Choosing the correct time step size for System Coupling is critical, since an incorrect time step size can lead to mesh folding and other potential issues. Mesh deformation is one aspect of an appropriate participant timescale and should always be evaluated against other relevant timescales in each participant.
To determine the mesh deformation time scale, calculate the time step Δt size based on:
The length of the smallest cell, Δs
The maximum expected velocity of the mesh, v
The maximum expected velocity of the mesh can be determined through Mechanical by examining the total deformation occurring due to the explicit motion. The maximum total deformation can be divided by the time step size to estimate the expected mesh velocity of the CFD mesh.
As a starting point, attempt to define the boundary so that it can move only a single cell in each time step. Then, once a successful run has been established, the time step size can be increased.
For participant-specific details, see the corresponding product documentation:
Performing Time-Dependent Calculations in the Fluent User's Guide
Timestep Selection in the CFX-Solver Modeling Guide
Steps and Step Controls for Static and Transient Analyses in the Mechanical User's Guide