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Simple question : Simple Vs Coupled solver

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Old   March 3, 2017, 05:31
Red face Simple question : Simple Vs Coupled solver
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Maria Angela - Roma
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Hello,

i am working on a simulation around an airfoil at very high Rey, the Simple solver doesn't converge even i decreased the under-relaxation factors but the coupled V-P works. my question is why ?


Thank you for your help & support

Maria
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Old   March 4, 2017, 13:10
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Is it diverging or what do you mean by converged?
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Old   March 6, 2017, 11:44
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Maria Angela - Roma
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Hey first of all thank you,

Well the idea is that i am runing a same mesh with SIMPLE & COUPLED solvers :

- The simple solver diverged.
- The coupled solver converged.

i just wanted to know why ?

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Old   March 6, 2020, 07:05
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Hi, have you figured it out yet? My solutions were converged too when I used coupled solver. I just can't understand the reason
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Old   March 6, 2020, 07:29
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SIMPLE and Coupled are just two numerical approaches. Whichever works is good. The reason why one works and other not is more of physical nature. If pressure and velocity are very tightly coupled in a scenario, such as in rotating machines, then Coupled works better. If that is not the case, SIMPLE is alright. Coupled is almost always better than SIMPLE but there are a few scenarios where SIMPLE works better, such as for low Gr number Natural convection flows.
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Old   March 6, 2020, 15:38
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Well there are non-physical reasons as well.


Always first you need to state your entire model. It's hard to say why your case diverged when you don't state what you're doing. The P-V coupling solver is merely a means of solving one part of the entire problem, namely how to solve the momentum equation for pressure and velocity. Your solution can fail to converge due to problems with coupling from other equations, i.e. the energy equation. If you are not running any energy equation at all, which means you're doing an incompressible simulation, then we can actually blame it from the P-V algorithm.


SIMPLE solves a pressure correction problem and uses this to update the velocity field. Once it does this, the pressure field is in an inconsistent state with the velocity field. You are guaranteed to have the wrong pressure field, every time until your solution converges. If you carry this wrong pressure field into another equation (e.g. the energy equation) then you're done-zo. In SIMPLE you rely on iterating many times to and pray that these inconsistencies get smaller.


Next, SIMPLE doesn't use the non-orthogonal correctors and this tends to make it even more unstable for non perfect meshes. That's why the default urf's for SIMPLE are as low as they are. The bottomline is, SIMPLE is not designed to be stable, it's designed to be fast by exploiting weak coupling. For strong coupling, SIMPLE will be smelly. For bad meshes, SIMPLE will stink. For bad initial guesses, SIMPLE will really really stink.
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