Дисертації: "Turbulent fluid flows"

1

Leeming, Angus David. "Particle deposition from turbulent flows." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.242996.

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2

Godden, Paul James. "Turbulent buoyant fluid flows in confined regions." Thesis, University of Bristol, 2002. http://hdl.handle.net/1983/7f335a17-90bb-4229-b264-36c470f573f7.

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3

Mastorakos, Epaminondas. "Turbulent combustion in opposed jet flows." Thesis, Imperial College London, 1994. http://hdl.handle.net/10044/1/11820.

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4

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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5

Vassilicos, J. C. "Fractal and moving interfaces in turbulent flows." Thesis, University of Cambridge, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293384.

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6

Murray, Kevin B. "Wavelet transform analysis of turbulent wake flows." Thesis, Edinburgh Napier University, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.322272.

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7

Karim, Othman A. "Prediction of two and three dimensional turbulent flows." Thesis, University of Liverpool, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.266196.

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8

Neel, Reece E. "Advances In Computational Fluid Dynamics: Turbulent Separated Flows And Transonic Potential Flows." Diss., Virginia Tech, 1997. http://hdl.handle.net/10919/30677.

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Computational solutions are presented for flows ranging from incompressible viscous flows to inviscid transonic flows. The viscous flow problems are solved using the incompressible Navier-Stokes equations while the inviscid solutions are attained using the full potential equation. Results for the viscous flow problems focus on turbulence modeling when separation is present. The main focus for the inviscid results is the development of an unstructured solution algorithm. The subject dealing with turbulence modeling for separated flows is discussed first. Two different test cases are presented. The first flow is a low-speed converging-diverging duct with a rapid expansion, creating a large separated flow region. The second case is the flow around a stationary hydrofoil subject to small, oscillating hydrofoils. Both cases are computed first in a steady state environment, and then with unsteady flow conditions imposed. A special characteristic of the two problems being studied is the presence of strong adverse pressure gradients leading to flow detachment and separation. For the flows with separation, numerical solutions are obtained by solving the incompressible Navier-Stokes equations. These equations are solved in a time accurate manner using the method of artificial compressibility. The algorithm used is a finite volume, upwind differencing scheme based on flux-difference splitting of the convective terms. The Johnson and King turbulence model is employed for modeling the turbulent flow. Modifications to the Johnson and King turbulence model are also suggested. These changes to the model focus mainly on the normal stress production of energy and the strong adverse pressure gradient associated with separating flows. The performance of the Johnson and King model and its modifications, along with the Baldwin-Lomax model, are presented in the results. The modifications had an impact on moving the flow detachment location further downstream, and increased the sensitivity of the boundary layer profile to unsteady flow conditions. Following this discussion is the numerical solution of the full potential equation. The full potential equation assumes inviscid, irrotational flow and can be applied to problems where viscous effects are small compared to the inviscid flow field and weak normal shocks. The development of a code is presented which solves the full potential equation in a finite volume, cell centered formulation. The unique feature about this code is that solutions are attained on unstructured grids. Solutions are computed in either two or three dimensions. The grid has the flexibility of being made up of tetrahedra, hexahedra, or prisms. The flow regime spans from low subsonic speeds up to transonic flows. For transonic problems, the density is upwinded using a density biasing technique. If lift is being produced, the Kutta-Joukowski condition is enforced for circulation. An implicit algorithm is employed based upon the Generalized Minimum Residual method. To accelerate convergence, the Generalized Minimum Residual method is preconditioned. These and other problems associated with solving the full potential equation on an unstructured mesh are discussed. Results are presented for subsonic and transonic flows over bumps, airfoils, and wings to demonstrate the unstructured algorithm presented here.
Ph. D.

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9

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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10

Ramsay, Euan Grant. "Nonlinear microscopy of semiconductor devices and turbulent fluid flows." Thesis, Heriot-Watt University, 2005. http://hdl.handle.net/10399/260.

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11

Wilson, Dean Robert. "Computational modelling of turbulent magnetohydrodynamic flows." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/computational-modelling-of-turbulent-magnetohydrodynamic-flows(787a2a9d-f2f6-42e0-b218-a7b6e3041e04).html.

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The study of magnetohydrodynamics unifies the fields of fluid mechanics and electrodynamics to describe the interactions between magnetic fields and electrically conducting fluids. Flows described by magnetohydrodynamics form a significant aspect in a wide range of engineering applications, from the liquid metal blankets designed to surround and remove heat from nuclear fusion reactors, to the delivery and guidance of nanoparticles in magnetic targeted drug delivery. The ability to optimize these, and other, processes is increasingly reliant on the accuracy and stability of the numerical models used to predict such flows. This thesis addresses this by providing a detailed assessment on the performance of two electromagnetically extended Reynolds-averaged Navier-Stokes models through computations of a number of electromagnetically influenced simple channel and Rayleigh-Bènard convective flows. The models tested were the low-Re k-ε linear eddy-viscosity model of Launder and Sharma (1974), with electromagnetic modifications as proposed by Kenjereš and Hanjalić (2000), and the low-Re stress-transport model of Hanjalić and Jakirlić (1993), with electromagnetic modifications as proposed by Kenjereš and Hanjalić (2004). First, a one-dimensional fully-developed turbulent channel flow was considered over a range of Reynolds and Hartmann numbers with a magnetic field applied in both wall-normal and streamwise directions. Results showed that contributions from the electromagnetic modifications were modest and, whilst both models inherently captured some of the reduction in mean strain that a wall-normal field imposed, results from the stress-transport model were consistently superior for both magnetic field directions. Then, three-dimensional time-dependent Rayleigh-Bènard convection was considered for two different Prandtl numbers, two different magnetic field directions and over a range of Hartmann numbers. Results revealed that, at sufficiently high magnetic field strengths, a dramatic reorganization of the flow structure is predicted to occur. The vertical magnetic field led to a larger number of thinner, more cylindrical plumes whilst the horizontal magnetic field caused a striking realignment of the roll cells' axes with the magnetic field lines. This was in agreement with both existing numerical simulations and physical intuition. The superior performance of the modified stress-transport model in both flows was attributed to both its ability to provide better representation of stress generation and other processes, and its ability to accommodate the electromagnetic modifications in a more natural, and exact, fashion. The results demonstrate the capabilities of the stress-transport approach in modelling MHD flows that are relevant to industry and offer potential for those wishing to control flow structure or levels of turbulence without recourse to mechanical means.

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12

Li, Zhaorui. "Modeling and simulation of turbulent multiphase flows." Diss., Connect to online resource - MSU authorized users, 2008.

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13

Plasting, Stephen Christopher. "Turbulence has its limits : a priori estimates of transport properties in turbulent fluid flows." Thesis, University of Bristol, 2004. http://hdl.handle.net/1983/ca76fd77-e2a3-4eed-8a34-39203e11c84f.

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14

Mossi, Michele. "Simulation of benchmark and industrial unsteady compressible turbulent fluid flows /." [S.l.] : [s.n.], 1999. http://library.epfl.ch/theses/?nr=1958.

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15

Nemouchi, Zoubir. "The computation of turbulent thin shear flows associated with flow around multi-element aerofoils." Thesis, University of Manchester, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.480482.

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16

Fu, Song. "Computational modelling of turbulent swirling flows with second-moment closures." Thesis, University of Manchester, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.267917.

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This work focuses on the simulation of turbulent swirling flows within the framework of second-moment closure. The main objectives are to assess the performance of currently available turbulence models in predicting such flows, and to develop new closure models which would further enhance current predictive ability, and hence, to provide a reliable turbulence model for engineering applications that would help the design process and reduce the development costs of industrial combustion systems. Attention is confined to isothermal flows, and predictions have been carried out for three major swirling cases: a weakly and a strongly swirling free jet and a confined strongly swirling flow in which an annular swirling stream is discharged together with a non-swirling central jet into a suddenly enlarging circular chamber. In the last case, mass transfer has also been examined by predicting the behaviour of an inert scalar tracer with which the central jet has been laced. The existing turbulence models examined are the standard versions of the k — e Boussinesq-viscosity model, the algebraic stress closure and the differential stress closure (BVM, ASM and DSM, respectively), as well as modified ASM and DSM variants. One outcome of this study is that neither the standard versions of the BVM, ASM and DSM nor their previously modified forms examined here predict adequately swirling-flow behaviour. An important conclusion emerging from preliminary efforts has been that the algebraic approximation of stress transport in terms of the transport of turbulence energy—which is a widely used practice—is fundamentally flawed in the presence of swirl. Specifically, the method returns a physically unrealistic behaviour of the normal stresses. It is this conclusion which eventually led to the ASM methodology being discarded and to the exclusive use of the differential methodology. Within the framework of differential closures, two new pressure-strain models have been proposed, namely the Isotropization of Production and Convection Model (IPCM) and the Cubic Quasi-Isotropic Model (CQIM). The former emerged as an extension of the standard DSM approach with the inclusion of the convection tensor into the turbulence isotropization mechanism, whereas the latter follows from a more rational and fundamental approach in which non-linear anisotropy effects have been incorporated, with the resulting model made to satisfy the limit of two-dimensional turbulence. Comparisons between predicted solutions and measurements for swirling flow show that the IPCM produces a marked improvement over all the other models considered, while it does not significantly alter the behaviour of the standard stress closure in non-swirling conditions. Only very limited improvement is achieved by the CQIM, however, despite its success in predicting nearly homogeneous shear flows. The merits and weaknesses of all the models examined are discussed in detail, and the IPCM is recommended as the best approach for predictions of swirling flows. Within the study of the confined case, considerations were extended to the modelling of scalar transport by a second-moment flux closure, and comparisons are made between eddy-diffusivity and flux-closure calculations and experimental data. Computational results show that the distribution of the scalar field is primarily governed by aero-dynamic features. There are indications, however, that the flux model is superior to the eddy-diffusivity model.

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17

Duffy, Graham James. "A computational study of particulate deposition from turbulent gas flows." Thesis, University of Salford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.334338.

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18

Kvick, Mathias. "Transitional and turbulent fibre suspension flows." Doctoral thesis, KTH, Strömningsfysik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-153018.

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In this thesis the orientation of macro-sized fibres in turbulent flows is studied, as well as the effect of nano-sized fibrils on hydrodynamic stability. The focus lies on enabling processes for new materials where cellulose is the main constituent. When fibres (or any elongated particles) are added to a fluid, the complexity of the flow-problem increases. The fluid flow will influence the rotation of the fibres, and therefore also effect the overall fibre orientation. Exactly how the fibres rotate depends to a large extent on the mean velocity gradient in the flow. In addition, when fibres are added to a suspending fluid, the total stress in the suspension will increase, resulting in an increased apparent viscosity. The increase in stress is related to the direction of deformation in relation to the orientation of the particle, i.e. whether the deformation happens along the long or short axis of the fibre. The increase in stress, which in most cases is not constant neither in time nor space, will in turn influence the flow. This thesis starts off with the orientation and spatial distribution of fibres in the turbulent flow down an inclined plate. By varying fibre and flow parameters it is discovered that the main parameter controlling the orientation distribution is the aspect ratio of the fibres, with only minor influences from the other parameters. Moreover, the fibres are found to agglomerate into streamwise streaks. A new method to quantify this agglomeration is developed, taking care of the problems that arise due to the low concentration in the experiments. It is found that streakiness, i.e. the tendency to agglomerate in streaks, varies with Reynolds number. Going from fibre orientation to flow dynamics of fibre suspensions, the influence of cellulose nanofibrils (CNF) on laminar/turbulent transition is investigated in three different setups, namely plane channel flow, curved-rotating channel flow, and the flow in a flow focusing device. This last flow case is selected since it is can be used for assembly of CNF based materials. In the plane channel flow, the addition of CNF delays the transition more than predicted from measured viscosities while in the curved-rotating channel the opposite effect is discovered. This is qualitatively confirmed by linear stability analyses. Moreover, a transient growth analysis in the plane channel reveals an increase in streamwise wavenumber with increasing concentration of CNF. In the flow focusing device, i.e. at the intersection of three inlets and one outlet, the transition is found to mainly depend on the Reynolds number of the side flow. Recirculation zones forming downstream of two sharp corners are hypothesised to be the cause of the transition. With that in mind, the two corners are given a larger radius in an attempt to stabilise the flow. However, if anything, the flow seems to become unstable at a smaller Reynolds number, indicating that the separation bubble is not the sole cause of the transition. The choice of fluid in the core flow is found to have no effect on the stability, neither when using fluids with different viscosities nor when a non-Newtonian CNF dispersion was used. Thus, Newtonian model fluids can be used when studying the flow dynamics in this type of device. As a proof of concept, a flow focusing device is used to produce a continuous film from CNF. The fibrils are believed to be aligned due to the extensional flow created in the setup, resulting in a transparent film, with an estimated thickness of 1 um.

QC 20141003

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19

Smith, Thomas M. "Unsteady simulations of turbulent premixed reacting flows." Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/13097.

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20

Witz, Matthew J. "Mechanics of particle entrainment in turbulent open-channel flows." Thesis, University of Aberdeen, 2015. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=225690.

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An advanced understanding of particle entrainment is required to optimise the design and maintenance of numerous open channel hydraulic systems and structures placed in these systems; including river channels and canals. This study is on particle entrainment (defined as the movement of a particle from a stationary position to being mobile in the flow). Three aspects of particle entrainment were identified as the focus of this work: First, the waiting time for an exposed particle to entrain under constant flow conditions. Second, the flow features responsible for the entrainment of an individual exposed particle. Third, the motion of an entrained particle immediately after entrainment. Waiting time was found to be highly sensitive to protrusion, with a small increase in protrusion resulting in a significant decrease in waiting time. Contrary to previous suggestions the waiting time to entrainment was found to be poorly described by an exponential distribution; instead Weibull or gamma distributions provide an improved fit in both qualitative and quantitative terms. Ensemble averaged flow fields at the point of entrainment were computed to determine the features responsible for entrainment. The data from the transverse vertical PIV plane indicated the presence of two counter-rotating vortices, with the boundary between the vortices located directly over the entrainment particle. The streamwise vertical PIV measurements showed the presence of a structure extending for a considerable distance in the streamwise direction, the length of which appeared to be independent of submergence. Further, the inclination of the downstream end of the structure appeared to increase with submergence. From the point of entrainment particle dffusion in all three coordinate directions displays an exponent significantly greater than that of ballistic diffusion. From the point of entrainment particle diffusion in all three coordinate directions displays an exponent significantly greater than that of ballistic diffusion. The results highlight the clear difference in the local scale between the diffusion of an already mobile particle with one starting from a position of rest.

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21

Shipton, Jemma. "Balance, gravity waves and jets in turbulent shallow water flows." Thesis, University of St Andrews, 2009. http://hdl.handle.net/10023/708.

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This thesis contains a thorough investigation of the properties of freely decaying turbulence in a rotating shallow water layer on a sphere. A large number of simulations, covering an extensive range of Froude and Rossby numbers, have been carried out using a novel numerical algorithm that exploits the underly- ing properties of the flow. In general these flows develop coherent structures; vortices interact, merge and migrate polewards or equatorwards depending or their sign, leaving behind regions of homogenized potential vorticity separated by sharp zonal jets. In the first half of the thesis we investigate new ways of looking at these structures. In the second half of the thesis we examine the properties of the potential vorticity (PV) induced, balanced component and the residual, unbalanced component of the flows. Cyclone-anticyclone asymmetry has long been observed in atmospheric and oceanic data, laboratory experiments and numerical simulations. This asymmetry is usually seen to favour anticyclonic vorticity with the asymmetry becoming more pronounced at higher Froude numbers (e.g. Polvani et al. [1994a]). We find a similar result but note that the cyclones, although fewer, are significantly more intense and coherent. We present several ways of quantifying this across the parameter space. Potential vorticity homogenization is an important geophysical mechanism responsible for sharpening jets through the expulsion of PV gradients to the edge of flow structures or domains. Sharp gradients of PV are obvious in contour plots of this field as areas where the contours are bunched together. This suggests that we can estimate the number of zonal jets by performing a cluster analysis on the mean latitude of PV contours (this diagnostic is also examined by Dritschel and McIntyre [2007]). This provides an estimate rather than an exact count of the number of jets because the jets meander signficantly. We investigate the accuracy of the estimates provided by different clustering techniques. We find that the properties of the jets defy such simple classification and instead demand a more local examination. We achieve this by examining the palinstrophy field. This field, calculated by taking the gradient of the PV, highlights the regions where PV contours come closer together, exactly what we would expect in regions of strong jets. Plots of the palinstrophy field reveal the complex structure of these features. The potential vorticity field is even more central to the flow evolution than the strong link with jets suggests. From a knowledge of the spatial distribution of PV, it is possible to diagnose the balanced components of all other fields. These components will not contain inertia-gravity waves but will contain the dominant, large scale features of the flow. This inversion, or decomposition into balanced (vortical) and unbalanced (wave) components, is not unique and can be defined to varying orders of accuracy. We examine the results of four dfferent definitions of this decomposition, two based on truncations of the full equations and two based on an iterative procedure applied to the full equations. We find the iterative procedure to be more accurate in that it attributes more of the flow to the PV controlled, balanced motion. However, the truncated equations perform surprisingly well and do not appear to suffer in accuracy at the equator, despite the fact that the scaling on which they are based has been thought to break down there. We round off this study by considering the impact of the unbalanced motion on the flow. This is accomplished by splitting the integration time of the model into intervals τ < t < τ+dτ and comparing, at the end of each interval, the balanced components of the flow obtained by a) integrating the model from t = 0 and b) integrating the full equations, initialised at t = τ with the balanced components from a) at t = τ. We find that any impact of the unbalanced component of the flow is less than the numerical noise of the model.

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22

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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23

Törnblom, Olle. "Experimental and computational studies of turbulent separating internal flows." Doctoral thesis, KTH, Mekanik, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4071.

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The separating turbulent flow in a plane asymmetric diffuser with 8.5 degrees opening angle is investigated experimentally and computationally. The considered flow case is suitable for fundamental studies of separation, separation control and turbulence modelling. The flow case has been studied in a specially designed wind-tunnel under well controlled conditions. The average velocity and fluctuation fields have been mapped out with stereoscopic particle image velocimetry (PIV). Knowledge of all velocity components allows the study of several quantities of interest in turbulence modelling such as the turbulence kinetic energy, the turbulence anisotropy tensor and the turbulence production rate tensor. Pressures are measured through the diffuser. The measured data will form a reference database which can be used for evaluation of turbulence models and other computational investigations. Time-resolved stereoscopic PIV is used in an investigation of turbulence structures in the flow and their temporal evolution. A comparative study is made where the measured turbulence data are used to evaluate an explicit algebraic Reynolds stress turbulence model (EARSM). A discussion regarding the underlying reasons for the discrepancies found between the experimental and the model results is made. A model for investigations of separation suppression by means of vortex generating devices is presented together with results from the model in the plane asymmetric diffuser geometry. A short article on the importance of negative production-rates of turbulent kinetic energy for the reverse flow region in separated flows is presented. A detailed description of the experimental setup and PIV measurement procedures is given in a technical report.
QC 20100923

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24

Yates, Matthew. "The measurement of particle dispersions in turbulent, four-way coupled flows." Thesis, University of Nottingham, 2018. http://eprints.nottingham.ac.uk/55293/.

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This work contained in this thesis is the result of an industrial and academic collaboration, designed to investigate and further the present knowledge of dense turbulent dispersions. Experiments were conducted to provide support and experimental validation to a CFD code being simultaneously developed, which was able to give insight into these types of flows. Additional to this support, the aim of this thesis was to also further knowledge of key topics in this field. The experimental methodology chosen was to use a mixture of Particle Image Velocimetry and Particle Tracking Velocimetry. To discriminate between particle and liquid phases, two approaches were adopted, depending upon the experiment. In one approach, fluorescent dyes were used to tag one phase, whilst optical filters were applied to the camera lenses. In the second approach, a size-based binary mask was applied to a single image, in order to remove phase information and produce two sets of images. A number of different analysis techniques were researched and developed as part of this thesis. The performance of particle tracking algorithms was assessed to ascertain their most suitable usage. A number of different algorithms, designed to characterise particle positions, were validated against known test cases. These included the Box Counting Method, a Voronoi analysis, and Radial Distribution Functions. A further technique, known as the Particle Potential method, was also developed to characterise local clustering. Two experiments were undertaken throughout this project, both of which were developed from scratch so that full control was assured over all experimental parameters. A vertical channel experiment was designed to assess the injections of particles into a rectangular channel. These experiments allowed for an ideal test case of highly concentrated particles, without the need to achieve optical visibility through a dense solution. The experiments also provided an early test of a Refractive Index Matching candidate pair; hydrogel particles and water. The second experiment was known as the Circulating Dispersion Rig, which was designed to pump a slurry in a continuous loop in a cylindrical pipe. These experiments, due to the geometry used and dense nature of the slurry, were reliant upon trying to achieve optimum optical visibility, and so hydrogel/water mixtures were tested in advance against other, more well-utilised pairings. The experiments conducted have provided some insight into the nature of particles in turbulent flows, in particular their clustering properties. Clustering was assessed under various concentrations. Key results included analysis of these clusters using a Voronoi diagram technique, which identified four key types of cluster structure, and the parameters under which these form. Collision probabilities of particle pairs were also assessed, using Particle Tracking data and computation of relative velocities. Such information is of importance for experimental validation of CFD codes relating to dispersed two-phase flows, where particle-particle coupling must be assessed in order to provide accurate solutions. The key drive towards the future, should further experiments be desirable, would be to investigate the improvement of optically matching liquids and solids, which was felt to be the limiting factor towards achieving measurements at even higher concentrations. However, these experiments show some progress can be made in making measurements of four-way coupled turbulent flows.

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25

Fornari, Walter. "Suspensions of finite-size rigid particles in laminar and turbulent flows." Doctoral thesis, KTH, Mekanik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-217812.

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Dispersed multiphase flows occur in many biological, engineering and geophysical applications. Understanding the behavior of suspensions is a difficult task. In the present work, we numerically study the behavior of suspensions of finite-size rigid particles in different flows. Firstly, the sedimentation of spherical particles larger than the Taylor microscale in sustained homogeneous isotropic turbulence and quiescent fluid is investigated. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. We also investigate the settling in quiescent fluid of oblate particles. We find that at low volume fractions the mean settling speed of the suspension is substantially larger than the terminal speed of an isolated oblate. Suspensions of finite-size spheres are also studied in turbulent channel flow. First, we change the solid volume and mass fractions, and the solid-to-fluid density ratio in an idealized scenario where gravity is neglected. Then we investigate the effects of polydispersity. It is found that the statistics are substantially altered by changes in volume fraction. We then consider suspensions of solid spheres in turbulent duct flows. We see that particles accumulate mostly at the corners or at the core depending on the volume fraction. Secondary motions are enhanced by increasing the volume fraction, until excluded volume effects are so strong that the turbulence activity is reduced. The inertial migration of spheres in laminar square duct flows is also investigated. We consider semi-dilute suspensions at different bulk Reynolds numbers and duct-to-particle size ratios. The highest particle concentration is found around the focusing points, except at very large volume fractions. Finally we study the rheology of confined dense suspensions of spheres in simple shear flow. We focus on the weakly inertial regime and show that the effective viscosity varies non-monotonically with increasing confinement.

QC 20171117

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26

Shipton, Jemma. "Balance, gravity waves and jets in turbulent shallow water flows /." St Andrews, 2008. http://hdl.handle.net/10023/708.

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27

Mårtensson, Gustaf. "Analysis of laminar and turbulent flows with turbomachinery, biotechnology and biomechanical applications." Doctoral thesis, KTH, Mechanics, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3928.

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The goal of this study was initially to gain a better understanding of the effects of rotation on turbulent flow in ducts. Knowledge concerning the influence of rotation on the structures of turbulence is of fundamental importance in many applications, e.g. centrifugal separators, turbines or cooling channels in rotating machinery, as well as meteorology and oceanography. Rapidly rotating duct flow is studied experimentally with rotation numbers in the interval [ 0, 1] . To achieve this, in combination with relatively high Reynolds numbers (5000 – 30000 based on the hydraulic radius), water was used as the working medium. The influence of the rotation on the pressure drop in the duct was investigated and suitable scalings of this quantity were studied. Due to questions that arose in the experimental study, two numerical studies were initiated. The first study probed the effect of rotation and geometrical configuration on the development length for turbulent flow, while the second comprised a direct numerical simulation of turbulent flow in a rotating duct. It is shown that while system rotation does not have a marked effect on the development length in a plane channel, the development length is substantially shortened in a duct.

Additional systems subject to rotation or curvature effects were studied. The laminar flow of fluid in a rotating PCR-cone was analysed analytically and numerically to understand the increased mixing and temperature homogenization. The flow field in the cone was described and the increased mixing was due to a strong boundary layer flow incited by Coriolis and buoyancy effects. Comparisons of the numerical simulations with experiments yielded good results.

A study to quantify the flow of blood in cerebral malformations using three-dimensional videodensitometry was performed. Data from experiments with an idealized flow phantom, as well clinical pathologies, showed that the proposed methodology in conjunction with clinical injection protocols can yield mean flux data with an error less than 20%. Protocol improvements are proposed.

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28

Aichouni, Mohamed. "Development and decay of turbulent pipe flows : an experimental and computational study." Thesis, University of Salford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.305082.

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29

Huval, Danny J. "Heat transfer in variable density, low mach number, stagnating turbulent flows." Diss., Georgia Institute of Technology, 1990. http://hdl.handle.net/1853/12394.

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30

San, Omer. "Multiscale Modeling and Simulation of Turbulent Geophysical Flows." Diss., Virginia Tech, 2012. http://hdl.handle.net/10919/28031.

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The accurate and efficient numerical simulation of geophysical flows is of great interest in numerical weather prediction and climate modeling as well as in numerous critical areas and industries, such as agriculture, construction, tourism, transportation, weather-related disaster management, and sustainable energy technologies. Oceanic and atmospheric flows display an enormous range of temporal and spatial scales, from seconds to decades and from centimeters to thousands of kilometers, respectively. Scale interactions, both spatial and temporal, are the dominant feature of all aspects of general circulation models in geophysical fluid dynamics. In this thesis, to decrease the cost for these geophysical flow computations, several types of multiscale methods were systematically developed and tested for a variety of physical settings including barotropic and stratified wind-driven large scale ocean circulation models, decaying and forced two-dimensional turbulence simulations, as well as several benchmark incompressible flow problems in two and three dimensions. The new models proposed here are based on two classes of modern multiscale methods: (i) interpolation based approaches in the context of the multigrid/multiresolution methodologies, and (ii) deconvolution based spatial filtering approaches in the context of large eddy simulation techniques. In the first case, we developed a coarse-grid projection method that uses simple interpolation schemes to go between the two components of the problem, in which the solution algorithms have different levels of complexity. In the second case, the use of approximate deconvolution closure modeling strategies was implemented for large eddy simulations of large-scale turbulent geophysical flows. The numerical assessment of these approaches showed that both the coarse-grid projection and approximate deconvolution methods could represent viable tools for computing more realistic turbulent geophysical flows that provide significant increases in accuracy and computational efficiency over conventional methods.
Ph. D.

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31

Chen, Zhuo. "Scalar dispersion in turbulent open channel flows over smooth and rough beds." Thesis, Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/44896.

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Study of passive dispersion of a neutral scalar in turbulentflows is highly important due to its numerous applications in the areas of turbulent flow visualization, turbulent heat transfer and transport of pollutants and other substances in the environment. Over the past few decades, many analytical, numerical, and experimental studies have been conducted on this topic to obtain a better understanding of the physical process. In the present work, Large Eddy Simulations (LES) of scalar dispersion in turbulent flow over smooth and rough channels is conducted to contribute to the further understanding of the relation between the turbulent velocity field and the concentration field. The LES results from the present work showed good agreement with a recently com-pleted experimental study(Rahman and Webster [2005]). An in-depth comparison of in-stantaneous concentration and velocity fields revealed thecorrelation between scalar dis-persion and coherent structures of the turbulent flow. Also,a three dimensional visual-ization of concentration iso-surfaces at different instants provided a good picture of the concentration structures transported as a result of hairpin vortices of turbulent flow, which is quite difficult to accomplish using experimental studies.

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32

Farbos, De Luzan Charles. "Numerical Analysis of Turbulent Flows in Channels of Complex Geometry." University of Cincinnati / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1468511618.

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33

Strömgren, Tobias. "Model predictions of turbulent gas-particle shear flows." Doctoral thesis, KTH, Mekanik, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-12135.

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A turbulent two-phase flow model using kinetic theory of granularflows for the particle phase is developed and implmented in afinite element code. The model can be used for engineeringapplications. However, in this thesis it is used to investigateturbulent gas-particle flows through numerical simulations. The feedback from the particles on the turbulence and the meanflow of the gas in a vertical channel flow is studied. In particular,the influence of the particle response time, particle volumefraction and particle diameter on the preferential concentration ofthe particles near the walls, caused by the turbophoretic effect isexplored. The study shows that when particle feedback is includedthe accumulation of particles near the walls decreases. It is also foundthat even at low volume fractions particles can have a significant impacton the turbulence and the mean flow of the gas. The effect of particles on a developing turbulent vertical upward pipeflow is also studied. The development length is found to substantiallyincrease compared to an unladen flow. To understand what governs thedevelopment length a simple estimation was derived, showing that itincreases with decreasing particle diameters in accordance with themodel simulations. A model for the fluctuating particle velocity in turbulentgas-particle flow is derived using a set of stochastic differentialequations taking into account particle-particle collisions. Themodel shows that the particle fluctuating velocity increases whenparticle-particle collisions become more important and that increasingparticle response times reduces the fluctuating velocity. The modelcan also be used for an expansion of the deterministic model for theparticle kinetic energy.
QC20100726

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34

Tselepidakis, Demetrios P. "Development and application of a new second-moment closure for turbulent flows near walls." Thesis, University of Manchester, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.332657.

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35

Ciofalo, Michele. "Large-eddy simulation of turbulent flows with heat transfer in simple and complex geometries." Thesis, University of London, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.262005.

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36

Hayes, E. R. "The prediction of droplet motion and breakup using a vortex model for turbulent flows." Thesis, Cranfield University, 1988. http://dspace.lib.cranfield.ac.uk/handle/1826/10285.

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This document describes the development of a computational model to study the movement and breakup _of droplets in turbulent two-component flows. The aim is to produce a suitable model which will be economical of computing resources and practical for engineering applications. The application of particular interest here is that of water droplets in fully developed turbulent pipe flows of oil. The computational method uses Vortex filaments to produce, in a novel way, instantaneous fluctuating velocities within the flow domain. The trajectory of a particle within this field is predicted by integrating the theoretical law of motion for the particle. In addition, the breakup of a fluid particle in the turbulent field may be predicted using an empirical criterion formulated using data obtained from a series of experiments. The tests were designed to study the deformation and breakup of a single water droplet in oil subjected to shear. Wherever possible the results of each development stage of the model were compared with work published in the literature.

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37

Kupiainen, Marco. "Compressible Turbulent Flows : LES and Embedded Boundary Methods." Doctoral thesis, KTH, Numerisk analys, NA, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-10090.

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38

Azouz, Idir. "Numerical simulation of laminar and turbulent flows of wellbore fluids in annular passages of arbitrary cross-section /." Access abstract and link to full text, 1994. http://0-wwwlib.umi.com.library.utulsa.edu/dissertations/fullcit/9500702.

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39

Alhamdi, Sabah Falih Habeeb. "INTERMITTENCY EFFECTS ON THE UNIVERSALITY OF LOCAL DISSIPATION SCALES IN TURBULENT BOUNDARY LAYER FLOWS WITH AND WITHOUT FREE-STREAM TURBULENCE." UKnowledge, 2018. https://uknowledge.uky.edu/me_etds/116.

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Measurements of the small-scale dissipation statistics of turbulent boundary layer flows with and without free-stream turbulence are reported for Reτ ≈ 1000 (Reθ ≈ 2000). The scaling of the dissipation scale distribution is examined in these two boundary conditions of external wall-bounded flow. Results demonstrated that the local large-scale Reynolds number based on the measured longitudinal integral length-scale fails to properly normalize the dissipation scale distribution near the wall in these two free-stream conditions, due to the imperfect characterization of the upper bound of the inertial cascade by the integral length-scale. When a length-scale based on Townsend's attached-eddy hypothesis is utilized to describe the local large-scale Reynolds number near the wall, the description of the Reynolds number scaling was determined to be significantly improved and agreed with that found in homogeneous, isotropic turbulence. However, the scaling based on Townsend's attached-eddy hypothesis agreed best for the lowest 40% of the boundary layer thickness and then it degraded due to the loss of the validity of the attached eddy-hypothesis and the onset of external intermittency. A surrogate large-scale found from turbulent kinetic energy and mean dissipation rate improved the scaling of the dissipation scales, relative to the measured integral length-scale. The probability density functions of the local dissipation scales were calculated. When the three local large-scale Reynolds numbers are used for normalization, the one based on the longitudinal integral length-scale and the one based on the length-scale of attached-eddy hypothesis provide support for the existence of a universal distribution of the local dissipation scales up to the edge of the outer region of the turbulent boundary layer, which scales differently for inner and outer regions. However, the probability density functions of the local dissipation scales normalized by these two large-scale Reynolds numbers are deviated in interface locations for the flow without free-stream turbulence due to external intermittency. The surrogate large-scale provided the best agreement throughout the entire depth of the boundary layer. However, in the outer part of the boundary layer, a significantly reduced collapse in the scaled probability density functions was shown due to bias in the calculation introduced by the intermittent presence of laminar flow in the time series. To support that intermittency argument, injection of the free-stream turbulence was determined to improve the distribution of these normalized probability density functions in the intermittency locations for the flow regime without free-stream turbulence. In addition, unlike in channel flow, in the outer part of the turbulent boundary layer, the normalized distributions of the local dissipation scales were observed to be dependent on wall-normal position. This was found to be attributable to the presence of external intermittency in this outer part as the presence of free-stream turbulence was found to restore the scaling behavior by replacing the intermittent laminar flow with turbulent flow. Thus, the influence of external intermittency on the scaling of the dissipation scale distribution was examined in greater detail for the laminar free-stream condition. Probability density functions of the dissipative scales were compared with, and without, accounting for the external intermittency using an intermittency detection function. Results showed that accounting for the external intermittency produces restores universality in the shapes of the probability density functions at the same wall-normal location at different instances in time. In addition, properly scaling the dissipation-scale-distribution collapses the probability density functions calculated at different wall-normal locations. This improvement in the scaling of the dissipation-scale-distribution supports prior observations of universality of the small-scale description of the turbulence for wall-bounded flow.

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40

Niazi, Ardekani Mehdi. "Numerical study of non-spherical/spherical particles in laminar and turbulent flows." Licentiate thesis, KTH, Mekanik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-204421.

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The presence of solid rigid particles alters the global transport and rheological properties of the mixture in complex (and often unpredictable) ways. In recent years a few studies have been devoted to investigating the behavior of dense suspensions in the turbulent/inertial regime with the majority of theses analyses limited to mono-disperse rigid neutrally-buoyant spheres. However, one interesting parameter that is rarely studied for particles with high inertia is the particle shape. Spheroidal particles introduce an anisotropy, e.g. a tendency to orient in a certain direction, which can affect the bulk behavior of a suspension in an unexpected ways. The main focus of this study is therefore to investigate the behavior of spheroidal particles and their effect on turbulent/inertial flows. We perform fully resolved simulations of particulate flows with spherical/spheroidal particles, using an efficient/accurate numerical approach that enables us to simulate thousands of particles with high resolutions in order to capture all the fluid-solid interactions. Several conclusions are drawn from this study that reveal the importance of particle's shape effect on the behaviour of a suspension e.g. spheroidal particles tend to cluster while sedimenting. This phenomenon is observed in this work for both particles with high inertia, sedimenting in a quiescent fluid and inertialess particles (point-like tracer prolates) settling in homogenous isotropic turbulence. The mechanisms for clustering is indeed different between these two situations, however, it is the shape of particles that governs these mechanisms, as clustering is not observed for spherical particles. Another striking finding of this work is drag reduction in particulate turbulent channel flow with rigid oblate particles. Again this drag reduction is absent for spherical particles, which instead increase the drag with respect to single-phase turbulence.

QC 20170328

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41

Bussman, Wesley Ryan. "A theoretical and experimental investigation of near-wall turbulence in drag reducing flows /." Access abstract and link to full text, 1990. http://0-wwwlib.umi.com.library.utulsa.edu/dissertations/fullcit/9111869.

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42

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.

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43

Springer, Matthias [Verfasser], and Stefan [Gutachter] Becker. "Fluid–Structure–Acoustics Interaction of Turbulent Wall–Bounded Flows / Matthias Springer ; Gutachter: Stefan Becker." Erlangen : Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 2018. http://d-nb.info/1153203278/34.

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44

Hawkins, Marion Joyce. "A study of turbulent flows and curved jets, including application of the laser Doppler anemometry technique." Thesis, Durham University, 1988. http://etheses.dur.ac.uk/1535/.

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45

Wiberg, Roland. "A study of heat transfer from cylinders in turbulent flows by using thermochromic liquid crystals." Licentiate thesis, KTH, Mechanics, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-1695.

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In gas quenching, metal parts are rapidly cooled from hightemperatures, and the convection heat transfer coefficientdistributions are of importance for the hardness and thedistortion (the shape nonuniformities) of the quenched parts.Thermochromic liquid crystals (TLC) and a thin foil techniques,were investi- gated and used for studies of a circular cylinderin axial flows, affected and not affected by upstream owmodifying inserts. Quadratic prisms in cross ows were alsostudied, a single prism, two prisms arranged in-line, and forfour prisms arranged in a square pattern. In this study,particle image velocime- try (PIV) was used for visualizationof the flow, giving physical insight to the convection heattransfer data. Further, relations of the typeNu=CReewere established. The TLC and thin foil techniques werealso used to indicate the dimensions of separated flowregions.

Descriptors:Fluid mechanics, wind-tunnel, turbulence,gas quenching, con- vection heat transfer, thermochromic liquidcrystals, calibration, temperature measurement errors, thinfoils, particle image velocimetry, cylinder in axial flow, flowmodifying inserts, quadratic prisms in cross flow

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46

Ferro, Marco. "Experimental study on turbulent boundary-layer flows with wall transpiration." Doctoral thesis, KTH, Mekanik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-217125.

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Анотація:

Wall transpiration, in the form of wall-normal suction or blowing through a permeable wall, is a relatively simple and effective technique to control the behaviour of a boundary layer. For its potential applications for laminar-turbulent transition and separation delay (suction) or for turbulent drag reduction and thermal protection (blowing), wall transpiration has over the past decades been the topic of a significant amount of studies. However, as far as the turbulent regime is concerned, fundamental understanding of the phenomena occurring in the boundary layer in presence of wall transpiration is limited and considerable disagreements persist even on the description of basic quantities, such as the mean streamwise velocity, for the rather simplified case of flat-plate boundary-layer flows without pressure gradients. In order to provide new experimental data on suction and blowing boundary layers, an experimental apparatus was designed and brought into operation. The perforated region spans the whole 1.2 m of the test-section width and with its streamwise extent of 6.5 m is significantly longer than previous studies, allowing for a better investigation of the spatial development of the boundary layer. The quality of the experimental setup and measurement procedures was verified with extensive testing, including benchmarking against previous results on a canonical zero-pressure-gradient turbulent boundary layer (ZPG TBL) and on a laminar asymptotic suction boundary layer. The present experimental results on ZPG turbulent suction boundary layers show that it is possible to experimentally realize a turbulent asymptotic suction boundary layer (TASBL) where the boundary layer mean-velocity profile becomes independent of the streamwise location, so that the suction rate constitutes the only control parameter. TASBLs show a mean-velocity profile with a large logarithmic region and without the existence of a clear wake region. If outer scaling is adopted, using the free-stream velocity and the boundary layer thickness (δ99) as characteristic velocity and length scale respectively, the logarithmic region is described by a slope Ao=0.064 and an intercept Bo=0.994, independently from the suction rate (Γ). Relaminarization of an initially turbulent boundary layer is observed for Γ>3.70×10−3. Wall suction is responsible for a strong damping of the velocity fluctuations, with a decrease of the near-wall peak of the velocity-variance profile ranging from 50% to 65% when compared to a canonical ZPG TBL at comparable Reτ. This decrease in the turbulent activity appears to be explained by an increased stability of the near-wall streaks. Measurements on ZPG blowing boundary layers were conducted for blowing rates ranging between 0.1% and 0.37% of the free-stream velocity and cover the range of momentum thickness Reynolds number 10000<Reθ<36000. Wall-normal blowing strongly modifies the shape of the boundary-layer mean-velocity profile. As the blowing rate is increased, the clear logarithmic region characterizing the canonical ZPG TBLs gradually disappears. A good overlap among the mean velocity-defect profiles of the canonical ZPG TBLs and of the blowing boundary layers for all the Re number and blowing rates considered is obtained when normalization with the Zagarola-Smits velocity scale is adopted. Wall blowing enhances the intensity of the velocity fluctuations, especially in the outer region. At sufficiently high blowing rates and Reynolds number, the outer peak in the streamwise-velocity fluctuations surpasses in magnitude the near-wall peak, which eventually disappears.
Genom att använda sig av genomströmmande ytor, med sugning eller blåsning, kan man relativt enkelt och effektivt påverka ett gränsskikts tillstånd. Genom sin potential att påverka olika strömningsfysikaliska fenomen så som att senarelägga både avlösning och omslaget från laminär till turbulent strömning (genom sugning) eller som att exempelvis minska luftmotståndet i turbulenta gränsskikt och ge kyleffekt (genom blåsning), så har ett otaligt antal studier genomförts på området de senaste decennierna. Trots detta så är den grundläggande förståelsen bristfällig för de strömningsfenomen som inträffar i turbulenta gränsskikt över genomströmmande ytor. Det råder stora meningsskiljaktigheter om de mest elementära strömningskvantiteterna, såsom medelhastigheten, när sugning och blåsning tillämpas även i det mest förenklade gränsskiktsfallet nämligen det som utvecklar sig över en plan platta utan tryckgradient. För att ta fram nya experimentella data på gränsskikt med sugning och blåsning genom ytan så har vi designat en ny experimentell uppställning samt tagit den i bruk.Den genomströmmande ytan spänner över hela bredden av vindtunnelns mätsträcka (1.2 m) och är 6.5 m lång i strömningsriktningen och är därmed betydligt längre än vad som använts i tidigare studier. Detta gör det möjligt att bättre utforska gränsskiktet som utvecklas över ytan i strömningsriktningen. Kvaliteten på den experimentella uppställningen och valda mätprocedurerna har verifierats genom omfattande tester, som även inkluderar benchmarking mot tidigare resultat på turbulenta gränsskikt utan tryckgradient eller blåsning/sugning och på laminära asymptotiska sugningsgränsskikt. De experimentella resultaten på turbulenta gränsskikt med sugning bekräftar för första gången att det är möjligt att experimentellt sätta upp ett turbulent asymptotiskt sugningsgränsskikt där gränsskiktets medelhastighetsprofil blir oberoende av strömningsriktningen och där sugningshastigheten utgör den enda kontrollparametern. Det turbulenta asymptotiska sugningsgränsskiktet visar sig ha en medelhastighetsprofil normalt mot ytan med en lång logaritmisk region och utan förekomsten av en yttre vakregion. Om man använder yttre skalning av medelhastigheten, med friströmshastigheten och gränsskiktstjockleken som karaktäristisk hastighet respektive längdskala, så kan det logaritmiska området beskrivas med en lutning på Ao=0.064 och ett korsande värde med y-axeln på Bo=0.994, som är oberoende av sugningshastigheten. Om sugningshasigheten normaliserad med friströmshastigheten överskrider värdet 3.70x10^-3 så återgår det ursprungligen turbulenta gränsskiktet till att vara laminärt. Sugningen genom väggen dämpar hastighetsfluktuationerna i gränsskiktet med upp till 50-60% vid direkt jämförelse av det inre toppvärdet i ett turbulent gränsskikt utan sugning och vid jämförbart Reynolds tal. Denna minskning av turbulent aktivitet verkar härstamma från en ökad stabilitet av hastighetsstråken närmast ytan. Mätningar på turbulenta gränsskikt med blåsning har genomförts för blåsningshastigheter mellan 0.1 och 0.37% av friströmshastigheten och täcker Reynoldstalområdet (10-36)x10^3, med Reynolds tal baserat på rörelsemängds-tjockleken. Vid blåsning genom ytan får man en stark modifiering av formen på hastighetesfördelningen genom gränsskiktet. När blåsningshastigheten ökar så kommer till slut den logaritmiska regionen av medelhastigheten, karaktäristisk för turbulent gränsskikt utan blåsning, att gradvis försvinna. God överens-stämmelse av medelhastighetsprofiler mellan turbulenta gränsskikt med och utan blåsning erhålls för alla Reynoldstal och blåsningshastigheter när profilerna normaliseras med Zagarola-Smits hastighetsskala. Blåsning vid väggen ökar intensiteten av hastighetsfluktuationerna, speciellt i den yttre regionen av gränsskiktet. Vid riktigt höga blåsningshastigheter och Reynoldstal så kommer den yttre toppen av hastighetsfluktuationer i gränsskiktet att överskrida den inre toppen, som i sig gradvis försvinner.

QC 20171101

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47

Martins, Ramon Silva. "Numerical simulation of turbulent viscoelastic fluid flows : flow classification and preservation of positive-definiteness of the conformation tensor." Thesis, Lille 1, 2016. http://www.theses.fr/2016LIL10127/document.

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Анотація:

Le but de ce travail est de fournir une amélioration de la connaissance sur le phénomène de la réduction de la traînée induite par polymère en considérant certains aspects de sa simulation numérique et les changements qui se produisent dans la cinématique de l’écoulement. Dans un premier temps, les transformations du type racine carrée et kernel racine-k pour le tenseur de conformation du modèle FENE-P ont été implémentées afin d’assurer la positivité du tenseur de conformation. Cependant, ces approches divergent en raison du caractère non-borné du tenseur de conformation. Cette contrainte n’a pas été respectée, même avec l’inclusion de diffusion artificielle. L’effet d’amortissement de la diffusion artificielle a permis d’assurer la stabilité numérique, mais il aboutit à une réduction de la traînée relative de 22% à 42% plus faible que prévue par les approches standards. Dans un second temps, les modes hyperboliques, paraboliques et elliptiques des écoulements turbulents viscoélastiques ont été évalués en utilisant de différents critères de classification d’écoulements. Certains avantages concernant les critères objectifs ont été discutés. On a observé que les domaines hyperboliques contribuent de manière significative à la cinématique de l’écoulement. Enfin, on a observé une tendance des domaines elliptiques et hyperboliques à devenir paraboliques et que cette tendance augmente avec l’élasticité
The purpose of this work is to provide an enhancement of the knowledge about the polymer-induced drag reduction phenomenon by considering some aspects of its numerical simulation and the changes that occur in the flow kinematics. In the first part, the square root and kernel root-k formulations for the conformation tensor in the FENE-P model were implemented and showed to preserve the positiveness of the conformation tensor. However, they led to numerical divergence due to the loss of boundedness of the conformation tensor. This constraint was violated even with the inclusion of artificial diffusion. The damping effect of artificial diffusion helped to ensure numerical stability, but led to relative drag reduction from 22% to 42% lower than expected from traditional methods. In the second part, the hyperbolic, parabolic and elliptic modes of turbulent viscoelastic flows were evaluated by means of different flow classification criteria. Some advantages of considering objective criteria were discussed. It was shown that the hyperbolic domains significantly contribute to the flow kinematics. Finally, a tendency of both elliptic and hyperbolic domains to become parabolic was observed and found to increase with the elasticity

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48

Buckland, Hannah. "Combined current, wave and turbulent flows and their effects on tidal energy devices." Thesis, Swansea University, 2014. https://cronfa.swan.ac.uk/Record/cronfa42509.

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Анотація:

This thesis considers the effect of disruptive waves and turbulence on a horizontal axis tidal stream turbine (TST), in terms of performance and survivability. The TST reaction to transient fluid flow is modelled analytically using Blade Element Momentum Theory (BEMT). Standard BEMT corrections are evaluated for the TST application and an alternative optimisation method is proposed for the steady state BEMT, improving compatibility with transient and depth dependent inflow, as well as the non-dimensionalisation constant needed to calculate the performance coefficients. Also, an alternative BEMT tip and hub loss implementation has led to a significant improvement of the turbine axial force prediction and in the high induction region. Validation studies are presented for BEMT coupled with regular, nonlinear wave theory and good agreement is found with published experimental data. A novel method to simulate irregular sea states is developed to couple with BEMT and a combined reactive coupling of waves and current is implemented. The TST performance in an irregular sea state is considered against turbine performance with real ADCP data and a good agreement is found. This work evaluates the BEMT implementation for the specific application of modelling TST's and significantly improves the fundamental theory, applicability and quality of results in this case.

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49

Schulz, Joseph C. "A study of magnetoplasmadynamic effects in turbulent supersonic flows with application to detonation and explosion." Diss., Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/53971.

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Анотація:

Explosions are a common phenomena in the Universe. Beginning with the Big Bang, one could say the history of the Universe is narrated by a series of explosions. Yet no matter how large, small, or complex, all explosions occur through a series of similar physical processes beginning with their initiation to their dynamical interaction with the environment. Of particular interest to this study is how these processes are modified in a magnetized medium. The role of the magnetic field is investigated in two scenarios. The first scenario addresses how a magnetic field alters the propagation of a gaseous detonation where the application of interest is the modification of a condensed-phase explosion. The second scenario is focused on the aftermath of the explosion event and addresses how fluid mixing changes in a magnetized medium. A primary focus of this thesis is the development of a numerical tool capable of simulating explosive phenomenon in a magnetized medium. While the magnetohydrodynamic (MHD) equations share many of the mathematical characteristics of the hydrodynamic equations, numerical methods developed for the conservation equations of a magnetized plasma are complicated by the requirement that the magnetic field must be divergent free. The advantages and disadvantages of the proposed method are discussed in relation to explosion applications.

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50

Sigfrids, Timmy. "Hot wire and PIV studies of transonic turbulent wall-bounded flows." Licentiate thesis, KTH, Mechanics, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-1577.

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Анотація:

The compressible turbulent boundary layer developing over atwo-dimensional bump which leads to a supersonic pocket with aterminating shock wave has been studied. The measurements havebeen made with hot-wire anemometry and Particle ImageVelocimetry (PIV).

A method to calibrate hot-wire probes in compressible ow hasbeen developed which take into account not only the ow velocitybut also the inuence of the Mach number, stagnation temperatureand uid density. The calibration unit consists of a small jetow facility, where the temperature can be varied. The hot wiresare calibrated in the potential core of the free jet. The jetemanates in a container where the static pressure can becontrolled, and thereby the gas density. The calibration methodwas verfied in the at plate zero pressure gradient turbulentboundary layer in front of the bump at three different Machnumbers, namely 0.3, 0.5 and 0.7. The profiles were alsomeasured at different static pressures in order to see theinuence of varying density. Good agreement between the profilesmeasured at different pressures, as well as with the standardlogarithmic profile was obtained.

The PIV measurements of the boundary layer ow in front ofthe 2D bump showed good agreement with the velocity profilesmeasured with hotwire anemometry. The shock wave boundary layerinteraction was investigated for an inlet Mach number of 0.69.A lambda shock wave was seen on the downstream side of thebump. The velocity on both sides of the shock wave as measuredwith the PIV was in good agreement with theory. The shock wavewas found to cause boundary layer separation, which was seen asa rapid growth of the boundary layer thickness downstream theshock. However, no back ow was seen in the PIV-data, probablybecause the seeding did not give enough particles in theseparated region. The PIV data also showed that the shock wavewas oscillating, i.e. it was moving approximately 5 mm back andforth. This distance corresponds to about five boundary layerthicknesses in terms of the boundary layer upstream theshock.

Descriptors:Fluid mechanics, compressible ow,turbulence, boundary layer, hot-wire anemometry, PIV, shockwave boundary layer interaction, shape factor.

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Дисертації з теми "Turbulent fluid flows"

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