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-   -   time-step <dtmin. (http://www.cfd-online.com/Forums/flow-3d/127034-time-step-dtmin.html)

ounifiras December 2, 2013 06:14

time-step <dtmin.
 
Dear experts

I am trying to simulate a flow of labyrinth weir. I have put the finish-time 20 second. I utlusť the model of turbulence ke, I put The Turbulent mixing length as 7% of the flow depth ho over the spillway. i put minimum time step as the 10th-6 goal stop the simulation at the 5 second and this post APPEAR: time-step <dtmin. I have not found an explanation?

dmilano December 2, 2013 12:07

The simulation is stopping because the time step size (which changes every computational cycle) is dropping below the minimum allowable time step size that you set.

Try looking at the short print data in the solver summary file (hd3out) to see if you can determine why the time step is becoming so small. For most simulations an unexpected small time step is caused by fluid splashing, cells with large aspect ratios, or a setup error (a typo in an input, mismatching boundary and initial conditions, etc.)

ounifiras December 2, 2013 12:51

thanks

do you think if i put the minimum time step 1e-40 may lead to a result

ounifiras December 2, 2013 12:58

how can I get the solver summary file (hd3out) ?

dmilano December 2, 2013 13:02

Doing this will likely let the simulation run to completion. However, you will need to be cautious when obtaining a solution in this manner since an unexpected small time step usually indicates a mistake in the setup.

ounifiras December 2, 2013 13:05

thank you greatly

how can I get the solver summary file (hd3out) ?

dmilano December 2, 2013 13:05

It should be accessible from the Diagnostics drop down menu

ounifiras December 2, 2013 13:12

thank you greatly :)

ounifiras December 2, 2013 16:11

DEAR dmilano

When i run the same simulation with the model of laminar ; the simulation converge and it give good results ; but when i use the turbulence model K-e with a Turbulent mixing length as 7% of the flow depth ho over the spillway the model does not converge and the simulation stop before the finish time and this post APPEAR: time-step <dtmin. I think the problem is related to the model of turbulence.

dmilano December 2, 2013 17:17

It could definitely be the turbulence model. You can try reducing the turbulent mixing length, using the dynamically computed mixing length, or using the RNG k-e model.

ounifiras December 2, 2013 17:29

thank you greatly

ounifiras December 3, 2013 04:26

dear dmilano
when i have use minimum step Time as 1e-40 . the simulation stop at 8 second an this message appear
''end of calculation at t= 8.001E+00 cycle = 7189
excessive pressure iteration failures''


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