Surface pressure spectrum variation with Mach number on a CD airfoil

Abstract

The surface pressure spectrum characteristics are studied on a Controlled Diffusion airfoil at varying Mach numbers and a fixed Reynolds number. Compressible large eddy simulation datasets have been used for this analysis. The study focusses on the noise generation mechanism and the source locations responsible for it. A mid-high frequency hump is observed for the higher Mach number cases in the pressure spectrum and the amplitude of this observed hump increases with Mach number. To investigate the cause of this hump, different hydrodynamic and acoustic pressure modes traveling at specific speeds are obtained by a wavenumber–frequency decomposition. The acoustic component originating from the separation bubble region near the leading edge is found to be responsible for the hump in the pressure spectrum. The interaction of hydrodynamic and acoustic waves also plays a significant role in the appearance of the hump in the spectrum. There is also an increase in the amplitude of the high frequency region in the acoustic spectrum as we go towards the trailing edge for higher Mach number which can be attributed to an increase in turbulent fluctuation amplitude in the leading edge region. Furthermore, stability analysis has been performed on one-dimensional mean boundary-layer profiles. The higher reverse velocity in the separation bubble with increasing Mach number seems to be causing higher growth rates of instabilities, ultimately resulting in the significant surface spectrum variations. The directivity in the farfield also confirms a significant contribution of the hump in the spectrum in the overall sound pressure level.