# 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: 88 Rep Power: 7 Sponsored Links 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: 88 Rep Power: 7 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: 13,738 Rep Power: 106 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: 88 Rep Power: 7 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: 13,738 Rep Power: 106 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: 4,331 Blog Entries: 6 Rep Power: 45 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 http://www.cfd-online.com/Forums/flu...lat-plate.html 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: 13,738 Rep Power: 106 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: 88 Rep Power: 7 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: 13,738 Rep Power: 106 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.

 August 16, 2017, 01:22 #10 New Member   meysam Join Date: Oct 2015 Posts: 21 Rep Power: 3 Hello I am simulation a moving mesh case. It is a turbulent, free surface, moving mesh, and transient problem. I set 1 to 4 for COEFFICIENT LOOP ITERATION and 4 for COUPLING/STAGGER ITERATION which a parameter related to moving mesh. So Every time step is about 16 (4*4) iteration which seems a bit weird. Do you have any idea? Since it takes a lot to solve such time-consuming problem, I think I am doing something wrong? Do you have any other idea to increase the solution speed? Actually, I have to choose 0.001 or 0.002 for time step because for larger ones it gives me a negative volume.

 August 16, 2017, 06:24 #11 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,738 Rep Power: 106 Coupling/stagger iterations are used in FSI simulations. Are you intending to do a FSI simulation? If you just need moving mesh you do not need FSI and the simulation becomes much easier.

 August 16, 2017, 07:58 #12 New Member   meysam Join Date: Oct 2015 Posts: 21 Rep Power: 3 Thanks Glenn. Actually, I want to have some degree of freedom to see the rotational and horizontal movement of an object in the presence of surface waves. Based on buoy example in CFX tutorial I thought it should be FSI. Is it true? The deformation of the solid object is not important for me. I am just going to investigate movement of a given object.

 August 16, 2017, 08:00 #13 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,738 Rep Power: 106 If the body is rigid, and its motion is just translation and rotation, then this is not FSI and should be modelled as a rigid body motion. You only need FSI if the body deforms due to the forces applied by the fluid. m.khatoonabadi likes this.

 August 16, 2017, 12:22 #14 New Member   meysam Join Date: Oct 2015 Posts: 21 Rep Power: 3 Yes you are right. Thank you

 Tags coefficient loop, time step

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