Direct numerical simulations of aeolian noise generated by a circular cylinder in subsonic and transonic conditions

Abstract

The aeolian noise generated by a circular cylinder is an important benchmark aero-acoustic problem. The overall goal of this thesis is to identify sound sources, isolate the radiating components from the non-radiating counterparts, assess their propagating capacities and to quantify the radiating acoustic sound sources versus the non-radiating hydrodynamic pseudo-sounds on and around a circular cylinder in subsonic and transonic conditions. Here, we adopt direct noise computations and the wavelet decomposition technique of Mancinelli et al. (JFM, Vol. 813, 2017), previously used in jet-noise experiments, to investigate the sound sources and pseudo-sounds generated by a cylinder. Rigorous independence and convergence analyses of the decomposition procedure are performed. In subsonic conditions, it is found that the radiating acoustic component strongly dominates over the non-radiating hydrodynamic component at near-field locations above and upstream of the cylinder. In the oscillating near-wake region, the hydrodynamic component dominates over the acoustic component at most frequencies, except at the vortex-shedding frequency where they exhibit comparable strengths. Furthermore, within the oscillating near-wake region, the overall sound pressure level associated with the hydrodynamic pressure fluctuation exceeds that associated with the acoustic pressure fluctuation. Away from the oscillating near-wake region, the hydrodynamic pressure fluctuations decrease dramatically while the acoustic counterparts decay slowly, demonstrating that the hydrodynamic pressure fluctuation does not propagate, and that the acoustic pressure fluctuation is the only component to propagate to the far field. Besides, it is found that the organized large-scale vortices shed from the cylinder remember their initial characteristics better as the compressibility increases. In the transonic condition, a dominant tone at the fluctuating frequency of the separated shear layers is observed in the downstream far field. In the region of fluctuating separated shear layers, it is found that the pressure fluctuation mainly consists of acoustic component at the fluctuating frequency of the separated shear layers. Thus, the dominant acoustic waves are generated primarily by the fluctuation of the separated shear layers. It is also found that, among the various-scale vortex structures, the structure induced by the fluctuation of the separated shear layers is most closely related with both sound and pseudo-sound signatures.