Boundary-layer transition triggered by hairpin eddies at subcritical Reynolds numbers

Abstract

Subcritical transition in a flat-plate boundary layer is examined experimentally through observing its nonlinear response to energetic hairpin eddies acoustically excited at the leading edge of the boundary-layer plate. When disturbed by the hairpin eddies convecting from the leading edge, the near-wall flow develops local three-dimensional wall shear layers with streamwise vortices. Such local wall shear layers also evolve into hairpin eddies in succession to lead to the subcritical transition beyond the x-Reynolds number Rx = 3.9 × 104, where the momentum thickness Reynolds number Rθ is 127 for laminar Blasius flow without excitation, and is about 150 under the excitation of energetic hairpin eddies. It is found that in terms of u- and v-fluctuations, the intensity of the near-wall activity at this critical station is of almost the same order as or slightly less than that of the developed wall turbulence. The development of wall turbulence structure in this transition is also examined.