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-   -   Please help!! Need to Confirm something -Transient (http://www.cfd-online.com/Forums/fluent/30266-please-help-need-confirm-something-transient.html)

Julie September 4, 2002 00:22

Please help!! Need to Confirm something -Transient
 
Hi,

Sorry for posting this out again but I was afraid the previous thread was missed out..and I am really pressed for time.

I just wanna confirm the basic steps in solving for tranisent simulations.

I am modeling the spread of ammonia gas in a room.The ammonia gas occupies a certain small volume in the domain initially.

Basic Steps:

1.Solve the steady state solution for velocity and temp distribution in the room without the species transport

2.After converegd, use this converged solution, turn on transient mode, turn on species transport, and patch a small volume of cells in the domain with ammonia gas mass fraction.

3.Click off the "check for convergence" in the monitors for residuals for ALL entities such as x-velocity,v-velocity,z-velocity,continuity etc etc.

4.Specify the number of time steps and step size and the number of iterations in one time-step.

5.And WITHOUT initialising again, run the iterations

Am I getting it right? Especially the part where in any transient simulation, we must use the converged result obtained in steady state solution.

Also, can someone tell me how to determine the sufficient number of iterations used in each time-step? I was told that the number of iterations should not exceed 30. And I'm running at 20, how would I know if it is sufficient?

Please help, Thanks.


Ashu September 4, 2002 00:39

Re: Please help!! Need to Confirm something -Trans
 
Monitor the variation of temperature or other variables to check whether the results are converged or not! Method is okay and works well. But are you interested in transeint phenomena? If the variation is smooth as per your expectation then you needn't worry.

Ashu

Julie September 4, 2002 01:31

Re: Please help!! Need to Confirm something -Trans
 
Hi Ashu,

So I have to set a monitor for the temperature of a plane in my domain in my transient modellinf or steady state modelling, and if monitor in tranisent, the variation of temp with respect to flow time or iterations?

And what kinda plots do I look for to determine if my solution has convereged in each time-steps?

Thank you very much for replyin

Ashu September 4, 2002 10:59

Re: Please help!! Need to Confirm something -Trans
 
Try monitoring variable like velocity/temperature with flow time in case of transient simulations. See that how the variable is varying per iteration also. THis way you can see whether the solution is going on right direction or not!

Ashu


sri September 5, 2002 12:34

Re: Please help!! Need to Confirm something -Trans
 
Hi Julie,

Make sure that the solution is converged for each timestep.The idea of the maximum number of iterations required for this can be had by intially giving the number of time steps equal to zero and then note the maximum number of iterations to converge.I suppose you mentioned that Fluent allows you to give a maximum of 30 iterations per time step.As such there is no limit I am aware of.

sri

Shankar September 5, 2002 12:45

Re: Please help!! Need to Confirm something -Trans
 
For this kind of problem do we deactivate the flow equation after the solution has converged for the steady-state?Do we just need to use the species-transport equation for the unsteady problem after that? Shankar.


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