Thèses : « Turbulent shear flows »

1

COHEN, JACOB. « INSTABILITIES IN TURBULENT FREE SHEAR FLOWS ». Diss., The University of Arizona, 1986. http://hdl.handle.net/10150/188143.

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The evolution of the large scale structures and the mean field were investigated in axisymmetric and plane mixing layers. Some aspects of the linear instability of an axisymmetric jet have been demonstrated. The axisymmetric geometry admits two additional length scales with relation to the two-dimensional shear layer: the radius of the jet column and the azimuthal wavelength. The importance of these two length scales in governing the instability of an axisymmetric jet was explored. The special case of a thin axisymmetric shear layer was analyzed and the results stressing the evolution of different azimuthal modes were compared with some phase-locked data which was produced by subjecting the jet to axisymmetric and helical excitation. The importance of the initial spectral distribution in a natural jet was demonstrated when it is used as an input to the amplification curve obtained from linear stability theory to predict a measured spectral distribution at a further downstream location. The inclusion of the nonlinear terms in the stability analysis reveals two main interactions: mean flow-wave interaction and wave-wave interaction. The modification of the mean flow of an axisymmetric jet was examined by exciting two azimuthal modes simultaneously. The interaction resulted in an azimuthal modulation of the mean velocity profile having a cosine shape. Effectively, the geometry of the jet was modified without changing the geometry of the nozzle. The coupling between an excited periodic disturbance and the mean flow was analyzed and the spatial evolution of both were compared with experimental results obtained in a plane mixing layer. The behavior of the concommittant Reynolds stresses is discussed in detail. The conditions under which one disturbance will transfer energy to another were derived and demonstrated in an axisymmetric jet. The interaction between a large amplitude plane wave with a weak subharmonic component was shown to enhance the amplification rate of the subharmonic. It was further shown that the nonlinear interaction between two azimuthal modes can produce a third azimuthal mode which was not initially present in the flow. The coupling between a fundamental wave and its subharmonic in a parallel plane mixing layer was demonstrated numerically.

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Buxton, Oliver R. H. « Fine scale features of turbulent shear flows ». Thesis, Imperial College London, 2011. http://hdl.handle.net/10044/1/9080.

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This thesis presents an investigation into kinematic features of fine scale turbulence in free shear flows. In particular it seeks to examine the interaction between the different length scales present in shear flow turbulence as well as the interaction between the strain-rate tensor and the rotation tensor, which are the symmetric and skew-symmetric components of the velocity gradient tensor respectively. A new multi-scale particle image velocimetry (PIV) technique is developed that is capable of resolving the flow at two different dynamic ranges, centred on inertial range scales and on dissipative range scales, simultaneously. This data is used to examine the interaction between large-scale fluctuations, of the order of the integral scale, and inertial and dissipative range fluctuations. The large-scale fluctuations are observed to have an amplitude and frequency modulation effect on the small scales, and the small scales are shown to have a slight effect on the large scales, illustrating the two way nature of the energy cascade. A mechanism whereby integral scale rollers leave behind a wake of intense small-scale fluctuations is proposed. The interaction between strain and rotation is examined with regards to the rate of enstrophy amplification (ωiSijωj). It is found that the mechanism that is responsible for the nature of enstrophy amplification is the alignment tendency between the extensive strain-rate eigenvector and the vorticity vector. This mechanism is also observed to be scale dependent for ωiSijωj > 0, but independent for ωiSijωj < 0. This is subsequently confirmed with new dual-plane stereoscopic PIV experiments performed as part of this study. Finally, computational data is used to examine the effect of experimental noise and variation of spatial resolution on the observation and understanding of this strain - rotation interaction.

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Naaseri, Masud. « Studies of complex three-dimensional turbulent flows ». Thesis, Imperial College London, 1990. http://hdl.handle.net/10044/1/7379.

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4

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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Raiford, John Phillip. « Numerical and physical modeling of turbulent shear flows ». Connect to this title online, 2007. http://etd.lib.clemson.edu/documents/1181669456/.

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El-Baz, A. M. « The computational modelling of free turbulent shear flows ». Thesis, University of Manchester, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.509038.

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Pantano-Rubino, Carlos. « Compressibility effects in turbulent nonpremixed reacting shear flows / ». Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2000. http://wwwlib.umi.com/cr/ucsd/fullcit?p9981965.

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8

Horender, Stefan. « Experiments and simulations of particle-laden turbulent shear flows ». Thesis, Imperial College London, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.401859.

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Li, Li. « Modelling of dispersive transport in turbulent free shear flows ». Thesis, University of the West of Scotland, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.430898.

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Lindgren, Björn. « Flow facility design and experimental studies of wall-bounded turbulent shear-flows ». Doctoral thesis, KTH, Mechanics, 2002. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3454.

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The presen present thesis spans a range of topics within thearea of turbulent flows, ranging from design of flow facilitiesto evaluation aluation of scaling laws and turbulence modelingdeling aspects through use of experimental data. A newwind-tunnel has been designed, constructed and evaluated at theDept. of Mechanics, KTH. Special attention was directed to thedesign of turning vanes that not only turn the flow but alsoallow for a large expansion without separation in the corners.The investigation of the flow quality confirmed that theconcept of expanding corners is feasible and may besuccessfully incorporated into low turbulence wind-tunnels. Theflow quality in the MTL wind-tunnel at the Dept. of Mechanics,KTH, was as also in investigated confirming that it still isvery good. The results are in general comparable to thosemeasured when the tunnel was as new, with the exception of thetemperature variation ariation that has decreased by a factorof 4 due to an improved cooling system.

Experimental data from high Reynolds number zeropressure-gradient turbulent layers have been investigated.These studies have primarily focused on scaling laws withe.g.confirmation of an exponential velocity defect lawin a region, about half the size of the boundary layerthickness, located outside the logarithmic overlap region. Thestreamwise velocity probability density functions in theoverlap region was found to be self-similar when scaled withthe local rms value. Flow structures in the near-wall andbuffer regions were studied ande.g. the near-wall streak spacing was confirmed to beabout 100 viscous length units although the relative influenceof the near-wall streaks on the flow was as found to decreasewith increasing Reynolds number.

The separated flow in an asymmetric plane diffuser wasdetermined using PIV and LDV. All three velocity componentswere measured in a plane along the centerline of the diffuser.Results for mean velocities, turbulence intensities andturbulence kinetic energy are presented, as well as forstreamlines and backflow coefficientcien describing theseparated region. Instantaneous velocity fields are alsopresented demonstrating the highly fluctuating flow. Resultsfor the above mentioned velocity quantities, together with theproduction of turbulence kinetic energy and the secondanisotropy inariant are also compared to data from simulationsbased on the k -wformulation with an EARSM model. The simulation datawere found to severely underestimate the size of the separationbubble.

Keywords:Fluid mechanics, wind-tunnels, asymmetricdiffuser, turbulent boundary layer, flow structures, PDFs,modeling, symmetry methods.

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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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12

Grosse, Sebastian. « Development of the micro-pillar shear stress sensor MPS3 for turbulent flows / ». Aachen : Shaker, 2008. http://d-nb.info/991820029/04.

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13

Zhang, Zhao. « Unstructured mesh methods for stratified turbulent flows ». Thesis, Loughborough University, 2015. https://dspace.lboro.ac.uk/2134/16617.

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Developments are reported of unstructured-mesh methods for simulating stratified, turbulent and shear flows. The numerical model employs nonoscillatory forward in-time integrators for anelastic and incompressible flow PDEs, built on Multidimensional Positive Definite Advection Transport Algorithm (MPDATA) and a preconditioned conjugate residual elliptic solver. Finite-volume spatial discretisation adopts an edge-based data structure. Tetrahedral-based and hybrid-based median-dual options for unstructured meshes are developed, enabling flexible spatial resolution. Viscous laminar and detached eddy simulation (DES) flow solvers are developed based on the edge-based NFT MPDATA scheme. The built-in implicit large eddy simulation (ILES) capability of the NFT scheme is also employed and extended to fully unstructured tetrahedral and hybrid meshes. Challenging atmospheric and engineering problems are solved numerically to validate the model and to demonstrate its applications. The numerical problems include simulations of stratified, turbulent and shear flows past obstacles involving complex gravity-wave phenomena in the lee, critical-level laminar-turbulence transitioning and various vortex structures in the wake. Qualitative flow patterns and quantitative data analysis are both presented in the current study.

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Najafi-Yazdi, Alireza. « Large eddy simulation of sound generation by turbulent reacting and nonreacting shear flows ». Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=107761.

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The objective of the present study was to investigate the mechanisms of sound generation by subsonic jets. Large eddy simulations were performed along with bandpass filtering of the flow and sound in order to gain further insight into the pole of coherent structures in subsonic jet noise generation.A sixth-order compact scheme was used for spatial discretization of the fully compressible Navier-Stokes equations. Time integration was performed through the use of the standard fourth-order, explicit Runge-Kutta scheme. An implicit low dispersion, low dissipation Runge-Kutta (ILDDRK) method was developed and implemented for simulations involving sources of stiffness such as flows near solid boundaries, or combustion. A surface integral acoustic analogy formulation, called Formulation 1C, was developed for farfield sound pressure calculations. Formulation 1C was derived based on the convective wave equation in order to take into account the presence of a mean flow. The formulation was derived to be easy to implement as a numerical post-processing tool for CFD codes.Sound radiation from an unheated, Mach 0.9 jet at Reynolds number 400, 000 was considered. The effect of mesh size on the accuracy of the nearfield flow and farfield sound results was studied. It was observed that insufficient grid resolution in the shear layer results in unphysical laminar vortex pairing, and increased sound pressure levels inthe farfield. Careful examination of the bandpass filtered pressure field suggested that there are two mechanisms of sound radiation in unheated subsonic jets that can occur in all scales of turbulence. The first mechanism is the stretching and the distortion of coherent vortical structures, especially close to the termination of the potential core. As eddies are bent or stretched, a portion of their kinetic energy is radiated. This mechanism is quadrupolar in nature, and is responsible for strong sound radiation at aft angles. The second sound generation mechanism appears to be associated with the transverse vibration of the shear-layer interface within the ambient quiescent flow, and has dipolar characteristics. This mechanism is believed to be responsible for sound radiation along the sideline directions.Jet noise suppression through the use of microjets was studied. The microjet injection induced secondary instabilities in the shear layer which triggered the transition to turbulence, and suppressed laminar vortex pairing. This in turn resultedin a reduction of OASPL at almost all observer locations. In all cases, the bandpass filtering of the nearfield flow and the associated sound provides revealing details of the sound radiation process. The results suggest that circumferential modes are significant and need to be included in future wavepacket models for jet noise prediction.Numerical simulations of sound radiation from nonpremixed flames were also performed. The simulations featured the solution of the fully compressible Navier-Stokes equations. Therefore, sound generation and radiation were directly captured in the simulations. A thickened flamelet model was proposed for nonpremixed flames. The model yields artificially thickened flames which can be better resolved on the computational grid, while retaining the physically currect values of the total heat released into the flow. Combustion noise has monopolar characteristics for low frequencies. For high frequencies, the sound field is no longer omni-directional. Major sources of sound appear to be located in the jet shear layer within one potential core length from the jet nozzle.
L'objectif de cette étude est d'obtenir la meilleure compréhension des mécanismesde géneration de bruit par des jet subsoniques. Cette étude est basée sur simulations aux grandes échelles de jets réactifs et sans réactifs. Des calculs numériques employant des schéme compacts de sixiéme ordre. L'integration temporelle fut éxéciteé à l'aide de schéme Runge-Kutta de de quatrième ordre. Des schéme à faible dispersion et dissipation numérique. Un formulation intégrale basée sur les analogies acoustiques fut développées pour la prédiction du champ acoustique lointain pour les sources et observateure en mouvement dans un ﬂuide avec vitesse uniforme. La formulation fut implémentée à l'aide d'algorithmes facilitant l'implémentation pour le traitement de données d'écoulement en haute performance utilisant des outils de simiulation à grande échelle. Les champs sonore produit par un jet turbulent non-réactif avec nombre de Mach de 0.9, et un nombre de Reynolds ReD = 400, 000 fut étudié. L'eﬀect de la taille du maillage sur la précision de l'écoulement en champs proche et e champs sonore loin de source fut analysé. La sous-résolution de la couche de cisaillement à la sortie du jet méne à l'apparition de structures cohérentes et forte radiation qui ne sort pas physiquement réalistes. Deux mécanismes principaux de génération sonore par jets subsoniques furent identiﬁés. Le premier mécanisme est l'étirement et la distorsion de structures tourbillonnaires cohérentes, en particulier prés de la ﬁn du coere potentiel. Ce mécanisme est quadripolaire, et émet principalement vers l'arriére du jet dans la direction de l'écoulement. Le seconde mécanisme semble être constitué de vibration transversale de la couche de cisaillement en réponse à la présemce de structures cohérentes dans la jet. Semblable à la radiation d'une plaque à bonds ﬁnis, la contribution de ce méchanisme est dipolaire et domine la champs sonore dans la direction transversale, perpendiculaire au jet. L'utilisation de plusieurs microjet fut investiguée pour la réduction du bruit. L'injection à l'aide de microjets précipite la transition à la turbulence, favorisent le mélange et la destrcutction de structures cohérentes de grande échelle. Un ﬁltrage en bandes de étroites fut eﬀectué. Ce traitement des données numériquepermet de visualiser les relations complexes entre l'écoulement et les onds sonores émises. Les résultats démontrent l'importance de modes circumférenciele, ce qui a des conséquenecs pour les modiles dits de paquets d'onde pour la preédiction du bruit du jet. Des simulation numériques d'écoulement et champs sonore d'une ﬂame sans prémélange furent aussi éxécutées. Les simulations incluent encore une fois l'écoulement et le champ sonore associé, obtenus directement des équations de Navier Stokes compressibles. Un modèle ﬂammelette épaissie fut proposé que donne ﬂammes épaissies artiﬁciellement qui peuvent être mieux résolus sur le maillage. Le bruit de combustion a des caractéristiques monopolaires aux basses fréquences. Principales sources de bruit semblent être situé dans la couche de cisaillement.

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Liu, Yiou [Verfasser]. « Measuring the Wall-shear Stress for Turbulent Flows Using the Micro-Pillar Shear-Stress Sensor MPS3 / Yiou Liu ». München : Verlag Dr. Hut, 2021. http://d-nb.info/1232847798/34.

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Semeraro, Onofrio. « Feedback control and modal structures in transitional shear flows ». Licentiate thesis, KTH, Stabilitet, Transition, Kontroll, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-29754.

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Two types of shear flows are investigated in this thesis; numerical simulations are performed for the analysis and control of the perturbation arising in a boundary layer over a flat plate, whereas PIV measurements are analysed for the investigation of a confined turbulent jet. Modal structures of the flows are identified: the aim is to understand the flow phenomena and to identify reduced-order models for the feedback control design. The attenuation of three-dimensional wavepackets of streaks and Tollmien-Schlichting (TS) waves in the boundary layer is obtained using feedback control based on arrays of spatially localized sensors and actuators distributed near the rigid wall. In order to tackle the difficulties arising due to the dimension of the discretized Navier-Stokes operator, a reduced-order model is identified, preserving the dynamics between the inputs and the outputs; to this end, approximate balanced truncation is used. Thus, control theory tools can be easily handled using the low-order model. We demonstrate that the energy growth of both TS wavepackets and streak-packets is substantially and efficiently mitigated, using relatively few sensors and actuators. The robustness of the controller is investigated by varying the number of actuators and ensors, the Reynolds number and the pressure gradient. The configuration can be possibly reproduced in experiments, due to the localization of sensing and actuation devices. A complete analysis of a confined turbulent jet is carried out using timeresolved PIV measurements. Proper orthogonal decomposition (POD) modes and Koopman modes are computed and analysed for understanding the main features of the flow. The frequencies related to the dominating mechanisms are identified; the most energetic structures show temporal periodicity.
QC 20110214

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Semeraro, Onofrio. « Active Control and Modal Structures in Transitional Shear Flows ». Doctoral thesis, KTH, Stabilitet, Transition, Kontroll, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-117916.

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Flow control of transitional shear flows is investigated by means of numerical simulations. The attenuation of three-dimensional wavepackets of Tollmien-Schlichting (TS) and streaks in the boundary layer is obtained using active control in combination with localised sensors and actuators distributed near the rigid wall. Due to the dimensions of the discretized Navier-Stokes operator, reduced-order models are identified, preserving the dynamics between the inputs and the outputs of the system. Balanced realizations of the system are computed using balanced truncation and system identification. We demonstrate that the energy growth of the perturbations is substantially and efficiently mitigated, using relatively few sensors and actuators. The robustness of the controller is analysed by varying the number of actuators and sensors, the Reynolds number, the pressure gradient and by investigating the nonlinear, transitional case. We show that delay of the transition from laminar to turbulent flow can be achieved despite the fully linear approach. This configuration can be reproduced in experiments, due to the localisation of sensing and actuation devices. The closed-loop system has been investigated for the corresponding twodimensional case by using full-dimensional optimal controllers computed by solving an iterative optimisation based on the Lagrangian approach. This strategy allows to compare the results achieved using open-loop model reduction with model-free controllers. Finally, a parametric analysis of the actuators/ sensors placement is carried-out to deepen the understanding of the inherent dynamics of the closed-loop. The distinction among two different classes of controllers – feedforward and feedback controllers - is highlighted. A second shear flow, a confined turbulent jet, is investigated using particle image velocimetry (PIV) measurements. Proper orthogonal decomposition (POD) modes and Koopman modes via dynamic mode decomposition (DMD) are computed and analysed for understanding the main features of the flow. The frequencies related to the dominating mechanisms are identified; the most energetic structures show temporal periodicity.

QC 20130207

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Khossousi, A. A. « A theoretical investigation of an averaged-structure eddy viscosity model applied to turbulent shear flows ». Thesis, London Metropolitan University, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.378944.

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Große, Sebastian [Verfasser]. « Development of the Micro-Pillar Shear-Stress Sensor MPS³ for Turbulent Flows / Sebastian Große ». Aachen : Shaker, 2008. http://d-nb.info/1161308083/34.

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Umbel, Matthew R. « Prediction of Turbulent Mixing at the Interface of Density Stratified, Shear Flows Using CFD ». Ft. Belvoir : Defense Technical Information Center, 1998. http://handle.dtic.mil/100.2/ADA477048.

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21

Alberti, Luca. « Statistical Breakdown of Numerically Simulated Shear-Free Flows ». Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020.

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A statistical analysis of several DNSs of incompressible turbulent shear-free flows (with Reλ ranging from ≈ 86 to ≈ 245) was performed, aimed to study the small scale processes that promote turbulent entrainment at the TNTI. After a certain transient, the shearless flows reached a statistically steady state. From that time instant, the main features of the various flow configurations were statistically investigated by assessing the classical and conditional mean profiles of total kinetic energy (K) and anisotropy ratio (urms/ξ), the classical and conditional mean profiles of the terms of the total kinetic energy budget (advection C, pressure strain Π, viscous diffusion Dv, dissipation ε and power injection Pi), the classical PDFs of the velocity components (u, v and w), the classical and conditional mean profiles of the invariants of the velocity gradient (R and Q), rate-of-strain (RS and QS) and rate-of-rotation (QW) tensors and the classical and conditional J PDFs of the same invariants. Conditional statistical quantities were computed in relation to the distance from the TNTI. To avoid the accidental mixing of turbulent and non-turbulent points in the conditional statistics, all the nodes belonging to irrotational bubbles and turbulent islands were neglected. All the conditional mean profiles are characterized by very sharp gradients at the TNTI, meanwhile in the classical mean profiles the variations are distributed on a much wider portion of the domain, indicating how the conditional averages are able to successfully discern turbulent and irrotational regions, avoiding the smoothening of the mean profiles via large scale intermittency. The similarity between the conditional statistics obtained here for shearless flows, and those extracted from temporal jets in previous works, is very interesting, since it suggests some sort of universality in the TNTI dynamics, as well asin the turbulent entrainment processes.

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Marstorp, Linus. « Subgrid-scale modelling for large-eddy simulation invluding scalar mixing in rotating turbulent shear flows ». Licentiate thesis, KTH, Mechanics, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3916.

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The aim of the present study is to develop subgrid-scale models that are relevant for complex flows and combustion. A stochastic model based on a stochastic Smagorinsky constant with adjustable variance and time scale is proposed. The stochastic model is shown to provide for backscatter of both kinetic energy and scalar variance without causing numerical instabilities. A new subgrid-scale scalar flux model is developed using the same kind of methodology that leads to the explicit algebraic scalar flux model, EASFM, for RANS. The new model predicts the anisotropy of the subgrid-scales in a more realistic way than the eddy diffusion model. Both new models were tested in rotating homogeneous shear flow with a passive scalar. Rogallo’s method of moving the frame with the mean flow to enable periodic boundary conditions was used to simulate homogeneous shear flow.

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Marstorp, Linus. « Subgrid-scale modelling for large-eddy simulation including scalar mixing in rotating turbulent shear flows ». Licentiate thesis, KTH, Mekanik, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3916.

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The aim of the present study is to develop subgrid-scale models that are relevant for complex flows and combustion. A stochastic model based on a stochastic Smagorinsky constant with adjustable variance and time scale is proposed. The stochastic model is shown to provide for backscatter of both kinetic energy and scalar variance without causing numerical instabilities. A new subgrid-scale scalar flux model is developed using the same kind of methodology that leads to the explicit algebraic scalar flux model, EASFM, for RANS. The new model predicts the anisotropy of the subgrid-scales in a more realistic way than the eddy diffusion model. Both new models were tested in rotating homogeneous shear flow with a passive scalar. Rogallo’s method of moving the frame with the mean flow to enable periodic boundary conditions was used to simulate homogeneous shear flow.
QC 20101119

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Helvey, Jacob. « Experimental Investigation of Wall Shear Stress Modifications due to Turbulent Flow over an Ablative Thermal Protection System Analog Surface ». UKnowledge, 2015. http://uknowledge.uky.edu/me_etds/57.

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Modifications were made to the turbulent channel flow facility to allow for fully developed rough quasi-2D Poiseuille flow with flow injection through one surface and flow suction through the opposing surface. The combination of roughness and flow injection is designed to be analogous to the flow field over a thermal protection system which produces ablative pyrolysis gases during ablation. It was found that the additional momentum through the surface acted to reduce skin friction to a point below smooth-wall behavior. This effect was less significant with increasing Reynolds number. It was also found that the momentum injection modified the wake region of the flow.

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Cuthbertson, Alan J. S. « The motion of fine sand particles in turbulent open channel shear flows over porous bed conditions ». Thesis, University of Glasgow, 2001. http://theses.gla.ac.uk/2489/.

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The current study aims to investigate the physical mechanisms controlling fine sediment transport within open channel shear flows over porous beds, with particular emphasis on the role of flow turbulence in particle settling and deposition processes. Preliminary visualisation experiments used a VHS camera to observe the near-bed motion of sand particles and their behaviour within the surface layer of a rhombically-packed bed of uniform spheres. Measurement of near-bed particle trajectories indicate that turbulent particle fall velocities w's are generally larger than fall velocities measured in still water ws, most notably for finer sand grades. Distinctive modes of particle behaviour observed at the bed interface also suggest that flow-separation eddies, generated within surface interstices, have a primary influence on subsequent particle motion, i.e. deposition or re-entrainment. Similar particle behaviour is also displayed in a natural gravel bed. A more detailed analysis of sand particle motion in turbulent open channel flow was carried out employing a high-speed camera and particle-tracking technique to record and analyse particle trajectories within different flow regions. The non-dimensional ratio of measured particle fall velocity w's and still water fall velocity ws was used to indicate the relative enhancement of vertical particle motion within the turbulent flow conditions. Experiment-averaged values of this ratio reveal that particle fall velocities are generally enhanced (i.e. w's/ws > 1) in recorded near-bed and intermediate flow regions (z/H 0.5) and hindered (i.e. w's/ws < 1) in a recorded outer flow region (z/H 0.5). The ratio w's/ws also reveals a general tendency to increase with decreasing grain size di. Vertical profiles of the normalised particle fall velocity w's/u* are shown to be analogous to turbulence intensity distributions (u' rms/u* and w'rms/u*), with the highest values of w's/u* occurring in the near-bed region and coinciding approximately with the regions of highest turbulence activity. This clearly implies the existence of turbulence-enhanced particle fall velocities within the flow conditions considered. Application of a quadrant analysis technique reinforces this notion, revealing further similarities between conditioned turbulent fluid fluctuations and particle motions, in particle, the dominance of 'inrush' events (quadrant 4) in the near-bed flow and 'ejection' events (quadrant 2) away from the bed.

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Rolling, August Jameson. « Design of Gages for Direct Skin Friction Measurements in Complex Turbulent Flows with Shock Impingement Compensation ». Diss., Virginia Tech, 2007. http://hdl.handle.net/10919/28093.

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This research produced a new class of skin friction gages that measures wall shear even in shock environments. One test specimen separately measured wall shear and variable-pressure induced moment. Through the investigation of available computational modeling methods, techniques for accurately predicting gage physical responses were developed. The culmination of these model combinations was a design optimization procedure. This procedure was applied to three disparate test conditions: 1) short-duration, high-enthalpy testing, 2) blow-down testing, and 3) flight testing. The resulting optimized gage designs were virtually tested against each set of nominal load conditions. The finalized designs each successfully met their respective test condition constraints while maximizing strain output due to wall shear. These gages limit sources of apparent strain: inertia, temperature gradient, and uniform pressure. A unique use of bellows provided a protective shroud for surface strain gages. Oil fill provided thermal and dynamic damping while eliminating uniform pressure as a source of output voltage. Two Wheatstone bridge configurations were developed to minimize temperature effects first from temperature gradient and then from spatially varying heat flux induced gradient. An inertia limiting technique was developed that parametrically investigated mass and center of gravity impact on strain output. Multiple disciplinary computational simulations of thermal, dynamic, shear, moment, inertia, and instrumentation interaction were developed. Examinations of instrumentation error, settling time, filtering, multiple input dynamic response, and strain gage placement to avoid thermal gradient were conducted. Detailed mechanical drawings for several gages were produced for fabrication and future testing.
Ph. D.

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Shirai, Katsuaki [Verfasser]. « Investigation and Application of Laser Doppler Velocity Profile Sensors toward Measurements of Turbulent Shear Flows / Katsuaki Shirai ». Aachen : Shaker, 2010. http://d-nb.info/1122546777/34.

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Bagheri, Shervin. « Analysis and control of transitional shear flows using global modes ». Doctoral thesis, Stockholm : Department of Mechanics, Royal Institute of Technology, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-11894.

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Hyensjö, Marko. « Fibre Orientation Modelling Applied to Contracting Flows Related to Papermaking ». Doctoral thesis, KTH, Mekanik, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4762.

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The main goal of this work was to develop numerical models for studying the behaviour of fibres in an accelerated flow. This is of special interest for e.g. papermaking. The early stage of the paper manufacturing process determines most of the final properties of a paper sheet. The complexity of studying the flow of fibre suspensions both experimentally and numerically emphasises a need for new ideas and developments. By means of solving the evolution of a convective-dispersion equation, i.e. the Fokker-Planck equation, a fully 3D approach with respect to the position and the two fibre angles, polar and azimuthal angles, following a streamline is presented. As an input to the fibre orientation model the turbulent flow field is solved by Computational Fluid Dynamics (CFD) with second-order closure in the turbulence model. In this work two new hypotheses have been presented for the variation of the non-dimensional rotational diffusivity with non-dimensional fibre length, Lf /η and the Reynolds number based on the Taylor micro-scale of the turbulence, Reλ Parameters for the two new hy- potheses and earlier models are determined with the aim of achieving a general relation and a value of the rotational dispersion coeffcient of stiff fibres in an anisotropic turbulent fluid flow. Earlier modelling work has been focused on solving the planar approach, i.e. assuming all fibres to be in one plane. This planar approach is discussed and compared with the fully 3D approach and its validity is evaluated. The optimization of parameters for the different hypotheses correlated on a central streamline, showed a good agreement with an independent experimental result in the undisturbed region. Moreover, it is particularly interesting that the boundary layer region and the wake region are predicted fairly well and the phenomena are well described, which has not been the case earlier. It seems that the new hypothesis based on the variation of the non-dimensional fibre length, Lf /η gives the best correlation in these shear-layer regions. Further- more it was established that the planar approach fails to predict shear layers, i.e. the boundary layer and the wake regions. As emphasized in the theory section, the planar formulation is strictly valid only if all fibres are oriented in one plane, which is not the case in the shear layers. In the undisturbed region, the 3D and the planar approaches, agree in their results. This leads to the conclusion that both approaches are suitable when shear layers are not studied.
QC 20100812

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Louis, Stephen. « Continuer à irriguer quand les lacs-réservoirs de barrage souffrent de taux de sédimentation sévères - Recommandations d'amélioration de la gestion du principal canal d'irrigation alimenté par l'ouvrage répartiteur de Canneau (Haïti) ». Doctoral thesis, Universite Libre de Bruxelles, 2019. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/288808.

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L’État haïtien, pour faire face à l’insécurité alimentaire que connaît sa population (à croissance rapide et à faible revenu), s’appuie particulièrement sur la Vallée du département de l’Artibonite qui constitue depuis toujours le véritable grenier agricole du pays, en fournissant, à elle seule, plus de 80% de la production rizicole nationale. Cette production agricole assure non seulement les besoins alimentaires de la population locale, mais également ceux particuliers des départements voisins (Ouest, Nord et Centre).L’irrigation de cette vaste plaine agricole (32400 ha de terres agricoles irrigables) est garantie, à plus de 75%, par le réseau d’irrigation établi en rive Gauche de l'ouvrage-partiteur de Canneau, alimenté par un Canal principal (CMRG) ayant un débit nominal de 40 m3/s.Néanmoins, ce Partiteur, source d’approvisionnement exclusive du CMRG, est lui-même régulé par le Barrage-réservoir de Péligre qui se trouve à 70 km plus en amont sur le fleuve de l’Artibonite. En termes hydrauliques, nos travaux ont d'abord examiné la situation du réservoir de Péligre. Alors qu'au moment de la construction il était initialement prévu d’y stocker 607 Mm3, il ne reste plus aujourd'hui qu'à peine 40% de cette capacité utile, en raison des dépôts sédimentaires qui se sont constitués année après année derrière le barrage.Cette sédimentation spectaculaire (due à l’érosion des bassins versants amont fort dégradés), combinée aux déficits pluviométriques, provoque en période d’étiage une rareté d’eau, des lâchers insuffisants et donc des déficits en eau utile envoyée en amont de ce Partiteur de Canneau. Cela impacte significativement le réseau d’irrigation aval, dont le CMRG. Nos travaux ont montré que la situation est d'autant plus critique que les débits eux-mêmes, en amont comme en aval du Partiteur, sont en fait très mal connus et devraient faire l'objet d'approches méthodologiques plus rigoureuses que celles déployées sur site actuellement.Aussi, face à ces constats, de nouvelles règles de distribution de l’eau s’imposent, pour continuer à irriguer et espérer obtenir un rendement agricole acceptable (souhaitable).Notre travail s'est ainsi donné pour objectif de contribuer à la mise en place des nouvelles règles de gestion de l’eau (répartition) au sein du réseau d’irrigation alimenté par le CMRG, pour continuer à fournir l’eau à l’irrigant en quantité acceptable (et connue). Cette amélioration de gestion a été envisagée à la fois en amont et en aval de ce réseau d’irrigation, particulièrement en ses différents nœuds-clés (Canal principal et Canal secondaire).La démarche méthodologique adoptée pour relever ce défi majeur s’appuie notamment sur un système d’information hydro-morpho-sédimentaire actualisé et de qualité. Il s’agit d’une base de données, riche en observations de hauteurs d’eau (lues aux stations limnimétriques), vitesses de surface au flotteur, champs de vitesse explorés au moyen d’un courantomètre et en données bathymétriques et granulométriques des tronçons des canaux étudiés, appréciées respectivement au moyen d’un GPS différentiel et du tamisage à sec.Les résultats fort encourageants obtenus permettent d'acquérir une meilleure compréhension du système et une amélioration particulière du réseau d’irrigation en rive gauche du Partiteur de Canneau. En s’appuyant sur les historiques de sédimentation du Lac-réservoir de Péligre (de 1960 à 2016), nous présentons un document de synthèse sur la sédimentation du Lac-réservoir de Péligre. Ce document met notamment en exergue le taux de sédimentation sévère de ce dernier (5.47 Mm3/an), qui continue d’augmenter encore aujourd’hui, ainsi que les conséquences de celui-ci sur les débits turbinés et la disponibilité de l’eau en amont du Partiteur de Canneau.Nous mettons également en évidence les formes irrégulières (Lit-non prismatique) des tronçons des canaux étudiés, via une vue axonométrique des profils en travers (issus de l’étude bathymétrique) des canaux d’irrigation en terre battue étudiés. Puis, nous présentons de manière détaillée le caractère très hétérogène des dépôts sédimentaires de ces derniers, à partir d’une analyse des représentations en Log-Probit des résultats du tamisage, construites au moyen du logiciel GrandPlots.En nous appuyant sur les mesures expérimentales des contraintes de Reynolds et des profils instantanés de vitesse (pris à intervalle de 64 ms), tirés de la base de données EPFL, nous avons montré qu’il faut absolument travailler dans les 18% inférieurs de la colonne d’eau (z/h<0.18) et en mode déficitaire, dans un écoulement turbulent comme celui-là, pour extraire de façon représentative et pertinente une pente expérimentale u*/, comme indicateur de u*.À l’issue d’un examen détaillé de la distribution verticale de la vitesse au canal secondaire FNE, nous validons un DMLWL (Dip-Modified-log-wake-law) à la fois en amont et en aval du réseau. Nous montrons que ceci permet de modéliser le Dip-phenomenon observé systématiquement au sein des profils explorés in situ. Nous proposons une relation entre le coefficient d’inégale répartition de la vitesse à la verticale αv (de Prony) et l’aspect ratio (W/h) pour tout le réseau d’irrigation étudié ;ceci afin d’obtenir une vitesse débitante (Ū), simplement à partir d’une prise de vitesse au flotteur, dans l’axe central d’écoulement.À partir des débits quantifiés à la section de référence du CMRG, via l’équation de continuité (Q=AŪ), nous fournissons un Abaque, diagramme à 3 entrées (débit (Qp), charge amont (H0) et ouverture de vanne (hv)), permettant aux vanniers de connaitre les débits au pont de fer correspondant aux différentes ouvertures de vanne et celui pour lequel le trop-plein (retour des eaux excédentaires vers le fleuve de l’Artibonite) commence à fonctionner.À l’égard des opérateurs locaux et gestionnaires du système, nous mettons enfin à disposition, des méthodes/outils simples et efficaces leur permettant de quantifier finement le débit au Canal principal en amont ainsi qu’au canal secondaire en aval, simplement à partir d’une mesure de hauteur d’eau (h) et de vitesse de surface au flotteur (Us).
Doctorat en Sciences agronomiques et ingénierie biologique
info:eu-repo/semantics/nonPublished

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Scharnowski, Sven [Verfasser], Christian J. [Akademischer Betreuer] Kähler, Fulvio [Akademischer Betreuer] Scarano et Christoph [Akademischer Betreuer] Garbe. « Investigation of turbulent shear flows with high resolution PIV methods / Sven Scharnowski. Universität der Bundeswehr München, Fakultät für Luft- und Raumfahrttechnik. Gutachter : Christian J Kähler ; Fulvio Scarano ; Christoph Garbe. Betreuer : Christian J Kähler ». Neubiberg : Universitätsbibliothek der Universität der Bundeswehr München, 2014. http://d-nb.info/1049833686/34.

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Huret, Thomas. « Génération en soufflerie d'écoulements cisaillés représentatifs des écoulements environnementaux de couches limites atmosphériques par des dispositifs passifs ». Electronic Thesis or Diss., Centrale Lille Institut, 2024. http://www.theses.fr/2024CLIL0011.

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Dans le domaine de l'ingénierie du vent, les dispositifs les plus couramment utilisés à la fois en milieux académiques et industriels pour générer des écoulements de couches limites atmosphériques en soufflerie consistent à associer des rugosités de paroi à un alignement de «spires» (flèches) en amont. Si des modèles permettent effectivement de concevoir a priori ce type de dispositifs passifs pour généré un champs moyen donné, le contrôle du profil d’intensité turbulente nécessite un processus d’ajustements par essai-erreur long et coûteux à réitérer chaque nouvelle configuration souhaitée.En parallèle, des études récentes sur les écoulements turbulents en aval de grilles régulières et fractales ont établis qu’un modèle d’interaction de sillages permet de prédire l’intensité turbulente générée, dans des configurations sans cisaillement moyen. L’extension de ces lois d’échelles à des écoulements cisaillés non-uniformes rendrait possible un contrôle indépendant des profils de vitesse moyenne et d’intensité turbulente dès la phase de conception, contrairement aux dispositifs de « spires » actuels. Pour étudier expérimentalement cette possibilité, notre travail s’intéresse à des grilles passives d'un type particulier –des Grilles Multi-échelles Inhomogènes (MIG) – qui permettent l’ajustement local des paramètres du modèle d’interaction de sillages en fonction de l’altitude, tout en maitrisant le champs moyen généré. Ils se trouvent que les dispositifs de « spires » peuvent conceptuellement être assimilés à un cas particulier de grilles MIG « continues ».Dans un premier temps, un algorithme de conception de grilles MIG est développé pour générer un champs moyen donné, première étape avant une tentative de contrôle de la turbulence. Cet algorithme est validé expérimentalement par la conception de grilles MIG et de « spires » dédiés à la génération d’écoulements moyens représentatifs de tout ou partie d’une couche limite atmosphérique neutre. En particulier, ces études préliminaires révèlent l’existence d’un régime d’écoulement défavorable à l’établissement du champs moyen souhaité pour certaines configurations, ce qui amène à suggérer un ensemble de critères sur les paramètres géométriques locaux de grille pour les éviter.En outre, les résultats montrent que le modèle d’interaction de sillages établi initialement en écoulement uniforme permet effectivement un bon « collapse » des mesures d’intensité turbulente en aval de chaque niveau horizontal de grille MIG discrètes, même en présence d’un cisaillement moyen. Pour autant, une variabilité non-négligeable subsiste en fonction de l’intensité de ce cisaillement. Par ailleurs, même si un « collapse » des mesures en aval de « spires » est également observé sur une gamme étendue d’altitudes, la courbe résultant de cette mise à l’échelle ne coïncide pas avec celle obtenue en aval de grilles MIG discrètes.Afin de tenter d’expliquer à la fois l’influence du cisaillement moyen sur le déclin de turbulence et la différence de régime observée entre grilles MIG discrètes et spires, un modèle est développé à partir d’hypothèses simplificatrices sur l’équation sur l’énergie cinétique turbulente. Ce modèle est complété par des modèles empiriques décrivant l’évolution des contraintes de Reynolds et celle des longueurs intégrales. Un modèle similaire pour le taux de dissipation turbulente reste à établir.L’analyse des différentes hypothèses utilisées pour établir notre modèle de turbulence révèlent des variations de cisaillements et de vorticité longitudinale qui sont inattendues à la fois en termes de complexité et de persistance en aval des dispositifs générateurs. Les structures d’écoulement mises au jour sont considérées comme la cause principale des différences observées entre spires et grilles MIG discrètes. Notre travail indique ainsi la nécessité d’orienter l’effort de modélisation vers les écoulements de « sillage proche », directement autour des dispositifs passifs étudiés
In the field of wind engineering, a common practice for both research and industrial applications involving the generation of a neutral atmospheric boundary layer (ABL) in a wind tunnel consists in associating a roughness fetch with an upstream array of “spires”. This passive method enables to tailor the mean velocity profile on the basis of quantitative guidelines. At the expense of a further time-consuming trial-and-error design process, these devices can be adjusted to generate representative turbulent intensity profiles.In parallel, the downstream evolution of regular and fractal grid-generated turbulence has been recently shown to scale with a wake-interaction model which can be used to predict basic properties of downstream turbulence profiles in case of zero mean shear. If this scaling law were to apply to grid-generated shear flow, it would enable the independent tailoring of turbulent intensity and mean velocity profiles without trial-and-error, contrary to previous spires devices. In order to investigate these ideas experimentally, our work makes use of Multiscale Inhomogeneous Grids (MIG), a new type of passive device defined to vary the turbulence scaling parameters with altitude, while enabling a tailoring of the mean velocity profile. In particular, it is shown that "spires" represent a specific case of "continuous" MIG grids.A general MIG design algorithm is developed for the generation of a prescribed mean flow profile. It is experimentally validated for the design of both discrete MIG grids and spires aiming to generate full-depth and part-depth neutral atmospheric boundary layer configurations. The devices are studied over both smooth and rough walls in the SCL-PIV wind tunnel of ONERA Lille using both Hot-Wire Anemometry (HWA) and stereo Particle Image Velocimetry (S-PIV). This validation process reveals the existence of a defective grid regime for specific local geometric configurations of MIG grids. A set of criteria on local geometric parameters (i.e. local obstruction and local mesh aspect ratio) is suggested to avoid this defective grid regime.The turbulence intensity decay downstream of the designed MIG grids is then studied by scaling its streamwise profile measured downstream of each horizontal grid level by the corresponding wake-interaction parameters for zero shear grid flows. A very good collapse of the turbulence decay is observed for all discrete MIG grids, with however a slight remnant effect of the local mean shear. Moreover, a collapse of spires-generated curves is also observed over a specific range of altitudes, but on a different collapse curve than for discrete MIG grids.An attempt to take into account both the remaining effect of mean shear on the turbulence decay and the observed collapse discrepancy between discrete and continuous MIG grids leads to the development of a simplified Turbulent Kinetic Energy (TKE) model. Several terms of this model remain to be closed in order to provide an a priori prediction. Successful closing empirical models are developed for both Reynolds stress and integral length scales, but not for the turbulent dissipation rate due to a lack of experimental data.The experimental investigation of the different hypotheses of the TKE model surprisingly revealed complex mean shear and streamwise vorticity patterns persisting far downstream of both spires and discrete MIG grids. These diverse and complex structures, originating from the near-wake flows around the upstream devices, are acknowledged as the main cause of discrepancy between spires and MIG grids. Tailoring turbulence intensity for ABL generation in wind tunnel appears to require an understanding and a modeling of this near-wake complexity

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Babarutsi, Sofia. « Modelling quasi-two-dimensional turbulent shear flow ». Thesis, McGill University, 1991. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=70223.

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A two-length-scale turbulence model is introduced in this thesis for the computation of quasi-two-dimensional turbulent shear flow with two distinct length scales of motion. In the model, the turbulence motions of the two distinct length scales are computed separately. The small-scale turbulence is treated as a background component locally in equilibrium while the large-scale turbulence is simulated using a second-order closure procedure. The development of the turbulent shear flows depends on the rate of energy transfer from the large-scale to the small-scale turbulence. Two mechanisms are identified to have a significant effect on the rate of this energy transfer. On one hand, the rate is reduced due to the confinement of the large-scale turbulence to two-dimensional motion, since the nonlinear energy cascade process is less efficient in two-dimensional turbulent motion. On the other hand, the rate is enhanced due to the work done by the large-scale turbulent motion against the friction forces. The energy transfer rate due to friction is derived in the model using a two-step averaging procedure, whereas the transfer rate due to nonlinear cascade process is determined using a model equation. The data from a number of experimental investigations of quasi-two-dimensional turbulent shear flows are analyzed. These data support the notion of the two-length-scale turbulence model, that (i) the maintenance of the turbulent motion depends on the transfer of energy from the large-scale turbulence to the small-scale turbulence, and (ii) the transfer rate is subjected to confinement and friction influences as specified in the model. Numerical computations are conducted using the two-length-scale model and a single-length-scale model. The results are compared with the experimental data. The two-length-scale model is superior in performance compared with the single-length-scale model, particularly in the intermediate region of the flow where both length scales of the turbule

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Alathur, Srinivasan Prem Anand. « Deep Learning models for turbulent shear flow ». Thesis, KTH, Numerisk analys, NA, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-229416.

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Deep neural networks trained with spatio-temporal evolution of a dynamical system may be regarded as an empirical alternative to conventional models using differential equations. In this thesis, such deep learning models are constructed for the problem of turbulent shear flow. However, as a first step, this modeling is restricted to a simplified low-dimensional representation of turbulence physics. The training datasets for the neural networks are obtained from a 9-dimensional model using Fourier modes proposed by Moehlis, Faisst, and Eckhardt [29] for sinusoidal shear flow. These modes were appropriately chosen to capture the turbulent structures in the near-wall region. The time series of the amplitudes of these modes fully describe the evolution of flow. Trained deep learning models are employed to predict these time series based on a short input seed. Two fundamentally different neural network architectures, namely multilayer perceptrons (MLP) and long short-term memory (LSTM) networks are quantitatively compared in this work. The assessment of these architectures is based on (i) the goodness of fit of their predictions to that of the 9-dimensional model, (ii) the ability of the predictions to capture the near-wall turbulence structures, and (iii) the statistical consistency of the predictions with the test data. LSTMs are observed to make predictions with an error that is around 4 orders of magnitude lower than that of the MLP. Furthermore, the flow fields constructed from the LSTM predictions are remarkably accurate in their statistical behavior. In particular, deviations of 0:45 % and 2:49 % between the true data and the LSTM predictions were obtained for the mean flow and the streamwise velocity fluctuations, respectively.
Djupa neuronät som är tränade med rum-tids utveckling av ett dynamiskt system kan betraktas som ett empiriskt alternativ till konventionella modeller som använder differentialekvationer. I denna avhandling konstruerar vi sådana djupinlärningsmodeller för att modellera en förenklad lågdimensionell representation av turbulensfysiken. Träningsdata för neuronäten erhålls från en 9-dimensionell modell (Moehlis, Faisst och Eckhardt [29]) för olika Fourier-moder i ett skärskikt. Dessa moder har ändamålsenligt valts för att avbilda de turbulenta strukturerna i regionen nära väggen. Amplitudernas tidsserier för dessa moder beskriver fullständigt flödesutvecklingen, och tränade djupinlärningsmodeller används för att förutsäga dessa tidsserier baserat på en kort indatasekvens. Två fundamentalt olika neuronätsarkitekturer, nämligen flerlagerperceptroner (MLP) och långa närminnesnätverk (LSTM), jämförs kvantitativt i denna avhandling. Utvärderingen av dessa arkitekturer är baserad på (i) hur väl deras förutsägelser presterar jämfört med den 9-dimensionella modellen, (ii) förutsägelsernas förmåga att avbilda turbulensstrukturerna nära väggar och (iii) den statistiska överensstämmelsen mellan nätverkets förutsägelser och testdatan. Det visas att LSTM gör förutsägelser med ett fel på ungefär fyra storleksordningar lägre än för MLP. Vidare, är strömningsfälten som är konstruerade från LSTM-förutsägelser anmärkningsvärt noggranna i deras statistiska beteende. I synnerhet uppmättes avvikelser mellan de sanna- och förutsagda värdena för det genomsnittliga flödet till 0; 45 %, och för de strömvisa hastighetsfluktionerna till 2; 49 %.

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Campana, Lorenzo. « Modélisation stochastique de particules non sphériques en turbulence ». Thesis, Université Côte d'Azur, 2022. http://www.theses.fr/2022COAZ4019.

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Le mouvement de petites particules non-sphériques en suspension dans un écoulement turbulent a lieu dans une grande variété d’applications naturelles et industrielles. Par exemple, ces phénomènes impactent la dynamique des aérosols dans l’atmosphère et dans les voies respiratoires, le mouvement des globules rouges dans le sang, la dynamique du plancton dans l’océan, la glace dans les nuages ou bien la combustion. Les particules anisotropes réagissent aux écoulements turbulents de manière complexe. Leur dynamique dépend ainsi d’un large éventail de para- mètres (forme, inertie, cisaillement du fluide). Les particules sans inertie, dont la taille est inférieure à la longueur de Kolmogorov, suivent le mouvement du fluide avec une orientation généralement gouvernée par le gradient local de vitesse turbulente. Cette thèse est axée sur la dynamique de tels objets en turbulence en ayant recours à des méthodes Lagrangienes stochastiques. Le développement d’un modèle qui peut être utilisé comme outil prédictif dans le cadre de la dynamique de fluides numérique (CFD) au niveau industriel est d’un grand intérêt pour les applications concrètes en ingénierie. Par ailleurs, pour progresser dans le domaine de la médecine, de l’environnement et des procédés industriels, il est nécessaire que ces modèles atteignent un compromis acceptable entre simplicité et précision. La formulation d’un modèle stochastique pour l’orientation de telles particules est tout d’abord présentée dans le cadre d’un écoulement turbulent bidimensionnel avec un cisaillement homogène. Des simulations numériques directes (DNS) sont produites pour guider et évaluer la proposition de modèle. Les questions abordés dans ce travail portent sur la représentation de formes analytiques du modèle, sur les effets des anisotropies inclues dans le modèle, et sur l’extension de la notion de dynamique rotationnelle dans le cadre de cette approche stochastique. Les résultats obtenus avec le modèle, comparés avec la DNS, produisent une réponse qualitative acceptable, même si ce modèle diffusif n’est pas conçu pour reproduire les caractéristiques non-gaussiennes des expériences numériques (DNS). L’extension au cas tridimensionnel du modèle d’orientation pose le problème de son implé- mentation numérique efficace. Dans ce travail, un schéma numérique capable de simuler la dynamique d’orientation de telles particules, à un coût de calcul raisonnable, est introduit. La convergence de ce schéma est également analysée. Pour ce faire, un schéma fondé sur la décomposition de la dynamique a été développé pour résoudre les équations différen- tielles stochastiques (EDS) de rotation de ces particules. Cette décomposition permet de surmonter les problèmes d’instabilité typiques de la méthode Euler–Maruyama; on a ainsi obtenu une convergence en norme L2 d’ordre 1/2 et une convergence faible d’ordre 1, comme classiquement attendu. Enfin, le schéma numérique a été implémenté dans un code CFD industriel (Code_Saturne). Ce modèle a ensuite été utilisé pour étudier l’orientation et la rotation de particules anisotropes sans inertie dans le cas d’un écoulement turbulent inhomogène, à savoir un écoulement de canal plan turbulent. Cette application dans un cas pratique a permis de mettre en evidence deux difficultés liées au modèle : d’abord, l’implémentation numérique dans un code industriel, ensuite la capacité du modèle à reproduire les expériences numériques obtenues par DNS. Ainsi, le modèle stochastique Lagrangien pour l’orientation de sphéroïdes implémenté dans Code_Saturne permet de reproduire, avec certaines limites, les statistiques d’orientation et de rotation de sphéroïdes mesurées dans la DNS
The motion of small non- spherical particles suspended in a turbulent flow is relevant for a large variety of natural and industrial applications such as aerosol dynamics in respiration, red blood cells motion, plankton dynamics, ice in clouds, combustion, to name a few. Anisotropic particles react on turbulent flows in complex ways, which depend on a wide range of parameters (shape, inertia, fluid shear). Inertia-free particles, with size smaller than the Kolmogorov length, follow the fluid motion with an orientation generally defined by the local turbulent velocity gradient. Therefore, this thesis is focused on the dynamics of these objects in turbulence exploiting stochastic Lagrangian methods. The development of a model that can be used as predictive tool in industrial computational fluid dynamics (CFD) is highly valuable for practical applications in engineering. Models that reach an acceptable compromise between simplicity and accuracy are needed for progressing in the field of medical, environmental and industrial processes. The formulation of a stochastic orientation model is studied in two-dimensional turbulent flow with homogeneous shear, where results are compared with direct numerical simulations (DNS). Finding analytical results, scrutinising the effect of the anisotropies when they are included in the model, and extending the notion of rotational dynamics in the stochastic framework, are subjects addressed in our work. Analytical results give a reasonable qualitative response, even if the diffusion model is not designed to reproduce the non-Gaussian features of the DNS experiments. The extension to the three-dimensional case showed that the implementation of efficient numerical schemes in 3D models is far from straightforward. The introduction of a numerical scheme with the capability to preserve the dynamics at reasonable computational costs has been devised and the convergence analysed. A scheme of splitting decomposition of the stochastic differential equations (SDE) has been developed to overcome the typical instability problems of the Euler–Maruyama method, obtaining a mean-square convergence of order 1/2 and a weakly convergence of order 1, as expected. Finally, model and numerical scheme have been implemented in an industrial CFD code (Code_Saturne) and used to study the orientational and rotational behaviour of anisotropic inertia-free particles in an applicative prototype of inhomogeneous turbulence, i.e. a turbulent channel flow. This real application has faced two issues of the modelling: the numerical implementation in an industrial code, and whether and to which extent the model is able to reproduce the DNS experiments. The stochastic Lagrangian model for the orientation in the CFD code reproduces with some limits the orientation and rotation statistics of the DNS. The results of this study allows to predict the orientation and rotation of aspherical particles, giving new insight into the prediction of large scale motions both, in two-dimensional space, of interest for geophysical flows, and in three-dimensional industrial applications

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Schmiegel, Armin. « Transition to turbulence in linearly stable shear flows ». [S.l. : s.n.], 1999. http://archiv.ub.uni-marburg.de/diss/z2000/0062.

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Lefauve, Adrien Sébastien Paul. « Waves and turbulence in sustained stratified shear flows ». Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/277352.

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The speed and efficiency of stratified turbulent mixing in homogenising temperatures, chemical composition and flow speeds makes it one of farthest reaching fluid mechanical phenomenon for life on earth. It is an aesthetically beautiful phenomenon, rich in complex physical behaviours and extremely challenging to model mathematically. Laboratory experiments have a valuable role to play to guide theoretical and numerical work towards a better understanding of this phenomenon by providing insight into real flows under controlled conditions. This dissertation addresses some aspects of the laboratory buoyancy-driven exchange flows through an inclined duct connecting two reservoirs containing fluids of different densities. We employ a novel experimental technique to perform near-instantaneous, volumetric measurements of the three-component velocity field and density field simultaneously, providing an unprecedented quantitative picture of these sustained stratified shear flows. We start by characterising the variety of observed behaviours, or flow regimes, as we vary the density difference between the two reservoirs, the angle of inclination of the duct with respect to the horizontal, the way the density difference is achieved (solutions of salt/fresh water or cold/warm water) and the geometry of the duct. These empirical observations allow us to formulate a number of specific research questions, guiding the work of the next chapters. We then focus on the regime in which Holmboe waves are observed, and demonstrate that these well-known interfacial travelling disturbances have a distinct structure when confined by solid boundaries. We characterise this structure and identify the physical mechanisms at its origin by means of linear stability theory. Since Holmboe waves are found in the intermediate, transitional regime between laminar and turbulent flows, we conjecture that their structure may be relevant to more turbulent flows, where resembling structures are indeed observed. Next, we tackle the quantitative analysis of universal transition curves separating the observed flow regimes (laminar, waves, intermittently turbulent or fully turbulent) as well and the net mass flow rate exchanged by the reservoirs. We show that these long-lasting questions in the study of exchange flows can be addressed in the framework of frictional hydraulic theory, and we derive detailed scaling laws involving only a few nondimensional parameters. Finally, we overcome some of the limitations of hydraulic theory by performing a more detailed, time-resolved, three-dimensional analysis of the energetics of the wave, intermittent and turbulent regimes. We identify and quantify the sources and sinks of energy in each regime, and identify some of the structures responsible for viscous energy dissipation and mixing. We also suggest possible future directions for the present work given recent progress in the literature.

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38

Etebari, Ali. « Wall shear measurements in arterial flows ». Diss., Virginia Tech, 2006. http://hdl.handle.net/10919/27326.

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Cardiovascular disease is responsible for the majority of morbidity and mortality in the United States. Physiologically healthy flow rarely displays turbulent behavior, thereby maintaining normal shear levels. The presence of vortical flow structures, however, alters the hemodynamical characteristics within the system, which has significant effect upon shear stress (SS) and wall shear stress (WSS) levels, as well as particle residence times. The relationship between these hemodynamic parameters and vascular injury response is of great relevance to understanding the cardiovascular disease process. In this work, new methods and algorithms are developed and presented for resolving, both globally and locally, the spatial and temporal variations of shear stress (SS) and WSS for in vitro models of the human cardiovascular system. Advancements in global measurements are based on improving the accuracy of SS and WSS estimation from time-resolved Digital Particle Image Velocimetry (DPIV) velocity measurements. A new velocity derivative method, the fourth-order noise-optimized compact-Richardson implicit scheme, has been developed, overcoming the obstacle of minimizing both the bias and random error in temporal/spatial derivative estimations. The resulting error is on the same order as the velocity measurement error for global measurements which results in an order of magnitude accuracy improvement. The method has been extended to WSS measurements, and combined with a new method of mirroring/reflecting a flow field over its boundary in order to achieve higher-order estimation. For moving boundaries an edge detection cross-correlation algorithm has been developed and characterized, yielding sub-pixel accuracy in measuring dynamic wall position prior to estimating WSS. An original microelectromechanical system (MEMS) WSS sensor capable of delivering high sensitivity, frequency response and accurate WSS measurements has been developed and characterized in this work.
Ph. D.

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39

Ciochetto, David S. « Analysis of Three Dimensional Turbulent Shear Flow Experiments with Respect to Algebraic Modeling Parameters ». Thesis, Virginia Tech, 1997. http://hdl.handle.net/10919/36808.

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The extension of the theory for two dimensional turbulent boundary layers into three dimensional flows has met with limited success. The failure of the extended models is attributed to the anisotropy of the turbulence. This is seen by the turbulent shear stress angle lagging the flow gradient angle and by the behavior of the Reynolds shear stresses lagging that of the mean flow. Transport equations for the turbulent shear stresses were proposed to be included in a modeling effort capable of accounting for the lags seen in the flow. This study is aimed at developing algebraic relationships between the various Reynolds-averaged terms in these modeling equations. Particular emphasis was placed on the triple products that appear in the transport equations. Eleven existing experimental data sets were acquired from the original authors and re-examined with respect to developed and existing parameters. A variety of flow geometries were collected for comparison. Emphasis was placed on experiments that included all six components of the Reynolds stress tensor and triple products. Parameters involving the triple products are presented that appear to maintain a relatively constant value across regions of the boundary layer. The variation of these parameters from station to station and from flow to flow is discussed. Part of this study was dedicated to parameters that were previously introduced, but never examined with respect to the data that was collected. Results of these parameters are presented and discussed with respect to agreement or disagreement with the previous results. The parameters presented will aid in the modeling of three dimensional turbulent boundary layers especially with models that employ the transport equations for the Reynolds stresses.
Master of Science

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40

Lazeroms, Werner. « Explicit algebraic turbulence modelling in buoyancy-affected shear flows ». Licentiate thesis, KTH, Turbulens, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-122468.

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Turbulent flows affected by buoyancy forces occur in a large amount of applica-tions, from heat transfer in industrial settings to the effects of stratification inEarth’s atmosphere. The two-way coupling between the Reynolds stresses andthe turbulent heat flux present in these flows poses a challenge in the searchfor an appropriate turbulence model. The present thesis addresses this issueusing the class of explicit algebraic models. Starting from the transport equations for the Reynolds stresses and the tur-bulent heat flux, an explicit algebraic framework is derived for two-dimensionalmean flows under the influence of buoyancy forces. This framework consistsof a system of 18 linear equations, the solution of which leads to explicit ex-pressions for the Reynolds-stress anisotropy and a scaled heat flux. The modelis complemented by a sixth-order polynomial equation for a quantity relatedto the total production-to-dissipation ratio of turbulent kinetic energy. Sinceno exact solution to such an equation can be found, various approximationmethods are presented in order to obtain a fully explicit algebraic model. Several test cases are considered in this work. Special attention is given tothe case of stably stratified parallel shear flows, which is also used to calibratethe model parameters. As a result of this calibration, we find a critical Richard-son number of 0.25 in the case of stably stratified homogeneous shear flow,which agrees with theoretical results. Furthermore, a comparison with directnumerical simulations (DNS) for stably stratified channel flow shows an excel-lent agreement between the DNS data and the model. Other test cases includeunstably stratified channel flow and vertical channel flow with either mixed con-vection or natural convection, and a reasonably good agreement between themodel and the scarcely available, low-Reynolds-number DNS is found. Com-pared to standard eddy-viscosity/eddy-diffusivity models, an improvement inthe predictions is observed in all cases. For each of the aforementioned test cases, model coefficients and additionalcorrections are derived separately, and a general formulation has yet to be given.Nevertheless, the results presented in this thesis have the potential of improvingthe prediction of buoyancy-affected turbulence in various application areas.

QC 20130530

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Zhu, Ying. « Modelling and calculation for shear-driven rotating turbulence, with multiscale and directional approach ». Thesis, Lyon, 2019. http://www.theses.fr/2019LYSEC002/document.

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Les écoulements cisaillés en rotation sont fréquents en ingénierie — par exemple en turbo- machines et dans la production d’énergie hydraulique — et en géophysique et astrophysique. L’étude de leurs propriétés de stabilité en lien avec la production de turbulence est donc essentielle. Dans la présente étude, nous ne considérons pas d’éventuels effets inhomogènes, et nous nous concentrons sur la complexité de la dynamique anisotrope, qui ne peut se représenter facilement par les seuls modèles statistiques en un point. La thèse porte donc sur l’étude des propriétés de la turbulence homogène anisotrope (HAT) avec champ moyen uniforme et effet Coriolis, à l’aide de modèles statistiques en deux points. Un modèle orig- inal est proposé qui permet de prédire la dynamique de la turbulence cisaillée en rotation, et sépare les effets de déformation linéaire de la dynamique turbulente non linéaire, afin de proposer un traitement adapté pour chaque contribution. Le modèle proposé porte sur les équations qui régissent l’évolution du tenseur spectral du second ordre des corrélations de vitesse en deux points. Il permet d’aborder les gradients de vitesse moyenne arbitraires, avec ou sans rotation d’ensemble du système. L’effet direct linéaire des gradients moyens est exact dans le modèle, alors que les effets non linéaires con- stitués des corrélations d’ordre trois en deux points sont fermés par un modèle anisotrope de type EDQNM. Dans ce modèle de fermeture, l’anisotropie est restreinte à un développe- ment tronqué en termes d’harmoniques angulaires d’ordre bas Mons et al. (2016). Notre nouveau modèle est validé pour le régime linéaire par comparaison à une solution trés pré- cise de distorsion rapide visqueuse (vRDT) dans plusieurs cas de cisaillement: stabilisant, déstabilisant ou neutre. Le modèle diffère des approches de simulation numérique directe (DNS) pseudo-spectrale pour les écoulements cisaillés proposées par Rogallo (1981) en ingénierie et par Lesur & Longaretti (2005) en astrophysique, en ce que l’opérateur de convection n’est pas résolu en suivant les courbes caractéristiques moyennes spectrales ou physiques, mais grâce à un schéma original de type différences finies d’ordre élevé qui permet de calculer les dérivées ∂ i iv par rapport au vecteur d’onde k. On évite ainsi la déformation du maillage et l’obligation de remailler, ce qui autorise l’obtention aisée des harmoniques angulaires à chaque instant, grâce au fait que l’espace physique ou spectral n’est pas déformé. La capacité de prédiction de cette nouvelle approche est significativement améliorée par rapport au modèle de Mons et al. (2016), pour lequel la solution linéaire peut être remise en cause à grand temps d’évolution, particulièrement pour le cas non tournant. Le nouveau modèle est suffisamment universel puisqu’il est implémenté pour plusieurs cas de gradients de vitesse moyenne compatibles avec l’approximation homogène. Les validations ont notamment été réalisées dans des cas de déformation plane. Pour la turbulence cisaillée, dont la modélisation est demeurée jusqu’à présent un point dur des approches en un point et aussi de l’approche en deux points de Mons, nous proposons une version adaptée de notre modèle en deux points, en l’hybridant avec un modèle de retour à l’isotropie proposé par Weinstock (2013). Ce nouveau modèle hybride pour la turbulence cisaillée fournit des résultats extrêmement satisfaisants
Stability and turbulence in rotating shear flows is essential in many contexts ranging from engineering—as in e.g. turbomachinery or hydraulic energy production—to geophysics and astrophysics. Apart from inhomogeneous effects which we discard in the present study, these flows are complex because they involve an anisotropic dynamics which is difficult to represent at the level of one-point statistics. In this context, the properties of these flows, such as scale-by-scale anisotropy or turbulent cascade can be studied via two-point statistical models of Homogeneous Anisotropic Turbulence (HAT), in which the distorting mean flow is represented by uniform mean velocity and density gradients, and by body forces as the Coriolis one. The context of HAT can be relevant for flows in a plane channel with spanwise rotation, or for a Taylor-Couette flow. We propose a new model for predicting the dynamics of homogeneous sheared rotating turbulence. The model separates linear distortion effects from nonlinear turbulent dynamics, so that each contribution can be treated with an adapted model. Our model deals with equations governing the spectral tensor of two-point second-order velocity correlations, and is developed for arbitrary mean velocity gradients with or with- out system rotation. The direct linear effect of mean gradients is exact in our model, whereas nonlinear effects come from two-point third-order correlations which are closed by an anisotropic EDQNM model. In the closure, the anisotropy is restricted to an expansion in terms of low-degree angular harmonics (Mons et al., 2016). The present model has been validated in the linear regime, by comparison to the accurate solution of viscous Rapid Distortion Theory (vRDT), in several cases, stabilizing, destabilizing or neutral. In contrast with pseudo-spectral DNS adapted to shear flow by Rogallo (1981) in en- gineering and by Lesur & Longaretti (2005) in astrophysics, the advection operator is not solved by following characteristic lines in spectral or physical space, but by an original high- order finite-difference scheme for calculating derivatives ∂ i with respect to the wave vector k. One thus avoids mesh deformation and remeshing, thus one can easily extract angular ii harmonics at any time since physical or spectral space are not distorted. With this new approach, we are able to improve the prediction of the previous model by Mons et al. (2016), in which the linear resolution is questioned at large time, especially in the case without rotation. The proposed new model is versatile since it is implemented for several cases of mean velocity gradients consistent with the homogeneity approximation. Validations have been done for several cases of plane deformations. In the case of sheared turbulence, whose modelling resists most one-point approaches and even the two-point model by Mons, we propose an adaptation of our two-point model in a new hybrid model, in which return-to- isotropy is explicitly introduced in the guise of Weinstock (2013)’s model. Predictions of the new hybrid model are extremely good

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Ramesh, Chandra D. S. « Turbulent Mixed Convection ». Thesis, Indian Institute of Science, 2000. https://etd.iisc.ac.in/handle/2005/236.

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Turbulent mixed convection is a complicated flow where the buoyancy and shear forces compete with each other in affecting the flow dynamics. This thesis deals with the near wall dynamics in a turbulent mixed convection flow over an isothermal horizontal heated plate. We distinguish between two types of mixed convection ; low-speed mixed convection (LSM) and high-speed mixed convection (HSM). In LSM the entire boundary layer, including the near-wall region, is dominated by buoyancy; in HSM the near-wall region, is dominated by shear and the outer region by buoyancy. We show that the value of the parameter (* = ^ determines whether the flow is LSM or HSM. Here yr is the friction length scale and L is the Monin-Obukhov length scale. In the present thesis we proposed a model for the near-wall dynamics in LSM. We assume the coherent structure near-wall for low-speed mixed convection to be streamwise aligned periodic array of laminar plumes and give a 2d model for the near wall dynamics, Here the equation to solve for the streamwise velocity is linear with the vertical and spanwise velocities given by the free convection model of Theerthan and Arakeri [1]. We determine the profiles of streamwise velocity, Reynolds shear stress and RMS of the fluctuations of the three components of velocity. From the model we obtain the scaling for wall shear stress rw as rw oc (UooAT*), where Uoo is the free-stream velocity and AT is the temperature difference between the free-stream and the horizontal surface.A similar scaling for rw was obtained in the experiments of Ingersoll [5] and by Narasimha et al [11] in the atmospheric boundary layer under low wind speed conditions. We also derive a formula for boundary layer thickness 5(x) which predicts the boundary layer growth for the combination free-stream velocity Uoo and AT in the low-speed mixed convection regime.

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43

Ramesh, Chandra D. S. « Turbulent Mixed Convection ». Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/236.

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Turbulent mixed convection is a complicated flow where the buoyancy and shear forces compete with each other in affecting the flow dynamics. This thesis deals with the near wall dynamics in a turbulent mixed convection flow over an isothermal horizontal heated plate. We distinguish between two types of mixed convection ; low-speed mixed convection (LSM) and high-speed mixed convection (HSM). In LSM the entire boundary layer, including the near-wall region, is dominated by buoyancy; in HSM the near-wall region, is dominated by shear and the outer region by buoyancy. We show that the value of the parameter (* = ^ determines whether the flow is LSM or HSM. Here yr is the friction length scale and L is the Monin-Obukhov length scale. In the present thesis we proposed a model for the near-wall dynamics in LSM. We assume the coherent structure near-wall for low-speed mixed convection to be streamwise aligned periodic array of laminar plumes and give a 2d model for the near wall dynamics, Here the equation to solve for the streamwise velocity is linear with the vertical and spanwise velocities given by the free convection model of Theerthan and Arakeri [1]. We determine the profiles of streamwise velocity, Reynolds shear stress and RMS of the fluctuations of the three components of velocity. From the model we obtain the scaling for wall shear stress rw as rw oc (UooAT*), where Uoo is the free-stream velocity and AT is the temperature difference between the free-stream and the horizontal surface.A similar scaling for rw was obtained in the experiments of Ingersoll [5] and by Narasimha et al [11] in the atmospheric boundary layer under low wind speed conditions. We also derive a formula for boundary layer thickness 5(x) which predicts the boundary layer growth for the combination free-stream velocity Uoo and AT in the low-speed mixed convection regime.

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Sreedhar, Madhu K. « Large eddy simulation of turbulent vortices and mixing layers ». Diss., This resource online, 1994. http://scholar.lib.vt.edu/theses/available/etd-06062008-163324/.

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Shen, Peiquing. « Numerical study of fluid particle dispersion in homogeneous turbulent shear flow ». Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/12486.

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Johnstone, Henry Webb 1956. « CONFINED JET-INDUCED MIXING AT A DENSITY INTERFACE (TURBULENT, SHEAR FLOW) ». Thesis, The University of Arizona, 1987. http://hdl.handle.net/10150/292003.

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Baum, Bryan Alan. « The extension of rapid distortion theory to stratified shear flows ». Diss., Georgia Institute of Technology, 1989. http://hdl.handle.net/1853/25971.

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48

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.

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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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49

Deshpande, Kiran B. « Studies On Phase Inversion ». Thesis, Indian Institute of Science, 2001. https://etd.iisc.ac.in/handle/2005/285.

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Agitated dispersions of one liquid in another immiscible liquid are widely used in chemical industry in operations such as liquid-liquid extraction, suspension polymerisation, and blending of polymers. When holdup of the dispersed phase is increased, in an effort to increase the productivity, at a critical holdup, the dispersed phase catastrophically becomes the continuous phase and vice versa. This phenomenon is known as phase inversion. Although the inversion phenomenon has been studied off and on over the past few decades, the mechanism of phase inversion (PI) has yet not become clear. These studies have however brought out many interesting aspects of PI, besides unravelling the effect of physical and operational variables on PL Experiments show that oil-in-water (o/w) and water-in-oil (w/o) dispersions behave very differently, e.g water drops in w/o dispersions contain oil droplets in them, but oil drops in o/w dispersions contain none, dispersed phase hold up at which inversion occurs increases with agitation speed for w/o dispersions but decreases for o/w dispersions. A common feature of both types of dispersions however is that as agitation speed is increased to high values, inversion holdups reach a constant value. A further increase in agitation speed does not change inversion hold up. Although this finding was first reported a long time ago, the implications it may have not received any attentions. In fact, the work reported in the literature since then does not even mention it. The present work shows that this finding has profound implications. Starting with the finding that at high agitation speed inversion hold up does not change with agitation speed, the present work shows that inversion hold up also does not change with agitator diameter, type of agitator and vessel diameter. In these experiments, carried out in agitated vessel, energy was introduced as a point source. The experiments carried out with turbulent flow in annular region of two coaxial cylinders, inner one rotating, in which energy is introduced nearly uniformly throughout the system, show that the inversion holdup remains unchanged. These results indicate that constant values of inversion holdups for a given liquid-liquid systems (o/w and w/o) are properties of the liquid-liquid systems alone, independent of geometrical and operational parameters. A new hypothesis is proposed to explain the new findings. Phase inversion is considered to occur as a result of imbalance between breakup and coalescence of drops. Electrolytes, which affect only coalescence of drops, were therefore added to the system to investigate the effect of altering coalescence of drops on phase inversion. The experiments performed in the presence of electrolyte KI at various concentrations indicate that addition of electrolyte increases the inversion holdup for both o/w and w/o dispersions for three types of systems: non polar-water, polar-water and immiscible organic-organic. Higher the concentration of electrolyte used, higher was the holdup required for phase inversion. These findings indicate that while the addition of electrolyte increases coalescence of drops in lean dispersions, it has exactly opposite effect on imbalance of breakage and coalescence of drops at high holdups near phase inversion point. The opposite effect of electrolytes in lean and concentrated dispersions could be explained qualitatively, but only in part in the light of a new theory, involving multi-particle interactions. The phase inversion phenomenon is quantified in a simple manner by testing the breakage and coalescence rate expressions available in literature. It has been found that, equilibrium drop size (where breakage and coalescence events are in dynamic equilibrium) approaches infinity near phase inversion holdup which is not an ex perimentally observed fact. To capture the catastrophic nature of phase inversion, two steady state approach is proposed. The two steady states namely the stable steady state and unstable steady state, are achieved by modifying the expression for coalescence frequency on the basis of (i) shear coalescence mechanism and, (ii) recognising the fact that at high dispersed phase holdup the droplets are already in contact with each other at all times and hence rendering the second order coales cence process to a first order one. Using two steady states approach, catastrophic phase inversion is shown to occur at finite drop size.

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50

Deshpande, Kiran B. « Studies On Phase Inversion ». Thesis, Indian Institute of Science, 2001. http://hdl.handle.net/2005/285.

Texte intégral

Résumé :

Agitated dispersions of one liquid in another immiscible liquid are widely used in chemical industry in operations such as liquid-liquid extraction, suspension polymerisation, and blending of polymers. When holdup of the dispersed phase is increased, in an effort to increase the productivity, at a critical holdup, the dispersed phase catastrophically becomes the continuous phase and vice versa. This phenomenon is known as phase inversion. Although the inversion phenomenon has been studied off and on over the past few decades, the mechanism of phase inversion (PI) has yet not become clear. These studies have however brought out many interesting aspects of PI, besides unravelling the effect of physical and operational variables on PL Experiments show that oil-in-water (o/w) and water-in-oil (w/o) dispersions behave very differently, e.g water drops in w/o dispersions contain oil droplets in them, but oil drops in o/w dispersions contain none, dispersed phase hold up at which inversion occurs increases with agitation speed for w/o dispersions but decreases for o/w dispersions. A common feature of both types of dispersions however is that as agitation speed is increased to high values, inversion holdups reach a constant value. A further increase in agitation speed does not change inversion hold up. Although this finding was first reported a long time ago, the implications it may have not received any attentions. In fact, the work reported in the literature since then does not even mention it. The present work shows that this finding has profound implications. Starting with the finding that at high agitation speed inversion hold up does not change with agitation speed, the present work shows that inversion hold up also does not change with agitator diameter, type of agitator and vessel diameter. In these experiments, carried out in agitated vessel, energy was introduced as a point source. The experiments carried out with turbulent flow in annular region of two coaxial cylinders, inner one rotating, in which energy is introduced nearly uniformly throughout the system, show that the inversion holdup remains unchanged. These results indicate that constant values of inversion holdups for a given liquid-liquid systems (o/w and w/o) are properties of the liquid-liquid systems alone, independent of geometrical and operational parameters. A new hypothesis is proposed to explain the new findings. Phase inversion is considered to occur as a result of imbalance between breakup and coalescence of drops. Electrolytes, which affect only coalescence of drops, were therefore added to the system to investigate the effect of altering coalescence of drops on phase inversion. The experiments performed in the presence of electrolyte KI at various concentrations indicate that addition of electrolyte increases the inversion holdup for both o/w and w/o dispersions for three types of systems: non polar-water, polar-water and immiscible organic-organic. Higher the concentration of electrolyte used, higher was the holdup required for phase inversion. These findings indicate that while the addition of electrolyte increases coalescence of drops in lean dispersions, it has exactly opposite effect on imbalance of breakage and coalescence of drops at high holdups near phase inversion point. The opposite effect of electrolytes in lean and concentrated dispersions could be explained qualitatively, but only in part in the light of a new theory, involving multi-particle interactions. The phase inversion phenomenon is quantified in a simple manner by testing the breakage and coalescence rate expressions available in literature. It has been found that, equilibrium drop size (where breakage and coalescence events are in dynamic equilibrium) approaches infinity near phase inversion holdup which is not an ex perimentally observed fact. To capture the catastrophic nature of phase inversion, two steady state approach is proposed. The two steady states namely the stable steady state and unstable steady state, are achieved by modifying the expression for coalescence frequency on the basis of (i) shear coalescence mechanism and, (ii) recognising the fact that at high dispersed phase holdup the droplets are already in contact with each other at all times and hence rendering the second order coales cence process to a first order one. Using two steady states approach, catastrophic phase inversion is shown to occur at finite drop size.

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Thèses sur le sujet « Turbulent shear flows »

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