Jet noise from instability mode interactions

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Air travel continues to expand in volume and noise disturbance from aircraft jet engines is a continuing issue for those living and working near airports. Engine manufacturers have an excellent tack record of reducing the noise from engines, but further progress depends on better underdstanding of the mechanisms of sound production. This project will develop a recently-proposed idea that some of the most important noise may come from nonlinear interactions between instability modes that propagate along the jet. Previous research has only considerd a linear mechanism, with limited success for high speed jets. Since the instability modes themselves can be efficiently computed with well-known techniques of stability analysis, all the quadratic nonlinear interactions can in principle be computed relatively quickly. This project will develop the technique by coupling a base flow with the acoustic field via combination of stability modes. A series of direct numerical simulations will be run so that the technique can be evaluated for accuracy over a range of test cases, including binary mode interactions, transition to turbulence and fully turbulent cases where the effect of the nozzle lip can be included. Published laboratory data will also be used for reference. The tests will determine direction and frequency ranges in which the nonlinear interaction mechanism is an important source of sound. Understanding this mechanism should then allow other sources to be isolated.