time step and # of coefficient loops
 

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.

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?

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.

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

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.

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.

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.

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

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.

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.:(

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.

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.

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.

Yes you are right. Thank you