The Effect Of Periodic Perturbation On Multi Scales In A Turbulent Boundary Layer Flow Under Drag Reduction

The active control actuators made of piezoelectric ceramic oscillators apply periodic disturbances with different frequencies to the turbulent boundary layer.The controlled and uncontrolled turbulent boundary layer was measured by using a miniature boundary layer hot-wire probe of a single sensor.The time series of the velocity of different wall-normal positions at 2mm downstream of the piezoelectric ceramic oscillators is obtained.The multi-scale flow field of the drag-reduction case was studied as well.The drag reduction rates were calculated by the wall frictional resistance.By comparing the average velocity profiles at the same free-stream velocity,an optimal excitation frequency could be found at the same excitation voltage,which can provide the most outstanding drag reduction effect.A larger excitation voltage can produce a large drag reduction rate when the excitation frequency is invariant.The same excitation condition provides different drag reduction rates at different free-stream velocities.The streamwise velocity fluctuations across the boundary layer are decomposed into large-and small-scale fluctuations by using a spectral filter.The RMS values of the turbulent fluctuations at different scales in a control case of 80V and 160Hz and the canonical turbulent boundary layer at different scales in a control case of 80V and160Hz and the canonical turbulent boundary layer flow were obtained.It was found that the perturbation produced by piezoelectric oscillators affects the near-wall region,which attenuates the large-scale intensity and enhances the small-scale turbulence.Furthermore,by visualizing the conditional average results of large-and small-scale fluctuations of controlled and uncontrolled cases respectively,the influences on small-scale fluctuations are not even in the time phase under the control case.The conditional average results of small-scale amplitude increase more during the positive large-scale fluctuations.The maximum correlation coefficient and the time shift were obtained in the correlation coefficients between large-scale fluctuations and the envelope of small-scale fluctuations in controlled and uncontrolled cases.The maximum correlation coefficient increased,while the time shift corresponding to the maximum correlation coefficient decreased.This indicated that the perturbation of piezoelectric oscillators shortened the time shift between the large-scale structures and small-scale structures in the turbulent boundary layer.The mu-level method was used to detect the burst events in small-scale fluctuations,and the burst period and burst duration under different large-scale fluctuations were compared.In the near-wall region,the perturbation significantly reduces the duration of the burst event when the large-scale fluctuations are negative and shortens the burst period of the burst event when the large-scale fluctuations are positive.Comparing the conditional average results of the burst events at y~+=5,it is found that the reason for the reduced duration in the negative large-scale fluctuations is that the disturbance of piezoelectric oscillators accelerates the attenuation of the burst events.