S. Lee et al., PREDICTION OF ROTOR HIGH-SPEED IMPULSIVE NOISE WITH A COMBINED CFD-KIRCHHOFF METHOD, Journal of sound and vibration, 207(4), 1997, pp. 453-464
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.