Experimental Studies of the Localized Disturbances and their Secondary High-Frequency Instability in the Flat Plate, Unswept and Swept Wing Boundary Layer (Review)

Results of experimental studies of the streaky structures instability at nonlinear stage of this process in the shear flows are shown in this work. Flow patterns of the streaky structures with secondary high-frequency disturbances generated on them during its spatial evolution are discussed. Different scenarios of the coherent vortical structures occurrence and downstream evolution in the flat plate, unswept and swept wing boundary layer are considered. Features of the sinusoidal and varicose instability of the longitudinal stationary streaky structure at nonlinear stage of its downstream development are shown. To these features concern: modulation of streaky structure in transverse and streamwise direction by frequency of secondary disturbances, occurrence of new streaky structures downstream and occurrence and development of the nonstationary localized vortices such as Λ-structures in both cases. Development of a nonlinear stage of flow instability is considered both in the region of the adverse and zero pressure gradient

Generation Mechanism of Streaky Structure in Natural Transition

In a transitional boundary layer subjected to free-stream turbulence, streaky structures develop and eventually break down to turbulence after wavy motions. The purpose of the present study is to reveal the dominant scales of the streaky structures using a novel stereo PTV system measuring the streamwise and wall-normal velocities. The correlation of the streamwise velocity exhibit a negative peak at a certain spanwise separation. If normalized with the local boundary layer thickness, the spanwise separation minimizing the correlation decreases downstream and reaches the boundary layer thickness at the most downstream position. The streamwise correlation coefficient is asymptotically decreasing to zero and the streamwise integral scales are increasing downstream, indicating the large streamwise elongation of the streaky structure. The normalized spanwise scale of the wall-normal disturbance velocity also decreases downstream and it is always narrower than the width of the disturbances of the streamwise velocity. In contrast to the elongating streamwise velocity disturbance, the integral scale in the streamwise direction of the wall-normal disturbance is constant with downstream position. The length scales of the wall-normal disturbance are more less constant with the streamwise position and this result concludes that the wall-normal disturbance is simply convected by the mean flow after the receptivity process.