Comparison of surface and volume integral methods for transonic propeller acoustic predictions

Abstract

A precise comparison of the surface and volume approaches of the Ffowcs Williams-Hawkings acoustic integral formulation is conducted for a propeller in transonic operating conditions. For both approaches, the calculations are carried out directly starting from CFD input data provided in the propeller rotating frame, i.e. with supersonically moving emission points. No approximation in the volume calculations, which could distort the comparison between both methods, is made. The principle and the calculation algorithm on which this particular integration technique is based are reviewed. Then calculations are carried out for four increasingly refined CFD meshes. They first confirm that both acoustic integral methods provide identical results when the numerical dissipation is negligible in the aerodynamic calculation. These calculations also show that the volume method is slightly less sensitive to the numerical dissipation than the surface method. However, the gain seems low compared to the computational cost of the volume integration. In addition, two techniques for determining the regions of the dominant acoustic sources are explored. With the first one, a rather conventional technique based on the local quadrupole term, the results show that specific terms, chosen according to the phenomenon concerned, may be better indicators of the real noise sources than the original shear and entropy terms. The second one, less known and consisting in calculating the elementary acoustic time signature radiated by each cell of the grid, seems more effective but may turn out to be costly in terms of data storage with the volume method.