Fluent Computation
 

Hi,

I'm facing a sttrange problem- I have a 3d geometry which was meshed partially in Gambit and the remaining in TGrid. I'm trying to run a steady laminar flow case. I input a velocity of 0.175 m/s at the inlet and want to maintain the outlet at atmospheric pressure. These are the only Boundary consitions that I imposed. All the remaining faces are treated as WAll.

When I ran the case in Fluent, it just goes on computing and the solution never converged. I also tried reducing the Under- Relaxation factors, changed to second- order upwinding, etc. Nothing worked.

Another thing that I noted is that in the plot of residuals, continuity is staying put at 100% error. The X,Y,Z velocities appear to be converging (at least it looks so in the plot), but continuity just remains unchanged.

I just cannot understand the reasons for these problems. Any comments/ suggestions?

Thanks.

Re: Fluent Computation
 

A few general tips:

Check your all boundary conditions carefully. Check you donot overspecify things e.g both velocity and pressure at a BC.

Check dimensions (units) of your domain as well if they are consistent.

Such a problem, you must get a converged solution if code is bug free ( I guess it is)

Re: Fluent Computation
 

Hi,

I checked all the parameters. I input a steady velocity of 0.175 m/s at the inlet and the outlet is at atmospheric pressure. The grid was created in mm and I scaled it after importing into Fluent.

So what could be the problem? It goes on computing for days and finally hangs up because the memory ran out.

Thanks.

Re: Fluent Computation
 

I m quite certain that there is something wrong with your boundary conditions. You may do the following test:

Set reference pressure in your domain 0 Pa (everywhere and also as initial guess before simulation)

Now at outlet set the static pressure BC i.e. P_static = 0 Pa.

Your problem may be that at inlet you are specifying a velocity and at outlet you are specifying a pressure of 1 atm which does not allow the liquid to come out and therefore continuity is not fullfilled. For an in compresible flow in a CFD solver absolute pressure has no meaning. Only the relative pressure i.e. P - P_ref is what a code cares for. Where P is the pressure in a control volume and P_ref is the reference presure at some point in your domain. A person who has never written a CFD code, unfortunately, do not realize this problem.

Hope this helps.

VK

Re: Fluent Computation
 

Hi,

I set up the initial test values that you suggested...... and that's what I did earlier too. But the same problem continues. I don't find any error, but the computation just keeps going on.

Thanks, VR

Re: Fluent Computation
 

In addition to initial test values you have to set reference and boundary pressure equal to 0. I hope you had also done this step.

Re: Fluent Computation
 

Yes, I did that too. Nothing happened even then.

Re: Fluent Computation
 

Would it help in any way if I specify pressure at both the inlet and outlet? Then I have a very small value (about 1mm Hg at the inlet). So I think velocity at the inlet and pressure at the outlet is the best possible combination. Or will specifying OUTFLOW at the outlet help?

Thanks, VR