Transition in separation bubbles: physical mechanisms and passive control techniques

Abstract

A thesis subm itted to the Faculty of G raduate Studies and Research in partial fulfillment of the requirements for the degree of D o cto r o f P h ilo so p h y in Aerospace Engineering Direction du Patrimoine de I'edition Bien que ces formulaires aient inclus dans la pagination, il n'y aura aucun contenu manquant. i * i Canada Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.Dedicated to m y wife Rakhi and to m y daughter P riya E m m a.This thesis documents complementary experimental and com putational studies aimed at identifying th e physical mechanisms associated with lam inar-to-turbulent transition in separation bubbles, and at examining passive techniques for m anipulation of this process.Measurements were made over the suction surface of two airfoil models using particle image velocimetry (PIV), and were performed in a low-Reynolds-number tow -tank facility. One of the models, which consists of an unconventional airfoil design, was developed specifically for the purpose of examining passive m anipulation of separation-bubble transition. The com putational studies were performed using direct numerical simulation (DNS), providing detailed resolution of the spatial and tem poral scales of the flow. Separation-bubble transition is observed to occur in a different manner depending on the level of freestream turbulence. In a low-disturbance environment, transition is shown to be initiated by the inviscid Kelvin-Helmholtz instability mechanism. This instability results in the roll-up and subsequent shedding of vorticity in the separated shear layer, downstream of which break-down to small-scale turbulence occurs in the region of high shear between sequential rollers and initiates reattachm ent of the shear layer to the surface. Under low-Reynolds-number conditions, a vortex-pairing phenomenon is identified and associated with a subharmonic instability of the separated shear layer. A possible link between the inviscid Kelvin-Helmholtz and the viscous Tollmien-Schlichting instabilities is identified. Under elevated-freestream-turbulence conditions, separation-bubble transition is shown to occur through the production, growth, and merging of turbulent spots. The spots, which consist of a series of vortex loops, are shown to be initiated through a secondary inviscid instability, promoted by the presence of streamwise streaks th a t originate in the upstream laminar boundary layer disturbed by the freestream turbulence. i Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Two-dimensional surface modifications were made to the unconventional airfoil to provide passive m anipulation of the separation-bubble-transition process. Most of the configurations examined are shown to provide reduced boundary-layer losses downstream of reattachm ent. The spatially-resolved PIV results identify various means through which the transition process can be altered to improve th e performance of airfoils over which separation bubbl...