To ensure that a mesh definition is fine enough without being so fine that computational resources are wasted, the following steps are recommended when performing any CFD analysis:
- First, determine if there are any symmetries, and divide the geometry in the CAD system as appropriate. Look for geometric symmetries, but be sure that the flow will be symmetric as well.
- Determine if the analysis can be modeled as a 2D or an axisymmetric geometry. A 2D approximation may be a good place to start, especially if you are unsure of how to solve a particular type of flow problem.
- Examine the geometry, identifying probable high and low gradient regions for all solution variables.
- Identify solid material zones and fluid zones and keep them as separate geometric entities or parts.
- If there are areas with small, repeating geometric details (such as perforated plates or baffles), replace with distributed resistances to model these zones, instead of meshing the detail.
- Assign mesh sizes to all volumes in the model, and then apply finer sizes to surfaces and edges where necessary in order to capture strong flow gradients or to represent complicated geometric features.
- Perform an analysis on a coarse mesh (no more than 50,000 nodes) to qualitatively assess the flow features present and identify meshing needs in high gradient regions without a severe time penalty.
- Looking at the results on the coarse mesh, refine the mesh in the high gradient regions.
- To ensure that the final solution is not "mesh-dependent," compare the two solutions from the coarse and fine meshes. If they are substantially different, then it is a good idea to construct a mesh that has at least 10% fewer nodes than the fine mesh, obtain a solution and compare. The idea is to have two meshes that vary in number of nodes by 10% or more and that give the same solution. This solution is then said to be “mesh-independent”.
In any finite element analysis, more elements are required in areas where spatial gradients of the solution variables are high. In CFD, an additional physical phenomenon called velocity-pressure coupling must also be accurately represented on the mesh to ensure continuity of fluid mass over the entire solution domain.
This distinction elicits the following two requirements:
- Many more elements must occupy the domain than in a typical structural analysis.
- Transitions in element size must be relatively smooth so that the area or volume of adjacent elements does not vary substantially.