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December 27, 2011, 06:33 |
weird Cd of CFX result
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#1 |
Member
^^
Join Date: Aug 2011
Posts: 70
Rep Power: 14 |
Dear All,
I simulated steady 3D cylinder to compute drag coefficient. I attached two figures - one is grid and another is Pressure coefficient. Boundary condition is like as; Diameter :0.08 m Inlet velocity 2.0 m/s turbulence model : k-e model When I compute Cd using CCL, the value is too low, it is around 0.25. I also check force_x@cylinder, the value of force_x@cylinder is 0.04N. I thought the value of Cd should be around 1.0 but the result is too low. Do you have any idea or comments on this problem? Best Regards, |
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December 28, 2011, 04:36 |
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#2 |
New Member
anonymous
Join Date: Oct 2011
Posts: 7
Rep Power: 14 |
Hello,
I assume your fluid is air? Then your Reynolds-Nummer is about Re=11000 and your flow is still laminar (but you use a turbulence model). At this Re-Number you also get "vortex-street instabilities", so i think a transient run would catch your drag more accurate. And maybe check the area of your cylinder in z-direction or other parameters of the cd determination. Furthermore your grid seams a little bit coarse to me. Maybe you could create a 2-dimensional model if you not have enough hardware power to run with a fine mesh in 3d. This problem is a well known example in cfd though. You should find a lot of further information on the internet or the common literature. Best Regards Paul |
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December 28, 2011, 05:15 |
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#3 | |
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Quote:
I will do that . |
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December 28, 2011, 05:21 |
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#4 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,700
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"Why is my simulation inaccurate" has been asked thousands of times, so have a look at the FAQ:
http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F And yes, your mesh is almost certainly too coarse. If the fluid is air then Re=1e5 is in the transitional region. This is the hardest region to simulate accurately and will require great attention to detail to get accurate results. |
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December 28, 2011, 05:22 |
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#5 | |
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Quote:
Thank you !! Last edited by colopolo; December 28, 2011 at 05:44. |
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December 28, 2011, 05:36 |
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#6 | |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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Quote:
So I have no idea what you comment is referring to. |
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December 28, 2011, 05:43 |
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#7 | |
Member
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Join Date: Aug 2011
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Quote:
At this time, I mean I fully agreed your opinion because I finally understood what I was wrong through literature review and your comments. Thanks you for your quick response. |
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