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Too small time step?

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Old   May 18, 2015, 18:16
Default Too small time step?
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I'm calculating heat transfer from hot air flow to a thin metal wire. Model is transient, and what I'm trying to find is a temperature response (step response) of mentioned wire.
In theory I should get something like this:

Model is defined quite precisely, but results are strongly depenent on time step size.
Air flow speed is constant almost everywhere and equal to 5m/s.
The smallest cell has size of 0.001mm. It is so small because metal wire is 0.01mm in diameter.
According to this data I set up the time step to 2e-7s, but results were completly ridiculous. I have prepared comparison of solutions with different time step sizes. It seems that solution which is the closest to experimental data is for time step much bigger than 2e-7s.

Could you tell me why it is so, and how should I find the proper time step?
Thank you for any of your help
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time step size

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