Mesh independence studies involve systematically refining the mesh and observing how the solution changes to determine the appropriate mesh density for accurate results. The approach is the same for Steady or Transient results. Established mesh independence for a steady case may not be appropriate for transient case since you also need to account for temporal resolution in the latter, while steady-state cases may average transient effects (for example, using RANS).
For transient cases, you need to establish that your results are both:
- mesh independent: you are spatially resolving flow features that matter to you and those result do not change significantly with finer mesh;
- time step independent: you are resolving important transient effects and further reducing the time step does not change the results significantly.