Experimental Investigation Of Active Control And Drag Reduction Mechanism For Turbulent Boundary Layer Over A Flat Plate

In this paper,active control system with piezoelectric oscillators is developed.Active control and drag reduction mechanism of coherent structures in turbulent boundary layers is experimentally investigated using hot-wire anemometry.Open-loop active control of a turbulent boundary layer has been achieved with a view to skin-friction reduction and suppression of coherent structure bursting process by means of periodic oscillating of single piezoelectric oscillator and double piezoelectric oscillators embedded on the surface of a flat plate wall.Ten experimental cases of single piezoelectric oscillator were carried out under variable input voltage amplitudes and frequencies,while the double-piezoelectric-oscillators cases included synchronous and asynchronous cases.At 2 mm downstream of the piezoelectric oscillator,the time sequence of streamwise velocity component within different active control cases,along different wall-normal positions in a flat-plate turbulent boundary layer has been finely measured by IFA300 hot-wire anemometer and a TBL single-sensor probe with resolution higher than Kolmogorov scale.The effects of piezoelectric oscillation on the mean velocity profile,friction velocity,frictional shear stress and friction coefficient were investigated.Wavelet analysis is used to decompose the time sequence of velocity.The scales of multi-scale coherent structures are determined by self-correlation function.It is represented that the coherent structures and intermittency are identified by multi-scale flatness factor based on locally averaged velocity structure functions.Conditional phase-average waveforms for multi-scale coherent eddy structures in turbulent boundary layer are extracted by this conditional sampling technique.The dynamic evolution of multi-scale coherent structures is investigated.An upward shift in the log-law and thickening in viscous sublayer 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 controlled and uncontrolled cases,the waveform for controlled cases 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 is 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.