Wall Pressure Spectra and Convection: Two-Dimensional Analysis Under Mean Pressure Gradients

Abstract

Wall pressure fluctuations beneath turbulent boundary layers subjected to a pressure gradient are studied in a subsonic channel flow, up to , where the test section’s ceiling is adjusted to create the desired pressure gradients with the Clauser parameter ranging from to 0.8. A rotating antenna of 63 nonuniformly distributed remote microphones enables a fine spatial resolution, down to 1 mm. The focus is put on the convective velocity, coherence scales, and two-dimensional wavenumber–frequency spectra. The convective velocity is increased by adverse pressure gradients, with values up to 14% higher than without gradient, which goes against the trend reported in previous studies. Both anisotropy and wall friction are increased by a favorable pressure gradient. In the subconvective region of the wavenumber–frequency spectra, classical models fail to match the data in terms of both levels and decay rates. Finally, the aspect ratio of these spectra’s convective ridges is increased from adverse to zero, and then favorable pressure gradients. These results are discussed in light of the literature on coherent motion in boundary layers and how it is affected by pressure gradients. Mechanisms that could explain the observed pressure gradients’ effects are finally proposed.