Effects of Pressure Gradients on Turbulent-Boundary-Layer Wall-Pressure Fluctuations

Spectral density, magnitude of the normalized longitudinal and lateral cross-spectral-density functions, and convection-velocity ratio as a function of longitudinal separation and frequency of wall-pressure fluctuations were measured with small flush-mounted transducers. These measurements were accomplished in both mild adverse and mild favorable pressure gradients in a low-turbulence subsonic wind tunnel. To establish a basis of comparison, similar measurements were made for the zero pressure gradient, and agreement with published measurements was excellent. The effect of an adverse pressure gradient on the nondimensionalized spectral density was an increase in the low-frequency content without influencing the high-frequency portion appreciably; a sharp decrease in the high-frequency portion was observed for the favorable pressure gradient. At similar nondimensionalized longitudinal separations and frequencies, the convection velocity ratio was higher in the favorable and lower in the adverse pressure gradients. The longitudinal decay of a particular eddy was more rapid in the adverse and slower in the favorable pressure gradients. No differences were found in the lateral cross-spectral density for the different pressure gradients.