# Computational aeroacoustics

The Computational Aero Acoustic (CAA ), the international term for the numerical aeroacoustics, used methods that numerically calculate the Umströmungsgeräusch with different simplifying assumptions.

## Calculation methods

An important backbone for CAA forms the computational fluid dynamics computational fluid dynamics ( CFD), the method for the simulation of aerodynamics provides. These are commercially available for some time and give statements on alternating pressures on the surfaces of, and fluctuations in the flow around, which may be significant acoustic sources.

The methods for simulating the Umströmungsgeräusches can in principle be in the direct numerical simulation (DNS) and the hybrid approaches divide. The DNA is based on the compressible Navier -Stokes equations and looking at both the flow relevant (small lengths, large amounts of energy ) and the acoustic scale (large lengths, small energy) simultaneously. It will calculate all turbulent scales directly, so there is no parameterization or modeling instead. A serious drawback, however, arises from the enormous computational effort that is required. Thus, the DNA remains at present still limited to academic cases.

The hybrid approaches using CFD methods with different turbulence models. That is, in different ways, the turbulence intensities are calculated on different length scales. The common methods are:

- Reynolds averaged Navier - Stokes ( RANS ) with statistical turbulence model: stationary process, all turbulence scales are modeled and not calculated directly

- Unsteady Reynolds averaged Navier - Stokes ( URANS ): unsteady RANS

- Large Eddy Simulation (LES ): Calculation of the large and modeling of the small scales

- Very Large Eddy Simulation ( VLES ): LES in which the boundary between computation and modeling at larger scales is

- Detached Eddy Simulation ( DES): RANS method with higher resolution areas where such LES is expected

From the directly calculated or modeled pressure and velocity fluctuations, which serve as source terms, then with the help of the linearized Euler equations ( LEE ) or acoustic analogies is determined, the sound field. Some common acoustic analogies include among others the method according to Lighthill, Ffowcs Williams and Hawkings.

To calculate the sound radiation into the interior of body parts or aircraft panels, which are excited by the flow vibrations, called boundary element methods ( Boundary Element Methods, BEM ) are particularly so. Finite element methods (FEM ) can account for the fluid-structure coupling and possible leaks beyond. However, they require significantly more computer resources.

In the hybrid approach, the acoustic quality of the results is highly dependent on the quality of the turbulence modeling. This therefore has to be chosen with care. Especially for the determination of absolute values aeroacoustic simulations, however, are ( so far) only partially suitable. The accuracy is often sufficient only low requirements in the simpler method.

Reason for these problems is that the error produced by the numerical methods in the worst case overlay the acoustics directly.

## Software

Meanwhile number of powerful commercial software tools are available for the CAA. Especially exist in the aerospace industry, mostly at universities and research facilities, and specially developed programs.