Assessment of the Contribution of Surface Roughness to Airframe Noise

Abstract

The generation of sound by turbulent boundary-layer flow at low Mach number over a rough wall is investigated by applying a theoretical model that describes the scattering of the turbulence near field into sound by roughness elements. Attention is focused on the numerical method to approximately quantify the absolute level of far-field radiated roughness noise. Models for the source statistics are obtained by scaling smooth-wall data by the increased skin friction velocity and boundary-layer thickness for a rough surface. Numerical integration is performed to determine the roughness noise, and it reproduces the spectral characteristics of the available empirical formula and experimental data. Experiments are conducted to measure the radiated sound from two rough plates in an open jet. The measured noise spectra of the rough plates are above that of a smooth plate in 1-2.5 kHz frequency and exhibit reasonable agreement with the predicted level. Estimates of the roughness noise for a Boeing 757 sized aircraft wing with idealized levels of surface roughness show that in the high-frequency region the sound radiated from surface roughness may exceed that from the trailing edge, and higher overall sound pressure levels are observed for the roughness noise. The trailing edge noise is also enhanced by surface roughness somewhat. A parametric study indicates that roughness height and roughness density significantly affect the roughness noise with roughness height having the dominant effect. The roughness noise directivity varies with different levels of surface roughness.

Measurement and simulation of surface roughness noise using phased microphone arrays

“…This is equivalent to comparing theory and experiment after applying a filter which suppresses much the extraneous noise in the experiments. A distribution of incoherent dipoles was simulated over a rigid plate with the source strengths determined equivalently by the prediction scheme of Liu and Dowling [7]. Comparison of the results of beamforming between measurement and simulation provides indirect validation of the predicted source type, magnitude and distribution.…”

“…The theoretical model of Howe [1] was recently extended by Liu and Dowling [7] to quantify numerically the far-field radiated roughness noise. They approximated the rough-wall wavenumber-frequency spectrum by smooth-wall models [8][9][10][11][12] with increased friction velocity u τ and boundary-layer thickness δ appropriate for a rough surface.…”

“…However, the data are insufficient for building an empirical model, as aforementioned, due to the unknown effects of acoustic refraction by the free-jet shear layers downstream of the nozzle exit. Liu and Dowling [7] measured the noise spectra of two rough plates with different roughness heights in an open jet. The measured noise spectra were significantly contaminated by background noise, but the roughness noise was detected in 1-2.5 kHz frequency range.…”

“…Acoustic measurements were performed at three streamwise locations to explore the directivity features of possible dipole sources. Theoretical predictions for the source distributions on the two rough plates were also obtained using Liu and Dowling's model [7].…”

“…Liu and Dowling [7] then applied the numerical method to the assessment of the contribution of surface roughness to airframe noise, and estimated the roughness noise for a Boeing-757 sized aircraft wing during approach or take-off with idealized levels of surface roughness. They found that in the high frequency region sound radiated from surface roughness may exceed that from the trailing edge depending on the size of roughness elements, and that the trailing-edge noise is also enhanced by surface roughness to some extent.…”