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Cd and Cl graphs are not smooth

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Old   November 7, 2007, 09:11
Default Cd and Cl graphs are not smooth
  #1
Sham
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Hi all,

I am simulating flow past cylinder. My domain is a circular domain with triangular mesh and quad mesh attached as the cylinder's boundary layer. I prescribed and up and down motion to the cylinder and also its boundary layer.

When I monitor the Cd and Cl, the graphs I got is not so smooth. Although it assumes a consistent pattern over time, but I can see jagged line. Is this normal for a moving cylinder or is it a sign of instability?

I hope someone can explain to me what happen and how to tackle the problem.

To have a better picture, go to:

http://img135.imageshack.us/my.php?image=graphspf4.jpg

Thanks.

Sham.
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Old   November 7, 2007, 10:30
Default Re: Cd and Cl graphs are not smooth
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fluid
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Hi, maybe it can be caused by the Reynolds number you are simulating. Also in steady simulation, RANS can capture the Von Karman vortex street. It can easily explain the behaviour of your Cl and Cd plot.... fluid
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Old   November 7, 2007, 17:42
Default Re: Cd and Cl graphs are not smooth
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Sham
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However when I simulate the exact same problem with fixed cylinder, I did get a smooth graph. It only happens when I simulate a motioned cylinder. I use kw model.
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Old   November 8, 2007, 05:20
Default Re: Cd and Cl graphs are not smooth
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fluid
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I'm sorry, I didn't get it! In this case i don't know, maybe it is intrinsecally an unsteady phenomenon and for this reason physical quantity are oscillating... fluid

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