# time step and # of coefficient loops

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 September 22, 2012, 02:58 time step and # of coefficient loops #1 Member   Hamed Abdul Majeed Join Date: May 2012 Posts: 76 Rep Power: 6 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.

 September 22, 2012, 03:00 #2 Member   Hamed Abdul Majeed Join Date: May 2012 Posts: 76 Rep Power: 6 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?

 September 24, 2012, 22:06 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,726 Rep Power: 99 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. hamed.majeed likes this.

 September 25, 2012, 22:07 #4 Member   Hamed Abdul Majeed Join Date: May 2012 Posts: 76 Rep Power: 6 Hi, In time dependence you said that the results must be withing "tolerance". How do we specify this tolerance value?

 September 26, 2012, 08:08 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,726 Rep Power: 99 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. hamed.majeed likes this.

 September 27, 2012, 01:54 #6 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,301 Blog Entries: 6 Rep Power: 43 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.

 September 27, 2012, 07:47 #7 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,726 Rep Power: 99 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.

 September 28, 2012, 06:53 #8 Member   Hamed Abdul Majeed Join Date: May 2012 Posts: 76 Rep Power: 6 Hi, I remember reading in ANSYS Help that the solution resolution must be increased by increasing the time steps rather increasing the coefficient loops

 September 28, 2012, 07:33 #9 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,726 Rep Power: 99 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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