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Numerical prediction of noise production and propagation

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VKI PHDT 2012-05, Lilla Kapa Koloszár, Numerical prediction of noise production and propagation, ISBN 978-2-87516-026-3, 253 pgs

Numerical prediction of noise production and propagation

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Numerical prediction of noise production and propagation
By Lilla Kapa Koloszár

PhD Thesis from the von Karman Institute/Université Libre de Bruxelles, November 2011, 978-2-87516-026-3, 253 pgs

Abstract

The thesis deals with numerical simulation of sound generation and propagation by turbulent flows. Numerical simulation of noise production  and propaga- tion is a very complex problem. A methodology fitting for one particular  problem can fail for an other one. So there are no general guideline how to deal with such phenomena.   In the present work, a Con- trolled Diffusion (CD-) airfoil placed in the potential core of a jet is considered. Noise is generated mainly by the fluctuating body forces on the profile and it is scattered by the solid walls, as well as, by the shear layers of the jet. The flow velocity is low and it can be assumed that there are no coupling  between acoustic and hydrodynamic. Therefore, in the source region an incompressible LES simulation is adequate. Moreover, due to the low Mach-number, the acoustic propagation  can be considered as linear,  so a hybrid approach can be an efficient  way of dealing with this problem. In the propagation field there is a rather significant region, where the mean flow is not uniform, but the sound production  is negligible compared to the noise emitted by the airfoil.  In this nearfield a linear set of propagation equation may be considered (LEE). Finally, the noise in the uniform flow can be propagated further by an integral method (if required).

For the noise simulation  produced by a CD airfoil in a jet flow the following 3-step approach seems to be the most adequate:
1. Incompressible LES simulation in the source region.
2. Linearized Euler Equations  to propagate  the noise through the shear-flow.
3. Acoustic analogy in the farfield, if necessary.

This thesis deals with the second item of this system (LEE), including interfacing with the other two steps. The discretization of the LEE is done through Residual Distribution Method (RDM). Since this method was not applied before for acoustic problem, an extensive theoretical study was performed to discover its properties if wave propagation phenomenon is considered. The implementation  is than verified with different analytical  testcases both in 2D and 3D. Through the verification an important  issue was discovered. The basic non-reflecting boundary conditions in the literature  was giving rather significant  reflections if vortical structures were passing the boundaries. A special focus was given to these problem  and new boundary formulation  was proposed. In the verification process the source of waves were was always analytical expressions. In order to be able to handle aerodynamically  generated noise problems a reconstruction method was implemented.  Such way LES/DNS  simulations resolving noise production can be directly used as sources for the propagation simulation. The developed hybrid method was finally applied to compute the wave propagation pattern of the noise emitted by a Controlled-Diffusion  CD-airfoil through shear  flow due to the windtunnel.

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