In CFD-calculations computational speed is very important. In order to avoid having to check whether nodes and/or elements are really part of the fluid (e.g. because there are gaps in the numbering, or part of the nodes belongs to the structure in fluid-structure calculations), the fluid nodes and fluid elements are renumbered in ascending order without any gaps. This is done in subroutine rearrangecfd.f. This also includes the renumbering of the boundary conditions (SPC's and MPC's) and the loading. In the CBS method the size of the equation system for the conservation laws is the number of nodes, except for the pressure equation for incompressible fluids.