problem with residual

yaseen wsu · May 15, 2016, 03:39

Dear all
this is the second time I post this problem
I simulate free surface flow (spillway) using RNG k-e model, at coarse mesh (500000) at 250 iteration reach (10-4) and converge but in fine mesh (2.5 million) it doesnot (10-4) even after 1200 iteration (mesh quality in fine mesh is better than coarse in terms of skewness orthogonal ..) is that related to time scale in coarse mesh I select auto but in fine auto does not work I use physical 0.03 sec. I add residual from backup in cfx pre but the it does not appear in variable list in CFX post.
also depth of water from experiment and CFX has difference (1 cm)!!, I read (why my result is inaccurate)
thanks for your help

http://www.cfd-online.com/Forums/att...1&d=1463297661
http://www.cfd-online.com/Forums/att...1&d=1463297714
http://www.cfd-online.com/Forums/att...1&d=1463297758
http://www.cfd-online.com/Forums/att...1&d=1463297793

ghorrocks · May 15, 2016, 06:06

Free surface simulations often do not converge well steady state. This is because of small transient surface waves. As you refine the mesh this effect will get stronger.

Free surface flows therefore frequently need to be run transient, even when the flow is actually steady state.

yaseen wsu · May 15, 2016, 11:21

Quote:

Originally Posted by ghorrocks

Free surface simulations often do not converge well steady state. This is because of small transient surface waves. As you refine the mesh this effect will get stronger.

Free surface flows therefore frequently need to be run transient, even when the flow is actually steady state.

(Free surface flows therefore frequently need to be run transient, even when the flow is actually steady state) very interested answer, because this is not only my problem, my friends who worked in CFD in MSc has the same problem.
So by monitoring value of interest in free surface like (mass flow rate, drag, shear stress ......) we can say we reach steady even the residuals does not reach.

ghorrocks · May 16, 2016, 01:31

Yes, if the parameters of importance to you have converged then it might be that your convergence is too tight. Have a look at the FAQ for a discussion on this.

But I was saying that to get tight convergence on free surface simulations you often need to run transient.

May 16, 2016, 01:31		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Yes, if the parameters of importance to you have converged then it might be that your convergence is too tight. Have a look at the FAQ for a discussion on this. But I was saying that to get tight convergence on free surface simulations you often need to run transient. yaseen wsu likes this.

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May 15, 2016, 06:06		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Free surface simulations often do not converge well steady state. This is because of small transient surface waves. As you refine the mesh this effect will get stronger. Free surface flows therefore frequently need to be run transient, even when the flow is actually steady state.