# time step of unsteady particle tracking!

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 April 27, 2003, 23:42 time step of unsteady particle tracking! #1 winnie Guest   Posts: n/a Hi I am using unsteady particle tracking. I am confused by two time steps which are 1ˇ˘integration time step size used to integrate the equations of motion for the particle controled by length scale or step length factor in Discrete Phase Model panel and 2ˇ˘the time step size in Iterate panel respectively. Hope anyone here can tell me what these two time steps mean? And their relationships??? Regards winnie

 April 28, 2003, 17:19 Re: time step of unsteady particle tracking! #2 Alex Munoz Guest   Posts: n/a Hi Good questions It seems to me that you have a good understanding of the Fluent code, therefore i will be very specific. The time step for the iteration of the continuous flow field is overall calculate through the strouhal number (str=U/tD,where t period time , U free flow velocity, and D diameter of the obstacle). Let me give an example. if an infinite cylinder of diameter D=2.0m is placed in an undisturbed flow with U=1.0 m/s, and the Strouhal number expected from experiments is 0.21, then the period time is 0.1s. Therefore this is the time step for your iteration. Now, the time step for Discrete phase should be less than the relaxation time(tp), which is defined as the proportion of tp=4/3*p/mu*Dp^2/CdRep where p density mu viscosity Dp particle diameter Cd drag coeeficcient Rep Reynolds number based on the particle diameter and relative particle velocity ABS(Uf-Up) Now you get both iteration times.

 April 20, 2011, 08:08 DPM time step #3 New Member   Join Date: Feb 2011 Posts: 16 Rep Power: 15 Hello Sorry to dig such an old topic but i don´t understand Alex Munoz second equation regardind the time step of discrete phase. Can anyone please tell me where i can find this equation, and if possible give an example like first equation. Real thanks