Numerical aeroacoustics

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Computational Aeroacoustic ( CAA ), the international term for numerical aeroacoustics , uses methods that numerically calculate the noise around the airflow using various simplifying assumptions .

Calculation method

Methods for the numerical calculation of flow noise

An important backbone for CAA forms the CFD (computational fluid dynamics, CFD), the procedure for simulating the aerodynamic provides. These have been commercially available for some time and provide information about alternating pressures on the surfaces and fluctuations in the flow, which can be significant acoustic sources.

The methods for simulating the flow noise can be divided into direct numerical simulation (DNS) and hybrid approaches. The DNS is based on the compressible Navier-Stokes equations and considers both the flow-relevant (small lengths, large energies) and the acoustic scales (large lengths, small energies) at the same time. All turbulent scales are calculated directly, so there is no parameterization or modeling. A serious disadvantage, however, results from the enormous computational effort that is required. The DNS is currently limited to academic cases.

The hybrid approaches use CFD methods with different turbulence models. I.e. The degrees of turbulence are calculated in different ways on different length scales. The common procedures are:

  • Reynolds averaged Navier-Stokes (RANS) with statistical turbulence model: stationary method, all turbulence scales are modeled and not calculated directly
  • Unsteady Reynolds averaged Navier-Stokes (URANS): unsteady RANS
  • Very Large Eddy Simulation (VLES): LES in which the boundary between calculation and modeling lies with larger scales
  • Detached Eddy Simulation (DES): RANS method with higher resolution areas in which calculations are carried out like LES

The sound field is then determined from the directly calculated or modeled pressure and speed fluctuations, which serve as source terms, with the help of linearized Euler equations (LEE) or acoustic analogies. The common acoustic analogies include the Lighthill , Ffowcs Williams and Hawkings methods .

For the calculation of the sound radiation into the interior of body parts or aircraft panels that are excited by the flow to vibrations, are especially so-called boundary element methods (Boundary Element Methods, BEM). Finite element methods (FEM) can also take into account the fluid-structure coupling and possible leaks. However, they require significantly more computer resources.

With the hybrid approaches, the quality of the acoustic results is strongly dependent on the quality of the turbulence modeling. Accordingly, this must be selected with care. However, aeroacoustic simulations are (so far) only suitable to a limited extent for determining absolute values. With the simpler methods, the accuracy often only meets low requirements.

The reason for these problems is that the errors produced by the numerical method, in the worst case, directly overlay the acoustics.

software

Several powerful commercial software tools are now available for the CAA. There are especially in aviation technology, mostly at universities and research institutes, but also specially developed programs.

literature

  • Goldstein, ME: Aeroacoustics . New York: McGraw-Hill Book Company, 1976
  • Schütz, T .; Green, N; Blumrich, R .: Numerical Methods . In: Schütz, T. (Ed.): Hucho - Aerodynamics of the automobile. Berlin Springer, 2013, ISBN 978-3834819192 .

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