Limited Bandwidth Aeroacoustic Schemes with Applications to Rotorcraft

Acoustic prediction for transonic aerodynamic sources is a tough problem. The highly nonlinear aerodynamics as well as the singular acoustic integrals feature enough diculties for themselves, and their coupling does not simplify the procedure. This is the case for example for helicopter rotors. The blade surface moves at high speeds, and part of the grid is even supersonic. Therefore it is necessary to handle the integrals of the acoustic analogy in a most general and robust manner, appropriate for subsonic as well as sonic and supersonic sources. The concept of limited bandwidth early in the integration process is essential to overcome numerical as well as conceptual singularities of the acoustic integrals in transonic cases. The uid physics are used as a guiding principle. The concept of ecien t evaluation is generalised in this paper to wind tunnel tests of forward igh t cases, where the uid is in motion with respect to the observer. While we are most interested in helicopter noise prediction, our scheme can be applied to any exterior acoustic computation with sucien t accurate aerodynamic data avaliable. It is especially suited for surfaces at nearly sonic speeds, while little gain is expected for stationary or only slowly moving cases.