calculate the time step

serrar · May 23, 2019, 08:28

Dear expert,
How do I calculate the time step of an unsteady flow?
Best regards,
SER

LuckyTran · May 23, 2019, 10:59

2 things:

1) The unsteadiness in the flow has some time-scale. Let's say there is a 10 Hz oscillation in your flow. To resolve this in time, you want to be like 10x or more smaller than this scale so you'd pick a timestep size of 0.01s or 0.001s. The smaller the time-step, the more you resolve this oscillation in time.

2) There are stability constraints due to numerics. You are limited to a maximum time-step of a certain size which is dictated by the maximum Courant number of say around 10. Normally people limit the Courant number of around 2-5 or even smaller (less than 0.5) if they want really high quality results. Usually this Courant number constraint is much more restrictive than the flow time-scales. One of the reasons is because, on a very fine grid, the time-step size needed to meet this Courant number constraint becomes smaller.

serrar · May 23, 2019, 12:06

Dear LuckyTan,
Transient flow, not unsteady flow sorry.
After talking about the frequency, I want to explain my situation.
I try to control the turbulent boundary layer separation, I used a jet is the motion of this jet is due to a sinusoidal function that depends on the frequency = 30 and Amplitude and its via UDF. so I'm confused about the value of no time
Best regards,
SER

LuckyTran · May 23, 2019, 15:40

Same answer. Read it again, a million times if you have to, until you get it.

serrar · May 23, 2019, 16:09

Thanks a lot for your help

May 23, 2019, 08:28	calculate the time step	#1
serrar Member Join Date: Sep 2018 Posts: 31 Rep Power: 8	Dear expert, How do I calculate the time step of an unsteady flow? Best regards, SER

May 23, 2019, 10:59		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	2 things: 1) The unsteadiness in the flow has some time-scale. Let's say there is a 10 Hz oscillation in your flow. To resolve this in time, you want to be like 10x or more smaller than this scale so you'd pick a timestep size of 0.01s or 0.001s. The smaller the time-step, the more you resolve this oscillation in time. 2) There are stability constraints due to numerics. You are limited to a maximum time-step of a certain size which is dictated by the maximum Courant number of say around 10. Normally people limit the Courant number of around 2-5 or even smaller (less than 0.5) if they want really high quality results. Usually this Courant number constraint is much more restrictive than the flow time-scales. One of the reasons is because, on a very fine grid, the time-step size needed to meet this Courant number constraint becomes smaller. Last edited by LuckyTran; May 23, 2019 at 15:39.

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May 23, 2019, 12:06		#3
serrar Member Join Date: Sep 2018 Posts: 31 Rep Power: 8	Dear LuckyTan, Transient flow, not unsteady flow sorry. After talking about the frequency, I want to explain my situation. I try to control the turbulent boundary layer separation, I used a jet is the motion of this jet is due to a sinusoidal function that depends on the frequency = 30 and Amplitude and its via UDF. so I'm confused about the value of no time Best regards, SER

May 23, 2019, 15:40		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	Same answer. Read it again, a million times if you have to, until you get it.

May 23, 2019, 16:09		#5
serrar Member Join Date: Sep 2018 Posts: 31 Rep Power: 8	Thanks a lot for your help