Broadband shock-associated noise predictions

Broadband shock-associated noise (BBSAN) is generated when a jet operates off-design. This can occur in both military fighter aircraft turbojet engines and commercial turbofan engines, particularly at cruise conditions. The noise is generated by the interaction between the turbulence in the jet shear layer and the jet's shock cell structure. The present paper describes a model for BBSAN that uses steady RANS CFD to describe the jet flow and to provide information on the turbulence scales. The acoustic model is based on a rearrangement of the equations of motion into a propagation component (the linearized Euler equations) and a source component, which depends on products of the turbulent fluctuations and the shock cell pressure perturbations. A model is introduced to describe the statistical properties of the turbulence and the shock cell structure is determined from the RANS simulations. Predictions are made for circular and rectangular single stream jets, both unheated and heated, and both under- and over-expanded conditions. Comparisons of the predicted BBSAN spectra are made with experiments.