first order converged--second order diverged
 

Dear all,

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

Any one has some suggestion to resolve this problem?:confused:

Thx in advance.

Sy

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

RE
 

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:D...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.

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...:mad:

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. :mad: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,:D
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

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. :D