Immersed boundary method

Immersed Boundary Method (IBM) is one of standard CFD tools for cases, where detailed mesh generation is either impractical or inappropriate. While the traditional IBM method relies on Cartesian background grids, a new implementation in OpenFOAM combines the flexibility of unstructured polyhedral background with immersed boundary handling. In this manner, a number of simulations can be performed, where a mesh "partially fits" the boundary where feasible and the complex geometrical features are handled by an immersed boundary patch.

An important issue of immersed boundary implementation is handling of high-Re flows, where the wall function near-wall treatment is a must.  The new implementation not only supports the wall functions on the immersed boundary but shows accuracy equivalent to that of a body-fitted mesh.

Implementation of IBM in OpenFOAM uses the Discrete Forcing Approach with direct imposition of boundary conditions.  Here, the value of the dependent variable in the IB cell centres is calculated by interpolation using neighbouring cell values and the boundary condition at the corresponding IB point.

Interpolation stencils for the immersed boundary patch have a polynomial basis of second order and do not violate the overall order of accuracy of the solver.  The quadratic polynomial implementation supports both the Dirichlet boundaries as well as Neumann and generalised gradient boundary condition, using Hermitian function fitting.

Implementation of Immersed Boundary Method in OpenFOAM is achieved in a way that requires no intervention in basic physical modelling, such as material property handling, turbulence modelling or free surface flow.
The Immersed Boundary Method developed by Wikki supports parallel processing in domain decomposition mode and shows linear scaling to a substantial number of processors.