# Fluent FAQ

(Difference between revisions)
 Revision as of 02:38, 24 November 2005 (view source)Zxaar (Talk | contribs)← Older edit Revision as of 02:39, 24 November 2005 (view source)Zxaar (Talk | contribs) Newer edit → Line 1: Line 1: This section is empty. This is just a suggestion on how to structure it. Please feel free to add questions and answers here! This section is empty. This is just a suggestion on how to structure it. Please feel free to add questions and answers here! - == General purpose codes == - === FLUENT === + == FLUENT == == Solver Related == == Solver Related == ==== What does the floating point error mean? How can I avoid it? ==== ==== What does the floating point error mean? How can I avoid it? ====

## Revision as of 02:39, 24 November 2005

This section is empty. This is just a suggestion on how to structure it. Please feel free to add questions and answers here!

## Solver Related

#### What does the floating point error mean? How can I avoid it?

The floating point error has been reported many times and discussed a lot. Here are some of the answers found in the Fluent Forum:

SOLVER AND ITERATION -----I think if you set shorter time step, it may be good. Or changing little Under-Relaxiation-Factors, it may be good. In my experience, I set 1/3 Under-Relaxiation-Factors as default.� -----�also lower the values of under relaxation factor and use the coupled implicit solver� -----�Try to change under-relaxation factors and if it is unsteady problem maybe time step is to large.� -----�you can improve the ratio in the solve--control--limits, maybe that can help.� -----�you will need to decrease the Courant number� -----�If you still get the error, initialize the domain with nothing to 'Compute from...' Then click 'init'. Again select the surface from which you want to compute the initial values & iterate. This should work.� -----�Another reason could be a to high courant number - that means, that the steps between two iterations are too large and the change in the results is too large as well (high residuals)�

GRID PROBLEMS -----�this error comes when I start scaling grid. in gambit, all my dimension is in mm, when in fluent i convert it in meter using buttone SCALE. after it, when i iterate, about hundred iteration, this error appeared. but when i not scale my drawing to m...and let it be as in gambit..then the iteration is success. -----�hi I think you should check your mesh grid mesh is very high. your problem solve by selection a low mesh.� -----�Your mesh is so heavy that your computers resources are not enough. try to use coarser mesh.�

BOUNDARY CONDITIONS -----�In my case I had set a wall boundary condition instead of an axis boundary condition and then FLuent refuses to calculate telling me 'floating point error'.� -----�Your Boudary Conditions do not represent real physis.� -----�wrong boundary condition definition might cause the floating point error. For example setting an internal boundary as interior� -----�Once I had the problem, simulating a 2D chamber with a symmetry BC. I set the symmetry somewhere as �axe symmetric� and the floating point error occur� -----�check the turbulence parameter you set. reduce the turbulence intensity to less that one for first, say 50 iterations.

USING A UDF -----�What I mean is really often when people creates UDF they generally forget that for the first iteration some variable can be zero. Therefore if you are divided a number by zero your solver will blow up telling you 'non floating error'. 'non' means 'not a number'. Depending on your UDF this kind of error does not effectively happens at the first iteration. An example, if you are simulated a domain with a stagnant water as initial condition and you are calculated for the first iteration something like 1/Re therefore lets call it BOOM !!! because Re=0 . To find this kind of think there a simple way : reread your UDF.�

MULTI PROCESSOR ISSUES -----"I've had similar problems recently with floating point errors on a multi processor simulation. The solution for my problem seems to be to run on a single processor, where it runs fine....?�

WRONG INITIATION ----- Initiating the case with wrong conditions may lead to floating point error when the iterations start.

## Solution Methodology

#### What is the role of under-relaxation parameters? What should be the optimum choice of these parameters?

They limit the influence of the previous iteration over the present one. If you choose small values it may prevent oscillations in residuum developing. At the same time the solution may need more time to converge. Keep the default values as they are given in FLUENT. You can decrease them gradually if necessary. Momentum 0.6, pressure 0.1, k 0.4, eps 0.4, mass source 1, viscosity 1.

## Tips

#### How to merge two mesh files and make one?

To merge two mesh files the suggested utility is tmerge. The syntax of tmerge is simple.
utility tmerge -3d file1 file2 finalfile
To join the two interior faces use:
Grid->Fuse

#### How to run multiple cases in batch mode

This could be achieved by running it from journal file. The example journal file that runs two cases is given as
solve dual-time-iterate yyy1
file write-case-data zzz1.cas
yes (comment: for discard cas dialog)
solve dual-time-iterate yyy2
file write-case-data zzz2.cas

#### What does the abbreviation mean?

CFD = Computational Fluid dynamics FEM = Finite element model UDF = User defined function