# convection from rotating cylinder without crossflow

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 June 6, 2016, 05:41 convection from rotating cylinder without crossflow #1 New Member   Prabul Chandran Join Date: Nov 2015 Posts: 12 Rep Power: 9 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

 June 6, 2016, 06:23 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,326 Rep Power: 138 This is related to FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

 June 8, 2016, 04:50 Size of the smallest element #3 New Member   Prabul Chandran Join Date: Nov 2015 Posts: 12 Rep Power: 9 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

June 8, 2016, 05:58
#4
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Glenn Horrocks
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Location: Sydney, Australia
Posts: 17,326
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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.

 June 10, 2016, 05:40 #5 New Member   Prabul Chandran Join Date: Nov 2015 Posts: 12 Rep Power: 9 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 ??

 June 10, 2016, 06:22 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,326 Rep Power: 138 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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