| 講演要旨： |
Transition to turbulence in laminar boundary layers subjected to high levels of free stream turbulence (FST) has usually been denoted, somewhat mysteriously, ``by-pass transition''. We have tried to elucidate the nature of FST-induced transition, using both hot-wire anemometry, PIV and flow visualisation. The cases under study is both the Blasius boundary layer and the asymptotic suction boundary layer, which give us some insight in both receptivity issues as well as disturbance growth/decay. The free stream turbulence was generated by grids (both passive and active) of various size placed at the start of the test section, giving rise to turbulence levels in the range 1-7 %.
Simultaneous Particle Image Velocimetry measurements and flow visualization show that a boundary layer subjected to moderate levels of free stream turbulence develops unsteady streamwise orientated streaky structures with high and low streamwise velocity. This leads to large amplitude, low frequency fluctuations inside the boundary layer. The current understanding of the streak development is that so called non-modal growth (sometimes referred to as transient growth) is responsible for the streak growth phase and that the breakdown occurs through a secondary instability developing on the spanwise shear layer between a high and low velocity streak. We will also point out that transition to turbulence in other flows seem to originate from similar streaky structures, which may breakdown when a critical Reynolds number based on their size is sufficiently large.
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