The structure of wall-pressure fluctuations in turbulent boundary layers with adverse pressure gradient and separation

Space-time correlations and frequency spectra of wall-pressure fluctuations , obtained from direct numerical simulation, are examined to reveal the eff ects of pressure gradient and separation on the characteristics of wall-pre ssure fluctuations. In the attached boundary layer subjected to adverse pre ssure gradient, contours of constant two-point spatial correlation of wall- pressure fluctuations are more elongated in the spanwise direction. Convect ion velocities of wall-pressure fluctuations as a function of spatial and t emporal separations are reduced by the adverse pressure gradient. In the se parated turbulent boundary layer, wall-pressure fluctuations are reduced in side the separation bubble, and enhanced downstream of the reattachment reg ion where maximum Reynolds stresses occur. Inside the separation bubble, th e frequency spectra of wall-pressure fluctuations normalized by the local m aximum Reynolds shear stress correlate well compared to those normalized by free-stream dynamic pressure, indicating that local Reynolds shear stress has more direct influence on the wall-pressure spectra. Contour plots of tw o-point correlation of wall-pressure fluctuations are highly elongated in t he spanwise direction inside the separation bubble, implying the presence o f large two-dimensional roller-type structures. The convection velocity det ermined from the space-time correlation of wall-pressure fluctuations is as low as 0.33U(0) (U-0 is the maximum inlet velocity) in the separated zone, and increases downstream of reattachment.