Shifting aeroacoustic source mechanisms of a transversely oscillating cylinder in lock-in region at various Mach numbers

Abstract

Direct numerical simulation of the aerodynamic noise generated by a flow past a transversely oscillating cylinder is conducted to investigate the effect of Mach number on the aeroacoustic characteristics. The results indicate that the coupling between the oscillating motion and vortex shedding in the lock-in region leads to a distribution of the surface pressure pulsation time derivative with four distinct peaks. The intensity of the radiated noise from the oscillating cylinder in the lock-in region is reduced compared to the stationary case. Although the oscillating frequency associated with this noise reduction is insensitive to Mach number, the reduction effect weakens at high Mach numbers. Furthermore, as the Mach number increases, the sound directivity at the lock-in boundary transitions from a figure-eight configuration to a butterfly-like configuration, featuring distinct lobes both upstream and downstream. To illustrate the changes in noise, the key finding is the modified scaling law governing noise intensity in relation to Mach number for the oscillating cylinder: it shifts from $M^{2.5}$ to $M^{3.5}$, characteristic of quadrupole sources. This alteration is further analyzed by applying the Ffowcs Williams-Hawkings equation to separate the contributions of different noise sources, and utilizing spectral proper orthogonal decomposition to extract the dominant coherent structures responsible for the oscillating cylinder at the lock-in boundary at high Mach numbers.