Attributes assigned to a model prior to meshing include element type and material number. Assign these attributes to volumes (VATT) to be meshed with 3D elements (FLUID30, FLUID220, and FLUID221) or areas (AATT) to be meshed with 2D elements (FLUID243 and FLUID244). Specify different material ID numbers for the various material regions.
For an acoustic FEA formulation, the mesh must be fine enough to minimize numerical dispersion effects from finite discretization. In general, the mesh should have at least ten low-order elements and five high-order elements per propagating or resonant wavelength of the material.
If different materials are present in the model, the mesh should have a smooth transition from the dense to the coarse. To obtain better accuracy and efficiency, use your best judgment and knowledge of the sound pressure field to determine appropriate mesh density. For example, if the pressure varies sinusoidally along the wide side of a rectangular cross section and is uniform along the narrow side, you might choose to mesh the cross section as shown in this figure:
Do not mix low-order and high-order elements together in a model. Unlike low-order elements, high-order elements have midside nodes to follow the curvature of a model.
If using MESH200 elements to generate 3D acoustic elements FLUID30, FLUID220, or FLUID221 (VEXT), select one of the following MESH200 options:
Example 6.1: Generating FLUID220 Elements
In a FLNS model, the mesh should be fine enough to resolve the viscous and thermal boundary layer near the wall. For more information on the evaluation of boundary layers, see The Full Linear Navier-Stokes (FLNS) Model in the Theory Reference.