### 5.1.2. Applying Large Displacements Gradually

In many simulations that require mesh deformation, the motion is known a priori. In these cases, the motion can be applied gradually, by relating it to the iteration or timestep counters, to reduce the likelihood of mesh folding. Mesh folding is often avoided with this strategy because the mesh displacement equations are assembled using the updated meshes from each deformation step (that is, outer iteration or timestep). In general, the desired total mesh deformation should be split up so that regions where motion is specified move through less than approximately 5 adjacent elements per step.

In some simulations, the motion is not known a priori. Fluid Structure Interaction is an excellent example of this. In these cases, mechanisms available for under-relaxing the displacements applied per deformation step should be used. For details, see Solver Controls, External Coupling Tab in the CFX-Solver Modeling Guide.

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