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Continuity divergence in simple gas flow

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Old   April 14, 2014, 06:03
Default Continuity divergence in simple gas flow
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Dear all,

I am modelling a simple waste heat boiler and I'm experiencing an unsuspected problem with the simulation.

The model is quite simple, as hot flue gas is entering from one end and exiting from the other. Inputs are as velocity inlet and pressure outlet.

Radiation: P1
Turbulence: rke, double precision
Solver: Simple
Momentum, turbulence and energy: 2nd order upwind

As I started the steady state calculation, I noticed that the continuity equation is not stabilizing and at the end it affected all the other residuals as well and the results were not as expected/not as my experience tells me. The continuity did not oscillate as it sometimes does, it was just "chaotic".

I refined the mesh and still the problem persisted. I coarsened the mesh, no effect. I switched to transient, but the continuity equation did not stabilize.

I tried to solve the problem as first order, I tried to switch the solver, and tried to calculate only the flow field. No success.

In the end I fooled around with Fluent and forgot to turn on gravity and BOOM - success! The calcutation converged as expected and the results were correct.

The question is: can anybody explain me why gravitation messed the calculation. As it is a simple gas flow calculation (desity at the inlet = 0.35kg/m3) gravitation should not practically affect the calculation at all.

I want to point out that I calculate similar cases in a daily basis and this was the first one which showed the above mentioned problem.

Regards,
O
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Old   April 14, 2014, 06:48
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Behrooz Jamshidi
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Hi
1-is your flow regime turbulent?
2-If you have correctly set the gravity acceleration, now just make gravity acceleration negative(if it is 9.8 put -9.8 and vise versa ) with the same direction as before. It seems somehow foolish, but test it and let me know the result.
Best regards
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Old   April 14, 2014, 07:31
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Hi cfd-seeker,

Yes, the flow regime is turbulent. I tried to reverse the gravity direction, but still the continuity equation diverged.

Regards,
O
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Old   April 15, 2014, 09:16
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Behrooz Jamshidi
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Hi
What about relaxation factors? Try solving the solution by 0.7 relaxation factor for pressure and 0.3 for momentum.
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Old   April 15, 2014, 11:17
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Hi,
I tried to tweak the relaxation factors up and down, but in vain.
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Old   December 8, 2016, 01:48
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Ramin
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Hi.
I had this problem and it was solved in this way:
you should "reorder" your mesh until achieving this notice in the command bar :

>> Reordering domain using Reverse Cuthill-McKee method:
zones, cells, faces, done.
Bandwidth reduction = 372525/670 = 556.01
Done.

>> Reordering domain using Reverse Cuthill-McKee method:
zones, cells, faces, done.
Bandwidth reduction = 670/670 = 1.00
Done.


after that you can initialize and run

*reorder :
in Ansys Fluent 17---> menu bar--->setting up domain--->reorder--->domain

Good Luck
Ramin
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Old   December 9, 2016, 05:02
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Kushal Puri
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Quote:
Originally Posted by Okka View Post
Dear all,

I am modelling a simple waste heat boiler and I'm experiencing an unsuspected problem with the simulation.

The model is quite simple, as hot flue gas is entering from one end and exiting from the other. Inputs are as velocity inlet and pressure outlet.

Radiation: P1
Turbulence: rke, double precision
Solver: Simple
Momentum, turbulence and energy: 2nd order upwind

As I started the steady state calculation, I noticed that the continuity equation is not stabilizing and at the end it affected all the other residuals as well and the results were not as expected/not as my experience tells me. The continuity did not oscillate as it sometimes does, it was just "chaotic".

I refined the mesh and still the problem persisted. I coarsened the mesh, no effect. I switched to transient, but the continuity equation did not stabilize.

I tried to solve the problem as first order, I tried to switch the solver, and tried to calculate only the flow field. No success.

In the end I fooled around with Fluent and forgot to turn on gravity and BOOM - success! The calcutation converged as expected and the results were correct.

The question is: can anybody explain me why gravitation messed the calculation. As it is a simple gas flow calculation (desity at the inlet = 0.35kg/m3) gravitation should not practically affect the calculation at all.

I want to point out that I calculate similar cases in a daily basis and this was the first one which showed the above mentioned problem.

Regards,
O
Try with coupled solver with pseudo transient turn on.
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Old   January 11, 2019, 01:01
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Trot
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Is this "reorder" option available in Ansys 15? Thanks in advance
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