Fluent FAQ

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Revision as of 06:02, 13 September 2005

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General purpose codes

FLUENT

Question 1

newbie of fluent, I wanna calculate the pressure drop of a channel, then after the simulation, i use Report->Surface Integrals->Facet Average, Field Variable->Total pressure, then I chose two surface in which I'm interested, and I got two total pressure value, and the difference between these two pressures is the pressure drop, am I right? if not, pls tell what should I do? ur kind help is appreciated in advance.

Answer 1

Question

What is the floating point error?

Answer

The floating point error has been reported many times and discussed a lot. Here are some of the answers I 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....?�

Question 3

What is turbulent viscosity ratio warning and how to deal with it?
Answer

Question 4

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

Question 5

How do i get better and faster convergence?
Answer

Question 6

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

Question 7

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

Question 8

How do I model heat conduction in a composite wall?
Answer

Question 9

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

Question 10

Are there any general guidelines on selecting a turbulence model?
Answer

Question 11

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

FloWizard

FIDAP

POLYFLOW

Pre-processors

Gambit

Gambit Turbo

TGrid

Application specific codes

Icepak

Airpak

MixSim

Educational codes

FlowLab