[boffin5]

Hello,


In the case of the motorbike tutorial, it is confusing to me how 'symmetryPlane' is used. In the boundary conditions, 'symmetryPlane' is used for the upperwall, and also for the "front/back".


To me, 'symmetry' implies a reflection, so I can envision such a plane being used for left and right symmetry, and another one for upper and lower. But in the motorbike tutorial, symmetry for both 'front' and 'back' does not make sense. I have seen this perplexing usage in other tutorials.


This is particularly important for me, as I am trying to use symmetry in its true sense, to model half of a vehicle, in order to reduce the computational load. And so far, without success. Can someone point me to a tutorial that shows how to do this? Or just explain how it is defined in blockMeshDict, and the BCs.


Thanks in advance, boffin5

[boffin5]

I always do this! In reality, for the motorbike tutorial, the use of 'symmetryPlane' for the upper wall doesn't make sense either. Wouldn't some other term be more understandable, like 'patch'?


Further, assuming that for the motorbike, only half of it was modelled, how would the real plane of symmetry (i.e. left/right) be dealt with?

[Tobermory]

Alan

the symmetryPlane boundary is also a "free slip" boundary condition, i.e. no flow across the boundary, zero gradient on all variables (check the description in 5.2.1 of https://cfd.direct/openfoam/user-gui...25-1790005.2.2).

So it's fairly standard practice to treat side boundaries as symmetry planes. Think of it as the walls of the wind tunnel, but frictionless.

[boffin5]

Thank you Tobermory!


You explained part of my questions, but I am still wondering: how do you define a reflective symmetry plane that actually mirrors half-body geometry that is supplied in constant/triSurface? Given that the symmetry BC doesn't actually do it?

[Tobermory]

I am not quite sure what you mean by "how do you define a reflective symmetry plane that actually mirrors half-body geometry that is supplied in constant/triSurface".

If you mean is it possible to take a piece of geometry, mirror it around a plane and then mesh around the whole mirrored geometry, then that's clearly a meshing task, and can't be achieved by boundary conditions.

Otherwise, the symmetry plane boundary is doing exactly what you are asking ... it is as if you have placed a mirror at the boundary face and are modelling the full mirrored geometry with just half the mesh. You can recreate the whole model easily in paraview with a transform.

If I still haven't understood what you meant above, can you explain more what you do mean?