Drag reduction in a turbulent boundary layer on a flexible sheet undergoing a spanwise traveling wave motion

The effect of a spanwise traveling-wave motion on a zero-pressure-gradient turbulent boundary layer over a flexible sheet was investigated at low Reynolds numbers using a single hot-wire anemometer for turbulence statistics and two laser displacement sensors for displacements of the flexible sheet. It was found that the log-law region of the mean velocity on the flexible sheet was slightly narrower compared with a rigid wall. The energy spectra of streamwise velocity fluctuations on the flexible sheet undergoing the spanwise traveling-wave motion were smaller in a region of frequency which corresponded to the bursting frequency in the canonical wall turbulence. This indicates that the bursting event near the flexible sheet was directly affected by the surface wave motion. It was revealed that a drag reduction of up to 7.5% could be obtained by the spanwise traveling-wave motion, estimating the friction coefficients through the growth rate of the momentum thickness.

Keywords:

• Turbulent Boundary Layer
• Drag Reduction
• Spanwise Traveling Wave Motion
• Flexible Sheet