On the wavenumber-frequency spectra of wall pressure fluctuations in turbulent channel flows.

The characteristics of the wavenumber-frequency spectra of the rapid, slow and total wall pressure fluctuations are investigated using direct numerical simulation (DNS) of turbulent channel flow up to $\Rey_\tau\approx 1000$. For the wavenumber-frequency spectra of the total wall pressure fluctuations, a valley-like behavior of contour lines in the sub-convective region is found, which may be linked to the Kraichnan-Phillips theorem. For the decomposition of the wall pressure spectra, it is commonly assumed in previous studies that the cross spectral density (CSD) between the rapid and slow components of the wall pressure fluctuations can be neglected. Yet no experimental or numerical evidence is available for either confirming or disproving this assumption. In this paper, we use DNS data to quantitatively evaluate this assumption. Emphasizes are put on the error in decibel scale caused by neglecting the CSD between the rapid and slow components of the wall pressure fluctuations. It is found that this assumption is approximately accurate for one- and two-dimensional spectra, but causes a large magnitude of error in the three-dimensional wavenumber-frequency spectra. An error of 5dB is observed in the sub-convective region and such a large error is observed for a wide range of Reynolds numbers ($180\le\Rey_\tau\le 1000$). The analyses show that the assumption of neglecting the CSD needs to be applied carefully at the scales falling in the sub-convective region.