Stability and Transition of Wall Boundary Layers Induced by Moving Waves.

Abstract

Abstract : The present study concerns a theoretical investigation of stability and an experimental investigation of transition for wall boundary layers developed within unsteady plane expansion waves and behind shock waves traveling into a gas at rest. The theoretical investigation involved three tasks: the use of approximate methods for the prediction of critical Reynolds numbers, a numerical integration of the ORR-Sommerfeld equation for the incompressible boundary layer flow at the expansion wave head, and the development of a multiple scales technique for nearly quasi-steady, quasi-parallel boundary layer flows. The boundary layer flow at expansion wave head has the same velocity profile as a semi-infinite flat plate given a sudden constant acceleration after being at rest. Expansion wave boundary layer transition times observed after the time of wave head arrival, are five to eight times larger than those previously reported and most of the transition Reynolds numbers observed are above or near the critical Reynolds numbers calculated by the approximate method. Shock wave boundary layer transition times observed are also larger than those previously reported and some of the transition Reynolds numbers observed based on distance to the shock wave front are above the critical values previously calculated.