# deciding time duration in transient simultion

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 March 3, 2012, 09:43 deciding time duration in transient simultion #1 New Member   Join Date: Jan 2012 Posts: 9 Rep Power: 7 Sponsored Links I am doing a transient simulation for my model , I have decided to use time steps as 0.0632 seconds,how do I decide the time duration ?I first gave time duration as 65 and time steps as 0.0632 but the solver ran continously ..has'nt converged at all

 March 4, 2012, 05:05 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,735 Rep Power: 106 How have you decided 0.0632s is the right time step size? If it is not converging it probably is not the right size. And as for the time duration - what are you modelling? An inkjet printer ejection (total time about 10 microseconds), or magma motion in a magma chamber (turn over time 140 million years). I have done work in both and guess what? They both use different total simulation times.

 March 4, 2012, 07:17 #3 New Member   Join Date: Jan 2012 Posts: 9 Rep Power: 7 I am modelling the flow of air through the intake manifold of a 4cyl diesel engine ,naturally aspirated ,upto before valve..now I calculated timestep by taking the whole length of manifold as distance and inlet velocity of air , that value I took multiplied by (1/20) to get 0.0632 sec ..my length is 0.349 m and intake velocity is 27.61m/s ..1/20 because that is the time required for a particle of fluid to traverse the length of object

 March 4, 2012, 17:41 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,735 Rep Power: 106 Hmmm, I don't think you have much idea of how to set a time step My PhD thesis was on IC engine modelling. Have a look at the time step size I used: http://hdl.handle.net/2100/248

 March 5, 2012, 08:47 #5 New Member   Join Date: Jan 2012 Posts: 9 Rep Power: 7 Thanks for that thesis..i have got new doubts...how is the CFL stability criterion related to timestep..and I am not able to comprehend what a timestep is and what are the parameters that affect it.

March 5, 2012, 18:16
#6
Super Moderator

Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 13,735
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Quote:
 how is the CFL stability criterion related to timestep
CFX is an implicit solver so it is not relevant.

Quote:
 and I am not able to comprehend what a timestep is and what are the parameters that affect it.
If you do not understand the basics you do not have much hope of doing a useful analysis. You need to do some studies or reading into numerical methods and CFD before you can start doing simulations.

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