Abstract

CIMNE coffee talks

New developments in computational aeroacoustics for the simulation of human phonation using the Variational Multiscale Method

by Arnau Pont (CIMNE)
    
Wednesday, June 14th, 2017 - 15h   
O.C. Zienkiewicz Room, C1, Campus Nord UPC, Barcelona
 
Abstract

The numerical calculation of fricative sounds with a FEM code in realistic 3D geometries requires a very fine spatial discretization in order to capture all relevant turbulent scales, which leads to heavy parallel computations. For low speed flows, like in the present case, conservative compressible flow formulations might fail, for which the more affordable incompressible approach is often used. However, most available formulations in Computational Aeroacoustics based on incompressible flow either require modeling acoustic source terms or are not valid at the near field, where flow perturbations are still non-negligible. Therefore, new developments in both incompressible and compressible formulations and the corresponding results will be presented.

https://web.cimne.upc.edu/groups/publicacions/mails/2017/coffee-talks/coffee-talks-2017-06-14.html

Abstract

The computation of flow-induced noise at low Mach numbers usually relies on a two-step hybrid methodolgy. In the first step, an incompressible fluid dynamics simulation (CFD) is performed and an acoustic source term is derived from it. The latter becomes the inhomogeneous term for an acoustic wave equation, which is solved in the second step, often resorting to boundary integral formulations. In the presence of rigid bodies, Curle’s acoustic analogy is probably the most extended approach. It has been shown that Curle’s boundary dipolar noise contribution does in fact correspond to the diffraction of the quadrupolar aerodynamic noise generated by the flow past the rigid body. In this work, advantage is taken from this fact to propose an alternative computational methodology to get the individual quadrupolar and dipolar contributions to the total acoustic pressure. For any linear acoustic wave operator, the unknown acoustic pressure can be split into its incident and diffracted components and be computed simultaneously to the incompressible flow field, in a single finite element computational run. This circumvents the problem found in Curle’s analogy of needing the total pressure at the body’s boundary, which includes the acoustic pressure fluctuations. The latter cannot be obtained from an incompressible CFD simulation. The proposed unified strategy could be beneficial for a large variety problems such as those involving noise generated from duct terminations, or those related with the simulation of fricatives in numerical voice production, among many others.

Abstract

The high computational cost of solving numerically the fully compressible Navier–Stokes equations, together with the poor performance of most numerical formulations for compressible flow in the low Mach number regime, has led to the necessity for more affordable numerical models for Computational Aeroacoustics. For low Mach number subsonic flows with neither shocks nor thermal coupling, both flow dynamics and wave propagation can be considered isentropic. Therefore, a joint isentropic formulation for flow and aeroacoustics can be devised which avoids the need for segregating flow and acoustic scales. Under these assumptions density and pressure fluctuations are directly proportional, and a two field velocity-pressure compressible formulation can be derived as an extension of an incompressible solver. Moreover, the linear system of equations which arises from the proposed isentropic formulation is better conditioned than the homologous incompressible one due to the presence of a pressure time derivative. Similarly to other compressible formulations the prescription of boundary conditions will have to deal with the backscattering of acoustic waves. In this sense, a separated imposition of boundary conditions for flow and acoustic scales which allows the evacuation of waves through Dirichlet boundaries without using any tailored damping model will be presented.

Abstract

A sibilant fricative /s/ is generated when the turbulent jet in the narrow channel between the tongue blade and the hard palate is deflected downwards through the space between the upper and lower incisors and then impinges the space between the lower incisors and the lower lip. The flow eddies in that region become a source of direct aerodynamic sound, which is also diffracted by the speech articulators and radiated outwards. The numerical simulation of these phenomena is complex. The spectrum of an /s/ typically peaks between 4 and 10 kHz, which implies that very fine computational meshes are needed to capture the eddies producing such high frequencies. In this work, a large‐scale computation of the aeroacoustics of /s/ has been performed for a realistic vocal tract geometry, resorting to two different acoustic analogies. A stabilized finite element method that acts as a large eddy simulation model has been adopted to solve the flow dynamics. Also, a numerical strategy has been implemented that allows the determination, in a single computational run, of the separate contribution of the sound diffracted by the upper incisors from the overall radiated sound. Results are presented for points located close to the lip opening showing the relative influence of the upper teeth depending on frequency.

Abstract

This paper investigates the fuid-structure interaction phenomena and their effect on the aerodynamic noise radiation observed for a membrane airfoil subjected to a turbulent flow at a fixed angle of attack α= 20° . For this purpose, two cases with different Reynolds numbers (Re = 53,100 and Re = 79,700) are simulated. The applied partitioned fluid-structure interaction approach uses large eddy simulations to predict the incompressible flow problem and includes geometric non-linear effects for the structural problem. Following a hybrid method based on a hydrodynamic/acoustic splitting technique, the incompressible flow field solution provides the acoustic source term for the aeroacoustic computation. The results suggest a coupling between the vortex shedding, the dynamic structural response and the radiated aerodynamic sound for this configuration.

Abstract