Tesi sul tema "Shear flow"
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1
Lemée, Thomas. "Shear-flow instabilities in closed flow". Thesis, Paris 11, 2013. http://www.theses.fr/2013PA112038.
Testo completoAbstract (sommario):
Cette étude se concentre sur la compréhension de la physique des instabilités dans différents écoulements de cisaillement, particulièrement la cavité entraînée et la cavité thermocapillaire, où l'écoulement d'un fluide incompressible est assuré soit par le mouvement d’une ou plusieurs parois, soit par des contraintes d’origine thermique.Un code spectral a été validé sur le cas très étudié de la cavité entrainée par une paroi mobile. Il est démontré dans ce cas que l'écoulement transit d'un régime stationnaire à un instationnaire au-delà d'une valeur critique du nombre de Reynolds. Ce travail est le premier à donner une interprétation physique de l'évolution non monotonique du nombre de Reynolds critique en fonction du facteur d'aspect. Lorsque le fluide est entraîné par deux parois mobiles, la cavité entraînée possède un plan de symétrie particulièrement sensible. Des solutions asymétriques peuvent être observés en plus de la solution symétrique au-dessus d'une certaine valeur du nombre de Reynolds. La transition oscillatoire entre la solution symétrique et les solutions asymétriques est expliquée physiquement par les forces en compétition. Dans le cas asymétrique, l'évolution de la topologie permet à l'écoulement de rester stationnaire avec l'augmentation du nombre de Reynolds. Lorsque l'équilibre est perdu une instabilité se manifeste par l'apparition d'un régime oscillatoire dans l'écoulement asymétrique.Dans une cavité thermocapillaire rectangulaire avec une surface libre, Smith et Davis prévoient deux types d'instabilités convectives thermiques: des rouleaux longitudinaux stationnaires et des ondes hydrothermales instationnaires. L'apparition de ses instabilités a été mis en évidence à plusieurs reprises expérimentalement et numériquement. Alors que les applications impliquent souvent plus d'une surface libre, il semble qu'il y ait peu de connaissances sur l'écoulement thermocapillaire entraînée avec deux surfaces libres. Un film liquide libre soumis à des contraintes thermocapillaires possède un plan de symétrie particulier comme dans le cas de la cavité entrainée par deux parois mobiles. Une étude de stabilité linéaire avec deux profils de vitesse pour le film liquide libre est présentée avec différents nombres de Prandtl. Au-delà d'un nombre de Marangoni critique, il est découvert que ces états de base sont sensibles à quatre types d'instabilités convectives thermiques qui peuvent conserver ou briser la symétrie du système. Les mécanismes qui permettent de prédire ces instabilités sont également découverts et interpréter en fonction de la valeur du nombre de Prandtl du fluide. La comparaison avec les travaux de Smith et Davis est faite. Une simulation numérique directe permet de valider les résultats obtenus avec l'étude de stabilité de linéaireThis study focuses on the understanding of the physics of different instabilities in driven cavities, specifically the lid-driven cavity and the thermocapillarity driven cavity where flow in an incompressible fluid is driven either due to one or many moving walls or due to surface stresses that appear from surface tension gradients caused by thermal gradients. A spectral code is benchmarked on the well-studied case of the lid-cavity driven by one moving wall. In this case, It is shown that the flow transit form a steady regime to unsteady regime beyond a critical value of the Reynolds number. This work is the first to give a physical interpretation of the non-monotonic evolution of the critical Reynolds number versus the size of the cavity. When the fluid is driven by two facing walls moving in the same direction, the cavity possesses a plane of symmetry particularly sensitive. Thus, asymmetrical solutions can be observed in addition to the symmetrical solution above a certain value of the Reynolds number. The oscillatory transition between the symmetric solution and asymmetric solutions is explained physically by the forces in competition. In the asymmetric case, the change of the topology allows the flow to remain steady with increasing the Reynolds number. When the equilibrium is lost, an instability manifests by the appearance of an oscillatory regime in the asymmetric flow. In a rectangular cavity thermocapillary with a free surface, Smith and Davis found two types of thermal convective instabilities: steady longitudinal rolls and unsteady hydrothermal waves. The appearance of its instability has been highlighted repeatedly experimentally and numerically. While applications often involve more than a free surface, it seems that there is little knowledge about the thermocapillary driven flow with two free surfaces. A free liquid film possesses a particular plane of symmetry as in the case of the two-sided lid-driven cavity. A linear stability analysis for the free liquid film with two velocity profiles is presented with various Prandtl numbers. Beyond a critical Marangoni number, it is observed that these basic states are sensitive to four types of thermal convective instabilities, which can keep or break the symmetry of the system. Mechanisms that predict these instabilities are discovered and interpreted according to the value of the Prandtl number of the fluid. Comparison with the work of Smith and Davis is made. A direct numerical simulation is done to validate the results obtained with the linear stability analysis
2
Marcos, Ph D. Massachusetts Institute of Technology. "Bacteria in shear flow". Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/65278.
Testo completoAbstract (sommario):
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 68-74).
Bacteria are ubiquitous and play a critical role in many contexts. Their environment is nearly always dynamic due to the prevalence of fluid flow: creeping flow in soil, highly sheared flow in bodily conduits, and turbulent flow in rivers, streams, lakes, and oceans, as well as anthropogenic habitats such as bioreactors, heat exchangers and water supply systems. The presence of flow not only affects how bacteria are transported and dispersed at the macroscale, but also their ability to interact with their local habitat through motility and chemotaxis (the ability to sense and follow chemical gradients), in particular their foraging. Despite the ubiquitous interaction between motility, foraging and flow, almost all studies of bacterial motility have been confined to still fluids. At the small scales of a bacterium, any natural flow field (e.g. turbulence) is experienced as a linear velocity profile, or 'simple shear'. Therefore, understanding the interaction between a simple shear flow and motility is a critical step towards gaining insight on how the ambient flow favors or hinders microorganisms in their quest for food. In this thesis, I address this important gap by studying the effect of shear on bacteria, using a combination of microfluidic experiments and mathematical modeling. In chapter 2, a method is presented to create microscale vortices using a microfluidic setup specifically designed to investigate the response of swimming microorganisms. Stable, small-scale vortices were generated in the side-cavity of a microchannel by the shear stress in the main flow. The generation of a vortex was found to depend on the cavity's geometry, in particular its depth, aspect ratio, and opening width. Using video-microscopy, the position and orientation of individual microorganisms swimming in vortices of various intensities were tracked. We applied this setup to the marine bacterium Pseudoalteromonas haloplanktis. Under weak flows (shear rates < 0.1 s 1), P. haloplanktis exhibited a random swimming pattern. As the shear rate increased, P. haloplanktis became more aligned with the flow. In order to study the detailed hydrodynamic interaction between shear and bacteria, we developed a mathematical model employing resistive force theory. In general, the modeling of a bacterium requires consideration of two factors: the rotating flagellar bundle and the cell body to which the flagella are attached. To make the problem analytically tractable, we study the hydrodynamics around the head and the flagellum separately. In chapter 3, we present a combined theoretical and experimental investigation of the fluid mechanics of a helix exposed to a shear flow. In addition to classic Jeffery orbits, resistive force theory predicts a drift of the helix across streamlines, perpendicular to the shear plane. The direction of the drift is determined by the direction of the shear and the chirality of the helix. We verify this prediction experimentally using microfluidics, by exposing Leptospira biflexa flaB mutant, a non-motile strain of helix-shaped bacteria, to a plane parabolic flow. As the shear in the top and bottom halves of the microchannel has opposite sign, we predict and observe the bacteria in these two regions to drift in opposite directions. The magnitude of the drift is in good quantitative agreement with theory. We show that this setup can be used to separate microscale chiral objects. In chapter 4, a theoretical and experimental investigation of a swimming bacterium in a shear flow is presented. The presence of the cell body results in a novel phenomenon: chiral forces induce not only a lateral drift, but also a reorienting torque on swimming bacteria. For typical flagellated bacteria, the magnitude of this drift velocity is much smaller (-0.7 gm s-1) than typical swimming speeds of bacteria (-50 [mu]m s-1). However, with the addition of a head, the chirality-dependent forces that lead to a lateral drift also lead to a reorienting torque. The model based on resistive force theory predicts that the drift velocity of swimming bacteria is in the same order of magnitude as the swimming speed. Experimental observations of the motile bacteria Bacillus subtilis exposed to shear flows show good agreement with the theoretical prediction. This process is a purely passive hydrodynamic effect, as demonstrated by further experiments showing that bacteria do not behaviorally (i.e. actively) respond to shear. This newly discovered hydrodynamic reorientation can significantly affect any process that involves changes of swimming direction, so that bacterial 'steering' in a flow cannot be understood unless the effects of chiral reorientation are quantified. Because swimming and reorientation are central to the chemotaxis used by many bacteria for foraging, we expect this coupling of motility and flow to play an important role in the ecology of many bacterial species.
by Marcos.
Ph.D.
3
Rychkov, Igor. "Block copolymers under shear flow". 京都大学 (Kyoto University), 2004. http://hdl.handle.net/2433/145457.
Testo completoAbstract (sommario):
Kyoto University (京都大学)0048
新制・課程博士
博士(理学)
甲第11046号
理博第2824号
新制||理||1421(附属図書館)
22578
UT51-2004-J718
京都大学大学院理学研究科物理学・宇宙物理学専攻
(主査)教授 吉川 研一, 教授 小貫 明, 助教授 瀬戸 秀紀
学位規則第4条第1項該当
4
Yato, Hiroki. "Flow pattern transition in curvilinear shear flows of viscoelastic fluids". 京都大学 (Kyoto University), 2010. http://hdl.handle.net/2433/131910.
Testo completo
5
Miller, Joel C. "Shear flow instabilities in viscoelastic fluids". Thesis, University of Cambridge, 2006. https://www.repository.cam.ac.uk/handle/1810/245318.
Testo completoAbstract (sommario):
This dissertation is concerned with the theoretical study of the stability of viscoelastic shear flows. It is divided into two parts: part I studies inertialess coextrusion flows at large Weissenberg number where the instabilities are due to discontinuities in the elastic properties, and part II studies the effect of elasticity on the well-known inertial instabilities of inviscid flows with inflection points. We begin part I with a previously known short-wave instability of Upper Convected Maxwell and Oldroyd–B fluids at zero Reynolds number in Couette flow. We show that if the Weissenberg number is large, the instability persists with the same growth rate when the wavelength is longer than the channel width. Intriguingly, surface tension does not modify the growth rate. Previous explanations of elastic interfacial instabilities based on the jump in normal stress at the interface cannot apply to this instability. These results are confirmed both numerically and with asymptotic methods. We then consider Poiseuille flow and show that a new class of instability exists if the interface is close to the center-line. We analyse the scalings and show that it results from a change in the boundary layer structure of the Couette instability. The growth rates can be large, and the wavespeed can be faster than the base flow advection. We are unable to simplify the equations significantly, and asymptotic results are not available, so we use numerics to verify the results. In studying these instabilities we encounter some others which we mention, but do not analyse in detail. In part II we study the effect of elasticity on the inertial instability of flows with inflection points. We show that the elasticity modifies the development of cat’s eyes. The presence of extensional flow complicates the analysis. Consequently we use the FENE–CR equations.
6
Paraschiv, Ioana. "Shear flow stabilization of Z-pinches". abstract and full text PDF (free order & download UNR users only), 2007. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3264527.
Testo completo
7
Wilson, Helen Jane. "Shear flow instabilities in viscoelastic fluids". Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.625082.
Testo completoAbstract (sommario):
The dissertation is concerned with the stability of channel flows of viscoelastic fluids. The content is primarily theoretical. The dissertation begins with a review of instabilities observed in experiments and then attempts to elucidate possible mechanisms using linear stability theory. The first section considers a previously known interfacial instability in coextrusion flows, whose mechanism is purely elastic. This instability is investigated in different parameter régimes for an Oldroyd-B fluid. The next section generalises the study to a continuously stratified fluid, and finds that a class of models with rapid variation in their elastic properties will also show the instability. These results are confirmed both numerically and using asymptotic methods. The fundamental mechanism of this "coextrusion" instability is the same as for the interfacial instability above. The next part concerns a shear-thinning White-Metzner fluid (i.e. a viscoelastic fluid having a relaxation time that is an instantaneous function of the local shear-rate). Evidence for another instability is found where the degree of thinning in the shear viscosity is high. The mechanism for this instability is fundamentally different from that in coextrusion. In the final section of the dissertation a study of two fluids of different constitutive types but identical base-state velocity and stress profiles shows that the criterion for the "coextrusion" instability depends on properties of the model itself. The flows in question are relevant to the practical problem of extrusion of polymeric liquids. The two instabilities found may provide mechanisms for experimental observations of helical distortions of extrudates. The demonstration that the constitutive type of a model has a crucial effect on its stability may have implications for future constitutive modelling in this field.
8
Ogino, Yoshiko. "Crystallization of Polymers under Shear Flow". 京都大学 (Kyoto University), 2006. http://hdl.handle.net/2433/77789.
Testo completo
9
Guvenen, Haldun. "Aerodynamics of bodies in shear flow". Diss., The University of Arizona, 1989. http://hdl.handle.net/10150/184917.
Testo completoAbstract (sommario):
This dissertation investigates spanwise periodic shear flow past two-dimensional bodies. The flow is assumed to be inviscid and incompressible. Using singular perturbation techniques, the solution is developed for ε = L/ℓ ≪ 1, where L represents body cross-sectional size, and ℓ the period of the oncoming flow U(z). The singular perturbation analysis involves three regions: the inner, wake and outer regions. The leading order solutions are developed in all regions, and in the inner region higher order terms are obtained. In the inner region near the body, the primary flow (U₀, V₀, P₀) corresponds to potential flow past the body with a local free stream value of U(z). The spanwise variation in U(z) produces a weak O(ε) secondary flow W₁ in the spanwise direction. As the vortex lines of the upstream flow are convected downstream, they wrap around the body, producing significant streamwise vorticity in a wake region of thickness O(L) directly behind the body. This streamwise vorticity induces a net volume flux into the wake. In the outer region far from the body, a nonlifting body appears as a distribution of three-dimensional dipoles, and the wake appears as a sheet of mass sinks. Both singularity structures must be included in describing the leading outer flow. For lifting bodies, the body appears as a lifting line, and the wake appears as a sheet of shed vorticity. The trailing vorticity is found to be equal to the spanwise derivative of the product of the circulation and the oncoming flow. For lifting bodies the first higher order correction to the inner flow is the response of the body to the downwash produced by the trailing vorticity. At large distances from the body, the flow takes on remarkably simple form.
10
Carter, Katherine Anne. "Shear banding in polymeric fluids under large amplitude oscillatory shear flow". Thesis, Durham University, 2016. http://etheses.dur.ac.uk/11746/.
Testo completoAbstract (sommario):
In this thesis, I theoretically explore shear banding of entangled linear polymer solutions and melts in large amplitude oscillatory shear strain (LAOStrain) and stress (LAOStress) protocols. This work moves beyond that of Moorcroft and Fielding [2013, 2014] who showed time-dependent shear banding in shear startup and step stress protocols. These protocols are only transiently time-dependent. LAOStrain and LAOStress have a sustained time-dependence. I consider the criteria derived in [Moorcroft and Fielding 2013] to predict the onset of shear banding in the transient material response for shear startup and step stress, relative to the triggers of shear banding in LAOStrain and LAOStress. I find that stability to the formation of shear banded flow in the LAOS protocols can be understood - to a good approximation - by the known triggers of shear banding in these simpler transiently time-dependent protocols. I employ the Rolie-Poly (RP) model [Graham et al. 2003] to investigate the existence of shear banding in LAOStrain and LAOStress over a wide range of imposed amplitudes and frequencies. I find shear banding to occur in the alternance state (where time-translational invariance is achieved), even in materials that are known to remain homogeneous at the steady state. For each protocol I consider the relative influence of the constraint-release stress relaxation RP parameter and entanglement number (Z) on the intensity of shear banding across the phase space. I find significant shear banding to occur in both LAOStrain and LAOStress for experimentally-realistic values of Z, both in materials that shear band to steady state, and those that don't. The main results of these investigations are submitted for publication in the Journal of Rheology [Carter et al. 2016]. Finally, I consider the shortcomings of using a single-mode RP model when characterising the full chain dynamics of entangled linear polymers in flow. I employ a multimode approach and fit a power-law spectrum to experimental linear rheology data and investigate time-dependent shear banding in the presence of higher-order relaxation dynamics. For this, I use the simpler shear startup protocol and investigate the limits under which significant shear banding exists for well-entangled polymers and discuss the possible importance of considering edge fracture as a mechanism for shear banding.
11
Voronkov, Igor. "Shear Alfvén waves and shear flow instabilities in the Earth's magnetosphere". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape15/PQDD_0003/NQ34851.pdf.
Testo completo
12
Boulay, Fabienne. "Suspension-flow modeling : curvilinear flows and normal stress differences". Thesis, Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/11689.
Testo completo
13
Sadrizadeh, Sasan. "Instabilities in Pulsating Pipe Flow of Shear-Thinning and Shear-Thickening Fluids". Thesis, Linköpings universitet, Mekanisk värmeteori och strömningslära, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-82037.
Testo completoAbstract (sommario):
In this study, we have considered the modal and non-modal stability of fluids with shear-dependent viscosity flowing in a rigid straight pipe. A second order finite-difference code is used for the simulation of pipe flow in the cylindrical coordinate system. The Carreau-Yasuda model where the rheological parameters vary in the range of 0.3 < n < 1.5 and 0.1 < λ < 100 is represents the viscosity of shear- thinning and shear thickening fluids. Variation of the periodic pulsatile forcing is obtained via the ratio Kω/Kο and set between 0.2 and 20. Zero and non-zero streamwise wavenumber have been considered separately in this study. For the axially invariant mode, energy growth maxima occur for unity azimuthal wave number, whereas for the axially non-invariant mode, maximum energy growth can be observed for azimuthal wave number of two for both Newtonian and non-Newtonian fluids. Modal and non-modal analysis for both Newtonian and non-Newtonian fluids show that the flow is asymptotically stable for any configuration and the pulsatile flow is slightly more stable than steady flow. Increasing the maximum velocity for shear-thinning fluids caused by reducing power-low index n is more evident than shear-thickening fluids. Moreover, rheological parameters of Carreau-Yasuda model have ignored the effect on the peak velocity of the oscillatory components. Increasing Reynolds number will enhance the maximum energy growth while a revers behavior is observed by increasing Womersley number.
14
La, Vecchia Miriam. "Bacterial chemotaxis in non-homogeneous shear flow". Thesis, KTH, Mekanik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-36075.
Testo completo
15
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.
Testo completoAbstract (sommario):
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
16
Feng, Yanhua. "Stably stratified shear flow over complex terrain". Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.264359.
Testo completo
17
Terry, Ann Elizabeth. "Shear flow studies of liquid crystalline polymers". Thesis, University of Bristol, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.390373.
Testo completo
18
Gipon, Matthew. "Shear flow instabilities in pipes and channels". Thesis, Imperial College London, 2018. http://hdl.handle.net/10044/1/60643.
Testo completoAbstract (sommario):
Two broad problems are considered in this thesis. The first investigation focuses on the spatial stability of pressure driven flow in a pipe, while the second problem is concerned with temporal stability of Couette flow in a parallel wall channel. A similar approach is taken with both halves of this thesis: an asymptotic analytical model is developed and this is then solved with numerical methods. The prominent problem in the first chapter is Hagen-Poiseuille flow with suction and injection on the pipe wall boundary. The flow is fully developed in the axial direction and the suction acts radially at the wall while the other boundary conditions are no-slip. The flow is perturbed by a small non-axisymmetric disturbance and this configuration is solved in the radial-azimuthal plane. The results are provided for 2pi-periodic suction and it is found that the non-parallel base flow is unstable in certain conditions. The suction coefficient is varied but the critical Reynolds number is found to be the same. A weakly nonlinear stability analysis reveals that there is a finite amplitude solution in the supercritical region. The second chapter presents the vortex-wave interaction equations and a special case of the model is created to seek stationary, equilibrium solutions of the sinuous wave disturbance in Couette flow. The flow is initiated with an artificial forcing which has the sinusoidal symmetries embedded. From this initial condition the 'roll' problem is solved and the 'streak' can be found from this solution. The wave on this streak has Reynolds stresses which now force the 'roll'. The amplitude of the wave is varied and the system is iterated until the wave is neutral. This equilibrium configuration is then marched forward in time and studied. The solutions agree with numerical calculations and experiments.
19
Günther, Katrin, Kristin Laube e Michael Mertig. "Shear-flow mediated changes in DNA morphology". Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-191757.
Testo completo
20
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.
Testo completoAbstract (sommario):
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 %.
21
Günther, Katrin, Kristin Laube e Michael Mertig. "Shear-flow mediated changes in DNA morphology". Diffusion fundamentals 11 (2009) 110, S. 1-2, 2009. https://ul.qucosa.de/id/qucosa%3A14084.
Testo completo
22
Ames, Danielle 1974. "Shear flow visualization at high Reynolds Numbers". Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/50478.
Testo completoAbstract (sommario):
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1998.Includes bibliographical references (leaves 92-94).
The mechanisms responsible for various disturbances in the wakes of ships have been investigated for some time. Efforts to define and characterize the contributing factors and resulting turbulent wake manifestations conclude that ship length, speed and geometry play integral and interdependent roles. Previous experimentation in the Ocean Engineering Towing Tank at MIT on small scale model ships supplied data for Reynold's Numbers up to ~ 0(106). The work included in this thesis represents a continuation of those efforts up to Re O(107) using a model DDG51 (5514) Destroyer. Through endeavors to identify wake phenomena and closely examine possible sources, previously unvisualized charicteristics were revealed and the calibration, comparison and validation of numerical simulations were made possible. Experimental efforts were concentrated on the study of flow in the wake and near the bow of the model DDG-51 Destroyer (5514). Qualitative and quantitative flow visualization methods were adapted, designed and implemented including ship-fixed and tank fixed streak videography and Digital Particle Image Velocimetry (DPIV). In addition, the experimental apparatus was modified for similar flow visualization near live fish, and a description of this endeavor and its progress are included.
by Danielle Ames.
S.M.
23
Topayev, Sultan. "Taylor-Couette flow for shear-thinning fluids". Electronic Thesis or Diss., Université de Lorraine, 2021. http://www.theses.fr/2021LORR0301.
Testo completoAbstract (sommario):
On s’intéresse aux instabilités secondaires dans un écoulement de Taylor-Couette en grand entrefer pour un fluide rhéofluidifiant. Des études théorique, expérimentale et numérique ont été mises en œuvre. D’un point de vue théorique, une analyse faiblement non linéaire a été développée en régime dit TVF (Taylor Vortex Flow) pour rendre compte des premiers effets de la non linéarité de la loi de comportement sur la structure de l’écoulement. Le comportement rhéologique du fluide est décrit par le modèle de Carreau. Des effets significatifs du caractère rhéofluidifiant ont été mis en évidence : Les rouleaux de Taylor ont une taille plus petite et sont écrasés contre le cylindre intérieur. Le jet radial sortant est plus fin et beaucoup plus intense que le jet radial entrant. Par conservation de débit, la zone de jet radial entrant est plus étendue. Ces modifications sont probablement à l’origine des instabilités des rouleaux de Taylor observées expérimentalement et numériquement. Le dispositif expérimental utilisé est constitué de deux cylindres coaxiaux, où le cylindre intérieur est en rotation et le cylindre extérieur est fixe. Le rapport des rayons est "eta = 0.4" et le rapport d’aspect "L = 32". Les fluides utilisés sont des solutions de xanthane à différentes concentrations ainsi qu’une solution de glycérole, comme fluide newtonien de référence. La structure de l’écoulement est déterminée par visualisation et par mesures de vitesse par PIV 2D. Pour la solution de glycérole, après la bifurcation primaire à "Re = Re_c", le régime TVF stationnaire reste stable jusqu’à pratiquement "7 Re_c". A partir de cette dernière valeur, les rouleaux de Taylor perdent leur stabilité vis-à-vis de perturbations azimutales. Dans le cas des solutions de xanthane, les valeurs du nombre de Reynolds à partir desquelles, les rouleaux de Taylor apparaissent sont en accord avec la théorie linéaire comme dans le cas Newtonien. En augmentant le nombre de Reynolds, les rouleaux de Taylor deviennent instables, mais cette-fois-ci vis-à vis de perturbations axiales. Ces instabilités peuvent être considérées comme des instabilités d’Eckhaus généralisées. Elles se caractérisent par un processus récurrent de création et d’appariement de rouleaux. L’augmentation du nombre de sites où se produit ce processus conduit à un écoulement chaotique (turbulence de phase). Il convient de noter que plus les effets rhéofluidifiants sont importants, et plus la gamme de Re où le régime TVF est stable, est réduite. Ces résultats ont été confirmés par une simulation numérique 2D des équations de conservation instationnaires, en utilisant le solveur de FreeFem++. Le cas des fluides rhéofluidifiants avec seuil de contrainte a été entamé , en se focalisant sur le cas particulier où il existe une zone non-cisaillée attachée au cylindre extérieurThis work deals with secondary instabilities in a Taylor-Couette flow with a wide gap in the case of shear-thinning fluids. Theoretical, experimental and numerical approaches are used. From theoretical point of view, a weakly nonlinear analysis has been done to account for the nonlinear effects of constitutive law on the flow structure of the Taylor Vortex Flow (TVF) regime. The shear-thinning behavior of the fluid are characterized by the Carreau model. Significant effects of shear-thinning have been demonstrated: Taylor vortices are smaller in size and shifted toward the inner cylinder. The radial outflow jet is thinner and stronger than the radial inflow jet. This asymmetry leads to an increase of the radial inflow zone. These changes in the flow structure are probably the origin of the secondary instabilities of Taylor vortices observed experimentally and numerically. The experimental setup consist of two coaxial cylinders where the inner cylinder is rotating and the outer one is at rest. The radius ratio is "eta = 0.4" and the aspect ratio is "L = 32". The fluids used are aqueous xanthan gum solutions at different concentrations and aqueous glycerol solution as a reference Newtonian fluid. The flow structure is analyzed through the visualization and by the 2D PIV velocity measurements. For the aqueous glycerol solution, once the primary bifurcation is reached at "Re = Re_c", the stationary TVF regime remains stable up to practically "Re = 7 Re_c". From this values the Taylor vortices lose its stability with respect to azimuthal disturbances. In the case of the aqueous xanthan gum solutions the values of the Reynolds number from which the Taylor vortices appear are in agreement with a linear theory as for the case of Newtonian fluid. By increasing the Reynolds number, the Taylor vortices become unstable, but with respect to axial disturbances. There instabilities can be considered as generalized Eckhaus instabilities. They are characterized by the continuous processes of creation and merging of vortices. The increase in the number of axial positions where these processes occur leads to the chaotic flow (phase turbulence). It should be noted that the stronger shear-thinning effects, the smaller the range of stable TVF regime. These results have been confirmed by a 2D numerical simulation of unsteady conservation equations, using PDE solver Freefem++. The case of shear-thinning with a stress-yield was started as well, focusing on the particular case when the unyielded zone is attached to the outer cylinder
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Abramson, Philip S. "Fluidic control of aerodynamic forces and moments on an axisymmetric body". Thesis, Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/31707.
Testo completoAbstract (sommario):
Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2010.Committee Chair: Ari Glezer; Committee Member: Bojan Vukasinovic; Committee Member: Mark Costello. Part of the SMARTech Electronic Thesis and Dissertation Collection.
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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.
Testo completoAbstract (sommario):
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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Lindsay, R. I. "Shear in nematic liquid crystal layers". Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.296747.
Testo completo
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Heitmann, Stefan. "Large eddy simulations of oceanic convective shear flow". [S.l. : s.n.], 2003. http://deposit.ddb.de/cgi-bin/dokserv?idn=968893325.
Testo completo
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Vassileva, Nikolina Dimitrova. "Behavior of two-dimensional aggregates in shear flow". Enschede : University of Twente [Host], 2006. http://doc.utwente.nl/58360.
Testo completo
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Phillips, Christopher George. "Transport in biological tissue and in shear flow". Thesis, University of Cambridge, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.257212.
Testo completo
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Hodgkinson, Richard. "The effect of extensional flow on shear viscosity". Thesis, University of Sheffield, 2017. http://etheses.whiterose.ac.uk/18327/.
Testo completoAbstract (sommario):
Shear rheology is conventionally studied under pure shearing flows, rather than more realistic mixed flows. Moving parallel surfaces and capillary rheometery are examples of the former, whilst the latter occurs whenever a flow accelerates or decelerates creating an additional component of extension, e.g. on passing through an industrial extrusion die. We postulate and gather supporting evidence that shear rheology is a function of not only shear, but both shear and extension rate, a factor with important consequences for fibre spinning and extrusion operations. The direction, as well as rate, of extensional deformation is important. A novel two-phase flow, planar extension experiment is developed and the surface coatings necessary to control the interface structure identified. Shear viscosity evolution is monitored, in-situ, under extensional flow, by optically measuring shear rates either side of a test fluid – reference fluid interface; issues due to optical refraction are critically addressed. Preliminary evidence is shown for a 1.2wt% 4x10^6 MW PEO solution that parallel (+ve) extensional flow, on the order of 11.5s-1 , causes a reduction in shear viscosity, and perpendicular (-ve) causes an increase in shear viscosity, supporting the hypothesis. A framework for a comparison experiment, with the same shear history but without extension, is presented. As part of this work, design criteria for planar hyperbolic extensional channels are critically assessed. In particular, expanding a hyperbola entrance region would maximise total Hencky strain, yet this region is almost never given rationalised consideration in literature. In this region the basis for the hyperbolic profile breaks down, and a new profiling strategy and channel form are presented, which is found to only differ significantly in this inlet region. A useful design limit of 130 degrees on channel inlet angle is identified. The new profile is compared to a hyperbolic profile through the use of CFD for wall slip flow, and a slight improvement in extension rate uniformity along the centreline found. Deviations are contrasted against assumptions made in the profiling strategy: comments are made with regards the possibility for “internal” shear to occur, and non-uniform extension rates are accordingly found to exist between streamlines in these channels despite the use of full wall slip in the simulations.
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Alaei, Kakhki Hossein. "Flow of shear-thinning fluids in geological media". Master's thesis, Alma Mater Studiorum - Università di Bologna, 2022.
Cerca il testo completoAbstract (sommario):
As the primary conduits for water, oil, and geothermal resources, fractures and fracture flow are important in a wide range of applications, including water reservoir development, subsurface contamination, and petroleum and geothermal reservoir exploitation. Fracture flow estimations are subject to significant uncertainty and the role of rheology when the fluid is non-Newtonian makes the task considerably more difficult. In such a complex phenomenon, simple models allow us to qualitatively assess the effect of rheological and geometric characteristics on the flow. This study aims to investigate the effect of shear-thinning fluid rheology on fracture flow rate using smooth parallel plate to present the fracture. By using power-law, Ellis and Carreau rheology models, we can also compare the results of using different rheology models. To achieve the aforementioned goals, we first introduced some equations to relate the three aforementioned rheology models together and then we introduced equations to calculate the fracture flow rate for each of the models. Using MATLAB software to solve these for four different fluids with different flow behavior indices, several diagrams were drawn to show the relation between dimensionless flow rate and dimensionless reduced pressure gradient. By comparing the produced graphs, one can see that fluid rheology and the way it is modeled has an important influence on calculated fracture flows. The way we present the fluids' rheology is also important. For example in higher pressure gradients, the power-law model calculates the flow rate incorrectly in both large and small pressure gradients because of its incapablity to simulate both low and infinite-shear viscosity. There is an excellent match between all three rheology models in the middle range of pressure gradient, especially between Ellis and power-law models. Finally, it should be noted that the pressure gradient rules which model and equation can be used to calculate the flow rate.
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Pinilla, Camilo Ernesto. "Numerical simulation of shear instability in shallow shear flows". Thesis, McGill University, 2008. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=115697.
Testo completoAbstract (sommario):
The instabilities of shallow shear flows are analyzed to study exchanges processes across shear flows in inland and coastal waters, coastal and ocean currents, and winds across the thermal-and-moisture fronts. These shear flows observed in nature are driven by gravity and governed by the shallow water equations (SWE). A highly accurate, and robust, computational scheme has been developed to solve these SWE. Time integration of the SWE was carried out using the fourth-order Runge-Kutta scheme. A third-order upwind bias finite difference approximation known as QUICK (Quadratic Upstream Interpolation of Convective Kinematics) was employed for the spatial discretization. The numerical oscillations were controlled using flux limiters for Total Variation Diminishing (TVD). Direct numerical simulations (DNS) were conducted for the base flow with the TANH velocity profile, and the base flow in the form of a jet with the SECH velocity profile. The depth across the base flows was selected for the' balance of the driving forces. In the rotating flow simulation, the Coriolis force in the lateral direction was perfectly in balance with the pressure gradient across the shear flow during the simulation. The development of instabilities in the shear flows was considered for a range of convective Froude number, friction number, and Rossby number. The DNS of the SWE has produced linear results that are consistent with classical stability analyses based on the normal mode approach, and new results that had not been determined by the classical method. The formation of eddies, and the generation of shocklets subsequent to the linear instabilities were computed as part of the DNS. Without modelling the small scales, the simulation was able to produce the correct turbulent spreading rate in agreement with the experimental observations. The simulations have identified radiation damping, in addition to friction damping, as a primary factor of influence on the instability of the shear flows admissible to waves. A convective Froude number correlated the energy lost due to radiation damping. The friction number determined the energy lost due to friction. A significant fraction of available energy produced by the shear flow is lost due the radiation of waves at high convective Froude number. This radiation of gravity waves in shallow gravity-stratified shear flow, and its dependence on the convective Froude number, is shown to be analogous to the Mach-number effect in compressible flow. Furthermore, and most significantly, is the discovery from the simulation the crucial role of the radiation damping in the development of shear flows in the rotating earth. Rings and eddies were produced by the rotating-flow simulations in a range of Rossby numbers, as they were observed in the Gulf Stream of the Atlantic, Jet Stream in the atmosphere, and various fronts across currents in coastal waters.
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Hellum, Aren. "Intermittency and the viscous superlayer in a single stream shear layer". Diss., Connect to online resource - MSU authorized users, 2006.
Cerca il testo completo
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Weldon, Matthew J. (Matthew Jacob). "Experimental studies of shear stress and flow separation in low Reynolds number flows". Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/39892.
Testo completoAbstract (sommario):
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.Includes bibliographical references (p. 77-78).
Presented here is an experimental investigation of the kinematic theory of separation in unsteady two-dimensional flows, and an evaluation of a novel optical shear stress sensor. Fixed separation in the rotor-oscillator flow is studied for steady, periodic, and quasi-periodic fluid motion. Experimental results are directly compared to numerical simulations, which provide the shear-stress and pressure data required for detecting fixed separation in an unsteady flow. Good agreement between theory and experiments in determining both the location of the separation point and the angle of the separation profile is found. With the goal of directly measuring shear stress to high accuracy, an optical shear stress sensor is evaluated on a flat plate boundary layer. Wall-shear measured with the sensor is compared to that derived from particle image velocimetry (PIV) velocity profiles, and the resulting discrepancy between the two measurements is discussed.
by Matthew J. Weldon.
S.M.
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Al-Mulla, Adam. "Droplet coalescence in the shear flow of model emulsions". Morgantown, W. Va. : [West Virginia University Libraries], 1998. http://etd.wvu.edu/templates/showETD.cfm?recnum=384.
Testo completoAbstract (sommario):
Thesis (M.S.)--West Virginia University, 1998.Title from document title page. Document formatted into pages; contains xxi, 153 p. : ill. Includes abstract. Includes bibliographical references (p. 68-72).
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Flannery, Conor James. "Thrombus Formation under High Shear in Arterial Stenotic Flow". Thesis, Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/6943.
Testo completoAbstract (sommario):
Acute thrombotic and thromboembolic occlusion of atherosclerotic vessels are events that precipitate most heart attacks and strokes. In arterial stenotic flow, thrombus formation is shear dependent and may or may not lead to complete occlusion of the vessel. Platelets in whole blood adhere to collagen-coated surfaces and as they accumulate the resistance of the stenosis increases because of the decreasing passageway of the occluded stenosis. As a model of blood clotting in stenoses, porcine blood is heparinized and perfused over tubular glass test sections that are coated with collagen type I. Each test section has a preexisting stenosis and its severity varies so that higher percent stenoses produce higher shear rates on the blood. The hypothesis of this thesis is that high shear rates due to stenosis in arteries are a necessary feature for occlusive thrombosis.
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Håkansson, Karl. "Orientation of elongated particles in shear and extensional flow". Licentiate thesis, KTH, Strömningsfysik, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-95282.
Testo completoAbstract (sommario):
Elongated particles in fluid flows are a big part of the world we are living in. Gaining knowledge on how particles behave in different fluid flows can potentially increase the efficiency of industrial processes and decrease the world's energy consumption as well as improve the properties of future materials. In this thesis, the orientation of elongated particles in two different flows are studied. The first case is a dilute fibre suspension in a turbulent flow and the second case is a semi-dilute fibril dispersion in a laminar flow. The fibres (cellulose acetate) are at least three orders of magnitude larger than the fibrils (nano-fibrillated cellulose). The turbulent flow case is half of a full channel flow, characterised by the friction Reynolds number, and is experimentally examined. This experiment is closely related to the papermaking process. Laser Doppler velocimetry measurements are preformed without fibres in order to make sure that the flow is turbulent and fully developed. Images of the fibres in the flow are acquired using a CCD-camera, from which it is possible to detect the fibres in an image processing step and extract both the positions and orientations of the fibres. A large parameter study is carried out, where the aspect ratio of the fibres, concentration and Reynolds number are changed. Short fibres are observed to align perpendicular to the flow, while the longer fibres are found to align in the flow direction. The fibres are also seen to accumulate in streamwise streaks, believed to be caused by velocity structures in the turbulent flow. The second flow case studied focusses on a semi-dilute dispersion in a laminar flow. It includes both experiments and numerical calculations of the fibril orientation. The aim of this study is to demonstrate that it is possible to control the fibril orientation with a fluid. In a semi-dilute dispersion, fibrils are interacting. However, no flocs or networks are formed. A flow focusing apparatus is used in order to hydrodynamically accelerate the dispersion with an outer fluid (sheath) flow. The mean orientation in the flow direction is experimentally studied by detecting the birefringence of the flowing dispersion. The orientation distribution is calculated by solving the Smoluchowski equation. The fibrils are seen to align in the flow direction both in the experiments and the calculations. Moreover, the alignment is found to increase with increasing acceleration.QC 20120607
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Brown, Nicholas J. "The effect of wall waviness on shear flow instabilities". Thesis, Imperial College London, 2007. http://hdl.handle.net/10044/1/1275.
Testo completoAbstract (sommario):
This thesis is concerned with the effect of wall waviness on shear flow instability, specifically for the incomprehensiblc flow in a channel. We investigate the stability of the flow in a channel with fixed wavy boundaries using two methods. Firstly the disturbance evolution is calculated using the parabolised stability equations (PSE), which apply to the flow stability at finite Reynolds number, and are solved using a finite-difference marching scheme, marching in the downstream direction. Secondly we employ the triple-deck formulation for channel flow which is valid at asymptotically high Reynolds number and the problem is solved using Floquet theory, making use of the periodic coefficients appearing in the disturbance equations. The mean flow for the PSE analysis is obtained by linearising the Navier-Stokes equations using a perturbation method, valid for small amplitude boundary waviness, Δ. We solve the linear PSE using this periodic mean flow, and it is found that increasing Δ stabilises plane Poiseuille flow near the nose of the neutral curve but has a destabilising effect on the lower branch for higher Reynolds numbers. The nonlinear PSE are used to study thc stability of 2-D finite amplitude waves, and are able to demonstrate the existence of suporcritical equilibrium amplitude solutions, as well as threshold amplitudes separating growing and decaying solutions in the subcritical regime. Wall waviness is found to have a stabilising effect on subcritical disturbances, raising the amplitude needed for instability to occur. Using Floquet theory and decomposing the disturbance equations into Fourier modes enables the high Reynolds number problem to be formulated as an eigenvalue problem. The waviness is found to be able to produce a destabilising effect in agreement with the results for the linear PSE near the lower branch. The method of multiple scales is used to study the wavy channel flow stability at high Reynolds number in the limit of small Δ, which gives an O(Δ 2) correction to the flat boundary eigenvalue, λ. When λ = ±i μ, for boundary wavenumber, μ, we find a degeneracy in the intermediate O(Δ) system of equations due to a resonance ktween a neutrally stable flat-boundary T-S wave and the boundary wave of equal wave-length. New asymptotic scalings are derived in this case to obtain a valid solution.
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Wilkins, Georgina Mary Heather. "Characterisation of a lyotropic lamellar phase under shear flow". Thesis, University of Leeds, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.426847.
Testo completo
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Krishnan, Anantha. "Numerical study of vorticity-combustion interactions in shear flow". Thesis, Massachusetts Institute of Technology, 1989. http://hdl.handle.net/1721.1/14188.
Testo completo
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Cunha, Lucas Hildebrand Pires da. "Magnetic emulsions in shear flow under external magnetic fields". reponame:Repositório Institucional da UnB, 2018. http://repositorio.unb.br/handle/10482/33876.
Testo completoAbstract (sommario):
Dissertação (mestrado)—Universidade de Brasília, Faculdade de Tecnologia, Departamento de Engenharia Mecânica, 2018.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
Este trabalho analisa a resposta de uma gota plana de ferrofluido imersa em um líquido newtoniano não magnético à ação combinada de um campo magnético externo e um escoamento cisalhante simples permanente. Uma metodologia numérica baseada no Método de Projeção e no Método Level-Set é desenvolvida para resolver o campo magnético, as equações completas de Navier-Stokes contemplando forças capilares e magnéticas, e capturar a interface líquido-líquido. Os resultados mostram que a força magnética exerce forte influência na inclinação da gota e na viscosidade da emulsão. O alinhamento da gota com a direção do campo magnético aplicado aumenta com a intensidade deste campo. Quando o campo externo é paralelo à direção do escoamento, a gota se alinha fortemente com as linhas de corrente, o que reduz sua contribuição na viscosidade da emulsão. Por sua vez, quando o campo externo é perpendicular à direção do escoamento, a inclinação da gota se torna mais alta, levando a um aumento dramático da viscosidade do fluido complexo resultante. Também mostramos que os campos magnéticos externos podem ser usados para controlar o processo de ruptura de gotas em termos de tempo para o rompimento e tamanho das gotas filhas. Campos externos aplicados na direção do escoamento atrasam o processo de ruptura e reduzem o tamanho da gota satélite. Notavelmente, há um número de capilaridade magnética crítico acima do qual a gota se torna tão alinhada com o escoamento que o rompimento não acontece. Alternativamente, quando o campo externo é aplicado perpendicularmente à direção do escoamento, dois mecanismos opostos ditam o processo de ruptura. Por um lado, a inclinação da gota cresce, o que aumenta as forças de cisalhamento que podem levar à ruptura. Por outro lado, a quantidade de líquido na região do pescoço da gota deformada também cresce, o que torna o processo de ruptura mais difícil. Assim, se a gota romper, o campo magnético aumenta o tamanho da gota satélite. Finalmente, também verificamos que os campos magnéticos externos aplicados perpendicularmente à direção do escoamento podem ser usados para induzir a ruptura de gotas que não iriam romper somente sob a ação do escoamento cisalhante. Em resumo, os resultados aqui apresentados destacam que os campos magnéticos externos podem ser potencialmente utilizados para controlar transformações topológicas de gotas de ferrofluido e projetar emulsões magnetoreológicas com funções específicas do material macroscópico.
This work has analyzed the response of a planar ferrofluid droplet immersed in a non-magnetic Newtonian liquid to the combined action of an external magnetic field and a simple shear flow. A numerical methodology based on the Projection Method and the Level-Set Method has been developed to solve the magnetic field, the full Navier-Stokes equations with additional capillary and magnetic forces, and capture the liquid-liquid interface. The results show that the magnetic force has a strong influence on the droplet inclination and emulsion viscosity. The drop alignment with the magnetic field direction increases with the field intensity. When the external field is parallel to the flow direction, the drop strongly aligns with the streamlines of the flow, which reduces its contribution to emulsion viscosity. In turn, when the external field is perpendicular to the flow direction, the droplet inclination becomes higher, leading to a dramatic increase in the two-phase liquid viscosity. We also show that external magnetic fields can be used to control the drop rupture process in terms of time to breakup and size of the daughter drops. External fields applied in the flow direction delay the breakup process and reduce the size of the satellite drop. Remarkably, there is a critical magnetic capillary number above which the drop becomes so aligned with the flow that breakup does not happen. Alternatively, when the external field is applied perpendicularly to the flow direction, two opposite mechanisms dictate the breakup process. One the one hand, the drop inclination grows, which increases the shear forces that might lead to the rupture. On the other hand, the amount of liquid in the neck region of the deformed droplet also grows, which makes the breakup process more difficult. Thus, if the drop breaks, the magnetic field leads to a larger satellite drop. Finally, we also verified that external magnetic fields applied perpendicularly to the flow direction can be used to induce the breakup of drops that would not break under the shear action only. In summary, the results we present here highlight that external magnetic fields can be potentially used to control topological transformations of ferrofluid droplets and design magnetorheological emulsions with specific macroscopic material functions.
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Lyman, Noah J. "Incorporating Shear Resistance into Debris Flow Triggering Model Statistics". DigitalCommons@CalPoly, 2020. https://digitalcommons.calpoly.edu/theses/2254.
Testo completoAbstract (sommario):
Several regions of the Western United States utilize statistical binary classification models to predict and manage debris flow initiation probability after wildfires. As the occurrence of wildfires and large intensity rainfall events increase, so has the frequency in which development occurs in the steep and mountainous terrain where these events arise. This resulting intersection brings with it an increasing need to derive improved results from existing models, or develop new models, to reduce the economic and human impacts that debris flows may bring. Any development or change to these models could also theoretically increase the ease of collection, processing, and implementation into new areas.
Generally, existing models rely on inputs as a function of rainfall intensity, fire effects, terrain type, and surface characteristics. However, no variable in these models directly accounts for the shear stiffness of the soil. This property when considered with the respect to the state of the loading of the sediment informs the likelihood of particle dislocation, contractive or dilative volume changes, and downslope movement that triggers debris flows. This study proposes incorporating shear wave velocity (in the form of slope-based thirty-meter shear wave velocity, Vs30) to account for this shear stiffness. As commonly used in seismic soil liquefaction analysis, the shear stiffness is measured via shear wave velocity which is the speed of the vertically propagating horizontal shear wave through sediment. This spatially mapped variable allows for broad coverage in the watersheds of interest. A logistic regression is used to then compare the new variable against what is currently used in predictive post-fire debris flow triggering models.
Resulting models indicated improvement in some measures of statistical utility through receiver operating characteristic curves (ROC) and threat score analysis, a method of ranking models based on true/false positive and negative results. However, the integration of Vs30 offers similar utility to current models in additional metrics, suggesting that this input can benefit from further refinement. Further suggestions are additionally offered to further improve the use of Vs30 through in-situ measurements of surface shear wave propagation and integration into Vs30 datasets through a possible transfer function. Additional discussion into input variables and their impact on created models is also included.
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Bremner, Sherry. "A granular flow model of an annular shear cell". Doctoral thesis, University of Cape Town, 2016. http://hdl.handle.net/11427/20304.
Testo completoAbstract (sommario):
Machinery such as an IsaMillTM used in communition to produce fine particle sizes that allow minerals to be extracted are best modelled using granular flows. A single rheological description that captures all the features of granular flows has not yet been realised, although considerable progress towards a complete theory has been made. Existing models of such horizontally stirred mills are empirical, tend to be extremely dependent on boundary conditions and do not allow for confident extrapolation beyond their window of design. As a first step to understanding the dynamics inside the IsaMillTM,a constitutive stress model of a horizontal annular shear cell is developed. This shear stress model was used in an athermal energy balance to develop a description of the power dissipation, which drives the communition purpose of the IsaMillTM. The key ingredients (velocity, shear rate and volume fraction distributions) to the granular ow model are extracted from experiments using Positron Emission Particle Tracking (PEPT), as well as Discrete Element Method (DEM) simulations. 5mm glass beads were used to fill an annulus 51mm wide. In the PEPT experiments, two different surfaces of the driving wall (the inner cylinder of the shear cell) were used, over two shearing velocities. The effect of two friction coefficients over a range of shearing wall velocities were examined in the DEM simulations. The data were examined over 3 selected radial lines and utilised to calculate the shear stress distribution and the power dissipation from the developed models. It was found that even the usually simple relations describing the dynamics within a vertical shear cell are greatly modified by changing the orientation of the rotation axis.
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Su, Siling. "Shear and extensional flow study of polymer/particle dispersions". Thesis, University of Sheffield, 2013. http://etheses.whiterose.ac.uk/4656/.
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Doty, Sherry D. "Fluid shear stress effects on fibronectin in endothelial cells". Thesis, Georgia Institute of Technology, 1989. http://hdl.handle.net/1853/19073.
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Selomulya, Cordelia Chemical Engineering & Industrial Chemistry UNSW. "The Effect of Shear on Flocculation and Floc Size/Structure". Awarded by:University of New South Wales. Chemical Engineering and Industrial Chemistry, 2002. http://handle.unsw.edu.au/1959.4/18226.
Testo completoAbstract (sommario):
The effect of shear on the evolution of floc properties was investigated to analyse the flocculation mechanisms. Little fundamental attention has been given to the shear influence that often creates compact aggregates, while the floc characteristics might differ in other aggregating conditions. It is thus crucial to understand how flocs evolve to steady state, if their properties are to be 'tailored' to suit subsequent solids-liquid separation processes. In this work, flocculation of monodisperse latex particles of various sizes (60, 380, and 810 nm diameter) via electrolyte addition was carried out in a couette-flow and also in shear fields generated by an axial-flow impeller (Fluid foil A310) and a radial-flow impeller (Rushton R100) in standard mixing tanks. A small-angle light scattering technique was used to acquire information regarding the time variation of floc properties in a non-intrusive manner. The structure was quantified by a measure of fractal dimension, signifying the degree of floc compactness. Estimates of the average floc mass were also obtained from the aggregate scattering patterns. By monitoring the changes in floc structure and mass, corresponding to the size evolution; mechanisms of floc formation, fragmentation, and restructuring were identified. Aggregates of 60 and 380 nm particles were observed to grew larger initially, before decreasing to their equilibrium sizes at moderate shear rates (32 - 100 s-1) in a homogeneous shear environment. Floc restructuring at large length scales occurred extensively, and was responsible for the drop in size, particularly at the early stage of the process. Aggregates of 810 nm particles did not, however, display this behaviour. Flocs of larger primary particles were presumably susceptible to breakage rather than deformation, as they were weaker under comparable conditions. Denser aggregates were found when restructuring transpired, while comparatively tenuous flocs were observed when formation and breakage kinetics were the governing mechanisms. The disparity in floc behaviour at higher shear rates (246 s-1 - 330 s-1) was less apparent. The intense hydrodynamic stresses in those instances inevitably caused fragmentation, regardless of the intrinsic particle properties; hence the observed floc compaction was the product of break-up and re-aggregation. A population balance model, incorporating variation in floc structure, displayed comparable trends in size evolution; verifying that restructuring indeed took an important role under certain flocculation conditions. Similar phenomena were likewise observed with the flocculation in stirred tanks. The results reinforced findings in literature; that while circulation time controlled the process kinetics; the floc size was determined by the turbulent stresses. In addition, the maximum shear levels also influenced the floc structures, with denser aggregates produced in a shear field generated using the radial-flow impeller at equivalent energy dissipation per-unit mass. A correlation between non-dimensional floc factor that embodied the aggregate size and structure, and aggregation factor comprising the significant parameters from flocculation conditions, was proposed. The proposed relationship takes into account aspects such as the aggregate structure, interparticle forces, and particle concentration that are often overlooked in existing relationships, which usually only relate the maximum floc size to the applied energy dissipation rate. It thus provides an improved manner of presenting general flocculation data, as well as a means to predict floc properties produced under a specific aggregation condition. Future studies with increasingly complex systems that resemble real conditions are recommended in order to establish a practical understanding of the flocculation mechanisms, for the purpose of optimising the aggregate properties.
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Carneal, Jason Bradley. "Integration and Validation of Flow Image Quantification (Flow-IQ) System". Thesis, Virginia Tech, 2004. http://hdl.handle.net/10919/35322.
Testo completoAbstract (sommario):
The first aim of this work was to integrate, validate, and document, a digital particle image quantification (Flow-IQ) software package developed in conjunction with and supported by Aeroprobe Corporation. The system is tailored towards experimental fluid mechanics applications. The second aim of this work was to test the performance of DPIV algorithms in wall shear flows, and to test the performance of several particle sizing algorithms for use in spray sizing and average diameter calculation. Several particle sizing algorithms which assume a circular particle profile were tested with DPIV data on spray atomization, including three point Guassian, four point Gaussian, and least squares algorithms. A novel elliptical diameter estimation scheme was developed which does not limit the measurement to circular patterns. The elliptic estimator developed in this work is able to estimate the diameter of a particle with an elliptic shape, and assumes that the particle is axisymmetric about the x or y axis. Two elliptical schemes, the true and averaged elliptical estimators, were developed and compared to the traditional three point Gaussian diameter estimator using theoretical models. If elliptical particles are theoretically used, the elliptical sizing schemes perform drastically better than the traditional scheme, which is limited to diameter measurements in the x-direction. The error of the traditional method in determining the volume of an elliptical particle increases dramatically with the eccentricity. Monte Carlo Simulations were also used to characterize the error associated with wall shear measurements using DPIV. Couette flow artificial images were generated with various shear rates at the wall. DPIV analysis was performed on these images using PIV algorithms developed by other researchers, including the traditional multigrid method, a dynamically-adaptive DPIV scheme, and a control set with no discrete window offset. The error at the wall was calculated for each data set. The dynamically adaptive scheme was found to estimate the velocity near the wall with less error than the no discrete window offset and traditional multigrid algorithms. The shear rate was found to be the main factor in the error in the velocity measurement. In wall shear velocity measurement, the mean (bias) error was an order of magnitude greater than the RMS (random) error. A least squares scheme was used to correct for this bias error with favorable results. The major contribution of this effort stems from providing a novel elliptical particle sizing scheme for use in DPIV, and quantifies the error associated with wall shear measurements using several DPIV algorithms. A test bed and comprehensive user's manual for Flow-IQ v2.2 was also developed in this work.Master of Science
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Monokrousos, Antonios. "Optimisation and control of shear flows". Doctoral thesis, KTH, Stabilitet, Transition, Kontroll, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-33771.
Testo completoAbstract (sommario):
Transition to turbulence and flow control are studied by means of numerical simulations for different simple shear flows. Linear and non-linear optimisation methods using the Lagrange multiplier technique are employed. In the linear framework as objective function the standard disturbance kinetic energy is chosen and the constraints involve the linearised Navier–Stokes equations. We consider both the optimal initial condition leading to the largest disturbance energy growth at finite times and the optimal time-periodic forcing leading to the largest asymptotic response for the case of the flat plate boundary layer excluding the leading edge. The optimal disturbances for spanwise wavelengths of the order of the boundary layer thickness are streamwise vortices exploiting the lift-up mechanism to create streaks. For long spanwise wavelengths it is the Orr mechanism combined with the amplification of oblique wave packets that is responsible for the disturbance growth. Also linear optimal disturbances are computed around a leading edge and the effect of the geometry is considered. It is found that two-dimentional disturbances originating upstream, relative to the leading edge of the plate are inefficient at generating a viable disturbance, while three dimentional disturbances are more amplified. In the non-linear framework a new approach using ideas from non-equilibrium thermodynamics is developed. We determine the initial condition on the laminar/turbulent boundary closest to the laminar state. Starting from the general evolution criterion of non-equilibrium systems we propose a method to optimise the route to the statistically steady turbulent state, i.e. the state characterised by the largest entropy production. This is the first time information from the fully turbulent state is included in the optimisation procedure. The method is applied to plane Couette flow. We show that the optimal initial condition is localised in space for realistic flow domains, while the disturbance visits bent streaks before breakdown. Feedback control is applied to the bypass-transition scenario with high levels of free-stream turbulence. The flow is the flat-plate boundary layer. In this scenario low frequency perturbations enter the boundary layer and streamwise elongated disturbances emerge due to non-modal growth. The so-called streaky structures are growing in amplitude until they reach high enough energy levels and break down into turbulent spots via their secondary instability. When control is applied in the form of wall blowing and suction, the growth of the streaks is delayed, which implies a delay of the whole transition process. Additionally, a comparison with experimental work is performed demonstrating a remarkable agreement in the disturbance attenuation once the differences between the numerical and experimental setup are reduced. Open-loop control with wall travelling waves by means of blowing and suction is applied to a separating boundary layer. For downstream travelling waves we obtain a mitigation of the separation of the boundary layer while for upstream travelling waves a significant delay in the transition location accompanied by a modest reduction of the separated region.QC 20110518
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Zhang, Xu. "A numerical and experimental study of a dynamic resonant shear stress sensor". Laramie, Wyo. : [University of Wyoming], 2006. http://proquest.umi.com/pqdweb?did=1221706921&sid=1&Fmt=2&clientId=18949&RQT=309&VName=PQD.
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Baker, William John Jr. "The effects of population doubling on the shear stress response of bovin aortic endothelial cells". Thesis, Georgia Institute of Technology, 1992. http://hdl.handle.net/1853/19600.
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