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time step and # of coefficient loops

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Old   September 22, 2012, 02:58
Default time step and # of coefficient loops
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Hamed Abdul Majeed
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Hi,

Guys I have some questions regarding value of time step needed and the # of coefficient loops specified. I have two questions:
1. How to perform time step dependence test.
2. What is the appropriate value of # of coefficient loops needed.
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Old   September 22, 2012, 03:00
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Hamed Abdul Majeed
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According to ansys cfx help I found the following..
Time step:
Time step dependence is done at a single time step,. doubling the time, and halving the time step.
ref: ANSYS Help // Modeling Guide // 15. Advice on Flow Modeling // 15.4. Timestep Selection // 15.4.2. Transient Timestep Control

Now, I have plotted curves for cd at a wall for (i) time step = 0.01 s
(ii) time step = 0.005 s, and (iii) time step = 0.02 s
I have another question
3. How to find the appropriate range of max and min %age error for time steps, and then finally select one?
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Old   September 24, 2012, 22:06
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Glenn Horrocks
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1. Time step dependance check - run a simulation, then do it again with half the time step size. If the two simulations are the same within a toelrance you are happy with then you are finished. If not the half the time step size and try again.
2. 3-5 coeff loops per time step for most applications, 5-10 for trickier (eg multiphase) simulations. There is no guide for steady state runs, only the tightness of convergence matters.
3. I do not understand your question.
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Old   September 25, 2012, 22:07
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Hamed Abdul Majeed
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Hi,
In time dependence you said that the results must be withing "tolerance". How do we specify this tolerance value?
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Old   September 26, 2012, 08:08
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The tolerance is how accurate you are happy to have that result. Are you doing a quick design check - maybe 20%. Want a good answer - maybe 5%. Need high accuracy - maybe 1%. You have to choose for your application.
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Old   September 27, 2012, 01:54
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I have seen an official Ansys CFX presentation on circular cylinder where he took the one coefficient loop per time step. Whereas I think there must be some coefficient loops per time step as you have already mentioned. Do you think they (CFX team) have done it for purpose?

I have shown some results on this thread Problem in getting mesh independent soluiton for flow over flat plate as well, if any one needs complete presenation, I am more than happy to provide one.
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Old   September 27, 2012, 07:47
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Doing even less coeff loops per time step is good for temporal resolution (ie fine time steps) but in most cases not optimal. Also be aware that I understand the second order components of the temporal terms only get enforced on the second coeff loop, so you need at least 2 coeff loops per iteration to get second order to work.
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Old   September 28, 2012, 06:53
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Hamed Abdul Majeed
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Hi,
I remember reading in ANSYS Help that the solution resolution must be increased by increasing the time steps rather increasing the coefficient loops
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Old   September 28, 2012, 07:33
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To clarify - increasing the number of time steps (ie smaller time steps) with 3-5 coeff loops per iteration is usually a better approach then larger time steps with more coeff loops. I think that is what you said.
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