THE EFFECT OF PERIODIC PERTURBATION ON MULTI SCALES IN A TURBULENT BOUNDARY LAYER FLOW UNDER DRAG REDUCTION

This study reports the drag reduction of a turbulent boundary layer flow under the active control by a pair of wall-mounted piezoelectric oscillators with different working frequency and amplitude. Under the maximum local skin drag reduction case of 80V and 160 Hz, a miniature boundary layer hot-wire probe of single sensor was used to measure velocity signals at the streamwise location 2 mm downstream of the oscillators. The time series of streamwise velocity signals at different wall normal positions were obtained. By comparing the turbulent fluctuations at different scales in the perturbed cases and canonical turbulent boundary layer flow, it was found that the perturbation produced by piezoelectric oscillators have an effect on the near-wall region, which attenuates the large-scale intensity and enhances the small-scale turbulence. Meanwhile, the outer layer is insensitive to the oscillator perturbation. Furthermore, conditional average of small-scale fluctuations shows that the impact of piezoelectric oscillators on small-scale fluctuations is not uniform in the condition of the large scales. The small-scale amplitude is increased more obviously as the large-scale fluctuations are positive than that the large-scale fluctuations are negative. It indicates that the current periodic perturbation dominantly modifies the high skin friction events by breaking the large-scale high-speed sweeping fluids into small-scale structures, which results in the local skin drag reduction.