Solvers for BEM Models
The default for The Pressure Acoustics, Boundary Mode Interface is to use an iterative solver for 3D models and a direct dense matrix solver for 2D problems. If the Far-Field Approximation is turned off, the direct dense matrix solver will be used in all situations. The dense matrix solver is adequate for small BEM problems but quickly runs out of memory for increasing number of DOFs. It is recommended to use the default settings.
In 3D two iterative solver suggestions are generated. The default iterative suggestion uses SCGS as preconditioner. This approach is memory efficient but for increasing frequencies the convergence is compromised. In this case the Stabilized formulation can be used (see below). As an alternative to using the stabilized formulation the second iterative suggestion can be used (enable the solver suggestion). This solver uses a Hierarchical LU (HLU) preconditioner which produces LU factors of high quality. This will ensure better convergence for the iterative GMRES solver. The memory consumption is higher for this approach.
When the Stabilized formulation is used, a different solver suggestion is generated. Make sure to reset the solver to default if changes are made. The solver uses PARDISO as a direct preconditioner (with hybridization splitting the BEM and the stabilized variables) and GMRES as the iterative method.
The boundary element method (BEM) used in the Pressure Acoustics, Boundary Elements interface is based on the direct method with Costabel’s symmetric coupling. The adaptive cross approximation (ACA) fast summation method is used for solving the resulting linear system. This method uses partial assembly of the matrices where the effect of the matrix vector multiplication is computed. The default iterative solver used is GMRES.
For multiphysics problems that combine BEM and FEM interfaces and are set up with one of the multiphysics couplings; the default approach is to use hybridization with ACA for BEM and an appropriate preconditioner for the FEM part of the problem (direct or multigrid). Several solver suggestions are given for the multiphysics FEM-BEM problems.
Further details about the BEM formulation are found in The PDE, Boundary Elements Interface documentation in the COMSOL Multiphysics Reference Manual.