibration levels were observed to return to levels that were observed when no baffle plate was employed. Power spectral densities of the accelerometer data from each baffle plate scenario were also computed and analyzed.
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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.
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Khayamyan, Shervin. "Transitional and turbulent flow in porous media." Licentiate thesis, Luleå tekniska universitet, Strömningslära och experimentell mekanik, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-26476.
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Fluid flow through porous media takes place in many natural processes such as ground water flows, capillary flows in plants and flow in human organs and muscles. It is also of outmost importance to have knowledge of this flow in a number of industrial processes such as paper making, making of fibre boards, composites manufacturing, filtering, forming and sintering of iron ore pellets and drying and impregnation of wood. Despite the significance of porous media flow and the vast amount of work that has been performed to investigate it, knowledge of some fundamentals is missing. Little is, for instance, known about transitional and turbulent flow in porous media on the microscopic level. On a macroscopic level Darcy law is extended to the so called Ergun or Forchheimer Equations when Re becomes larger than about 10 to fit experimental. The actual value depends both on the porous media and how Re is defined. The deviation from Darcy flow can for modest Re be explained by inertia but may, as Re increases, also be attributed to turbulence. The macroscopic way of modelling the transition from inertia dominated to turbulent flow is just to continue with the Forchheimer Equation or possibly some version of it. In any case experimental data yields that, on a macroscopic level, the transition from Darcy flow to inertia dominated and turbulent flow is smooth. To get a better understanding of this process the transition from laminar to turbulent flow in porous media is here studied with a new method. To mimic inter-connected pores, a simplified geometry is studied consisting of a pipe with a relatively large diameter that is split into two parallel pipes with different diameters. This is a pore-doublet set-up and the pressure drop over all pipes is recorded by pressure transducers for different flow rates. Statistical method and frequency analysis are performed to investigate collected data (Papers A and B). Positive skewness of pressure drop fluctuations indicates early stage of presence of turbulent patches in the flow for each pipe. The measured flow distribution and pressure drop fluctuations highlights six distinct flow patterns in the pipe network based on variation in flow regime of each pipe and the level of pressure fluctuations (Paper B). Correlation between the pressure drop between two pipes shows that two parallel pipes follow each other fluctuations much better before both of them become fully turbulent. Some detailed results are that the frequency analysis reveals two different frequency band events in the pipes. The gain factor shows that both frequency band events originate from the larger pipe until the early presence of turbulent patches in the smaller pipe (Paper B). The low frequency fluctuations makes the flow in the pipes to be out of phase while the high frequency band fluctuations try to bring the flow in the pipes back to equilibrium state.Godkänd; 2013; 20130521 (shekha); Tillkännagivande licentiatseminarium 2013-05-29 Nedanstående person kommer att hålla licentiatseminarium för avläggande av teknologie licentiatexamen. Namn: Shervin Khayamyan Ämne: Strömningslära/Fluid Mechanics Uppsats: Transitional and Turbulent Flow in Porous Media Examinator: Professor Staffan Lundström, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Diskutant: Professor Carl-Erik Grip, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Onsdag den 19 juni 2013 kl 09.00 Plats: E231, Luleå tekniska universitet
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Riahi, Ardeshir. "Turbulent swirling flow in short cylindrical chambers." Thesis, University of British Columbia, 1990. http://hdl.handle.net/2429/30810.
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The effects of aspect ratio (L/D) on the rate of decay of swirl in a cylindrical chamber were experimentally studied using the Laser-Doppler-Anemometry technique. Preliminary measurements revealed that water should be used as working fluid; the results pertaining to air were inferred from Reynolds number similarity.
The steady-state measurements revealed that a solid body type of rotation can be generated by a disc whose surface has been uniformly roughened. The effect of aspect ratio on the rate of decay of such flow field was studied in three chambers with aspect ratios in the range of interest to engine combustion. Experimental results showed a faster decay rate in the shorter chamber (i.e. smaller aspect ratio). This was attributed to the stronger swirl driven secondary flow pattern in the shorter chamber.
A mathematical model describing axi-symmetric, decaying, turbulent swirling flow inside a short cylindrical chamber was also developed. The model was numerically solved, using the control-volume analysis, to provide insight on swirl decay in engines. The model validation was based on experimental observations.
Turbulence parameters were represented by a two-equation turbulence model, modified for streamline curvature effects. The ad-hoc curvature modification of the standard k-e model proposed by Launder et al. and the mixing energy model developed by Saffman-Wilcox-Traci (SWT) were used to account for curvature effects. The analysis of steady flow between two long concentric cylinders, established the superiority of the latter over the former method. The SWT model was also successfully used in reproducing previously published experimental results, pertaining to decaying swirling flow field (mean velocity and turbulence intensity) in a short cylinder. The calculated turbulence intensity profile revealed that swirl promotes anisotropic turbulence.
The validated numerical model was used to predict the effect of aspect ratio on the rate of decay of the flow field observed by the experimental measurements in the present study. The overall prediction of decay rate was successful, leading to the conclusion that Wilcox and Chambers model can be used in predicting the behaviour of two-dimensional transient turbulent swirling flows.Applied Science, Faculty of
Mechanical Engineering, Department of
Graduate
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Babarutsi, Sofia. "Modelling quasi-two-dimensional turbulent shear flow." Thesis, McGill University, 1991. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=70223.
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A two-length-scale turbulence model is introduced in this thesis for the computation of quasi-two-dimensional turbulent shear flow with two distinct length scales of motion. In the model, the turbulence motions of the two distinct length scales are computed separately. The small-scale turbulence is treated as a background component locally in equilibrium while the large-scale turbulence is simulated using a second-order closure procedure. The development of the turbulent shear flows depends on the rate of energy transfer from the large-scale to the small-scale turbulence. Two mechanisms are identified to have a significant effect on the rate of this energy transfer. On one hand, the rate is reduced due to the confinement of the large-scale turbulence to two-dimensional motion, since the nonlinear energy cascade process is less efficient in two-dimensional turbulent motion. On the other hand, the rate is enhanced due to the work done by the large-scale turbulent motion against the friction forces. The energy transfer rate due to friction is derived in the model using a two-step averaging procedure, whereas the transfer rate due to nonlinear cascade process is determined using a model equation. The data from a number of experimental investigations of quasi-two-dimensional turbulent shear flows are analyzed. These data support the notion of the two-length-scale turbulence model, that (i) the maintenance of the turbulent motion depends on the transfer of energy from the large-scale turbulence to the small-scale turbulence, and (ii) the transfer rate is subjected to confinement and friction influences as specified in the model. Numerical computations are conducted using the two-length-scale model and a single-length-scale model. The results are compared with the experimental data. The two-length-scale model is superior in performance compared with the single-length-scale model, particularly in the intermediate region of the flow where both length scales of the turbule
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Muto, Yasunori. "Turbulent flow in two-stage meandering channels." Thesis, University of Bradford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.506139.
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Clark, N. R. "Kinematic simulation of a turbulent channel flow." Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597714.
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Kinematic simulation is a Lagrangian model of turbulence based on an underlying random Eulerian field. We detail a new method of generating kinematic simulation fields in a channel. We employ a new decomposition for kinematic simulation which ensures that the boundary conditions are automatically satisfied while preserving incompressibility. We impose statistics up to second order, including the Reynolds shear-stress and one-dimensional spectral densities. These fields are used as a Lagrangian model and the statistics of one particle, and particle pairs are calculated in a kinematic simulation of a fully developed stationary channel flow with a Reynolds number based on wall units, of 200. A dimensional analysis of the relative accelerations of particle pairs in the channel is presented. We argue that, in kinematic simulation, the relative acceleration correlation should be approximately stationary and that the variance should increase quadratically in time, for times much smaller than the Lagrangian integral time scales. This is used in an analysis of the second order moments of pair separation, and reltive velocity and compared to the simulation data. Finally we examine the flow structures present in the kinematic simulation fields, concentrating on the log region streak structures, and consider their effects upon the Lagrangian statistics.
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Smith, Sarah Elizabeth. "Turbulent duct flow of non-Newtonian liquids." Thesis, University of Liverpool, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399184.
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The turbulent flow of non-Newtonian fluids in straight ducts has been investigated. Specifically, the fully developed circular pipe flow, axisymmetric sudden expansion flow and fully developed square duct flow were studied. The pipe flow study analysed previous measurements of the mean velocity profiles and friction factor-Reynolds number characteristics of different non-Newtonian fluids in pipe fully developed pipe flows. An investigation of different nondimensional parameters permitted initial progress on developing a correlation between drag reduction and fluid rheology to be made. Comparison of the ranking orders of drag reduction, fluid extensional viscosity and fluid elasticity revealed that these fluid properties are most strongly correlated with drag reduction at low shear/strain rates (that is, in the buffer and outer regions of the boundary layer). The sudden expansion geometry was investigated for flows of aqueous Xanthan gum solution and two reference Newtonian fluids. A smooth contraction was placed at the inlet to the sudden expansion. Few significant differences were observed between the mean flow behaviours of the test fluids for the turbulent Reynolds numbers tested (26,000 and 80,000). These results may reflect the manner in which the rigid, rod-like molecules found in Xanthan gum influence the flow behaviour. Turbulence measurements indicated that all three turbulence components were suppressed for the polymer solution flow within the free shear layer downstream of the expansion. The turbulent flow of two non-Newtonian fluids (a blend ofXanthan gum and Carboxymethylcellulose in water and an aqueous solution of polyacrylamide) in a square duct were compared with a turbulent Newtonian square duct flow. Although suppression of the transverse turbulence components was noted, the polymer solutions also strongly affected the behaviour of the secondary flows found in turbulent non-circular duct flows of Newtonian fluids. Specifically, the secondary flows appeared to be weakened in the polymer blend flow and completely suppressed in the polyacrylamide solution flow. It is anticipated that fluid elasticity is influential in this suppression
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Humayun, Mohammed Adel. "A hybrid approach for turbulent flow calculations." Thesis, University of Salford, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.272623.
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Horlacher, Volker. "Gravity waves and turbulent flow over orography." Thesis, University of Leeds, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.400955.
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Gong, W. "Turbulent flow and dispersion over model hills." Thesis, University of Reading, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.374878.
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Jones, Dorian Phillip. "Parallel simulation of turbulent square duct flow." Thesis, University of Bristol, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.294548.
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Belcher, Stephen E. "Turbulent boundary layer flow over undulating surfaces." Thesis, University of Cambridge, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279606.
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YUPA, LUIS FERNANDO PALOMINO. "EXPERIMENTAL STUDY WAX DEPOSITION IN TURBULENT FLOW." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2010. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=17170@1.
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CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOUm dos principais problemas encontrados na produção de petróleo em águas profundas é a deposição de parafina em linhas de produção e transporte. À medida que o óleo escoa pelos dutos, perde calor para o ambiente marinho mais frio, reduzindo a solubilidade dos hidrocarbonetos mais pesados, podendo levar à deposição destes componentes nas paredes do duto. A deposição de parafina pode causar redução da capacidade de produção e até bloqueio total da linha. A capacidade de previsão da variação temporal destes depósitos, assim como a sua distribuição espacial ao longo do duto, são informações relevantes para o projeto e a operação das linhas. Uma revisão atualizada da literatura revela que os mecanismos responsáveis pela deposição de parafina ainda não são bem compreendidos. O presente trabalho faz parte de um programa de pesquisa em andamento voltado para o estudo dos mecanismos de deposição de parafina em dutos. Foram conduzidos experimentos de laboratório onde soluções de parafina e querosene com propriedades conhecidas escoavam por seção de testes especialmente projetada para permitir a medição da variação espacial e temporal da espessura dos depósitos. As medições foram realizadas para regime de escoamento turbulento utilizando um microscópio óptico. Os resultados obtidos mostraram que a espessura de depósito decresce com o aumento do número de Reynolds, sendo inferior aos valores obtidos para a condição de regime laminar. A boa qualidade das visualizações obtidas permitiu observar a remoção de depósito por cisalhamento. Os resultados obtidos estão disponíveis para validar soluções numéricas e auxiliar no entendimento dos mecanismos básicos responsáveis pela deposição de parafina.
One of the main problems encountered in oil production in deepwaters is the wax deposition in production and transportation lines. As the oil flows in the pipelines, it loses heat to the colder outside environment, reducing its solubility to heavy hydrocarbons, and potentially leading to deposition at the pipe walls. Wax deposition may cause loss of production and even the complete blockage of the line. The ability to predict the spatial and temporal variation of the deposits is relevant to the design and operation of subsea lines. An updated literature review revealed that the mechanisms responsible for wax deposition are still not fully understood. The present work is part of an ongoing research project aimed at studying the mechanisms responsible for wax deposition in pipelines. Laboratory experiments were conducted for turbulent flow of wax and querosene solutions through a specially designed test section that allowed for optical acces. An optical microscope was employed for measuring the wax deposit thicknesses which were shown to decrease with increasing Reynolds number. The deposits measured in the turbulent regime were smaller than those for the laminar regime. The good quality of the visualizations obtained allowed the observation of wax deposit removal by shear effects. The experimental results obtained are available for comparison with numerical solutions, what can contribute to the study of the basic mechanisms responsible for wax deposition.
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Alathur, Srinivasan Prem Anand. "Deep Learning models for turbulent shear flow." Thesis, KTH, Numerisk analys, NA, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-229416.
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Deep neural networks trained with spatio-temporal evolution of a dynamical system may be regarded as an empirical alternative to conventional models using differential equations. In this thesis, such deep learning models are constructed for the problem of turbulent shear flow. However, as a first step, this modeling is restricted to a simplified low-dimensional representation of turbulence physics. The training datasets for the neural networks are obtained from a 9-dimensional model using Fourier modes proposed by Moehlis, Faisst, and Eckhardt [29] for sinusoidal shear flow. These modes were appropriately chosen to capture the turbulent structures in the near-wall region. The time series of the amplitudes of these modes fully describe the evolution of flow. Trained deep learning models are employed to predict these time series based on a short input seed. Two fundamentally different neural network architectures, namely multilayer perceptrons (MLP) and long short-term memory (LSTM) networks are quantitatively compared in this work. The assessment of these architectures is based on (i) the goodness of fit of their predictions to that of the 9-dimensional model, (ii) the ability of the predictions to capture the near-wall turbulence structures, and (iii) the statistical consistency of the predictions with the test data. LSTMs are observed to make predictions with an error that is around 4 orders of magnitude lower than that of the MLP. Furthermore, the flow fields constructed from the LSTM predictions are remarkably accurate in their statistical behavior. In particular, deviations of 0:45 % and 2:49 % between the true data and the LSTM predictions were obtained for the mean flow and the streamwise velocity fluctuations, respectively.Djupa neuronät som är tränade med rum-tids utveckling av ett dynamiskt system kan betraktas som ett empiriskt alternativ till konventionella modeller som använder differentialekvationer. I denna avhandling konstruerar vi sådana djupinlärningsmodeller för att modellera en förenklad lågdimensionell representation av turbulensfysiken. Träningsdata för neuronäten erhålls från en 9-dimensionell modell (Moehlis, Faisst och Eckhardt [29]) för olika Fourier-moder i ett skärskikt. Dessa moder har ändamålsenligt valts för att avbilda de turbulenta strukturerna i regionen nära väggen. Amplitudernas tidsserier för dessa moder beskriver fullständigt flödesutvecklingen, och tränade djupinlärningsmodeller används för att förutsäga dessa tidsserier baserat på en kort indatasekvens. Två fundamentalt olika neuronätsarkitekturer, nämligen flerlagerperceptroner (MLP) och långa närminnesnätverk (LSTM), jämförs kvantitativt i denna avhandling. Utvärderingen av dessa arkitekturer är baserad på (i) hur väl deras förutsägelser presterar jämfört med den 9-dimensionella modellen, (ii) förutsägelsernas förmåga att avbilda turbulensstrukturerna nära väggar och (iii) den statistiska överensstämmelsen mellan nätverkets förutsägelser och testdatan. Det visas att LSTM gör förutsägelser med ett fel på ungefär fyra storleksordningar lägre än för MLP. Vidare, är strömningsfälten som är konstruerade från LSTM-förutsägelser anmärkningsvärt noggranna i deras statistiska beteende. I synnerhet uppmättes avvikelser mellan de sanna- och förutsagda värdena för det genomsnittliga flödet till 0; 45 %, och för de strömvisa hastighetsfluktionerna till 2; 49 %.
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Tsai, Ping-Ho. "Turbulent flow in a curved streamwise corner." Thesis, University of Iowa, 1985. https://ir.uiowa.edu/etd/2803.
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This thesis is concerned with a study of the turbulent boundary layer in the corner region of a curved duct. The pressure-driven secondary flow is investigated by experiments and calculations. Measurements of pressure distribution and mean-velocity components are made at three streamwise stations in the convex corner of a curved duct and the results are compared with calculations using a finite-difference numerical method.
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Cassisa, Cyril. "Optical flow estimation with subgrid model for study of turbulent flow." Phd thesis, Ecole Centrale de Lyon, 2011. http://tel.archives-ouvertes.fr/tel-00674772.
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The objective of this thesis is to study the evolution of scalar field carried by a flow from a temporal image sequence. The estimation of the velocity field of turbulent flow is of major importance for understanding the physical phenomenon. Up to now the problem of turbulence is generally ignored in the flow equation of existing methods. An information given by image is discrete at pixel size. Depending on the turbulent rate of the flow, pixel and time resolutions may become too large to neglect the effect of sub-pixel small-scales on the pixel velocity field. For this, we propose a flow equation defined by a filtered concentration transport equation where a classic turbulent sub-grid eddy viscosity model is introduced in order to account for this effect. To formulate the problem, we use a Markovian approach. An unwarping multiresolution by pyramidal decomposition is proposed which reduces the number of operations on images. The optimization coupled with a multigrid approach allows to estimate the optimal 2D real velocity field. Our approach is tested on synthetic andreal image sequences (PIV laboratory experiment and remote sensing data of dust storm event) with high Reynolds number. Comparisons with existing approaches are very promising.
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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
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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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Meleschi, Shangari B. "Ultrasonic technique in determination of grid-generated turbulent flow characteristics and caustic formation." Link to electronic thesis, 2004. http://www.wpi.edu/Pubs/ETD/Available/etd-0429104-153706.
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Lindgren, Björn. "Flow facility design and experimental studies of wall-bounded turbulent shear-flows." Doctoral thesis, KTH, Mechanics, 2002. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3454.
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The presen present thesis spans a range of topics within thearea of turbulent flows, ranging from design of flow facilitiesto evaluation aluation of scaling laws and turbulence modelingdeling aspects through use of experimental data. A newwind-tunnel has been designed, constructed and evaluated at theDept. of Mechanics, KTH. Special attention was directed to thedesign of turning vanes that not only turn the flow but alsoallow for a large expansion without separation in the corners.The investigation of the flow quality confirmed that theconcept of expanding corners is feasible and may besuccessfully incorporated into low turbulence wind-tunnels. Theflow quality in the MTL wind-tunnel at the Dept. of Mechanics,KTH, was as also in investigated confirming that it still isvery good. The results are in general comparable to thosemeasured when the tunnel was as new, with the exception of thetemperature variation ariation that has decreased by a factorof 4 due to an improved cooling system.
Experimental data from high Reynolds number zeropressure-gradient turbulent layers have been investigated.These studies have primarily focused on scaling laws withe.g.confirmation of an exponential velocity defect lawin a region, about half the size of the boundary layerthickness, located outside the logarithmic overlap region. Thestreamwise velocity probability density functions in theoverlap region was found to be self-similar when scaled withthe local rms value. Flow structures in the near-wall andbuffer regions were studied ande.g. the near-wall streak spacing was confirmed to beabout 100 viscous length units although the relative influenceof the near-wall streaks on the flow was as found to decreasewith increasing Reynolds number.
The separated flow in an asymmetric plane diffuser wasdetermined using PIV and LDV. All three velocity componentswere measured in a plane along the centerline of the diffuser.Results for mean velocities, turbulence intensities andturbulence kinetic energy are presented, as well as forstreamlines and backflow coefficientcien describing theseparated region. Instantaneous velocity fields are alsopresented demonstrating the highly fluctuating flow. Resultsfor the above mentioned velocity quantities, together with theproduction of turbulence kinetic energy and the secondanisotropy inariant are also compared to data from simulationsbased on the k -wformulation with an EARSM model. The simulation datawere found to severely underestimate the size of the separationbubble.
Keywords:Fluid mechanics, wind-tunnels, asymmetricdiffuser, turbulent boundary layer, flow structures, PDFs,modeling, symmetry methods.
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Chaplin, G. C. "Turbulent wind interactions with ventilated structures." Thesis, University of Nottingham, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.339663.
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Semlitsch, Bernhard. "Large Eddy Simulation of Turbulent Compressible Jets." Doctoral thesis, KTH, Mekanik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-156230.
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Acoustic noise pollution is an environmental aggressor in everyday life. Aero- dynamically generated noise annoys and was linked with health issues. It may be caused by high-speed turbulent free flows (e.g. aircraft jet exhausts), by airflow interacting with solid surfaces (e.g. fan noise, wind turbine noise), or it may arise within a confined flow environment (e.g. air ventilation systems, refrigeration systems). Hence, reducing the acoustic noise levels would result in a better life quality, where a systematic approach to decrease the acoustic noise radiation is required to guarantee optimal results. Computational predic- tion methods able to provide all the required flow quantities with the desired temporal and spatial resolutions are perfectly suited in such application areas, when supplementing restricted experimental investigations. This thesis focuses on the use of numerical methodologies in compressible flow applications to understand aerodynamically noise generation mechanisms and to assess technologies used to suppress it. Robust and fast steady-state Reynolds Averaged Navier-Stokes (RANS) based formulations are employed for the optimal design process, while the high fidelity Large Eddy Simulation (LES) approach is utilized to reveal the detailed flow physics and to investigate the acoustic noise production mechanisms. The employment of fast methods on a wide range of cases represents a brute-force strategy used to scrutinize the optimization parameter space and to provide general behavioral trends. This in combination with accurate simulations performed for particular condi- tions of interest becomes a very powerful approach. Advance post-processing techniques (i.e. Proper Orthogonal Decomposition and Dynamic Mode Decomposition) have been employed to analyze the intricate, highly turbulent flows. The impact of using fluidic injection inside a convergent-divergent nozzle for acoustic noise suppression is analyzed, first using steady-state RANS simulations. More than 250 cases are investigated for the optimal injection location and angle, amount of injected flow and operating conditions. Based on a-priori established criteria, a few optimal candidate solutions are detected from which one geometrical configuration is selected for being thoroughly investigated by using detailed LES calculations. This allows analyzing the unsteady shock pattern movement and the flow structures resulting with fluidic injec- tion. When investigating external fluidic injection configurations, some lead to a high amplitude shock associated noise, so-called screech tones. Such unsteady phenomena can be captured and explained only by using unsteady simulations. Another complex flow scenario demonstrated using LES is that of a high ve- locity jet ejected into a confined convergent-divergent ejector (i.e. a jet pump). The standing wave pattern developed in the confined channel and captured by LES, significantly alters the acoustic noise production. Steady-state methods failed to predict such events. The unsteady highly resolved simulations proved to be essential for analyzing flow and acoustics phenomena in complex problems. This becomes a very powerful approach when is used together with steady-state, low time-consuming formulations and when complemented with experimental measurements.QC 20141202
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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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Lakkadi, Navneeth Sagar Reddy. "Flow Measurements in Turbulent Flow Fields with Magnetic Resonance Phase Velovity Mapping." Cleveland State University / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=csu1251412647.
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