Aeroacoustic noise prediction of helicopter rotor in forward flight using Kirchhoff method

For modem helicopter rotor design, low noise radiation has been taken as one of the most important design targets. Upon use of CFD (Computational Fluid Dynamics) technologies and Kirchhoff formulation, an efficient method is developed as an analytical tool to reduce acoustic noise generated by rotor in forward flight. The flow field of rotor in forward flight is quite complex, especially when rotor blades work with unsteady loads. By use of dual time stepping method and moving grid-strategy, the 3D Euler's equations are numerically solved to obtain the near-field acoustic noise. Then, Kirchhoff formula is employed to calculate the acoustic pressure signal of far-field observing points base on the near-field solution. Kirchhoff formulation can be used to predict the noise radiation from moving surface, but the integration surface must be placed in the linear region of flow field because Kirchhoff formula is just the solution of linear wave equation. A nonrotating cylinder that covers the nonlinear flow region is generated as the integration surface and a high efficient 3D linear interpolation from CFD solutions is introduced. The integral is preformed on the integration surface, but the source variables are evaluated at the retarded time. To examine the correctness of the implementation of our method, the pressure signal of a simple point source flow and an AH-l/OLS model rotor in forward flight are calculated. The computed result shows good agreement with the theoretical solution and experimental data, respectively, and it indicates that our method is efficient and is capable of capturing the nonlinear quadrupole noise for transonic rotor.