A ray-tracing method applied to the propagation of jet noise

Aircraft noise remains a major obstacle for air traffic growth as new aircraft must meet ever more stringent certification requirements for noise emissions around airports. A large part of aircraft noise arises from the engine and manufacturers have taken steps to make them quieter. In the case of turbofan aircraft this has been achieved largely by increasing the bypass ratio. Today, there is little room left for further reduction of noise in this way and more novel solutions must be found. One such method is to redesign exhaust nozzles in order manipulate the flow in a way that gives an acoustic benefit, for example in non-circular nozzles a larger flow field may shield the noise sources more efficiently. In turn, this requires more advanced noise prediction tools. In this paper, a jet noise prediction method called LRT, which is based on Lighthill's Acoustic Analogy coupled with a Ray-Tracing theory is presented. A full 3D Ray-Tracing method is developed which provides information about the refraction effects due to wave propagation in a jet flow. Using the turbulence information obtained from a RANS CFD simulation and the refractions effects obtained from the Ray-Tracing, the LRT method calculates the far-field noise. The classic jet noise prediction method, known as MGBK, is also used here for comparison. Results are presented for subsonic single-stream jets operating at Mach 0.5, 0.75 and 0.9. The refraction results obtained using the ray tracing method are compared with those found using Lilley's wave propagation equation. Comparisons have shown that the ray tracing method works well for all polar angles outside the zone of silence. The far-field noise comparisons have shown that the LRT method is capable of capturing the peak frequency much better than the MGBK method. The general trend of the spectrum at high and low frequencies as obtained using the new method is also better than those found using MGBK.