On Transition to Turbulence in Boundary Layers

Abstract

This article is concerned with transition to turbulence in boundary layers and, by implication, in similar kinds of wall-bounded shear flows. There appear to be many types of boundary-layer transition observable in practice, depending on the disturbance environment, e.g. on how rough the solid surface supporting the boundary layer is upstream or on how unsteady the oncoming free stream is. Two possible extremes of transition are the following. First there is the “fast”, savage, transition that is epitomised by the wake-passing effect, from an upstream row of rotor blades, on the boundary layers on stator blades in turbines. Broadly, a characteristic measure of the stator boundary-layer properties, such as the heat transfer at a fixed station on the surface, shows a very rapid change taking place there, apparently from a fully laminar to a fully turbulent state almost immediately as the highly turbulent wake of a typical rotor hits the stator blade. When the wake moves on there is a slightly less rapid adjustment, back to laminar conditions, until the violent wake of the next rotor blade hits the stator and the rapidly changing temporal cycle repeats itself. The other possible extreme of boundary-layer transition is “slow” (civilised) transition. This is exemplified by extensions of the classical experiments by Schubauer & Skramstad in the 1940’s, on tiny unsteady controlled disturbances introduced upstream into a basic planar laminar boundary layer, the extensions being carried out in the recent experiments of Saric, Gaster, Kachanov and their co-workers, and others, who examine the effects of gradually increasing the size of the small input disturbance upstream.