Effect of pressure gradients on turbulent boundary layer noise and wall-pressure fluctuations

Abstract

Excitation of the walls of a vehicle by turbulent boundary layers constitutes a major source of interior noise in the field of transport industry. A turbulent boundary layer over a wall can provide a direct contribution to the noise and also an indirect one due to the excitation of the structure below. In the present study, simulations of turbulent boundary layers at Mach=0.5 are carried out using direct noise computation by solving the compressible NavierStokes equations. This method provides the direct contribution (despite its very low intensity) and the wall pressure fluctuations which are responsible for the indirect contribution to the aerodynamic noise. The low efficiency of the noise radiated inthe freestream can be explained by the fact that the sources are turbulence quadrupoles. In order to determine the effects of pressure gradients on both noise contributions, adverse and favorable cases are investigated. The results are compared to a reference case with zero pressure gradient computed by Gloerfelt 1,2 . Results show that an adverse pressure gradient leads to higher levels of the direct acoustic emission whereas lower levels are obtained in presence of a favorable pressure gradient in comparison to the zero gradient case. The same hierarchy of levels is visible in the wavenumber pressure spectra of wall pressure fluctuations. In the field of transport industry, boundary layer constitutes a fundamental source of aerodynamic noise (Blake 3 ). A turbulent boundary layer over a vehicle has two mechanisms of noise generation: an indirect contribution corresponding to structural excitation and a direct contribution consisting of propagation of pressure waves. Due to very low amplitudes of the direct contribution in subsonic flows compared to the overall pressure levels, it is difficult to evaluate the direct acoustic contribution experimentally and numerically. The noise radiated by a zero pressure gradient (ZPG) turbulent boundary layer has been investigated for subsonic flows (Mach 0.5, 0.7 and 0.9) by Gloerfelt 1,2,4 . In these studies, the author especially confirmed the U 8 power law dependence of the acoustic intensities and showed that the acoustic domain and the convective ridge merge at these relatively high speeds. The amplitude of the radiated pressure waves which propagate in the opposite direction of the flow is less than 1% of the aerodynamic pressure for the Mach 0.5 case. In this work, a numerical study of a Mach 0.5 turbulent boundary layer is investigated in presence of adverse and favorable pressure gradients (APG and FPG) using Large Eddy Simulation (LES). Pressure gradients are generated by means of curved geometries. An adverse pressure gradient leads to a deceleration � = − � @� + @� +