Computational aeroacoustics including fluid-structure-coupling
with the finite element and the Lattice Boltzmann Method

Siemens AG, Munich, Germany

In this presentation a bidirectional coupling procedure, enabling the calculation of aeroacoustic phenomena in connection with coupled effects that can occur if structure and fluid influence each other mutually, is discussed. The approach is useful to describe problems typically occurring e.g. in the area of aviation and railway, in devices containing fans like computers or household appliances, or in pipes or HVAC systems. Fluid flows can lead to sound generation often experienced as disagreeable, at the same time exiting vibrations of structures. Examples for such vibrating structures, in turn generating sound themselves, are i.e. pipe walls or components outside cars, trains or airplanes (like mirrors, antennas, pantographs).
Both flow and structure, as well as the interaction between them are to be considered. For the computation of the structural behavior, the finite element method was applied, calculating vibrations caused by fluctuating flow pressures acting onto the surfaces. On the fluid side, instead of so-called "classical CFD", where a macroscopic model is used by solving the discretized Navier-Stokes equations, an alternative approach was chosen : the lattice-Boltzmann method, a ’mesoscopic’ formulation based on a strongly simplified kinetic theory and likewise approximating the Navier-Stokes equations, enables time explicit and very fast computation of compressible flows. As it is also able to represent acoustic wave propagation, enabling the concurrent approximation of flow and acoustic field, it was considered as extremely suitable for the purpose.
It was decided to use commercially available programs both for the finite element part (ANSYS) and the Lattice Boltzmann part (ExaPowerflow) of the calculations in order to benefit from the large number of possibilities offered by this software packages. A special coupling routine was developed, that provides interfaces for the bidirectional exchange of result data, considering the typical differences between CFD grids and finite element meshes and controlling time step sizes as well as result output etc. Coupled computations have been performed for theoretical models like flows around certain obstacles (i.e. bars or cylinders, causing the well-known "von Kármàn vortex streets" or harmonic eddy separation as sources of sound). Furthermore, to demonstrate the suitability of the method for more complex models, representative industrial problems were examined.

Laboratoire de Mécanique des Structures et des Systèmes Couplés - LMSSC