# FLow past cylinder. Mesh independency test

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February 22, 2016, 13:47
FLow past cylinder. Mesh independency test
#1
New Member

Beata Czapla
Join Date: Feb 2016
Posts: 3
Rep Power: 8
Hi,

I’m trying to do grid independence test of flow past cylinder using Ansys Fluent 17.0. I carried out calculation for several meshes (from 134 993 to 395 098), But i still don’t get coverage of results. The flow is 2D, so I think the numer of 395 098 elements is really high. And I should get coverage of results for mesh with less numer of elements. I know that I do sth wrong, but I don’t know what. Could you help me ? I’m beginner in CFD. I present the conditions of the test below.

Conditions of the test:
• Diameter of cylinder is 20mm; Domain size 900mmx300mm.
• Simulation type 2D
• Turbulent model: k-omega SST
• Water as a fluid
• Reynolds numer: 19 904
• Boundary conditions:
o Inlet: Velocity inlet: 1m/s, turbulent intensivity 5%, Hydraulic Diameter 0,3m
o Outlet as outflow
o Top and bottom walls and cylinder : non slip walls
• Y+ =(0,2 -10)
• Time step: 0,0001s, Number of time steps: 100000

Below, I attached my results and image of my mesh

Thank you
Beata
Attached Images
 results.png (21.9 KB, 22 views) mesh.png (31.3 KB, 28 views)

 February 23, 2016, 08:20 #2 Senior Member   Join Date: Mar 2014 Posts: 375 Rep Power: 11 how do you know your results are not converged?

February 23, 2016, 08:35
#3
New Member

Beata Czapla
Join Date: Feb 2016
Posts: 3
Rep Power: 8
because cd and cl, and pressure aren't the same for each mesh
I attached my results in table below:
Attached Images
 results.png (21.9 KB, 24 views)

 February 24, 2016, 18:42 #4 Senior Member   Join Date: Mar 2014 Posts: 375 Rep Power: 11 mesh independence testing is not the same as convergence of a simulation. and mesh independence testing does not mean that the two meshes will have exact 0.0000001 and 0.0000001 cd or cl etc. when you go from 1 million cells to 2 million and get a cd of 0.5 in the first and 0.7 in the second, then you can make a mesh with 3 million cells but if the cd goes to 0.72 now and for a 4 million cell mesh 0.74, you have to stick to the 2 million cell mesh, since the improvement you are getting is small compared to the double resources and computer power you are using. Unless ofcourse you are in NASA and sending a space shuttle to pluto, then you can worry about the small difference.

 March 3, 2016, 14:10 #5 New Member   Pradip Join Date: Jan 2016 Location: Mumbai Posts: 13 Rep Power: 8 hi beattacz may i know y did u use K omega sst model???

 March 4, 2016, 13:06 #6 New Member   Beata Czapla Join Date: Feb 2016 Posts: 3 Rep Power: 8 yes I dod use, k-w SST model

April 26, 2020, 16:44
You mean the mesh in this cases has been completed, but you see the average pressure
#7
Senior Member

dega
Join Date: Dec 2017
Posts: 333
Rep Power: 7
Quote:
 Originally Posted by hwet mesh independence testing is not the same as convergence of a simulation. and mesh independence testing does not mean that the two meshes will have exact 0.0000001 and 0.0000001 cd or cl etc. when you go from 1 million cells to 2 million and get a cd of 0.5 in the first and 0.7 in the second, then you can make a mesh with 3 million cells but if the cd goes to 0.72 now and for a 4 million cell mesh 0.74, you have to stick to the 2 million cell mesh, since the improvement you are getting is small compared to the double resources and computer power you are using. Unless ofcourse you are in NASA and sending a space shuttle to pluto, then you can worry about the small difference.
You mean the mesh in this cases has been completed, but you see the average pressure of cylinder, it has changed a lot for the mesh models, why?

 Tags flow past cylinder, mesh independency