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Time step for unsteady simulation

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Old   June 29, 2011, 05:18
Default Time step for unsteady simulation
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Mohankumar.G
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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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Old   July 2, 2011, 14:40
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Nik
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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
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Old   July 3, 2011, 23:51
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Mohankumar.G
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Thank you Nikhil
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Old   July 4, 2011, 15:03
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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
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