Effect of Driven-Wall Motion on a Turbulent Boundary Layer

Summary

An experimental boundary layer study was conducted in low-speed flow over a driven wall model. Acoustic horn drivers were used to vibrate a thin membrane surface to obtain two-dimensional standing wave motion. The purpose of the study was to examine the influence of the unsteady wall boundary motion on the structure of a zero pressure gradient turbulent boundary layer. Measured quantities included velocity profiles, turbulence quantities (including Reynolds stress), and smoke wire flow visualization.

The turbulent boundary-layer flow was examined at several velocities. This gave a combination of flow and wall induced disturbances. Longitudinal and vertical surveys were made to determine the variations relative to the surface.

At small distances from the driven wall there were large variations in fluctuation quantities along each model wave. These longitudinal variations decreased rapidly with increasing distance from the driven wall. However, the average level of these fluctuations only decreased slightly even outside the boundary layer. The effects of the acoustically induced quantities (due to the driven wall) seemed to be essentially independent of the state of the boundary layer, but with the level related to the mean velocity. The acoustic field did not alter significantly the turbulent boundary layer structure for the model surface motions examined in the present study. When the acoustic fluctuations were subtracted out, the remaining fluctuations were essentially the same as for the undriven model.