NONLINEAR INTERACTIONS DURING EARLY STAGES OF BOUNDARY LAYER TRANSITION INDUCED BY FREE-STREAM TURBULENCE

etc15 Tracking Number 208

The experimental study of a disturbed flat plate boundary layer subjected to moderate free-stream turbulence (FST) is presented. All measurements were conducted in a flow region with zero intermittency. By means of bispectral analysis it was found that after initial linear growth of low-frequency streaks two distinct nonlinear processes arises in a boundary layer. The first one is represented by interactions between low frequencies in upper third of a boundary layer and in immediate vicinity of it and the second one is an interaction of streaks with high-frequency disturbances across whole layer. In present experimental setup the region of nonlinear development had taken length about two-thirds of the measurements domain. Inside boundary layer the critical r.m.s.-amplitude of disturbances needed to initiate nonlinear development was found to be about 2 per cent of free-stream velocity.