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Turbulent boundary-layer noise: direct radiation at Mach number 0.5
Published online by Cambridge University Press: 16 April 2013
Abstract
Boundary layers constitute a fundamental source of aerodynamic noise. A turbulent boundary layer over a plane wall can provide an indirect contribution to the noise by exciting the structure and a direct noise contribution. The latter part can play a significant role even if its intensity is very low, explaining why it is difficult to measure. In the present study, the aerodynamic noise generated by a spatially developing turbulent boundary layer is computed directly by solving the compressible Navier–Stokes equations. This numerical experiment aims at giving some insight into the noise radiation characteristics. The acoustic wavefronts have a large wavelength and are oriented in the direction opposite to the flow. Their amplitude is only 0.7 % of the aerodynamic pressure for a flat-plate flow at Mach 0.5. The particular directivity is mainly explained by convection effects by the mean flow, giving an indication about the compactness of the sources. These vortical events correspond to low frequencies and thus have a large lifetime. They cannot be directly associated with the main structures populating the boundary layer such as hairpin or horseshoe vortices. The analysis of the wall pressure can provide a picture of the flow in the wavenumber–frequency space. The main features of wall pressure beneath a turbulent boundary layer as described in the literature are well reproduced. The acoustic domain, corresponding to supersonic wavenumbers, is detectable but can hardly be separated from the convective ridge at this relatively high speed. This is also due to the low frequencies of sound emission as noted previously.
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- ©2013 Cambridge University Press
Footnotes
Present address: EDF R&D, MFEE, I84, 6 Quai Watier, 78400 Chatou, France.
References
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Gloerfelt and Berland supplementary movie
Colormap of the pressure fluctuations p' in the median plane (range ±5 Pa) for the fine-grid LES. Broadband noise is radiated from the turbulent boundary layer (TBL) with wavefronts oriented in the upstream direction. Important time-to-time variations are visible for the large-wavelength pressure lobes advected in the TBL.
Gloerfelt and Berland supplementary movie
Band-pass filtered pressure around ωU∞/δ*ref=0.033, corresponding to the peak value of the low-frequency bump. 2-D view in the median plane (range ±0.7 Pa). Near the wall, large lobes corresponding to aerodynamic pressure are advected, and the acoustic waves are clearly connected with their evolution.
Gloerfelt and Berland supplementary movie
Band-pass filtered pressure around ωU∞/δ*ref=0.20, corresponding to the end of the low-frequency bump. 2-D view in the median plane (range ±0.2 Pa). For this smaller wavelength, it is possible to identify approximately the location and duration of well-defined compact sources. The animation shows that these sources are not advected with the boundary layer, but are rather at fixed locations, and have a large life time. The structure and "breathing" behaviour of the near-field pressure lobes can also be observed.
Gloerfelt and Berland supplementary movie
Band-pass filtered pressure in the frequency band ωU∞/δ*ref∈[0.52;0.99], corresponding to the bounds of the first high-frequency peak. 2-D view in the median plane (range ±2 Pa). The radiation levels are significantly higher in the inlet region, where a small step is used to ignite the eruption of turbulence. Note also that the downstream-oriented part of the wavefronts is visible for these high frequencies, yielding a complex interference pattern.
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