Time step

prabulchandran · June 16, 2016, 04:28

i am doing a numerical simulation on flow past a rotating cylinder between reynolds 50 and 400. can anybody help me how to choose the time step for the analysis. pls reply

YuguiYo · June 16, 2016, 04:48

Hi Prabul,

You should choose your time step in function :
1. of what you want to see
2. of the fluid velocity
3. of the domain dimensions

Indeed if you have a velocity of 1 m/s and a domain of 1 meter length, the max timestep would be 0.1. (and in function of what you want to see you should decrease it or not.) With a time step of 0.1 (with this configuration) you can see 10 times in a sec a point moving from inlet to outlet.

I hope it helps

Hugo

ghorrocks · June 16, 2016, 07:04

I have a bit of a different take on time step size.

If the simulation is steady state:
* Start by using the auto time step.
* If the simulation is converging consistently but slowly then increase it (using edit run in progress, don't stop and restart)
* If the simulation is converging poorly and jumping around try a smaller time step
* This FAQ might help: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

If you are doing a transient simulation then use adaptive time steps, homing in on 3-5 coeff loops per iteration. Make sure the high and low limits are wide enough that you do not hit them and make sure the initial time step is reasonable. The the solver will find the time step size by itself.

prabulchandran · June 17, 2016, 05:35

thank for your reply sirs.

June 16, 2016, 04:28	Time step	#1
prabulchandran New Member Prabul Chandran Join Date: Nov 2015 Posts: 12 Rep Power: 10	i am doing a numerical simulation on flow past a rotating cylinder between reynolds 50 and 400. can anybody help me how to choose the time step for the analysis. pls reply

June 16, 2016, 07:04		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I have a bit of a different take on time step size. If the simulation is steady state: * Start by using the auto time step. * If the simulation is converging consistently but slowly then increase it (using edit run in progress, don't stop and restart) * If the simulation is converging poorly and jumping around try a smaller time step * This FAQ might help: http://www.cfd-online.com/Wiki/Ansys...gence_criteria If you are doing a transient simulation then use adaptive time steps, homing in on 3-5 coeff loops per iteration. Make sure the high and low limits are wide enough that you do not hit them and make sure the initial time step is reasonable. The the solver will find the time step size by itself. Far likes this.

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June 16, 2016, 04:48		#2
YuguiYo New Member Hugo Regina Join Date: Apr 2016 Posts: 23 Rep Power: 10	Hi Prabul, You should choose your time step in function : 1. of what you want to see 2. of the fluid velocity 3. of the domain dimensions Indeed if you have a velocity of 1 m/s and a domain of 1 meter length, the max timestep would be 0.1. (and in function of what you want to see you should decrease it or not.) With a time step of 0.1 (with this configuration) you can see 10 times in a sec a point moving from inlet to outlet. I hope it helps Hugo

June 17, 2016, 05:35		#4
prabulchandran New Member Prabul Chandran Join Date: Nov 2015 Posts: 12 Rep Power: 10	thank for your reply sirs.