Analysis of Jet-Noise-Reduction Concepts by Large-Eddy Simulation

The paper outlines the latest improvements to a CFD/CAA numerical system developed by the authors starting in 2001, and presents its application to the evaluation of three noise-reduction concepts. The improvements include a two-step RANS-LES approach to represent complex nozzles much more faithfully, and an accurate algorithm for shock capturing in LES, now based on local automatic activation of flux-limiters. The noise-reduction concepts considered are: beveled nozzles, dual nozzles with fan-flow deflection, and chevron nozzles. The simulations are carried out on PC clusters with at most six processors and on rather modest grids (2–4 million nodes). Nonetheless, in most cases the system is close to the 2–3 dB target accuracy both in terms of directivity and spectrum, while limited in terms of frequency (to a diameter Strouhal number that ranges from 2 to 4, depending on the grid used and the flow regime). Although this limitation is significant, especially for chevron nozzles, the overall message of the paper is that the available CFD/CAA numerical and physical models, properly combined, are capable of predicting the noise of rather complex jets with affordable computational resources, and already today can be helpful in the rapid low-cost analysis of noise-reduction concepts.