Abstract
In order to clarify the mechanism of wall turbulence generation, we have been studying a subcritical transition in a flat-plate boundary layer experimentally, through observing its nonlinear response to energetic hairpin eddies acoustically excited near the leading edge of boundary-layer plate. In this paper, our recent results on the development of wall turbulence structure in this transition (upto Rx =1.2 × 105 where the log-low velocity distribution appears) is presented. We also examined the effect of riblets on the development of wall turbulence and found that just as observed in the developed turbulent boundary layers, riblets cause the spanwise spacing of the near-wall stream wise vortices to increase and the intensity of the associated velocity fluctuation near the wall to decrease. Even in the manipulated boundary layer, the wall turbulence occurs through the subcritical transition.
