A New Finite-Difference Method for General Long-Range Rotorcraft Acoustics: Initial Comparisons with Intermediate-Range Data

A new method is developed for the computation of long-range rotorcraft acoustics. The approach is a hybrid of near and far-field methods, and is unique in its Eulerian treatment of the far-field propagation. The near-field employs a standard CFD solver (GT-hybrid) together with an acoustic analogy (WOP-WOP) to project an acoustic solution onto a spherical surface that surrounds a rotorcraft (at a distance of about 5-10 rotor radii). The acoustic field on this source surface is then extended to an arbitrarily large distance in an inhomogeneous far-field, which would normally require an Eulerian solution of the wave equation - except for the fact that conventional Eulerian methods (those that propagate a fully resolved wave) have prohibitive grid requirements. This problem is overcome by using "Wave Confinement" (WC), a new method that propagates wave-identifying phase fronts as nonlinear solitary waves. The method has been tested by comparisons with analytic, ray tracing and Green's function methods. Presently, first comparisons are being made with field test data.