If you are confident that the internal or external flow contains one or more mutually perpendicular symmetry planes that are parallel to the Global Coordinate System planes, you can separate a relevant flow region by resizing the computational domain. The flow symmetry planes can be utilized as computational domain boundaries with the Symmetry conditions specified on them. Since the computational domain size is reduced, both computer memory requirements and CPU time will be reduced. Note that sometimes symmetry of both model and the incoming (inlet) flow does not guarantee symmetry in other flow regions, e.g. a von Karman vortex street past a cylinder.
NOTE: If you specify an integral boundary condition (e.g., mass or volume flow, heat generation rate) at the opening, surface or volume crossed by a symmetry plane you must adjust the input value to the symmetry condition. Flow Simulation automatically applies the specified value to the calculated area. Since the symmetry condition reduces the calculated area you have to reduce the value specified for the whole opening (surface or volume) as well. For example, if a symmetry plane halves the opening you have to specify half of the actual mass/volume flow rate to satisfy your conditions.
To specify symmetry planes:
- In the Flow Simulation analysis tree, right-click the Computational Domain icon and select Edit Definition or click Flow Simulation, Computational Domain.
- In the graphics area click and drag the arrow handles to the desired symmetry planes positions or specify coordinates of the flow symmetry planes on the Size tab.
- Click the Boundary Condition tab and select the Symmetry condition for the corresponding boundaries (At X min, or At X max, etc.)
- Click OK.
2 comments:
Hmm - this could be dangerous. With structural static FE this is OK but with fluids you'd have to be ultra careful.
Well, I would agree that people do need to be careful. But symmetry in Flow Simulation is no more dangerous than symmetry in structural analysis.
In either case, you still need to ask yourself two very important question. In structural analysis, you need to ask yourself 1. Is my geometry symmetrical, and 2. Are my boundary conditions (loads and restraints) symmetrical. Then, you have to make sure you apply the symmetry restaint accordingly.
The same 2 questions need to be asked in a flow symmetry problem. 1. Is my geometry (fluid flow region) symmetrical, and 2. Are my boundary conditions (inlets and outlets) symmetrical. And once again, you have to apply the computational domain accordingly.
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