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The most complex CFD mesh modeling involves the use of moving and deforming grids, or dynamic meshing. Systems that would require this advanced technique include a piston moving in a cylinder, a co-rotating interpenetrating twin-screw extruder or a solid object oscillating in a fluid. In these systems, the mesh deforms as the parts rotate or move.

To accommodate the changing grid, three mesh schemes can be applied: layering, smoothing, and remeshing. Layering only applies to hexahedral structured meshes in which mesh cells on identified layers are compressed or expanded a prescribed amount until the whole layer is either deleted or a new one added. If the general shape of the fluid zone doesn’t change and the number of nodes remains the same, smoothing can be applied. However, if the mesh deforms considerably, both remeshing and smoothing need to be applied and the remeshing scheme can only be used for tetrahedral meshes.

Once the particular dynamic meshing scheme is chosen, the motion is prescribed, and the transient solution is calculated. Obviously, suitably small time-steps are required in order to capture the associated fluid motion in response to the moving boundaries.