Absorbing Layers for the Linearized Euler, Transient Interface
The Linearized Euler, Transient Interface includes so-called absorbing layers, also often referred to as sponge layers. The layers work by combining three techniques: a scaling system, damping, and a simple nonreflecting condition.
The layers are set up by adding the Absorbing Layer under the Definitions node. This adds a special scaled system. The scaling effectively slows down the propagating waves and ensures that they “align up” (normal) toward the outer boundary. This means that the waves will hit the outer boundary in a closer to normal direction. Damping in the form of artificial numerical viscosity, is automatically applied in the layer in a matched manner (increasing from zero at the boundary to the physical domain). The artificial viscosity attenuates or damps high-frequency components of the wave in the layer. The amount of damping is controlled under the Damping Parameters for Absorbing Layers section at the main physics level. Finally, at the outer boundary of the layer add a simple Specific Acoustic Impedance (Isentropic) boundary condition, which will work well to remove all remaining waves, as normal incidence has been ensured.
The physical thickness of the layer should be such that it contains a minimum of 10 structured mesh elements. In practice, this means that it should roughly have a thickness equal to the largest wavelength resolved in the model.
For the Absorbing Layers select the Type (Cartesian, cylindrical, spherical, or user defined) under the Geometry section. Enter values for the Physical Width and Pole Distance under the Scaling section.
For more details on the Geometry and Scaling see the Infinite Elements, Perfectly Matched Layers, and Absorbing Layers section in the COMSOL Multiphysics Reference Manual.