Boundary-layer noise induced by arrays of roughness elements

Abstract

Sound induced by arrays of 10 × 4 roughness elements in low-Mach-number turbulent boundary layers at Re θ = 3065 is studied with Lighthill's theory and large-eddy simulation. Three roughness fetches consisting of hemispheres, cuboids and short cylinders are considered. The roughness elements of different shapes have the same height of 0.124δ, the same element-to-element spacing of 0.727δ and the same flow blockage area. The acoustically compact roughness elements and their images in the wall radiate sound primarily as acoustic dipoles in the plane of wall. The dipole strength, orientation and spatial distribution show strong dependence on the roughness shape. Correlations between dipole sources associated with neighbouring elements are found to be small for these sparsely distributed roughness arrays. Correlations and coherence between roughness dipoles and surface pressure fluctuations are analysed, which reveals the importance of the impingement of upstream turbulence and surrounding vortical structures to dipole sound radiation, especially in the streamwise direction. For roughness shapes with sharp frontal edges, the edge-induced unsteady separation and reattachment also play important roles in sound generation. Large-scale turbulent structures in the boundary layer have a relatively low influence on roughness dipoles, except for the first row of elements.

Computational prediction and analysis of rotor noise generation in a turbulent wake

“…Note that for stationary turbulence, it can be shown theoretically (e.g. Yang & Wang 2013) that the dipole coherence is identical to the force coherence, and hence the results in figure 23 also apply to blade sectional dipoles. The small blade-to-blade sectional dipole correlations and coherence demonstrate that different blades act as nearly independent acoustic sources, at least for frequencies higher than the BPF.…”

“…The effect of subgrid-scale motions is modelled using the dynamic Smagorinsky model (Germano et al 1991). The accuracy of this code for flow-noise studies has been established in a number of previous configurations including rough-wall boundary layers (Yang & Wang 2013), tandem cylinders and airfoil in a cylinder wake (Eltaweel et al 2014).…”

Extension of roughness noise to bluff bodies using the boundary element method

“…The roughness elements act as compact dipole sources with streamwise and spanwise components. Several numerical studies have also been performed [12,13], focusing on the flow features in the vicinity of the roughness elements and their relationship to the far field noise.…”

“…The point pressure spectrum Φ(f ) and the wavenumber-frequency pressure spectrumΦ(k, f ) on the body surface are computed from Eqs. (A 13). and (A.14), respectively.…”

Numerical investigation of tandem-cylinder noise reduction using plasma-based flow control

“…Jacob et al 2005;Eltaweel & Wang 2011;Giret et al 2014). It is also discussed extensively by Yang & Wang (2009, 2013 in the context of noise induced by roughness elements in a turbulence boundary layer. For the controlled case, the downstream cylinder dominates sound generation at most frequencies including its own shedding frequency (the primary spectral peak).…”