ACTIVE CONTROL OF TURBULENT BOUNDARY LAYER BASED ON A SINGLE PIEZOELECTRIC OSCILLATOR

Open-loop active control of a turbulent boundary layer has been achieved in skin-friction reduction and suppression of coherent structure bursting process by means of periodic oscillating of a piezoelectric oscillator embedded on the surface of a flat plate wall. Ten experimental cases were carried out under variable input voltage amplitudes and frequencies. At 2 mm downstream of the piezoelectric oscillator, the simultaneous time series of streamwise velocity component at di erent wall-normal positions in the turbulent boundary layer were finely measured by hot-wire anemometer and a mini single-sensor probe. The e ects of piezoelectric oscillation on the mean velocity profile, drag-reduction rate and conditional phase-average waveform of coherent structure burst were investigated at Re_θ=2 183. An upward shift in the log-law of mean velocity profile is observed, which indicates the reduction of skin-friction. With the larger amplitude of vibration, the higher drag reduction rate is achieved. Furthermore, a maximum rate of 25% can be reached when the vibration frequency is very close to the burst frequency of maximum-energy scale, which indicates that the manipulation of energetic-scale coherent structure burst is the key of wall-bounded turbulence drag reduction. In addition, by comparing the conditional phase-average waveforms of manipulated and unmanipulated cases, the waveform for manipulated conditions has more decreased amplitude with its wave crest damping rapidly in the later stage of high-speed sweep event and the sweep process of high-speed fluids are shorten. The vibration of piezoelectric oscillator can suppress the coherent structure sweep process of high-speed fluids, weaken the shear process of the high-speed fluids with the surface of the wall, bate the amplitude of coherent structure burst in the near-wall region, and as a result, reduce the skin-friction drag.