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Old   February 24, 2020, 09:03
Default Roe upwind scheme
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Hi everyone, I am trying to computing some aerodynamic coefficients (drag, lift, coefficient of pressure) using the Roe upwind scheme, but it doesn't converge. I tried also to use the multigrid method, but I had no success. Are there any options that might I have missed in my .cfg file?
wingD.txt
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Old   February 24, 2020, 12:12
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Can you share the cfg file. Multigrid with Roe does not work well in SU2 (usually JST goes well) .
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Old   February 25, 2020, 15:55
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Hi aero_amit, I have shared the cfg file yet (it is the txt file named wingD). I must use Roe scheme for "academic issues", is there a way to reach convergence?
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Old   February 26, 2020, 06:09
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In my experience the multigrid works very well in hex-dominant grids, provided you are not using too many cores relative to problem size (see pull request 861 on GitHub)
My advice (based on RANS cases):
Use only pre-smoothing (1, 1, 2, 3), 2-3 levels should be enough.
0.7 damping coefficients.
Limiters tend to make the case flip flop, Venkatakrishnan-Wang works best but try using no limiter (your case is subsonic). If you really want limiters, increasing the Venkat coefficient may help.
Fixed CFL in the range 15-25, going higher sometimes makes convergence harder.
Increase the entropy fix factor until you get convergence, then worry about its influence on the solution (once you have a converged solution for initialization it is easier to converge with lower fix factors).
FGMRES + LU-SGS, no more than 10 linear iterations and tolerance of 0.05 or so should be more than enough, especially on inviscid grids.
I stay away from the low-Mach schemes / options, you can probably do without them too at Mach 0.2.
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Old   February 26, 2020, 09:14
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Hi pcg, my case is an Euler one, with a low Mach (0.2). Is your advice still valid for my situation?
I have another (maybe stupid) question: I have ever studied finite difference methods right now, and for upwind schemes the CFL condition is less or equal than 1. I noticed that either in SU2 tutorial or in the other threads the suggestions take in consideration CFL numbers of 10/20/30 and so on...are the upwind schemes (like Roe's scheme, for example) still stable at such a high CFL number?

I have started using SU2 and finite volume method since a very little time, so I ignore a lot of things! :S
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Old   February 27, 2020, 03:24
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I would say it is worth a shot.

The CFL limit depends on the (pseudo)time integration method, for explicit Euler (the integration, not the equations) it is very low like you say, Runge-Kutta methods allow slightly higher values, and implicit Euler is unconditionally stable (but the implicit part is based on an approximate linearization of the residual so there are practical limits).
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Old   February 27, 2020, 05:07
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That's right, but there is a thing that I can't understand: I have two grids, a coarse one and a finer one; if I try to do the computation on the coarser grid it converges, but if I use the same options on the finer one, it does not converge. Which could be the reason?
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