Experimental investigation on wall-bounded turbulence drag reduction by double piezoelectric vibrator active control

In order to manipulate large-scale coherent structures in the wall-bounded turbulence and reduce the skin-friction, an active-control experimental investigation was performed through synchronous and asynchronous vibrations of double piezoelectric vibrators embedded spanwisely on a smooth flat plate surface. A TSI-IFA300 hot-wire anemometer and TSI-1621A-T1.5 hot-wire probe were used to measure the time series of instantaneous velocity at different locations. Influence of the vibrations on wall-bounded turbulence are compared in a multi-scale point of view. A disturbance Reynolds Number Red = ρd2f/μ is introduced to represent the disturbance. A probability density functions (PDFs) of the multi-scale components of the turbulence velocity and multi-scale conditional phase-averaged waveform are studied in detail using the Wavelet transform. The results show that the maximum drag reduction rate 18.54% was obtained at 100V/160Hz and Red = 0.54 case with an asynchronous vibration mode. The disturbances generated by the vibrators have a significant influence on the sweep events of burst. The asynchronous vibration model is more effective than synchronous vibration one. A possible physical mechanism is recognized to explain why the disturbance frequency of 160Hz leads to an optimal parameter set for drag reduction.