Application of Omega vortex identification method in cavity buffeting noise

Abstract

The cavity buffeting noise is related to the free shear layer oscillation and the periodic vortex shedding, where weak vortices coexist with strong vortices and the strong shear phenomenon also exists at the opening of the cavity. Therefore, it is of great significance to accurately capture vortices at the opening for the control of the cavity buffeting noise. This paper first compares the Omega vortex identification method with the Q and λ2 criteria based on the large eddy simulation (LES) of the backward-facing step flow, and it is found that the Omega method enjoys the following advantages: it is not sensitive to a moderate threshold change and Ω = 0.52 can be used as a fixed threshold, it can capture both the strong and weak vortices at the same time; and it will not be contaminated by the shear. Then the Omega (Ω) method is applied to the LES of the cavity buffeting noise: the mechanism of the cavity buffeting noise is studied based on a simple cavity model firstly, and then the effects of the incoming boundary layer thicknesses and the incoming boundary layer shapes on the cavity buffeting noise are analyzed. The results show that: the Ω method clearly captures the processes of the vortex generation, development, collision and fragmentation, verifying that the generation of the cavity buffeting noise is related to the free shear layer oscillation and the periodic vortex shedding; as the thickness of the incoming boundary layer increases, the free shear layer becomes more stable and the Helmholtz resonance is avoided effectively, thereby the cavity buffeting noise is reduced effectively, adding a convexity upstream of the cavity opening to interfere the shape of the incoming boundary layer to reduce the acoustic feedback effect can reduce the cavity buffeting noise effectively.