Meshing
When solving a model using one of the Aeroacoustic interfaces, it is important to remember that this is a wave problem. This means that the wavelength should be resolved by an appropriate number of mesh elements. Using the same guidelines as for Pressure Acoustics is reasonable, that is, a first good mesh should use at least 5 elements per wavelength for second order shape functions. In the linearized Euler interface the default shape functions are first order and here using at least 15 to 20 elements per wavelength is a first good choice.
Meshing guidelines for the Pressure Acoustics interface is given in
Meshing (Resolving the Waves)
under the
Modeling with the Pressure Acoustics Branch (FEM-Based Interfaces)
section.
The Background Flow
Another important parameter to consider is resolving details in the background mean flow field. The acoustic mesh should capture gradients that exist in the background flow as these have a large influence on the acoustic propagation. Waves may, for example, be reflected and refracted in shear layers.
Meshing the Acoustic Boundary Layers in LNS
The Linearized Navier-Stokes interfaces just as the Thermoviscous Acoustics interface captures the physics of the acoustic boundary layer. When no-slip and isothermal conditions are used on walls a viscous and a thermal boundary layer will exist. In order for the solution to be well behaved an capture losses correctly it is important to mesh this layer, for example, by using a boundary layer mesh. If the model is large using a single boundary layer mesh, with the approximate extend of the acoustic boundary layer, is the minimum requirement for a good solution.
Resolving Vorticity and Physics in LE and LNS
In the LE and LNS interfaces the GLS stabilization is very efficient and can ensure smooth and converged solution even without resolving details like vorticity generation (the propagation of vorticity waves). If these are important processes in the model the mesh should of course be able to resolve these details. Either refine the mesh or switch to (P2, P2, P2) discretization (keep the stabilization turned on). For LNS models these phenomena are typically generated at walls where the no-slip condition generates vorticity when an acoustic wave is interacting with the background flow.
