Fluent Settings - General Stuff - Numerics - Algorithms
 

Dear all,



since 10 years I am using FOAM but right now, I have to use Fluent which is surprisingly interesting in the calculation. The set-up speed is slow for me but I guess this is related to a terminal behaved habit rather than the GUI. However, I do have a bunch of question regarding FLUENT and its numerical settings.

Numerical Algorithms

• It is a bit hard to get the idea of the algorithms FLUENT offers. Not all but some. For example, the SIMPLE is fine and others, however, what is «Pseudo-Transient»?
• In Pseudo-Transient I would expect to have the time-derivative included (so it is not zero) but as it is written «pseudo» I would expect to have a high Courant-Number while using the PIMPLE algorithm. I am not sure about that but if I would set-up a «pseudo-transient» analysis in FOAM, I would increase the Courant number to e.g., 50 or larger with a steady-state analysis between the time steps including under-relaxation and things like that.

Would anybody could proof this statement or give a clearer feedback/statement to pseudo-transient analysis in FLUENT?

• If it would be like a large transient time-step with SIMPLE(C) algorithm between the time steps (PIMPLE algorithm), how does fluent set the Courant number or time-step? Is it a local Courant number or a global one such as a local-time-stepping (LTS) in PIMPLE?

Numerical Schemes

• The numerical schemes are - again - a bit hard to understand. The second order upwind scheme for example should be either first or second order accurate. The scheme that is applied should be based on the flow regime and should be somehow comparable to «linearUpwind» in OpenFOAM.

I highly appreciate any information about that.

• The calculation of the pressure field using the «face-weighted» scheme should be similar to a recalculation of the pressure values from the faces back to the cell center.

Feedback is welcomed.


Continuity / Residuals

• I encounter strange behavior of the residual plots of FLUENT. The continuity is close to 1 or above 1 and it stays there. The residuals of U and the energy are dropping rapidly to e.g., 1e-3 and stays there for ever.
• Checking the fluxes at the inlets and outlets assume a continuity error to be lower 1e-5. However, in the residual plot of fluent it is around 1. There is some discrepancy and I don't know why

Solving the linear system of equation

• Using the output of the terminal, I only see any output for Ux, Uy, Uz and the energy as well as the turbulence quantities. However, I am missing the pressure equation to be solved. I cannot imagine that Fluent does not solve any pressure equation as everyone cooks with water and the fundamental equation are still the same :)

I would highly appreciate any feedback from you experts.
Thank you in advance.