An investigation was carried out with an aim to better understand the mechanism of turbulent
drag reduction with spanwise-wall oscillation, by carefully analyzing the experimental data
of the near-wall structure of the boundary layer modified by the wall motion. It was found that
the mean velocity gradient of the turbulent boundary layer was reduced close to the wall,
and the logarithmic velocity profile shifted upwards by the wall oscillation. We argue that
these changes in the mean velocity profile are mainly due to a negative spanwise vorticity …