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hamed.majeed September 22, 2012 02:58

time step and # of coefficient loops

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.

hamed.majeed September 22, 2012 03:00

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?

ghorrocks September 24, 2012 22:06

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 September 25, 2012 22:07

In time dependence you said that the results must be withing "tolerance". How do we specify this tolerance value?

ghorrocks September 26, 2012 08:08

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.

Far September 27, 2012 01:54

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 as well, if any one needs complete presenation, I am more than happy to provide one.

ghorrocks September 27, 2012 07:47

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.

hamed.majeed September 28, 2012 06:53

I remember reading in ANSYS Help that the solution resolution must be increased by increasing the time steps rather increasing the coefficient loops

ghorrocks September 28, 2012 07:33

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.

m.khatoonabadi August 16, 2017 01:22

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.:(

ghorrocks August 16, 2017 06:24

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.

m.khatoonabadi August 16, 2017 07:58

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.

ghorrocks August 16, 2017 08:00

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 August 16, 2017 12:22

Yes you are right. Thank you

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