hi.
I have a problem in my simulation work. I use flow-3d for simulating a surface vortex with an air core in a pipe-tank system. I have been successful simulating the vortex with 1cm mesh size, but I haven't been able to see the air core. Due to this problem I had to had to use finer mesh with size of 0.6cm.
When using finer mesh ,my streamlines of the near surface particles change dramatically in comparison with the streamlines I had when using coarser mesh with 1cm mesh cell size.
In coarser mesh my streamlines are strongly sucked and driven into my pipe but when i reduce my mesh cell to 0.6cm, with exactly the same setup my streamline suddenly get shorter and incomplete. It seems that after changing the mesh size the flow of water in to the pipe is not enough strong to create the enough suction for near surface particles of water and due to this there i see no air entrainment afterward.
I will upload the photos of my streamlines in both conditions, I'll be so delightful if anyone can tell what is the cause of this problem I have and how I can solve it?
In many cases similar to yours, the problem is related to the stability condition of the mesh. You have to study this condition carefully and find the upper bound of the stability.
Dear Amirhossein Taghizadeh
Have you done a mesh intendancy study to check which mesh suits your case?
Ahmed Al-Manea I
have to use use hexahedral mesh, do you mean that??
Giuseppe Altieri in that case I need stronger computer because the estimated simulation time increases too much....don't you thin 0.6cm would be enough to encounter the air entrance already?
Saleh Albashrawi i have done my meshing in flow 3d itself and according to the criteria's of the program, the quality of the mesh was good. what else can cause instability of the if it has a high quality?? the most interesting part is that I get this problem when refining the mesh.
Amirhossein Taghizadeh unfortunately the simulation needs a strong computer as preliminarily must be carried on a grid independence test, i.e. how the error of the solution increases when the mesh size increases but I've seen this rarely. The solution with 1.0 cm mesh size is probably wrong and this is demonstrated by the mesh size of 0.6 cm. In order to estimate the solution error you must compare the solutions (i.e. the relative error between two solutions) obtained with different mesh size, when the relative error goes below a certain threshold then you can use that mesh size. Hoping this can help.
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