# How to determine time steps?

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 July 11, 2016, 23:46 How to determine time steps? #1 New Member   Join Date: Jul 2016 Posts: 11 Rep Power: 3 my fyp topic is "investigating drag by modifying surface pattern". i need to study the lift of a sepak takraw ball under unsteady state. how can i determine the time steps and also the maximum inner itertaion? I know it deals with courant number. but i dont know how to determine the courant number. I am working on star ccm+.

 July 13, 2016, 14:49 #2 Senior Member   Matt Join Date: Aug 2014 Posts: 611 Rep Power: 9 you will want to chose a time step based on a target value of your courant number. this value depends on your solver. If it is an explicit solver you need to keep courant less than or equal to 1 for stability. For implicit solver you can go much higher, in some cases up to 100. once you establish a target for courant number then you look at your mesh and find the areas with high velocity and small cell size (where max CFL will likely lie). Then use the definition of courant number to calculate the time step size that satisfies your courant number target, local cells size and flow velocity...

July 13, 2016, 22:23
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 Originally Posted by MBdonCFD you will want to chose a time step based on a target value of your courant number. this value depends on your solver. If it is an explicit solver you need to keep courant less than or equal to 1 for stability. For implicit solver you can go much higher, in some cases up to 100. once you establish a target for courant number then you look at your mesh and find the areas with high velocity and small cell size (where max CFL will likely lie). Then use the definition of courant number to calculate the time step size that satisfies your courant number target, local cells size and flow velocity...
Hi, thank you for your reply. The solver that i used is implicit unsteady and the velocity is set to 6m/s. There are 2026119 elements for my model. I am suing star ccm+. Is it i need to determine the Courant number myself?

 July 16, 2016, 18:55 #4 Senior Member     Khamlaj Join Date: Nov 2010 Location: United States Posts: 241 Rep Power: 9 You need to use the following relation according to wikipedia https://en.wikipedia.org/wiki/Couran...Lewy_condition CFL = (u*delta_t)/delta_x where, delta_t = is the step size you aim to calculate delat_x = the dimension size where the maximum velocity occurs. u = is the free stream velocity in your case is 6m/s. CFL = Cournat number. Best,

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