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-   -   Adaptive Timestep in Eulerian Multiphase? (http://www.cfd-online.com/Forums/fluent/112168-adaptive-timestep-eulerian-multiphase.html)

bbmorales January 23, 2013 06:28

Adaptive Timestep in Eulerian Multiphase?
 
Hi there.

I am running a gas-solid euler-euler transient simulation in Fluent 14.
I am using the granular model. The transient formulation is the first order implicit.

I would like to use adaptive timesteps, but I cannot select it under the Time Stepping Method scroll list. It only has the fixed time step option available.

How can I do it?

I have found the following info in the manual:

Important:
Adaptive time stepping is available only with the pressure-based and density-based implicit formulations; it cannot be used with the density-based explicit formulation. In addition, it cannot be used with the discrete phase model, second-order time integration, Euler-Euler multiphase models (Approaches to Multiphase Modeling in the Theory Guide), or user-defined scalars (User-Defined Scalar (UDS) Transport Equations).

Is there any way that I can set an UDF or something to adapt the time step?

Regards!

msaeedsadeghi January 23, 2013 10:58

You can use DEFINE_DELTAT to adapt timestep in density-based mode.

vig January 24, 2013 07:29

DEFINE_DELTAT could be used irrespective of the solver.
You could write your own time step controlling mechanism.

bbmorales January 24, 2013 07:45

Hi guys - thanks for your help.

I haven't tried to do what you told me, so I may sound a little grumpy, but I have found this in the Fluent UDF manual:

DEFINE_DELTAT is a general-purpose macro that you can use to control the size of the time step
during the solution of a transient problem. Note that this macro can be used only if Adaptive is selected from the Time Stepping Method drop-down list in the Run Calculation task page in ANSYS FLUENT

In my case, the Adaptative option is not available in the drop-down list.

Did you guys go to run Euler-euler transient with adaptive?

Thanks!

bbmorales February 14, 2013 06:08

[SOLVED]

In short: I cannot use UDF to change the time-step in Euler-Euler, but using the journal, I can do something similar.

I have contacted the Ansys support and this was the answer.

------

If granular model does not allow for adaptive time stepping then you will not be able to use DEFINE_DELTAT. This is because the hook for this function is available only when we select adaptive time stepping.

Instead of UDF, you can use a jurnal file for this purpose. A jurnal file is a file which contains commands for Fluent. In journal file, you can put text menu interface (TUI) commands which tells Fluent to do some time steps and then change the time step size. You can find more details on TUI commands in ANSYS14 help guide at this location:

// User's Guide :: 2 // 3. Text User Interface (TUI) // 3.1. Text Menu System


For example if you want to run the case for 5 time steps using existing timestep size and then want to change the time steps size to 2e-3 sec, here are the commands:

*********************
solve dual-time-iterate 5 10
solve set time-step 2e-3
***********************
You can put these two commands in a file with extension as jou and read it in Fluent from File->read -> journal


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