Several Computational Aeroacoustics Solutions for the Ducted Diaphragm at Low Mach Number

Abstract

A hybrid method of aeroacoustic noise computation based on Lighthill’s Acoustic Analogy is applied to investigate the noise radiated by a low Mach number flow through a ducted diaphragm. The simulation method is a two-step hybrid approach relying on Lighthill’s acoustic analogy, assuming the decoupling of noise generation and propagation. The first step consists of an incompressible Large Eddy Simulation of the turbulent flow field, during which aerodynamic quantities are recorded. In the second step, a variational formulation of Lighthill’s Acoustic Analogy using a finite element discretization is solved in the Fourier space. The sensitive steps consisting in computing and transferring source term data from the fluid mesh to the acoustic mesh are reviewed. Indeed, as fluid and acoustic meshes have different constraints due to the different wavelengths to be resolved, interpolation is required. The method is applied to a three-dimensional ducted diaphragm with low Mach number flow. Although the configuration is symmetric, this study exhibits a very complex three-dimensional flow behavior. Four different aerodynamic solutions are compared with the Direct Noise Computation performed by Gloerfelt & Lafon. Good agreement is found in terms of mean flow as well as on instantaneous behavior and turbulent intensities. Acoustic computations are performed with different mesh refinement and interpolation methods. Comparison with literature data on similar cases shows the method relevancy.