Experimental Research On Active Control Of Turbulent Boundary Layer For Drag Reduction With Double Piezoelectric Oscillators Periodic Perturbation

Skin friction is the main source of total resistance.It is necessary to restrain the burst process of coherent structures to reduce skin friction.The self-sustaining process of strip and streamwise vortex is the key to generate and maintain turbulence.Controlling to any link can restrain turbulence and reduce friction.The large scale structures is verified by using HWA and wavelet transform.The drag reduction mechanism is also studied.Using open-loop and closed-loop active control with PZT arranged along the spanwise direction,active control was realized.The streamwise and normal velocity were measured by HWA and the hot-wire probe.By using wavelet transform,the distribution of kinetic energy at different scales are compared and analyzed.The maximum energy scale increases away from the wall.The burst period of coherent structures increases,which indicates that there are large scale structures in the outer region.The conditional waveforms of Reynolds stress are changed.The negative amplitude of Reynolds stress decreases and fluctuates.The open-loop active control with PZT was designed.Different voltage amplitude and vibration frequency was applied to PZT.The synchronous(SYN)and asynchronous(ASYN)vibration was realized to the multi-scale coherent structure.As the frequency(160 Hz)is close to the maximum energy scale,it reduces the friction and realizes the modulation to the streamwise vortex.A disturbance Reynolds NumberRe_d(28)?A~2f/?is introduced.The drag reduction rate 18.54%was obtained andRe_d(28)0.54at 100 V/160 Hz-ASYN case.The kinetic energy curves of 160 Hz-ASYN case have two maximum values and one minimum value,and the PDF curves present periodic effect.The vibration has a periodic modulation effect on the coherent structure.The conditional waveform are disturbed,the modulation effect is obviously enhanced and weakens gradually away from the wall.The velocity signal is divided into large and small scale.The disturbance of oscillators at 160 Hz increases the turbulence intensity of small scale,adjusts the energy distribution,and transfers energy from large scale to small scale.The disturbance reduce drag by breaking large scale of high-speed fluid into small scale structure.The effect of vibration weakens gradually away from the oscillator.The mu-level method was used as the detection criterion,and a closed-loop system was designed,which was composed of a hot-wire sensor,a SCM controller and a bimorph vibrator.Closed-loop active control was realized.The best drag reduction 16.03%is obtained in the 160 Hz-ASYN case.The closed-loop control saves about 75%energy,and achieves the same effect as the open-loop control.The kinetic energy from 6th to the 12th scales was changed.When 160 Hz is close to the frequency of the maximum energy scale,the disturbance has the best effect.In the ASYN-100 V/160 Hz case,the amplitude of the waveform increases and the period decreases.