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meaning of timescale

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Old   September 6, 2017, 01:33
Talking meaning of timescale
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heesang Yoo
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hello guys

I am confused with some basic questions. It is about timescale and residual...

First, I want to know about timescale and iteration...

As I understand, timescale in steady state means time step and max iteration means the number of steps. For an example it physical time scale is 2s and max iteration is 100 than time of your last step is 200s.
However in transient max coefficient loops of convergence control means number of calculation of each step.

I want someone could certify my understanding...

Second, I want to know about difference between RMS residual and MAX residual. Just I know about is MAX residual is usually set 10 times of RMS residual.

I would appreciate if someone answer to my questions.

Thank you!
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Old   September 6, 2017, 03:13
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Glenn Horrocks
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Time scale in steady state simulations - this is effectively the rate at which you advance the solution. It has the normal aspects of a numerical simulation - too slow and convergence will be steady, but slow; too fast and the simulation will diverge and crash.

Max iteration is simply the maximum number of iterations in the coefficient loops you allow the solver to make.

Residuals: The residual is calculated in every control volume. So how do you report a field of residuals to give a guide for convergence? One way is to do the RMS of the control volume values, a second way is to just report the maximum residual, which is the worst convergence in the whole model.

Which approach is better depends on what you are doing. Also note that the difference between the RMS and MAX residual can suggest whether your simulation is converging nicely across the whole domain or if there are regions which have very poor convergence while the rest is converging.
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Old   September 7, 2017, 00:49
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heesang Yoo
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Thank u so much for your explanation!!

I have a last question...

could u let me know about conservation target??

In ANSYS manual, it says that conservation means the fractional imbalance but, I can't understand what is the 'balance' mean in this statement..

thank you
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Old   September 7, 2017, 01:17
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Imbalance is the global sum of the quantities over the domain. For mass in steady state it means the integral of the mass flows over the inputs equals the integral of the mass flow over the outputs. The fractional imbalance is the mass imbalance divided by the amount of mass passing through. For transient simulations you have some additional quantities as the domain can gain or loose mass in time.

Likewise momentum is a global force balance, and temperature is a global heat balance.
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Old   September 7, 2017, 02:25
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Thank u so much Glenn!!!!!!!!!
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