Time step size, number of time steps and max iterations per time step

guido_88 · December 4, 2009, 06:07

I'm modeling a flow in an open-open pipe with a sinusoidal UDF. My frequency is 2000 Hz. How I can set time step size, number of time steps and max iterations per time step ? I want that it resonate so i find that it has to be 8.43cm long and 1cm of diameter

thecfduser · December 4, 2009, 13:34

Hi
Time step = 1/2*2000.
Number of time steps: u must continue ure simulation untill u obtain a steady (oscillatory of course) solution. U will need at least that ure fluid traverse the pipe for 2 times , i mean 2*V/L
Max iteration by time step: try to monitor variables...choose the max iter. as the number of iterations after what ure solution does not change anymore within a time step....(u can also add 1 iteration for security, this ure unsteadiness is not uniform)

If those condirtions are so strict (take a lot of time to finish ure simulation), use NITA solver: it will give u fairly accurate solution within 1 iter by time step

P.S be4 asking a question, u can say hello and at the end, u can say thanks no?

guido_88 · December 4, 2009, 13:40

Thanks a lot,you're absolutely right !
I'll try to follow your suggestions, thanks again !

thecfduser · December 4, 2009, 14:11

No problem comrade

Anyway i forgot to tell u, for the time step i told u 1/4000 to stay conform to shannon theorem...but if u have more unsteadiness, u must use a smaller time step.
URANS models are not sensible to high frequencies, so this will not be a big problem but if u are performing DNS or LES, u will need maybe to refine ure time step...
There is no universal formula: the best way is to perform ure simulations with 2 different time steps and compare so u can be sure.

With smaller time step, u will need less ietrations by time step. Sometimes using a smaller time step only influences rms values without influencing the mean field...

Good Luck

smak.quadri · August 30, 2012, 15:49

I am performing FSI analysis on a elastic tube.

Total time is 0.8sec and I am using timestep of 0.005sec.

How can we calculate the required no. of time steps for a total time. is there any thumb rule for this ..? or its always a guess..?

kindly reply

December 4, 2009, 06:07	Time step size, number of time steps and max iterations per time step	#1
guido_88 New Member Guido Join Date: Dec 2009 Posts: 17 Rep Power: 16	I'm modeling a flow in an open-open pipe with a sinusoidal UDF. My frequency is 2000 Hz. How I can set time step size, number of time steps and max iterations per time step ? I want that it resonate so i find that it has to be 8.43cm long and 1cm of diameter

December 4, 2009, 14:11		#4
thecfduser Senior Member karine Join Date: Nov 2009 Posts: 158 Rep Power: 17	No problem comrade Anyway i forgot to tell u, for the time step i told u 1/4000 to stay conform to shannon theorem...but if u have more unsteadiness, u must use a smaller time step. URANS models are not sensible to high frequencies, so this will not be a big problem but if u are performing DNS or LES, u will need maybe to refine ure time step... There is no universal formula: the best way is to perform ure simulations with 2 different time steps and compare so u can be sure. With smaller time step, u will need less ietrations by time step. Sometimes using a smaller time step only influences rms values without influencing the mean field... Good Luck chek321 likes this.

August 30, 2012, 15:49	how to calculate time steps required in a transient analysis	#5
smak.quadri New Member abdul khader Join Date: Dec 2010 Posts: 20 Rep Power: 15	I am performing FSI analysis on a elastic tube. Total time is 0.8sec and I am using timestep of 0.005sec. How can we calculate the required no. of time steps for a total time. is there any thumb rule for this ..? or its always a guess..? kindly reply

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December 4, 2009, 13:34		#2
thecfduser Senior Member karine Join Date: Nov 2009 Posts: 158 Rep Power: 17	Hi Time step = 1/22000. Number of time steps: u must continue ure simulation untill u obtain a steady (oscillatory of course) solution. U will need at least that ure fluid traverse the pipe for 2 times , i mean 2V/L Max iteration by time step: try to monitor variables...choose the max iter. as the number of iterations after what ure solution does not change anymore within a time step....(u can also add 1 iteration for security, this ure unsteadiness is not uniform) If those condirtions are so strict (take a lot of time to finish ure simulation), use NITA solver: it will give u fairly accurate solution within 1 iter by time step P.S be4 asking a question, u can say hello and at the end, u can say thanks no?

December 4, 2009, 13:40		#3
guido_88 New Member Guido Join Date: Dec 2009 Posts: 17 Rep Power: 16	Thanks a lot,you're absolutely right ! I'll try to follow your suggestions, thanks again !