MIKE 21 comprises three simulation engines: 
Single Grid: the -dependent non-linear equations of continuity and conservation of momentum are solved by implicit finite difference techniques with the variables defined on a space-staggered rectangular grid.
Multiple Grids: the Multiple Grids version uses the same simulation engine and numerical approach as the single grid version. However, it provides the possibility of refining areas of special interest within the model area (nesting). All domains within the model area are dynamically linked. 
Flexible Mesh: is an unstructured mesh and uses a cell-centred finite volume solution technique. The mesh is based on linear triangular elements.