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-   -   Error and Divergence (https://www.cfd-online.com/Forums/fluent/115912-error-divergence.html)

 ouce_2930 April 9, 2013 06:28

Error and Divergence

We are doing a simulation of a 2-phase nozzle(air-glassbeads) in fluent..
The error we got is floating point exception , Temperature limited to 1 in 997 cells on one zone ,one domain ....
Reversed flow in 12 faces on pressure outlet 7...etc etc
We entered the value of outlet pressure as 101325 pascal and inlet is 395900 pascal...Flow selected is laminar
Also tell us whats the significance of solution controls and what are the values that can be given and wats the significance of relaxation factors in this problem we used default values..
As of now we are gettin divergent solution

 vasava April 10, 2013 06:55

The floating point error is due to divergence. The warning of reversed flow is common as you are specifying pressure and not the direction of velocity on the boundary. By the way How big is your domain? You have a pressure drop of almost 4 atm. What exactly are you simulating?

 ouce_2930 April 12, 2013 04:49

We are doing a simulation of a CD nozzle in which we use air and glassbeads are the phases..Can you elaborate what exactly does domain mean ??
And we are facing one more problem where when we look at the plots of velocity the air velocity is showing some plot where as glass beads velocity is remaining constant in that plot.. That means it is not taking glass beads into the account
What may be the problem and what changes should be made so that we get plot for glass beads velocity even??

 RodriguezFatz April 12, 2013 05:44

"Domain" means "setup" or "experiment" here.
You should not think about any strange results you get since your solution does not converge.

It sounds quite unusual that you have a pressure drop of 3 atm but your flow is laminar. With a domain of moderate size you will get huge velocities with such a pressure drop.
Please post pictures of your geometry and you mesh and show where your outlets and inlets are!

 ouce_2930 April 12, 2013 08:24

2 Attachment(s)
Attachment 20685

Attachment 20686

These are the attachments... We are going with turbulent flow( k-epsilon) where the solution got converged but the problem is the glass beads were not taken into account by the processosor. We gave all the boundary conditions for mixture but its only taking air in the mixture.

 RodriguezFatz April 12, 2013 08:35

Quote:
 Originally Posted by ouce_2930 (Post 420123) These are the attachments...
What...!?:eek:

 ouce_2930 April 13, 2013 01:08

2 Attachment(s)
sorry v dint check upon d pics... here r d pics of the geometry and mesh
the left edge is the inlet
and the right edge is the outlet

Attachment 20693

Attachment 20694

 oj.bulmer April 13, 2013 04:47

It is never robust to specify just pressure values at inlet and outlet. It's preferable to use a quantifiable flow (mass flow rate/velocity etc) for a more stable setup.

Why do you need to simulate only converging-diverging portion of nozzle, why not extend it to few pipe diameter before and after the nozzle? This should be helpful in mitigating backflow. This will use fully developed flow before coming into nozzle giving a more accurate represenation.

Are you sure mixture model is setup properly? Have you had a go at the tutorials on this?

OJ

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