Theoretical Acoustic Prediction of the Aerodynamic Interaction for Contra-Rotating Fans

Abstract

The theoretical prediction formula of far-field interaction tonal noise radiation by contra-rotating fans in free space with point force approximation is derived as the extension of the method of William K. Blake for the conventional stator–rotor interaction. Two primary mechanisms for interaction noise reduction are easily found based on a simple analysis of the formula, which are either to increase the order of the Bessel function of the first kind or to reduce the unsteady lift force. The latter can be realized by reducing the magnitude of the wake centerline velocity deficit or broadening the wake width, which are both related to the interstage separation distance. The aerodynamic and acoustic effects of inter-rotor axial spacing are studied experimentally. It is found that the axial spacing has little effect on aerodynamic performance of the contra-rotating fan, when the nondimensional inter-rotor axial gap based on the axial rotor chord varies in the measured range of 0.18–1.36. Sound pressure level follows an exponential decay law with interstage spacing, from which it is predicted that the three-dimensional rotor wake follows an exponential decay law as well and it decays much faster than the two-dimensional cascade wake. When the nondimensional inter-rotor axial gap is larger than 0.41, it has little effect on the noise level. On the contrary, the interaction noise is very sensitive to the axial spacing in the range below 0.41.