Reduction of Radiation Efficiency in High-Speed Jets

The Lighthill acoustic analogy indicates that substantial noise reduction is possible in high-speed jets by reducing the radiation efficiency. This entails reducing the convective Mach number of the eddies whose footprint is sensed in the near pressure field. In dualstream jets, local reduction in convective Mach number is possible by inducing asymmetry in the plume that redistributes the most energetic eddies from the fast inner stream to the slower outer stream on the underside of the jet. Assuming that the convective velocity of the most energetic eddies is the mean velocity at the location of the peak turbulent kinetic energy, a RANS-based acoustic analogy predicts with reasonable fidelity the noise reduction measured experimentally. Importantly, it is shown that the reduction in the source strength is caused primarily by the reduction in radiation efficiency. A large eddy simulation of a single-stream high-subsonic jet allows determination of the convective velocity through space-time correlations throughout the computational domain. The convective velocity at the edge of the jet, defined as the boundary between the inner rotational field and the outer linear pressure field, is close to the mean axial velocity at the location of peak kinetic energy. This provides a tool for RANS-based acoustic analogy models to accurately predict the radiation efficiency and its reduction.