Time step for unsteady simulation

Mohankumarg12 · June 29, 2011, 05:18

Hi
I am doing a simulation for defrosting of wind shield using fluent 12.1. In the input I am giving a time dependent temperature profile for 40min. For the unsteady calculation I am giving a time step of 5 sec and with intermediate iteration of 5.
I am also getting solution for time step of 10 sec and I am getting a quicker solution as compare with 5 sec, but I want to know which is right and how to decide the time step for unsteady simulation.

Thanks in advance.

Mohan.G

nikhil · July 2, 2011, 14:40

I would suggest, Monitor the temperature or the heat flux from the wind shield after a particular time interval.

If the difference between these monitored parameters is within some desired critera, say 5-8%, then you can trust the results of the 10 second case.

Another way to deside whether your time step is correct as per grid and convection-diffusion of your physical problem is to check
1. CFL condition OR 2. Grid Fourier Number criterion ( Depending upon a physics of your problem)

CFD criterion needs to be satisfied for negligible diffusion.
Grid Fourier number criterion needs to be satisfied for negligible convection

Mohankumarg12 · July 3, 2011, 23:51

Thank you Nikhil

mecamor · July 4, 2011, 15:03

hello I wanted to see the shock wave generated by a piston. My problem I can not even move my piston with moving wall. thank you for helping me

June 29, 2011, 05:18	Time step for unsteady simulation	#1
Mohankumarg12 Member Mohankumar.G Join Date: Sep 2010 Location: Pune,India Posts: 44 Rep Power: 16	Hi I am doing a simulation for defrosting of wind shield using fluent 12.1. In the input I am giving a time dependent temperature profile for 40min. For the unsteady calculation I am giving a time step of 5 sec and with intermediate iteration of 5. I am also getting solution for time step of 10 sec and I am getting a quicker solution as compare with 5 sec, but I want to know which is right and how to decide the time step for unsteady simulation. Thanks in advance. Mohan.G

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July 2, 2011, 14:40		#2
nikhil New Member Nik Join Date: Mar 2010 Location: Michigan Posts: 25 Rep Power: 16	I would suggest, Monitor the temperature or the heat flux from the wind shield after a particular time interval. If the difference between these monitored parameters is within some desired critera, say 5-8%, then you can trust the results of the 10 second case. Another way to deside whether your time step is correct as per grid and convection-diffusion of your physical problem is to check 1. CFL condition OR 2. Grid Fourier Number criterion ( Depending upon a physics of your problem) CFD criterion needs to be satisfied for negligible diffusion. Grid Fourier number criterion needs to be satisfied for negligible convection

July 3, 2011, 23:51		#3
Mohankumarg12 Member Mohankumar.G Join Date: Sep 2010 Location: Pune,India Posts: 44 Rep Power: 16	Thank you Nikhil

July 4, 2011, 15:03		#4
mecamor New Member mecamor Join Date: Jul 2011 Posts: 23 Rep Power: 14	hello I wanted to see the shock wave generated by a piston. My problem I can not even move my piston with moving wall. thank you for helping me