Transonic Helicopter Noise

Abstract

Helicopter noise is an increasingly important issue, and at large forward-flight speeds transonic rotor noise is a major contributor. A method for predicting transonic rotor noise, which is more computationally efficient than previous methods and which furthermore offers physical insight into the noise generation, is developed. These benefits combine to make it of potential use to helicopter rotor designers. The permeable surface form of the Ffowcs Williams-Hawkings (FW-H) equation is used to express the sound field in terms of a distribution of monopole and dipole sources over a permeable control surface and a distribution of quadrupole sources over the volume outside of this surface. By choosing the control surface to enclose the transonic flow regions, the noise from the quadrupole distribution becomes negligible. Only the more straightforward surface sources then need be considered, making the acoustic approach computationally efficient. By locating the control surface close to the blade subject to enclosing the transonic flow regions, efficiency in the computational-fluid-dynamics (CFD) approach is also attained. To perform noise predictions, an Euler CFD method to calculate the flowfield was combined with an acoustic method incorporating the retarded time formulation of the FW-H equation. Several rotor blades in hover and steady forward flight were considered, all of which involved transonic flows but for which shock delocalization did not occur. The predictions showed very good agreement with experimental data and with predictions obtained using more computationally intensive methods.