Fluent FAQ

From CFD-Wiki

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

Contents 1 General purpose codes 1.1 FLUENT 1.1.1 What does the floating point error mean? How can I avoid it? 1.1.2 What is the turbulent viscosity ratio warning and how can I handle? 1.1.3 How do i carry out rotating body analysis, eg a rotating sphere or cylinder in flow? 1.1.4 How do i get better and faster convergence? 1.1.5 What is the role of under-relaxation parameters? What should be the optimum choice of these parameters? 1.1.6 How can I determine the inputs for a porous media or porous jump from flow versus pressure drop data? 1.1.7 How do I model heat conduction in a composite wall? 1.1.8 What pressures should be specified at inlets and outlets for buoyancy flow problems? 1.1.9 Are there any general guidelines on selecting a turbulence model? 1.1.10 How can both turbulent and laminar flow be included in one model? 1.1.11 How to start a 3D simulation with an compressible medium and temperature changes? What is important to consider 1.1.12 What does the abbreviation mean? 1.2 FloWizard 1.3 FIDAP 1.4 POLYFLOW 2 Pre-processors 2.1 Gambit 2.2 Gambit Turbo 2.3 TGrid 3 Application specific codes 3.1 Icepak 3.2 Airpak 3.3 MixSim 4 Educational codes 4.1 FlowLab

General purpose codes

FLUENT

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....?�

What is the turbulent viscosity ratio warning and how can I handle?

How do i carry out rotating body analysis, eg a rotating sphere or cylinder in flow?

How do i get better and faster convergence?

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

How can I determine the inputs for a porous media or porous jump from flow versus pressure drop data?

How do I model heat conduction in a composite wall?

What pressures should be specified at inlets and outlets for buoyancy flow problems?

Are there any general guidelines on selecting a turbulence model?

How can both turbulent and laminar flow be included in one model?

How to start a 3D simulation with an compressible medium and temperature changes? What is important to consider

What does the abbreviation mean?

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

FloWizard

FIDAP

POLYFLOW

Pre-processors

Gambit

Gambit Turbo

TGrid

Application specific codes

Icepak

Airpak

MixSim

Educational codes

FlowLab