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convection from rotating cylinder without crossflow

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Old   June 6, 2016, 06:41
Default convection from rotating cylinder without crossflow
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Prabul Chandran
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i am modelling convection from rotating cylinder in CFX at a reynolds number of 22000 which comes within the mixed convection regime. bouyancy is activated . i observe that my residual values go down upto 10-4 and the solution converges. but as i set my convergence criteria to 10-5 the residues rise unacceptably and solution looks bizzare.why could this be happening
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Old   June 6, 2016, 07:23
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This is related to FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria
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Old   June 8, 2016, 05:50
Default Size of the smallest element
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Does making the smallest element too small distort the results. While doing grid independence the solution doesnt seem to stop, goes on varying on and on. iam doing a natural convection problem
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Old   June 8, 2016, 06:58
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Quote:
Does making the smallest element too small distort the results.
As the mesh gets finer you will eventually reach the point where numerical round-off errors stop convergence.

Quote:
While doing grid independence the solution doesnt seem to stop, goes on varying on and on.
This is saying:
1) your mesh is miles too coarse, you need it much finer. That is why lots of the worlds supercomputers are doing CFD. OR

2) Your modelling approach is wrong. A better approach would allow convergence with a coarser mesh.
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Old   June 10, 2016, 06:40
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The cylinder is located in free space.i have used pressure inlet and outlet boundary conditions for the simulation. i also tried with wall boundary conditions with the walls located very far away from cylinder and set to ambient temperature. I find that using the wall boundary condition gives results which are more close to the experimental ones . But i am worried about the air recirculating within the domain. is it appropriate to use wall boundary conditions when simulating such a case ??
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Old   June 10, 2016, 07:22
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If you have done a sensitivity check to ensure that the proximity of the walls does not affect the results then yes, you can use them. But if you are trying to model a far field (ie: free air) then the normal way of doing this is with an inlet/outlet pair so if you cannot get convergence with this then it is very unlikely a wall boundary will have converged. A wall boundary should converge at a larger distance away than an inlet/outlet pair as it is less like a free field condition.
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