Sound emission in a quasi-steady transonic turbulent flow past a circular cylinder

Journal article, 2025

This study investigates the noise generation mechanism of a circular cylinder in a quasi-steady transonic condition using direct noise computation. Unlike a subsonic circular cylinder, where noise is generated by alternate vortex shedding, it is found that in a quasi-steady condition, where vortex shedding is suppressed, the cylinder noise is primarily generated by the oscillation of separated shear layers, also known as shear layer instability. The flow contains complex features such as weak oblique shocks, expansion fans, fluctuating separated shear layers, suppressed vortex shedding, λ shocks, and quasi-steady bow shocks. Near-wake pressure fluctuations are found to be more strongly correlated with far-field pressure fluctuations, whereas wall pressure fluctuations are uncorrelated with far-field pressure fluctuations, suggesting that the sound sources are located in the near wake rather than on the cylinder surface. In the downstream far field, a dominant tone at the frequency of the fluctuating separated shear layers is observed. In the near-field region just behind the λ shocks, as the distance from the fluctuating separated shear layer increases, the acoustic pressure fluctuation replaces the hydrodynamic pressure fluctuation as the dominant component. In the region of expansion fans, the pressure fluctuation is mainly low-frequency acoustic pressure fluctuation. In the region of fluctuating separated shear layers, while the pressure fluctuation includes similar amounts of acoustic and hydrodynamic components at most frequencies, it predominantly consists of acoustic component at the frequency of the fluctuating separated shear layers. Overall, the dominant acoustic waves are primarily generated by the oscillation of the separated shear layers. Among the various-scale vortical structures, the one induced by the oscillation of the separated shear layers is most closely associated with both sound and pseudo-sound signatures.