THE ROLE OF EXTERNAL DISTURBANCES INLAMINAR-TURBULENT TRANSITION

Abstract

Transition to turbulence in parallel or nearly parallel flows is known to be critically influenced by ambient perturbations. At relatively low intensity level, external perturbations initiate transition through receptivity process, i.e. by exciting instability waves, but without affecting linear and subsequent nonlinear amplification rate. At moderate to high intensity level, however, external disturbances may directly alter the instability properties. The present paper reports some recent theoretical progresses in understanding the role of external disturbances in transition. These include (a) a self-consistent theory for the vortical receptivity, which predicts accurately the experimental measurements, (b) the demonstration that the streaks formed due to the free stream vortical fluctuations modify the viscous Tollmien-Schlichting (T-S) waves, and indeed may even induce stronger intermittent inviscid instability, and (c) a theoretical model, which links the critical Reynolds number to the external perturbations, and shows, for reduced by even a minimal level of external perturbations. Several problems which require further studies are highlighted. the case of channel flow, that the critical Reynolds number can be substantially