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FLow past cylinder. Mesh independency test

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Old   February 22, 2016, 13:47
Default FLow past cylinder. Mesh independency test
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Beata Czapla
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Hi,

Im 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 dont 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 dont know what. Could you help me ? Im 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
Unsteady flow
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
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File Type: png results.png (21.9 KB, 13 views)
File Type: png mesh.png (31.3 KB, 14 views)
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Old   February 23, 2016, 08:20
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how do you know your results are not converged?
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Old   February 23, 2016, 08:35
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Beata Czapla
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because cd and cl, and pressure aren't the same for each mesh
I attached my results in table below:
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Old   February 24, 2016, 18:42
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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.
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Old   March 3, 2016, 14:10
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hi beattacz
may i know y did u use K omega sst model???
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Old   March 4, 2016, 13:06
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yes I dod use, k-w SST model
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