Vorticity Dynamics and Flow Statistics Near the Edge of High-Speed Multi-Stream Jets

摘要

We investigate the vorticity dynamics and flow statistics near the edge of multi-stream, high-speed jets for the purpose of developing linear surface-based models for the noise source. Those models would be informed by low-cost, Reynolds-Averaged Navier-Stokes (RANS) computations of the flow field. The study encompasses two triple-stream jets, one coaxial and the other eccentric, and a single-stream round jet. Large Eddy Simulations (LES) validate RANS-based models for the convective velocity U_c of the noise-generating turbulent eddies in the jet flow. In addition, the LES results help define a "radiator surface" on which the jet noise source model would be prescribed. The radiator surface is located near the boundary between the rotational and irrotational fields and is defined as the surface on which the U_c distribution, obtained from space-time correlations of the pressure, matches that inferred from the RANS model. This surface overlaps with a band of negative skewness of the pressure. Examination of the instantaneous vorticity field shows vortices peeling off from the main flow and migrating towards the radiator surface outside of which their strength vanishes. The vortical events near the radiator surface help explain the negative pressure skewness. The edge of the mean vorticity is nearly coincident with the radiator surface, which suggests a straight-forward RANS-based criterion for locating this surface.