Unsteady Surface Pressure Spectra with Small Levels of Wall Vibration

The spatial-temporal characteristics of the Unsteady Surface Pressure (USP) due to turbulent boundary layer flow is important for understanding forcing mechanisms in flow-induced vibration. Often, the USP is considered to be determined by the turbulent flow characteristics only, and is not influenced by wall vibration. This is sometimes referred to as the "blocked pressure assumption". An experiment was conducted in an anechoic wind tunnel in which the USP was measured directly downstream of a flexible membrane. The data indicated that changes in USP amplitude up to 5dB were observed over a broad range of frequencies when the wall was allowed to vibrate. This was found even when the wall motion was less than 20 viscous units in amplitude. Tonal acoustic forcing was considered, in which the wall vibration amplitude was in excess of 100 viscous units. These conditions led to increases in USP up to 8dB over a wide range of frequencies.