| Popis: |
Experiments were conducted to determine the effects of steady, lateral wall oscillations on turbulent boundary layer characteristics, particularly drag reduction. Hotfilm measurements of the mean velocity in the viscous sublayer were used to determine drag reduction. These measurements were confirmed by laser Doppler velocimeter measurements of the reduction of the turbulence shear stress. A maximum 25% drag reduction was achieved, and this drag reduction was found to scale with the peak oscillation velocity. The drag reduction induced by the oscillating wall was accompanied by decreases in u', v', and uv in the wall region (y+ < 100). The probability density functions for the velocity near the wall showed that the wall oscillations significantly decreased the influx of high velocities near the wall. There were also corresponding decreases in the frequencies and durations of bursts and sweeps near the wall. Stronger bursts and sweeps, i.e. bursts and sweeps with greater uv magnitude, were suppressed significantly more than the weaker bursts and sweeps. These effects were evident within one 5 of the start of the oscillating wall, and were sustained beyond 78. |
