Assessment of Sound Radiation of semi-buried Engine Intakes

Abstract

The sound radiation from the intake of unconventional aeroengine intakes was studied computationally. The configuration is characterized as a nacelle which is buried into an aerodynamic surface (fuselage or wing) with various burying levels. The influence of the flow field on the sound radiation was studied as well. After generating a curvilinear structured 3D multi-block grid for the given configuration, the precalculated solution of DLR’s CFD finite volume code Tau was interpolated from its unstructured mesh onto the structured one for the acoustic calculations. The simulation of aeroacoustic propagation out of the engine inlet was performed using DLR’s CAA code PIANO (perturbation investigation of aerodynamic noise), which solves the (non)linear Euler perturbation equation. PIANO is a high resolution, high order finite difference code advancing the numerical solution in time by the well known 4-stage Runge-Kutta scheme. Spatial derivatives are discretized by the dispersion relation preserving (DRP) scheme of C.K.W. Tam et. al. A 4th-order pade filtering is used to eliminate spurious waves. As a generic source plane waves of 1000 Hz (blade passing frequency) were enforced by a sponge-layer technique to enter the duct from downstream of the given fan face position (i.e. in the unphysical duct extension). For two levels of burying the case without flow served as a reference. The spatial structure of the sound field was evaluated on distinct sections (symmetry, i.e. y = 0, and ground plane, i.e. z = 0) of the three-dimensional computational domain. The directivities in the planes of three circles (perpendicular to each other) were determined by recording the pressure time histories. The influence of different mean flows as well as burying levels was proven. Contrary to the 1/2-buried case the 1/3-buried geometry causes a non-axi-symmetric lateral directivity without flow. It could be demonstrated how an only locally too poor grid resolution may result in a completely non-physical solution. The actual viscous mean flow plays an important role, especially because of its (even poorly resolved) boundary layers. The results underline the need of using a perturbation approach of LEE type to describe the physics of the sound radiation from a buried intake properly. No self sustained flow oscillations (resonances) were observed in any considered case.