Numerical Analysis of Airframe Noise Scattering Effects in Tilt Rotor Systems

Abstract

The present paper illustrates a computational aeroacoustic method for evaluating the effects of airframe scattering on the noise generated by a tilt-rotor system under realistic operating conditions. The physical model assumes ideal fluid, irrotational flow, and linear acoustic perturbation of the base flow. In a first step, the aerodynamic field about the rotating blades and the airframe is computed by solving a Laplace equation for the velocity potential, with vortical wakes shed from the blades and the wing, and freely convected in the field. In a second step, the unsteady pressure field on the blades is used to compute the noise signals around the prop rotor by means of an acoustic analogy approach. Then harmonic components are extracted from these signals and used to compute an equivalent source distribution in the prop-rotor region. In a third step, a convected wave equation for the acoustic velocity potential, with equivalent source terms, is solved in the frequency domain. This provides the tonal sound field generated by the prop rotor, propagating in the nonuniform flow past the airframe and reflected by structural components. In a final step, a Kirchhoff formulation is used to extrapolate the acoustic field past the aircraft in the far field, where the noise levels are compared to those generated by the same, but isolated prop rotor. As expected, significant scattering effects are observed at high blade passage frequencies.