Towards a Low-Cost Wavepacket Model of the Jet Noise Source

We discuss the development of a simplified model for the jet noise source that can be used in practical predictions of installed noise. In the proposed scheme, the source is prescribed on a radiator surface defining the boundary between the inner rotational jet flow and the outer linear pressure field. The source consists of wavepacket-type partial fields whose noise propagation can be computed using well-established linear methods. The complete solution entails synthesizing the partial-field solutions based on the probability density functions of the partial fields. The ability of low-cost Reynolds Averaged Navier Stokes (RANS) solution of the flow field to guide the wavepacket formulation is assessed. The RANS solution is accompanied by acoustic analogy based matching of the far-field spectra to determine the coefficients of the appropriate time and length scales. The RANS-derived velocity, length, and time scales are compared with those obtained using Large Eddy Simulation (LES) of a cold Mach 0.9 jet. The RANS quantities are assessed on the surface of peak turbulent kinetic energy while the LES statistics are computed on the radiator surface. There is good agreement in the convective velocity, however RANS under-predicts the correlation scales on the radiator surface. The LES provides additional physical insights into the noise source and suggests that the partial fields have limited azimuthal extent.