The suppression of flow-induced noise of finite wall-mounted harbour seal vibrissae

A finite wall-mounted cylinder (FWMC) refers to a cylinder mounted to a flat wall, characterised by flow over its free end and at the wall junction. In the present study, finite wall-mounted harbour seal vibrissal cylinders with different spanwise aspect ratios ( $\textit{AR} = L/D$ , where $L$ is the length of the cylinder and $D$ is its diameter) are examined at a Reynolds number of $\textit{Re} = 12\,000$ based on the cylinder diameter, with an incoming turbulent boundary layer thickness of $\delta /D = 0.16$ . The far-field acoustic spectra of the harbour seal vibrissal FWMCs are broadband for all aspect ratios and effectively suppress the tonal peaks observed in the spectra of circular FWMCs. However, the effectiveness of the suppression mechanism diminishes with decreasing aspect ratio. For the shortest harbour seal vibrissal FWMC ( $\textit{AR}= 3.2$ ), the overall sound pressure level exceeds that of its circular counterpart, and the tonal peak is replaced by broadband noise of comparable magnitude. Spanwise-intermittent recirculation zones and cellular vortex shedding are identified in the near wake of the harbour seal vibrissal FWMCs. For all harbour seal vibrissal FWMCs, as well as for circular FWMCs with $\textit{AR}$ = 6.5, hairpin vortex shedding is observed rather than classical von Kármán vortex shedding. This corresponds to the absence of tonal peaks in the far-field acoustic spectra. Wavy separation lines play a crucial role in the formation of hairpin vortices and in inducing phase differences in vortex shedding in the spanwise direction. Concentrated quadrupole streamwise vortical structures periodically appear along the span in the wake of the longest harbour seal vibrissal FWMC, which diminish with $\textit{AR}$ decreasing to $\textit{AR}$ = 6.5. This concentrated quadrupole arrangement of positive–negative streamwise vortices suppresses the vortex shedding observed along the span of circular FWMCs by disrupting the acoustic-coherent structures into smaller, spanwise-wavy organised elements. It also stabilises the flow and balances transverse ( $y$ ) forces.