Toward an understanding of fundamental mechanisms in transitional and turbulence flow control

Turbulence is an emergent phenomenon found throughout nature and engineering, alike. It plays a vital role in the aquatic locomotion of organisms, scalar mixing, fluid transport, shipping and transportation, and even the flow of biological fluids in the human body. Therefore, it is of utmost importance in both a practical and engineering sense to better understand turbulence with the goal of better controlling it. This dissertation focuses broadly on better understanding the underlying mechanisms behind wall-bounded turbulent flows, with an emphasis on exploiting those mechanisms for turbulence flow control. We developed a numerical simulation to study the effect of slip surfaces on the dynamics of transitional and turbulent flows. Slip surfaces were found to promote the return of a turbulent flow to the laminar state. They also impact the transition to and from turbulence depending upon flow structure. The simulation was extended to study composite drag reduction of slip surfaces and polymer additives. An additive effect was observed due to the distinct drag reduction mechanisms of each individual method. Using simulations and experiments, intermittent dynamics of turbulent flows were investigated which manifest in the form of low-drag events: events described by low levels of skin friction and three-dimensionality. Because these events exhibit desirable traits, they are targets for flow control techniques, and their characterization will hopefully inform more efficient flow control methods. The minimal flow unit (MFU) approach to simulating turbulent flows was first popularized by the seminal 1991 work of Jiménez and Moin. Since then, the technique has become a powerful tool in teasing out underlying mechanisms of turbulent flows due to its ability to resolve the many scales in turbulence. While the technique faithfully captures the dynamics of most flows, there are questions surrounding larger Reynolds numbers. We investigate the efficacy of MFUs in promoting healthy turbulence and show that additional criteria should be put in place when simulating higher Reynolds number flows with MFUs. Adviser: Jae Sung Par

Intermittency in Transitional Shear Flows

This book contains original peer-reviewed articles written by some of the most prominent international physicists active in the field of hydrodynamics. The topic is entirely devoted to the study of the transitional regimes of incompressible viscous flow found at the onset of turbulent flows. Nine articles written for this 2020 Special Issue of the journal Entropy (MDPI) have been gathered at the crossroads of fluid mechanics, statistical physics, complexity theory, and applied mathematics. They include experimental, analytic, and computational material of an academic level that has not been published anywhere else

Multiphase wall-bounded turbulence

In many geophysical situations and in all industrial applications, turbulent flows are wall-bounded. Many of these flows are multi-phase, i.e. flows consisting of one or multiple inclusions. The current understanding of these flows is still limited and this makes it important to study them. In this thesis we study these wall-bounded multi-phase flows in two canonical systems: Taylor-Couette flow (TC) and Rayleigh-Bénard convection (RBC). In this work we used spherical and cylindrical particles to investigate if we have reduced skin friction similar to bubbly drag reduction. The global torque measurements showed that these particles barely alter the drag, even at very large particle volume fractions. Surprisingly, we found a preferential alignment for the cylindrical particles with respect to the inner cylinder wall. Using oil and water we are able to create deformable inclusions. Increasing the oil volume fraction over a critical point results in phase inversion with water droplets in oil. In this regime we found drag reduction due to the large water droplets in the flow. This is confirmed with in-situ microscopic imaging. In the last two chapters of this thesis we study the effect of non-homogeneous boundaries in both TC and RBC. Using bands of sandgrain roughness we were able to control the secondary flows in TC. This means that for example roughness like barnacles on the hull of a ship can induce secondary flows that push air bubbles away and thereby, reducing the drag reducing effect

Direct Pore Level Simulation of Heat Transfer in Open Cell Reticulated Porous Ceramics

The project involved in studying the fluid transport, heat and mass transport inside various ceramic porous inserts by Direct Pore Level Simulations (DPLS). The geometric grid data required for the simulations are reconstructed from the computer tomographic scan images of the real porous media. The simulation results are used to study the influence of the structural properties of porous media on the fluid flow, heat transfer and mass transfer

New magnetic stimulation routes with magnetic nanoparticles from process intensification in chemical engineering

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2012-2013.Les nanoparticules magnétiques (NPM) suscitent un vif intérêt dans plusieurs branches de l’ingénierie et de la recherche. En effet, la taille de ces dernières ainsi que leur propriétés magnétiques lorsqu’en suspension permettent leur manipulation à distance en utilisant des champs magnétiques externes appropriés. Cela ouvre la voie à l’activation de fonctionnalités supplémentaires lorsqu’ancrées à des catalyseurs métalliques, des enzymes ou des agents thérapeutiques. Conséquemment, les NPM ont été impliquées au sein de plusieurs applications dans lesquelles le mélange à l’échelle microscopique est une problématique importante, par exemple dans les réactions catalytiques, la séparation et l’administration de médicaments. Le présent travail de thèse explore l’utilisation de NPM en tant que dispositifs nanométriques pour manipuler le mélange à l’échelle microscopique lorsque le système complet est soumis à des champs magnétiques. Toutes les expérimentations ont été menées à l’intérieur d’un électro-aimant à bobines tubulaire statique possédant deux pôles et trois phases. Ce dernier génère des champs magnétiques rotatifs uniformes (CMR), des champs magnétiques oscillatoires (CMO) ainsi que des champs magnétiques stationnaires (CMS). En premier lieu, une technique de mélange dans laquelle un CMR transforme des NPM en agitateurs nanométriques créant de petits tourbillons dans la phase liquide est présentée. L’utilisation de cette technique permet l’augmentation du coefficient de diffusion de l’eau quiescente dans une cellule de diffusion statique jusqu’à 200 fois. Les études systématiques des paramètres d’opération révèlent que l’ampleur de l’augmentation dépend de la fraction volumique en NPM ainsi que de la force et de la fréquence du champ magnétique. En second lieu, un écoulement convectif est utilisé afin de comprendre l’effet du couple hydrodynamique sur le comportement des NPM en champs magnétiques. Des tests de distribution de temps de séjour par impulsion sont effectués avec et sans champ magnétique dans le but d’examiner la dispersion axiale d’un écoulement laminaire de Poiseuille à l’intérieur d’un tube capillaire (Tests de dispersion de Taylor). Les résultats obtenus démontrent que le mélange latéral au long du tube est favorisé en présence de NPM et d’un champ magnétique. De plus, l’effet hydrodynamique observé de ce mélange latéral sur le profil de vitesse laminaire est interprété comme provenant d’une approche d’un profil de vitesse plat similaire à celui d’un écoulement piston. À l’aide de la même technique, l’effet des CMO et des CMS sur la dispersion de Taylor et sur le profil de vitesse laminaire est aussi examiné en écoulement capillaire. Alors que les CMO n’induisent pas de mélange nano-convectif dans le capillaire et ont un impact négligeable sur la dispersion axiale, les CMS pour leur part, détériorent le mélange latéral du traceur et créent des profils de vitesse déviant de la forme parabolique vers une forme plus saillie. Une discussion détaillée de la vorticité du fluide en fonction de l’orientation du champ magnétique est aussi présentée. Finalement, un écoulement multiphasique est étudié en ciblant le transfert de matière gaz-liquide entre des bulles de Taylor d’oxygène et la phase liquide, composée d’une solution diluée de NPM, à l’intérieur de tubes capillaires soumis à des CMR, des CMO et des CMS. Les résultats indiquent que les NPM qui tournent sous l’action d’un CMR améliorent le mélange dans le film lubrificateur qui entoure les bulles de Taylor comme cela est révélé par une augmentation mesurable du kLa. À l’opposé, les CMS immobilisent les NPM, menant à des taux de transfert de matière systématiquement plus faibles alors que les CMO n’ont pas d’effet détectable sur le coefficient de transfert de matière. Par ailleurs, l’interaction entre le couple magnétique et le couple hydrodynamique nécessaire pour dominer la direction de rotation des NPM est tirée de ces résultats.Magnetic nanoparticles (MNPs) have attracted significant interest in diverse areas of engineering and research. Particle size and magnetic properties of suspended MNPs in a suspension allow their manipulation at a distance using appropriate external magnetic fields. In particular by enabling additional functionality in forms anchored to metal catalysts, enzymes or therapeutic drug agents. Owing to this feature, MNPs have been involved in many applications where mixing in micro-scale is also a critical issue, e.g., catalytic reaction, separation and drug delivery. This thesis explores MNPs as nano-scale devices to manipulate mixing in micro-scale when the whole system is subject to magnetic fields. All the experiments were performed in tubular two-pole, three-phase stator winding magnet, generating uniform rotating magnetic field (RMF), oscillating magnetic field (OMF) and stationary magnetic field (SMF). Initially, we present a mixing technique in which a RMF converts MNPs into nano-stirrers generating small vortices in liquid phase. Using this technique, self-diffusion coefficient of motionless water in a static diffusion cell was intensified up to 200 folds. Systematic studies of operating parameters revealed that the extent of enhancement depends on MNP volume fraction, and strength and frequency in magnetic field. In order to understand the effect of hydrodynamic torque on the MNPs behavior under magnetic fields, convective flow was also included. As such, axial dispersion of pressure-driven laminar Poiseuille flows in a capillary tube (Taylor dispersion test) was examined through a series of impulse (residence time distribution) RTD tests with and without RMF. This resulted in lateral mixing along the channel that was promoted relative to that in absence of MNPs or magnetic field. Moreover, we interpreted the observed hydrodynamic effects of such lateral mixing on laminar velocity profile as resulting from an approach to plug flow-like flat velocity profile. Using the same technique, the effect of OMF and SMF on Taylor dispersion and laminar velocity profile was examined in capillary flows. OMF did not induce nano-convective mixing in the capillary and had negligible impact on axial dispersion. On the contrary, SMF deteriorated lateral mixing of solute tracer and led to velocity profiles deviating from parabolic shape towards more protruded ones. A detailed discussion of magnetic field orientation versus fluid vorticity vector was presented. Finally a multiphase flow case concerned gas-liquid mass transfer from oxygen Taylor bubbles to the liquid in capillaries which was studied using dilute concentration of MNPs as the liquid phase under RMF, OMF and SMF. Experimental results implied that spinning MNPs under RMF improved mixing in the lubricating film that surrounds Taylor bubbles which reflected in a measurable enhancement of kLa. On the contrary, SMF pinned MNPs leading to systematically degraded gas-liquid mass transfer rates whereas axial oscillating magnetic field had no detectable effects on the mass transfer coefficient. Moreover, interaction between magnetic torque and hydrodynamic torque to dominate MNP spin direction was conceived from these results

The influence of superhydrophobic surfaces on near-wall turbulence

Superhydrophobic surfaces are able to entrap gas pockets in-between surface roughness elements when submerged in water. These entrapped gas pockets give these surfaces the potential to reduce drag due to the overlying flow being able to locally slip over the gas pockets, resulting in a mean slip at the surface. This thesis investigates the different effects that slip and the texturing of the surface have on turbulence over superhydrophobic surfaces. It is shown that, after filtering out the texture-induced flow, the background, overlying turbulence experiences the surface as a homogeneous slip boundary condition. For texture sizes, expressed in wall units, up to $L^+ \lesssim 20$ the only effect of the surface texture on the overlying flow is through this surface slip. The direct effect of slip does not modify the dynamics of the overlying turbulence, which remains canonical and smooth-wall-like. In these cases the flow is governed by the difference between two virtual origins, the virtual origin of the mean flow and the virtual origin experienced by the overlying turbulence. Streamwise slip deepens the virtual origin of the mean flow, while spanwise slip acts to deepen the virtual origin perceived by the overlying turbulence. The drag reduction is then proportional to the difference between the two virtual origins, reminiscent of drag reduction using riblets. The validity of slip-length models to represent textured superhydrophobic surfaces can resultantly be extended up to $L^+ \lesssim 20$. However, for $L^+ \gtrsim 25$ a non-linear interaction with the texture-coherent flow alters the dynamics of the background turbulence, with a reduction in coherence of large streamwise lengthscales. This non-linear interaction causes an increase in Reynolds stress up to $y^+ \lesssim 25$, and decreases the obtained drag reduction compared to that predicted from homogeneous slip-length models.Funding from Engineering and Physical Sciences Research Counci

experimental studies in non-equilibrium physics

This work is a collection of three experiments aimed at studyingdifferent facets of non-equilibrium dynamics. Chapter I concernsstrongly compressible turbulence, which turns out to be verydifferent from incompressible turbulence. The focus is on thedispersion of contaminants in such a flow. This type of turbulencecan be studied, at very low mach number, by measuring the velocityfields of particles that float on a turbulently stirred body ofwater. It turns out that in the absence of incompressibility, theturbulence causes particles to cluster rather than to disperse.The implications of the observations are far reaching and includethe transport of pollutants on the oceans surface, phytoplanktongrowth, as well as industrial applications.Chapter II deals with the effects of polymer additives on dragreduction and turbulent suppression, a well-known phenomenon thatis not yet understood. In an attempt to simplify the problem, theeffects of a polymer additive were investigated in a vortex streetformed in a flowing soap film. Measurements suggest that anincrease in elongational viscosity is responsible for asubstantial reduction in periodic velocity fluctuations. Thisstudy also helps to illuminate the mechanism responsible forvortex separation in the wake of a bluff body.Chapter III describes an experiment designed to test a theoreticalapproach aimed at generalizing the classical fluctuationdissipation theorem (FDT). This theorem applies to systems drivenonly slightly away from thermal equilibrium, whereas ours, aliquid crystal undergoing electroconvection, is so stronglydriven, that the FDT does not apply. Both theory and experimentfocus on the flux in global power fluctuations. Physicallimitations did not permit a direct test of the theory, however itwas possible to establish several interesting characteristics ofthe system: the source of the fluctuations is the transient defectstructures that are generated when the system is driven hard. Itis found that the power fluctuations are spatially uncorrelated,but strongly correlated in time and even quasi-periodic