[lukasf]

Hi,


I am using the solver rhoPimpleFoam and I can choose between two pressure equations pEqn.H or pcEqn.H to be solved.


As default the pEqn.H is used but one can use the other Equation by setting "consistent yes;" in the fvSolutions.


What is the difference and which one should be used in which case?


Best regards,


Lukas

[lukasf]

Okay,


PIMPLE is a combination of the PISO and SIMPLE algorithm. Using the consistent approach one may use the PISO combined with the SIMPLEC algorithm.


In Tobias Holzmann's book one can find that the SIMPLEC algorithm includes a missing pressure term which is neglected in the SIMPLE algorithm.


Which one is better? Since SIMPLE is the default one, one may think that it is the best choice for most cases.


I will test the PIMPLE algorithm with SIMPLEC and check it's influence.

[Tobi]

Well, simply:

SIMPLE

• Needs under-relaxation for p and U/k/epsion/...
• Commonly for p ~ 0.3 - 0.5
• Commonly for other quantities between 0.3 to 0.7

SIMPLEC

• Commonly no under-relaxation for p
• Other quantities > 0.7

The difference is, that we need more iterations for the linear solver but less iterations of the algorithm (speaking about SIMPLEC). Hence, using SIMPLEC will - commonly - find the solution faster and less computational expensive.

However, this is only rules of thumbs and there are much more things to consider regarding stability etc. such as - pressure equation relaxation or making it at least diagonal dominant using an equation relaxation for p = 1. Was already discussed several times here. By the way, please trigger me to the thread the next time due to the fact that YouTube is not the place to share information.