Subconvective characteristics of wall-pressure fluctuations in low-Mach-number turbulent channel flow

Accurate prediction and modeling of flow-induced structural vibration and noise require a comprehensive understanding of turbulent wall-pressure fluctuations. In this study, compressible direct numerical simulations are conducted to investigate the spectral characteristics of wall-pressure fluctuations in turbulent channel flows at friction Reynolds number of 180 and bulk Mach numbers of 0.4, 0.2 and 0.1, with a focus on the subconvective wavenumber range. While acoustic peaks are barely visible in the one-dimensional streamwise wavenumber-frequency spectra, they are clearly observed in the two-dimensional wavenumber-frequency spectra at the zeroth spanwise wavenumber, with magnitudes several decades lower than the convective peak. At lower frequencies, the acoustic peaks correspond to propagating longitudinal and oblique waves that match the theoretical predictions of two-dimensional duct modes with a uniform mean flow. They decay with decreasing Mach number but remain distinctly identifiable even at the nearly incompressible Mach number of 0.1. At high frequencies, in contrast, no propagating waves are found, and the spectral level in the low-wavenumber range increases with decreasing Mach number.