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-   -   Floating point exception? (http://www.cfd-online.com/Forums/fluent/27514-floating-point-exception.html)

Ricky Wong February 23, 2000 07:15

Floating point exception?
 
Hello,

I'm trying to model a plain underexpanded jet (still), and I can't get the iterations started. Fluent keeps coming back with floating point exception, (underflow). I don't know what it's complaining about!.

I've tried reducing the skewness, checked the grid, re-entered the boundary conditions (trying to start off with a jet at a very low NPR, without turbulence), no joy.. am I grasping at straws or is it something blindingly obvious?

please help

Thanks

J. McKelliget February 23, 2000 09:59

Re: Floating point exception?
 
Your model is diverging.

The underexpanded jet is almost impossible to converge using Fluent.

Make sure you are using the coupled solver, and maybe some kind of initial guess.


LW February 23, 2000 12:21

Re: Floating point exception?
 
Can you explain "Underexpanded Jet"? Thanks!

J. McKelliget February 23, 2000 14:11

Re: Floating point exception?
 
Underexpanded jet = a compressible gas flow where the pressure in the jet is substantially higher than the pressure of the region into which it is discharging - leads to lots of shock and expansion waves etc

John C. Chien February 23, 2000 14:27

Re: Floating point exception?
 
(1). That's right. (2). Or, (p,ambient/ p,jet exit) < 1. (3). (p,jet exit) is the nozzle exit pressure but still inside the nozzle. (4). Further expansion will occur in the jet, downstream of the nozzle exit, until the pressure at the boundary of the jet matches the ambient pressure. In between, shocks and expansion waves will form in the jet (or plume).

Ricky Wong February 24, 2000 06:48

Re: Floating point exception?
 
Thanks,

Managed to get the model started, stripped it down completely to a laminar, incompressible flow at a very low NPR to begin with and gradually increased the NPR as the solution progressed.


Sung-Eun Kim February 24, 2000 22:06

Re: Floating point exception?
 
Have you tried to patch a non-zero initial guess for the velocity field ? Please contact your support engineer if the problem doesn't go away.

Ricky Wong February 25, 2000 11:39

Re: Floating point exception?
 
Thanks for the reply

Non-zero intial guesses also prompted the error, It wasn't until I took stripped down the model that it was able to start. Unfortunately I don't think I have a support engineer, I'm a post-grad and the University has an academic license to which I am not the primary licence holder, queries to fluent (europe) support are very slow in replying (if at all) because an academic license is has low priority. Hence I look to the fluent forum for possible help.

sheikh nasir February 7, 2012 23:31

Floating point error.
 
Hello.
I am getting floating point error:invalid number in my thesis. Ie train moving in tunnel. Can any help me. My email is sheikhnasir39@gmail.com
thanks

chngyian February 15, 2014 23:49

One reason could be that the factors are over-relaxed. So if you are confident that BCs, model and solver are appropriate, try reducing the relaxation factor in solution control.


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