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Mass flux is not converging

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Old   November 9, 2011, 07:57
Default Mass flux is not converging
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prince
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Hii everyone

I am using fluent as solver. From residuals plot, i have observed my solutions are converging (I have kept residuals of the order of 0.001). After some 300 iterations i got message from fluent interface also that my solution are converged.

But on post processing, i found my mass flux is not converged. What probably can be the reason ??

My problem involves high reynolds number. I am using k-epsilon RNG model. I have given velocity inlet and pressure outlet as Boundary condition.
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Old   November 9, 2011, 13:19
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How big are your final mass flux residuals? Also, could you clarify what inlet velocity or mach number you are using?
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Old   November 10, 2011, 02:51
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hii swetkyz
Thanks for taking interest.

I have kept inlet velocity as 2.8m/s which is converted to around 83 kg/s for my geometry. I am getting mass flux at outside which fluctuates in the range of around 70kg/s to 110kg/s.

My problem constitutes a big pool (8.37m*3.14m*9m). From inlet pipe water is supplied into pool through 10 nozzles which is mounted on pipe. In the mid of pool, there is small cube (1m* 1m*1m) from where pipe is installed to takeout the water from the pool.
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Old   November 10, 2011, 03:55
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You can - check your mesh quality
- try to reduce residuals to 0.0001 or less
- use double precision solver
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Old   November 10, 2011, 09:25
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I often turn off automatic convergence. You can watch the mass fluxes by creating monitors and setting them to plot and write.

It may help to turn on "pseudo transient" as well. Oscillations should damp out a little sooner using this feature. You can also try reducing the Under-relaxation factors to see if it will converge better. However, make sure that you finish your solution with the energy under-relaxation factor as 1.0.
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Old   November 11, 2011, 00:26
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Please try to reduce UDF's for momentum. Also try using a higher order scheme for pressure and momentum.also set your residuals to E-06. It should work.
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Old   November 11, 2011, 01:59
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Hii laurentb

I was using 3d and not double precision solver. Lemme check.. Mesh quality is around 28%. Is it good ?? I am using T/Grid type of mesh.

Thanks
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Old   November 11, 2011, 02:18
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Hii swetkyz

I am not able to find pseudo transient option. Can you guide me through it?? I have decreased the relaxation factor and waiting for results. Lets see

Thankss
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Old   November 11, 2011, 02:21
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hii Neville

I have decreased the URF value for momentum from default 0.7 to 0.3 and i also set the residuals below 1.0E-06. Lets see whether it will work or not

Thanks for taking interest.
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Old   November 11, 2011, 02:55
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Quote:
Hii swetkyz
I am not able to find pseudo transient option. Can you guide me through it??
You can turn on pseudo transient option by enabling the solution to be the unsteady with some specified time step and no of time steps. Check the convergence of your parameters of interest and when it reaches the steady state value, stop the run
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Old   November 14, 2011, 09:34
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Quote:
Originally Posted by prince_pahariaa View Post
Hii swetkyz

I am not able to find pseudo transient option. Can you guide me through it?? I have decreased the relaxation factor and waiting for results. Lets see

Thankss
Hi Far,

pseudo-transient is a check-box at the bottom of the "Solution Methods" panel. You can also turn it on through the TUI.
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Old   November 14, 2011, 09:59
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good addition to my knowledge. Is this available for Fluent 6.3 and Fluent 12.?
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Old   November 15, 2011, 07:41
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I couldnt find "Solution Methods" panel. I am using fluent 6.3.26 But was able to activate pseudo transient by the method described by Far.

As i described earlier also, in my problem, water from inlet pipe ( L shaped) is supplied into pool through 10 nozzles which is mounted on pipe. In the mid of pool, there is small cube from where pipe is installed to takeout the water from the pool. I am uploading picture of my geometry for better understanding.

After activating pseudo transient mode my overall mass flux is conserved but flow of water from nozzle into the tank is not what i expected.

I was getting the expected value for nozzle when pseudo transient were not activated. I also have tried other suggestions but not much of help.

Looking forward for any more help
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Old   November 16, 2011, 02:34
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Hii friends

Problem at last seems to converge. Mass flux is pretty okay and supply through nozzle also coming as expected.

thanks for all your's valuable input

Regards

Prince
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Old   November 16, 2011, 04:30
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Originally Posted by swetkyz View Post
Hi Far,

pseudo-transient is a check-box at the bottom of the "Solution Methods" panel. You can also turn it on through the TUI.
I am using Fluent 12.0 and 6.3, but I am unable to the locate the pseudo option in solution method panel. Can you please describe how to activate this option step by step?
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Old   November 16, 2011, 12:08
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Quote:
Originally Posted by Far View Post
I am using Fluent 12.0 and 6.3, but I am unable to the locate the pseudo option in solution method panel. Can you please describe how to activate this option step by step?
It may not be available in those versions. I am using version 13.
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