Streaky structures in a controlled turbulent boundary layer

This paper reports the modifications of near-wall low-speed streaks by local surface oscillations generated by a spanwise-aligned actuator array in a turbulent boundary layer over a flat plate at Re_θ = 1,000. The streaks were educed from PIV-measured fluctuating velocities in the viscous sublayer using a procedure proposed by Schoppa and Hussain (2002). The wall-based perturbations, corresponding to a large skin-friction drag reduction (about 50% at 17 wall units downstream of the actuator array), modified greatly the low-speed streaks, leading to a reduction by over 15% in both the averaged width and spacing while an increase by 17% in the streak center number. The alterations of velocity streak distributions are consistent with results from other techniques such as smoke-wire flow visualization and two-point cross-correlation, where the breakup of largescale coherent structures into small-scale ones was observed.