Wall Pressure Fluctuations in a Turbulent Boundary Layer

In the flow of air past a solid surface, a turbulent boundary layer is produced. The turbulence is ordinarily thought of as velocity fluctuation but in addition pressure, temperature and density fluctuations also occur. The turbulent boundary layer causes a sound field to be set up in the free stream and also induces fluctuating normal loads on the boundary. If the boundary is elastic, its motion will set the air on each side into motion. Thus, additional sound radiation is to be expected. In an initial investigation of this problem some of the properties of the wall pressure fluctuations in a turbulent boundary layer have been measured. The equipment used in the investigation includes: a specially designed low noise and turbulence level wind tunnel, a small barium titanate microphone for frequencies up to 50 kc and the necessary electronic equipment. The quantities measured were the root-mean-square pressure at the wall and the power spectrum of the wall pressure. It was found that the root-mean-square wall pressure was a constant portion (0.0035) of the free stream dynamic pressure for 0.2 M 1 and 106 R 13 × 106. Supported by the National Advisory Committee for Aeronautics.