Low-Reflecting Boundary Condition
The low-reflecting boundary condition is mainly intended for letting waves pass out from the model domain without reflection in time-dependent analyses. It is also available in the frequency domain, but then adding a perfectly matched layer (PML) is usually a better option.
As a default, the low-reflecting boundary condition takes the material data from the adjacent domain in an attempt to create a perfect impedance match for both pressure waves and shear waves, so that
where n and t are the unit normal and tangential vectors at the boundary, respectively, and cp and cs are the speeds of the pressure and shear waves in the material. This approach works best when the wave direction in close to the normal at the wall.
In the general case, you can use
where the mechanical impedance di is a diagonal matrix available as the user input, and by default it is set to
More information about modeling using low-reflecting boundary conditions can be found in Ref. 1.