time-step <dtmin.

ounifiras · December 2, 2013, 05:14

Dear experts

I am trying to simulate a flow of labyrinth weir. I have put the finish-time 20 second. I utlusé 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, 11:07

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, 11:51

thanks

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

ounifiras · December 2, 2013, 11:58

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

dmilano · December 2, 2013, 12:02

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, 12:05

thank you greatly

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

dmilano · December 2, 2013, 12:05

It should be accessible from the Diagnostics drop down menu

ounifiras · December 2, 2013, 12:12

thank you greatly

ounifiras · December 2, 2013, 15:11

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, 16:17

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, 16:29

thank you greatly

ounifiras · December 3, 2013, 03:26

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''

December 2, 2013, 05:14	time-step <dtmin.	#1
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	Dear experts I am trying to simulate a flow of labyrinth weir. I have put the finish-time 20 second. I utlusé 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?

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December 2, 2013, 11:07		#2
dmilano Member Join Date: Mar 2012 Posts: 72 Rep Power: 14	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.)

December 2, 2013, 11:51		#3
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	thanks do you think if i put the minimum time step 1e-40 may lead to a result

December 2, 2013, 11:58		#4
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	how can I get the solver summary file (hd3out) ?

December 2, 2013, 12:02		#5
dmilano Member Join Date: Mar 2012 Posts: 72 Rep Power: 14	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.

December 2, 2013, 12:05		#6
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	thank you greatly how can I get the solver summary file (hd3out) ?

December 2, 2013, 12:12		#8
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	thank you greatly

December 2, 2013, 15:11		#9
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	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.

December 2, 2013, 16:17		#10
dmilano Member Join Date: Mar 2012 Posts: 72 Rep Power: 14	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.

December 2, 2013, 16:29		#11
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	thank you greatly

December 3, 2013, 03:26		#12
ounifiras Member ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 12	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''