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-   -   Errors while iterating (http://www.cfd-online.com/Forums/fluent/39902-errors-while-iterating.html)

Vidya Raja March 1, 2006 09:40

Errors while iterating
 
Hi,

I am modeling a steady flow with a constant velocity of 0.175 m/s in the z- direction. But while iterating, Fluent constantly displays messages that read:

REVERSED FLOW IN 288 FACES ON OUTFLOW 4.

What does this mean? How can I correct this? And it is taking very very long to do even a single iteration. Is it because there are many nodes and cells?

Thanks, Vidya


Julian March 1, 2006 11:38

Re: Errors while iterating
 
It literally means that there is reversed flow in 288 faces on the face OUTFLOW 4. That wasn't very helpful so I shall try to elaborate.

I am assuming that outflow4 is either a pressure boundary or an outflow. Check you boundary conditions make sure that they are set correctly. You should also make an estimate of roughly what proportion of the whole face 288 cells is. If it is a small proportion, then don't worry too much.

What _may_ be happening is that due to a poor initial guess or sometimes due to recrculation in the model, there is inflow at the outlet. Normally this is not a problem, but if due to recirculation then you should consider moving the face further downstream to prevent errors being introduced by the boundary.

I cannot answer as to why the simulation is slow. However, if you are just starting out, then run it first order. If memory is tight on your machine, then run it segregated. If your machine is swapping, then reduce the number of cells, dramatically.

All the best, Julian

Vidya Raja March 1, 2006 17:58

Re: Errors while iterating
 
Hi Julian,

When I read my mesh file into Fluent, it said that there are 288 outflow faces on zone 4. So does the error mean that there is reversed flow on all the outflow faces? Which means, my modeling is incorrect??????????????

And how do you reduce the number of cells? Please give me a detailed explanation because I'm new to Fluent.

Thanks, Vidya

Julian March 2, 2006 03:55

Re: Errors while iterating
 
It means that there is reversed flow for all of the faces at present. Let the model run for a while and see what happens. Does the reversed flow stop, or reduce.

Move the outlet further downstream and see what happens.

You will need to remesh the model in GAMBIT to reduce the number of cells. Try putting in hex cells where the flow is mostly in one direction so that the sides of the hex cells are parallel to the flow direction. This will reduce the number of cells dramatically.

Is this the only outlet? What is the boundary condition? Read the Fluent manual - what does this boundary condition mean.

Julian


Vidya Raja March 2, 2006 10:23

Re: Errors while iterating
 
Hi Julian,

Thnaks a lot for the reply. The reversed flow is remaining the same and it's there for all the faces in the outlet. And there is only one outlet.The flow path is something like a cylinder. So do you mean that I should decrease the length of the cylinder?

I used the sizing functions for meshing......... so should I try to create coarser mesh? I really want fine mesh, especially near the walls and I'm modeling a biological flow.

Thanks, Vidya

Julian March 2, 2006 10:26

Re: Errors while iterating
 
Keep it running for a while. If there is only one inlet, and you have a velocity inlet and one outlet, then the flow has to go out eventually.

Start with a coarse mesh and refine it as you make the model more complicated. Right now you should model single phase until you have sorted this problem out.

Julian


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