The present study proposes to use a Conrad probe for transient growth study by measuring the two-dimensional velocity behind an array of roughness elements in the Blasius boundary layer. A look-up table approach is proposed to increase the accuracy of the data reduction process at low velocities, based on the results of the calibration performed in a round jet. A velocity correction method is proposed to minimize the errors due to high velocity shear and wall-proximity in the Blasius boundary layer by comparing Conrad probe results to previous hot-wire data. Measurements of the steamwise velocity perturbation obtained with the Conrad probe agree with previous studies. The measured spanwise perturbation confirms the transport process suggested by simulation. The results show that the perturbation amplitude increases by increasing both the freestream velocity and roughness elements height. However, the mechanism for changing the perturbation amplitude of influence differs for these two parameters.

The present study proposes to use a Conrad probe for transient growth study by measuring the two-dimensional velocity behind an array of roughness elements in the Blasius boundary layer. A look-up table approach is proposed to increase the accuracy of the data reduction process at low velocities, based on the results of the calibration performed in a round jet. A velocity correction method is proposed to minimize the errors due to high velocity shear and wall-proximity in the Blasius boundary layer by comparing Conrad probe results to previous hot-wire data. Measurements of the steamwise velocity perturbation obtained with the Conrad probe agree with previous studies. The measured spanwise perturbation confirms the transport process suggested by simulation. The results show that the perturbation amplitude increases by increasing both the freestream velocity and roughness elements height. However, the mechanism for changing the perturbation amplitude of influence differs for these two parameters.

No description available.

<p>Transition from laminar to turbulent flow has beentraditionally studied in terms of exponentially growingeigensolutions to the linearized disturbance equations.However, experimental findings show that transition may occuralso for parameters combinations such that these eigensolutionsare damped. An alternative non-modal growth mechanism has beenrecently identified, also based on the linear approximation.This consists of the transient growth of streamwise elongateddisturbances, mainly in the streamwise velocity component,called streaks. If the streak amplitude reaches a thresholdvalue, secondary instabilities can take place and provoketransition. This scenario is most likely to occur in boundarylayer flows subject to high levels of free-stream turbulenceand is the object of this thesis. Different stages of theprocess are isolated and studied with different approaches,considering the boundary layer flow over a flat plate. Thereceptivity to free-stream disturbances has been studiedthrough a weakly non-linear model which allows to disentanglethe features involved in the generation of streaks. It is shownthat the non-linear interaction of oblique waves in thefree-stream is able to induce strong streamwise vortices insidethe boundary layer, which, in turn, generate streaks by thelift-up effect. The growth of steady streaks is followed bymeans of Direct Numerical Simulation. After the streaks havereached a finite amplitude, they saturate and a new laminarflow, characterized by a strong spanwise modulation isestablished. Using Floquet theory, the instability of thesestreaks is studied to determine the features of theirbreakdown. The streak critical amplitude, beyond which unstablewaves are excited, is 26% of the free-stream velocity. Theinstability appears as spanwise (sinuous-type) oscillations ofthe streak. The late stages of the transition, originating fromthis type of secondary instability, are also studied. We foundthat the main structures observed during the transition processconsist of elongated quasi-streamwise vortices located on theflanks of the low speed streak. Vortices of alternating signare overlapping in the streamwise direction in a staggeredpattern.</p><p><strong>Descriptors:</strong>Fluid mechanics, laminar-turbulenttransition, boundary layer flow, transient growth, streamwisestreaks, lift-up effect, receptivity, free-stream turbulence,nonlinear mechanism, streak instability, secondary instability,Direct Numerical Simulation.</p> / QC 20100518

This work is concerned with the response of a Blasius boundary layer to dielectric-barrier-discharge (DBD) plasma actuators for the purpose of using these devices in bypass transition control. The plasma actuators consist of a spanwise-periodic array of high voltage electrodes, which are oriented to produce streamwise vortex pairs. The structure of actuator-induced streaks is measured using hot-wire anemometry over a streamwise distance of approximately 100 boundary layer thicknesses, and is decomposed into 4 spanwise Fourier modes. The modal content and corresponding streamwise growth characteristics are discussed for ten plasma actuator geometries over multiple excitation voltages and freestream velocities. Actuator power consumption was found to control the streak amplitude, whereas freestream velocity affected both amplitude and streamwise extent of the streaks. A common relationship between disturbance energy and power consumption was found among actuators of different dielectric thickness and similar electrode geometry.

This work is concerned with the response of a Blasius boundary layer to dielectric-barrier-discharge (DBD) plasma actuators for the purpose of using these devices in bypass transition control. The plasma actuators consist of a spanwise-periodic array of high voltage electrodes, which are oriented to produce streamwise vortex pairs. The structure of actuator-induced streaks is measured using hot-wire anemometry over a streamwise distance of approximately 100 boundary layer thicknesses, and is decomposed into 4 spanwise Fourier modes. The modal content and corresponding streamwise growth characteristics are discussed for ten plasma actuator geometries over multiple excitation voltages and freestream velocities. Actuator power consumption was found to control the streak amplitude, whereas freestream velocity affected both amplitude and streamwise extent of the streaks. A common relationship between disturbance energy and power consumption was found among actuators of different dielectric thickness and similar electrode geometry.

No description available.

Cette thèse vise à mettre en évidence les limites du contrôle passif en utilisant des éléments de rugosité miniaturisés. La topologie des écoulements induite par la présence d’une rugosité cylindrique et des générateurs de tourbillons miniaturisés a été étudiée pour analyser la dynamique pour des temps court et long. Différentes bifurcations supercritiques ont été examinées au moyen d’une analyse de stabilité globale. La bifurcation souscritique est déclenchée par des mécanismes de croissance transitoire de l'énergie ou par la réceptivité de modes globaux stables. Des structures deforçage optimal 3D sont extraites pour comprendre la distribution spatiale liée à la fréquence de résonance du système. La simulation numérique directe perturbée révèle le rôle central du mode global le moins stable dans les instationnarités non-linéaires observées. Une analyse détaillée des structures tourbillonnaires montre qu’elles sont principalement liées aux mécanismes linéaires sous-jacents. La principale caractéristique globale du mode propre est liée à la présence d’une zone de séparation en aval de la rugosité cylindrique. En utilisant des générateurs de tourbillon miniaturisés, cette zone de séparation est fortement diminuée et aucun mode global isolé est alors présent. La dynamique de l’écoulement se révèle être conduit non seulement par le nombre de Reynolds de rugosité et par son rapport d'aspect géométrique, mais aussi par le rapport entre la hauteur de la rugosité et l’epaisseur de couche limite. / This thesis aims at highlighting the limits of passive control by usingminiaturized roughness elements. The flow topology induced by the presence ofcylindrical roughness and miniaturized vertex generators has been studied to uncover asymptotic and short time dynamics. Supercritical bifurcations has been investigated by means of global stability analysis. Subcritical bifurcation are induced by transient growth of the energy or receptivity of stable global modes. 3D optimal forcing structures are extracted to figure out the spatial distribution linked to the resonant pulsation. Perturbed direct numerical simulation reveals the pivotal role of the less steady global mode in the non-linear unsteadiness. A detailed analysis of the flowstructures is provided and linked to the involved linear mechanisms. Global feature of the eigenmode are linked to the presence of the separation zone behind the cylindrical roughness. By using miniaturized vortex generators the separation zone is suppressed and no isolated global modes are present. The flow dynamics turns out to be driven not only by roughness Reynolds number and geometrical aspect ratio but also by the ratio between the roughness height and the boundary layer thickens.

In this thesis I investigate a number of problems in the nonlinear stability of density stratified plane Couette flow. I begin by describing the history of transient growth phenomena, and in particular the recent application of adjoint based optimisation to find nonlinear optimal perturbations and associated minimal seeds for turbulence, the smallest amplitude perturbations that are able to trigger transition to turbulence. I extend the work of Rabin et al. (2012) in unstratified plane Couette flow to find minimal seeds in both vertically and horizontally sheared stratified plane Couette flow. I find that the coherent states visited by such minimal seed trajectories are significantly altered by the stratification, and so proceed to investigate these states both with generalised Koopman mode analysis and by stratifying the self-sustaining process described by Waleffe (1997). I conclude with an introductory problem I considered that investigates the linear Taylor instability of layered stratified plane Couette flow, and show that the nonlinear evolution of the primary Taylor instability is not coupled to the form of the linearly unstable mode, in contrast to the Kelvin-Helmholtz instability, for example. I also include an appendix in which I describe joint work conducted with Professor Neil Balmforth of UBC during the 2015 WHOI Geophysical Fluid Dynamics summer programme, investigating stochastic homoclinic bifurcations.

Quand on augmente le nombre de Reynolds d’un écoulement, il passe progressivement d’un état ordonné, laminaire, à un état chaotique, turbulent. Lors de cette transition, on voit apparaître des structures hydrodynamiques ordonnées qui se désordonnent sous l’effet de déstabilisations successives. Ces structures peuvent poser problème lorsqu’on souhaite par exemple homogénéiser des suspensions car des particules peuvent s’y retrouver piégées et générer des zones sur ou sous-concentrées. La plupart des fluides naturels et industriels sont non newtoniens. Pour mieux comprendre et contrôler l’apparition des structures hydrodynamiques dans de tels fluides, nous nous sommes intéressés dans le cadre de cette thèse, aux mécanismes d’instabilités gouvernant la transition vers la turbulence de fluides complexes dans la configuration simplifiée de Taylor-Couette. Dans un premier volet théorique et numérique, les concepts d’instabilités hydrodynamiques ont été appliqués à l’écoulement de Taylor-Couette pour trois modèles rhéologiques : la loi de puissance, le modèle de Carreau et le modèle de Bingham. Deux rapports de rayons et différentes vitesses de rotation des deux cylindres ont été considérés. La déstabilisation de l’écoulement de Couette circulaire a été étudiée pour prédire l’apparition de structures telles que les rouleaux de Taylor-Couette aux temps longs. Une analyse aux temps courts a également été effectuée pour étudier les phénomènes de croissance transitoire dans l’écoulement. Dans un second volet expérimental, un dispositif de Taylor-Couette en plexiglas a été élaboré. Dans un premier temps, des mesures de couple avec un fluide newtonien (glycérol) ont été réalisées et comparées avec succès avec les résultats de la littérature. Des mesures rhéométriques ont ensuite été réalisées pour choisir des fluides répondant aux lois rhéologiques utilisées dans le volet théorique. Des visualisations ont été effectuées pour détecter l’apparition des instabilités et les conditions critiques ont été comparées aux prédictions de l’analyse linéaire de stabilité. Des mesures de vélocimétrie par image de particules (PIV) ont été aussi effectuées pour déterminer les champs de vitesse de l’écoulement. Ces différentes études ont permis de mettre en évidence le caractère sous-critique et l’apparition d’un écoulement chaotique engendrés par le caractère rhéofluidifiant des fluides. / When increasing the Reynolds number of a flow, it passes gradually from an ordered state, called laminar, to a chaotic state, called turbulent. During this transition, organized hydrodynamic structures appear which disorganize under successive destabilizations. These structures may cause problems for example in the relation to the homogenization of suspensions, because particles may be trapped in the structures, leading to under or over-concentrated zones. Furthermore, most of the fluids found in nature and in the industry are non-Newtonian. In order to better understand and control the apparition of hydrodynamic structures in such fluids, we focused during this PhD on the instability mechanisms governing the transition to turbulence of complex fluids in the simplified configuration of Taylor-Couette. In a theoretical and numerical approach, concepts of hydrodynamic instabilities were applied to the circular Couette flow for three rheological models: the power law, the Carreau and the Bingham models. Two radius ratios and different rotation velocities were considered for the cylinders. The destabilization of the circular Couette flow was studied to predict the apparition of structures such as Taylor vortex at long times. An analysis at short time was also performed to study the phenomena of transient growth. In an experimental approach, a Taylor-Couette device was designed. First, the torque on the inner cylinder was measured for a Newtonian fluid (glycerol) and successfully compared to previous results of the literature. Secondly, rheological measurements were conducted in order to select fluids showing rheological behaviors considered in the theoretical approach. Visualizations were then performed in order to detect the apparition of instabilities and the critical conditions were compared to the predicted by means of the linear stability analysis. Finally, PIV measurements provided velocity fields in the flow. These different studies evidenced a subcritical transition and chaotic flow due to the shear-thinning character of the fluids.