Configuration of Perfectly Matched Layers (PMLs) for Acoustic-Structure Interaction Models
Care should be taken when setting up perfectly matched layers (PMLs) in models that include fluid domains and solid domains. In general models with several different materials, that being different solids or different fluids.
Two configurations exist for the PMLs in these systems:
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The PML regions of two physics (or two different material models) are adjacent and in contact. In this case, a single PML feature should be used for the two PML domains. Set the Typical wavelength from to User defined and enter a value based on the longest wavelength. For example, for an air-steel system enter: 5770[m/s]/freq
The shorter wavelengths should be captured by increasing the mesh resolution and/or increase the PML scaling curvature parameter. This will ensure optimal damping of all wavelengths in the system. In any case control the convergence of the results by increasing the number of mesh layers in the PML.
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Infinite Elements, Perfectly Matched Layers, and Absorbing Layers in the COMSOL Multiphysics Reference Manual.
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See Perfectly Matched Layers (PMLs) in Modeling with the Pressure Acoustics Branch in the Acoustics Module User’s Guide.
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The PML regions of two physics (or two different material models) do not touch. In this case, use a PML feature for each PML domain. Keep the default setting of Typical wavelength from to Physics interface and select the relevant physics interface under Physics.
The reason that two PML features cannot be used when the PML domains a adjacent to each other is that the scaling inside the PMLs will differ (because of different speed of sounds). This will lead to an unphysical situation at the interface between the two PMLs which results in errors in the solution. Spurious waves may be generated.
See the Acoustic-Structure Interaction with a Perfectly Matched Layer (PML) model in the Application Gallery on www.comsol.com. It is located here: https://www.comsol.com/model/acoustic-structure-interaction-with-a-perfectly-matched-layer-pml-23521