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 created in the near-wall region of the boundary layer over the oscillating wall. The resultant near-wall velocity reduction seems to have weakened the near-wall turbulence activity by hampering the stretching of the quasistreamwise longitudinal vortices, leading to a reduction in skin-friction drag. Indeed, the signatures of the sweep events over the oscillating wall indicated that the duration and the strength were reduced by 78% and 64%, respectively. The reductions obtained by a 2D numerical model, incorporating the streamwise flow and associated stretching of the longitudinal vortical structure, were 47% for the sweep duration and 23% for the sweep strength, although these reductions were observed within the first cycle of wall oscillation. By phase averaging the conditionally sampled velocity data, we were able to show that the frequency of sweep events was reduced with a reduction in streamwise velocity when the negative spanwise vorticity is created by the wall oscillation in the near-wall region. It was also shown that the turbulent skin-friction reduction with spanwise-wall oscillation can be optimized with a nondimensional, spanwise wall velocity, and nearly 45% drag reduction can be obtained in the turbulent boundary layer at an optimum value of w+=15.

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