Perturbation Approaches to the Simulation of Airframe Noise

The application of perturbation approaches to simulate noise mechanisms of aircraft during landing approach is discussed. Euler-perturbation approaches that simulate sound generation at obstacles due to upstream injected test-vorticity allow to estimate the efficiency of a geometry to transform vortical energy into acoustic energy. Acoustic analogies with sources determined either by an unsteady flow simulation or described statistically yield hybrid two-step methods that in addition take into account the turbulent source strength. A hybrid method is presented that predicts trailing edge noise based on a Large Eddy Simulation (LES) of the compressible flow problem in the immediate acoustic source region and acoustic perturbation equations (APE) for the simulation of the acoustic radiation problem in space in time. The mean flow convection and refraction effects are part of the simulation of wave propagation such that the computational domain of the flow simulation in general has to comprise only the significant acoustic source region. Using linearized acoustic perturbation equations the unbounded growth of hydrodynamic instabilities in critical mean flows is prevented completely