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first order converged--second order diverged

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Old   November 26, 2009, 13:44
Default first order converged--second order diverged
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Shengyi Wang
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Dear all,

In my case, I use first order upwind scheme and get a very well converged solution; however, when I switch to second order upwind scheme, totally diverge!!!

Any one has some suggestion to resolve this problem?

Thx in advance.

Sy
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Old   November 27, 2009, 01:35
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Would you please tell me the implementation method of the second order upwind scheme? and have you try to change the relaxation parameter
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Old   November 27, 2009, 10:48
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Originally Posted by ztdep View Post
Would you please tell me the implementation method of the second order upwind scheme? and have you try to change the relaxation parameter
Hi Ztdep,

I am sorry I do not quite understand what you mean by the implementation. Could you please give a more detailed explanation?

Underralaxation factor has been tuned down, but it doesn't work.

I am using sst k-w with transition model, y+ is less than 1. The blade is actually a NACA0012 blade with an angle of attack of 8 degree.

Thank you for reply!

R, Shengyi
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Old   December 23, 2009, 06:57
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Dear gmwsy,

To get a stable second-order simulation you should do the following trick: start the simulation for the first let say 100 timesteps with a first-order scheme and after switch to the second-order one. The problem behind is that for high-order schemes is very hard to start from "bad" initial solution (the initial solution is illconditioned)...
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Old   December 23, 2009, 13:45
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Originally Posted by tele View Post
Dear gmwsy,

To get a stable second-order simulation you should do the following trick: start the simulation for the first let say 100 timesteps with a first-order scheme and after switch to the second-order one. The problem behind is that for high-order schemes is very hard to start from "bad" initial solution (the initial solution is illconditioned)...
Dear Tele,

Thank you for your reply. But what i am doing is actually a steady case...so there does not exist time steps or initial solution.
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Old   December 25, 2009, 02:34
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Originally Posted by gmwsy View Post
Dear Tele,

Thank you for your reply. But what i am doing is actually a steady case...so there does not exist time steps or initial solution.
I agree with tele's suggestion. For steady case, do some iteration with low order scheme, then swith to high order scheme.
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Old   December 25, 2009, 09:39
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I agree with tele's suggestion. For steady case, do some iteration with low order scheme, then swith to high order scheme.
Thank you! But this is just what I have done. The iteration diverged when switched to 2nd order from a converged solution using 1st order...
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Old   December 28, 2009, 04:50
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Dear gmwsy


I would suggest to carefully check your mesh.
Second order usually relies on an enlarged stencil, and will be much more sensitive to the mesh quality.

Try to find where the convergence problem comes from, and readjust your mesh in the area. You may also fix it using some classical mesh quality criteria (determinant, skewness, etc.).

BR


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Old   December 30, 2009, 05:37
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Originally Posted by koo View Post
Dear gmwsy


I would suggest to carefully check your mesh.
Second order usually relies on an enlarged stencil, and will be much more sensitive to the mesh quality.

Try to find where the convergence problem comes from, and readjust your mesh in the area. You may also fix it using some classical mesh quality criteria (determinant, skewness, etc.).

BR


koo
Dear Koo,

Thank you for your reply. I have checked the mesh and obviously the problem occurs at the boundary layer. Because I am not using the wall function and thus y+ is around 1, so the aspect ratio of the cells is very big. But this is usual in my area. I do not know how to change the mesh.

Best wishes to the new year,
Shengyi
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Old   January 1, 2010, 18:50
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Originally Posted by gmwsy View Post
Dear Koo,

Thank you for your reply. I have checked the mesh and obviously the problem occurs at the boundary layer. Because I am not using the wall function and thus y+ is around 1, so the aspect ratio of the cells is very big. But this is usual in my area. I do not know how to change the mesh.

Best wishes to the new year,
Shengyi
Hi Shengyi,

What meshing program are you using? What solver? NB FLUENT has a very useful mesh adaption criterion specially for boundary layers_: Boundary layer adaption
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Old   January 1, 2010, 19:08
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Originally Posted by kingjewel1 View Post
Hi Shengyi,

What meshing program are you using? What solver? NB FLUENT has a very useful mesh adaption criterion specially for boundary layers_: Boundary layer adaption
Dear Kingjewel1:

Thanks for reply. I know about the adaption function in Fluent, but it might not help a lot. I recently found a potential solution for this problem: using a k-e model to get a solution and then switch to k-w.
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