Physical processes influencing acoustic radiation from jet engine inlets

Abstract

Numerical simulations of acoustic radiation from a jet engine inlet are performed using advanced computational aeroacoustics algorithms and high-quality numerical boundary treatments. As a model of modern commercial jet engine inlets, the inlet geometry of the NASA Source Diagnostic Test is used. Fan noise consists of tones and broadband sound. This investigation considers the radiation of tones associated with upstream-propagating duct modes. The primary objective is to identify the dominant physical processes that determine the directivity of the radiated sound. Two such processes have been identified. They are acoustic diffraction and refraction. Diffraction is the natural tendency for an acoustic duct mode to follow a curved solid surface as it propagates. Refraction is the turning of the direction of propagation of a duct mode by mean flow gradients. Parametric studies on the changes in the directivity of radiated sound due to variations in forward flight Mach number, duct mode frequency, azimuthal mode number and radial mode number are carried out. It is found there is a significant difference in directivity for the radiation of the same duct mode from an engine inlet when operating in static condition versus one in forward flight. It will be shown that the large change in directivity is the result of the combined effects of diffraction and refraction.

Christopher Tam: Brief history and accomplishments

“…Tam et al 98 shed light on the physical processes that control noise radiation from engine inlets. Two important effects of diffraction and refraction on inlet fan tones were identified and explained.…”

Turbofan Inlet Distortion Noise Prediction with a Hybrid CFD-CAA Approach

“…In the lateral direction the 10 angle of attack case shows a strong tendency to radiate to one side (Figure 12, front and top view) with highly distorted waves leaving the nacelle. As discussed by Tam et al 17 , a major impact on the directivity for a given frequency and mode are diffraction and refraction effects caused by the mean flow orientation (stagnation points and overspeed regions). Tam et al 17 had investigated these effects separately and for a symmetric inlet contour.…”

“…As discussed by Tam et al 17 , a major impact on the directivity for a given frequency and mode are diffraction and refraction effects caused by the mean flow orientation (stagnation points and overspeed regions). Tam et al 17 had investigated these effects separately and for a symmetric inlet contour. Here, the sound field is composed of a large number of circumferential modes (see Figure 9) and the mean flow is non-uniform, resulting in a more complicated directivity pattern.…”

A Simplified Linearized Lattice Boltzmann Method for Acoustic Propagation Simulation

“…The phenomenon of acoustic waves propagating in complex flows such as shear layers or a vortex often exists in aerospace engineering [ 1 , 2 , 3 , 4 ]. Studies have shown that such flow structures will change the characteristics of acoustic wave propagation, leading to refraction, reflection, and scattering and thus affect the measurement and localization of sound source [ 5 , 6 ].…”