ROTOR HIGH-SPEED NOISE PREDICTION WITH A COMBINED CFD-KIRCHHOFF METHOD

A combined Computational Fluid Dynamics(CFD)-Kirchhoff method is presented for predicting high-speed impulsive noise generated by a hovering rotor blade. Two types of Kirchhoff integral formula are used: one for the classical linear Kirchhoff formulation and the other for the nonlinear Kirchhoff formulation. An Euler finite-difference solver, TURNS, is solved first to obtain the flow field close to the blade, and then this flow field is used as an input to a Kirchhoff formulation to predict the acoustic far field. These formulas are used at Mach numbers of 0.90 and 0.95 to investigate the effectiveness of the linear and nonlinear Kirchhoff formulas for delocalized flow. During these calculations, the retarded time equation is also carefully examined, in particular, for the cases of the control surface located outside of the sonic cylinder, where multiple roots are obtained. Predicted results of acoustic far field pressure with the linear Kirchhoff formulation agree well with experimental data when the control surface is well outside of the nonlinear aerodynamic flow field. Calculation based on the nonlinear Kirchhoff equation using a sonic cylinder as a data surface show a good agreement with experimental data for both tip Mach numbers of 0.90 and 0.95. The agreement supports the conclusion that a nonlinear formulation is necessary if the data surface is close to the blade and the flow is delocalized.