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Prakash October 12, 2006 18:44

Why Fluent Solver Wrong?
Dear CFD Users

I would like to ask one of the CFD, FLUENT solver problem which I faced in my project.

I am solving a problem with two gas mixing in 3-D, the 1st gas is mixing in 2nd gas by fraction of amount with time through small opening. The Boundry is fully closed.

Problem: suppose, I consider the 1st gas pressure higher that the 2nd gas preassure (P1>P2) and (T1 < T2) in these conditions the solver is showing unrealistic variation in pressure and (T1>T2), the results are realistic and convergency. Why this situation is happening in FLUENT solver? can you advise me, how to overcome this problem.

your suggestions are appereciate in advance



U October 13, 2006 15:11

Re: Why Fluent Solver Wrong?
1 - if you have an inlet, you must have an outlet if your case is not compressible.

2- don't use tet cells for compressible flows (from experience)


Ahmed October 13, 2006 23:03

Re: Why Fluent Solver Wrong?
If you are pressurizing your tank (inlet and no outlet) then this is unsteady analysis Explain your problem in better terms and more detail Good Luck Do not Jump to the conclusion that the FLUENT solver is wrong It all depends on your problem set up remember GIGO

prakash October 16, 2006 18:30

Re: Why Fluent Solver Showing Wrong?
Dear Ahmed

Thank you for your reply, absolutely this is unsteady problem.

i am explaining exact of my problem.

This problem has two volumes, one is larger than second one, around 50 times.

The larger volume is enclosed by smaller volume (by simple understanding insulated on larger volume). the samller volume thickness is very small.

Here there is no inlet and outlet. only the flow communication b/n the 2 volumes is through small leak on one of interface wall by pressure difference. All are wall boundary conditions.

as explained in my posting, whenever i consider pressure and temperature of larger volume higher than the smaller volume, the results are realistic and convergnecy, but

if temperature of smaller volume higher than larger volume the results are unrealistic. This is i am not understading why fluent is showing like that?

i am using species model. These two volumes contain two-different gases.

conditions, 1. unsteady, ideal gas law, species model, energy enable.

i hope we can understand my explanation.

if u have any idea on this problem,please suggest me



zxaar October 16, 2006 19:49

Re: Why Fluent Solver Showing Wrong?
check the interface, if you are using interface to join two volumes, if the mesh sizes of the interface face are very much different it can play with your results.

Ahmed October 16, 2006 21:13

Re: Why Fluent Solver Showing Wrong?
You have to keep in mind that the time scale for diffusion is very much smaller compared to the time scale for convection, so even if the smaller volume is at a higher temp than the larger volume, but the pressure difference (that will translates into a higher velocity through the connection between the two volumes) will be the dominant mecanism. If you have time, try to repeat the analysis with less pressure difference than your original case. Hope that will help Good Luck

Ahmed October 16, 2006 21:45

Re: Why Fluent Solver Showing Wrong?
I guess the interface between the two volumes ought to be modelled as a porous region (even if the thickness is small) Not sure about that, you know better, you have the exact dimensions of both and you are the only one to judge

Prakash October 17, 2006 02:46

Re: Why Fluent Solver Showing Wrong?
Thank you for your reply,

Dear Ahmed i tried all options by trial and error method it was not clicked any point of view, only my doubt is why, if temperature is higher at higher pressure, the pressure distribution is showing right, but temperature is lower at higher pressure, the pressure distribution is showing wrong? if u have any idea please suggest me

Dear Zxaar the interface mesh is common for both volumes. i tried the solution with coarse mesh as well as fine mesh.

Thank you very much for your suggestions.

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