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

A combined Computational Fluid Dynamics(CFD)-Kirchhoff method is prese nted for predicting high-speed impulsive noise generated by a rotor in hover. Two types of Kirchhoff integral formulas are used: one of them is a classical linear Kirchhoff formulation and the other a non-linea r Kirchhoff formulation. An Euler finite-difference solver is executed first, from which a flow field is obtained to be used as an input to the KirchhofF formulation to predict the acoustic far-field. The calcu lations are performed at Mach numbers of 0.90 and 0.95 to investigate the effectiveness of the linear and non-linear Kirchhoff formulas for delocalized flow. During these calculations, the retarded time equatio n is also carefully examined, in particular, for the cases where a con trol surface is located outside the sonic cylinder, for which multiple roots are obtained. Predicted results of acoustic far-field with the linear Kirchhoff formulation agree well with the experimental data whe n the control surface is at a particular location (R-cs/R = 1.46), but the correlation weakens as it moves away from this specific location of the control surface due to the delocalized non-linear aerodynamic f low field. Calculations based on the non-linear Kirchhoff equation usi ng the sonic cylinders as the control surfaces show reasonable agreeme nts with the experimental data in the negative amplitudes for both tip Mach numbers of 0.90 and 0.95, except for some computational integrat ion problems over a shock. It can be concluded that a non-linear formu lation is necessary if the control surface is close to the blade and t he flow is delocalized. (C) 1997 Academic Press Limited.