Helicopter noise is one of the most important issues facing the helicopter industry today, for reasons including environmental acceptability of ground noise levels, passenger comfort and acoustic detectability. At large forward flight speeds, transonic rotor noise is a major contributor. A method has been developed to predict this noise which is more computationally efficient than previous methods, with a greater physical insight into the noise generation and is therefore more useful to helicopter rotor designers.
When helicopter blades move at high subsonic speeds patches of supersonic flow occur near the blades and these greatly influence the character and amplitude of the far-field sound. Traditional approaches either involve calculating a volume integral with high resolution or extending a CFD solution to a mid-field Kirchhoff surface. In both cases computational time can be prohibitive. Instead, we use an extension of the acoustic analogy based on a control surface which is chosen to be as small as possible, while enclosing the blade and any local patches of supersonic flow. The far-field sound can then be expressed solely in terms of mass and momentum fluxes through this control surface. We use CFD, with high resolution shock capturing techniques, to calculate the flow field near the blade.
Particular emphasis is placed on developing simple models to describe and explain dynamic changes in the acoustic sources in forward flight, thereby reducing CFD requirements by an order of magnitude. This near-field flow has been validated against transonic wind-tunnel and full-scale data. Predictions have been obtained for the far-field sound both in hover and in forward flight, and compared with measured pressures, with good agreement.
Collaborators and Support
Westland helicopters, Thales Underwater Systems.