The mechanism of turbulent drag reduction with wall oscillation

An extensive study of the near-wall structure of turbulent boundary layer over a spanwise oscillating wall was conducted using hot-wire measurements in a wind tunnel in order to better understand the mechanisms involved in turbulent drag reduction under these conditions. The results showed that the logarithmic velocity profiles were shifted upwards and the turbulence intensities reduced, suggesting that the viscous sublayer was thickened as a result of drag reduction with wall oscillation. The probability density functions of velocity fluctuations exhibited long tails of positive probability, reflecting increases in the skewness and kurtosis within the viscous sublayer. The measured thickness of the Stokes layer was comparable to that of the viscous sublayer of the boundary layer when the turbulent drag reduction was observed with the wall oscillation. At the same time, the Reynolds number of the Stokes layer was found well below the critical value, so that the Stokes layer was expected to remain laminar if there were no boundary layers over the oscillating surface. These are considered to be important conditions in obtaining turbulent drag reduction with spanwise-wall oscillation. The present study also showed that a net spanwise vorticity was generated by the periodic Stokes layer over the oscillating wall just outside the viscous sublayer, which reduces the mean velocity gradient of the boundary layer near the wall. At the same time, the spanwise vorticity hampers the longitudinal vortices in the viscous sublayer to stretch in the streamwise direction, reducing the streamwise vorticity associated with these vortices. The near-wall burst activity was weakened as a result of this, resulting in the reduction of turbulent skin-friction drag. A remarkable change in the burst signature in the near-wall region of the boundary layer was observed.