free surface problem with capillary effect

ghorrocks · July 28, 2013, 07:39

I would not worry about it. I suspect it will disappear with tighter convergence. But as the interface is reconstructed every time step these minor wiggles should dissappear wuick enough.

A much more important issue is that you will never achieve grid convergence with simulations involving gas/liquid/solid interfaces and moving contact lines. It is impossible with a Navier Stokes solver like CFX (or Fluent, Star-CD or any of the others to my knowledge).

rabbitmelon · August 5, 2013, 05:03

hello, Mr. Horrocks. I've tried to solve the simulation with fine mesh. The results seem to be better. The copper flows to the end of the gap, when the gap is small. But there is always a small distance between the opening boundary at the end of the gap and the copper. I think that may because i have set the volume fraction at the opening for copper to 0. What does the "zero gradient" option for volume fraction at opening boundary mean?

ghorrocks · August 5, 2013, 06:47

Can you post an image of the small gap?

zero gradient means the boundary condition is a Neumann type.

rabbitmelon · August 5, 2013, 09:25

in the attach is the image of the small gap between the end and the molten copper
gap.jpg

ghorrocks · August 5, 2013, 18:27

Yes, I suspect that is due to your specification of VF=0 at the boundary. I would not worry about it, as long as it is far away from the area of interest it should not affect things.

rabbitmelon · August 16, 2013, 03:05

Hi, Mr. Horrocks. I've try various settings for the simulation and found out the things below.

1)For the modells with smaller gap size, where the capillary effect is significant, when the aspect ratio of the elements inside the gap bigger than 1 ist, for example 2.5 (the longer side of the element is in the flow direction of the copper), the molten copper will flow in the gap and form a symmetrical meniscus. The residual is stable and there are no periodical peaks. But when i run it with modells with the element aspect ratio of 1, there are peaks. There is no symmetrical meniscus, the copper flows either a litte more along the upper or the lower side of the gap at different time during the simulation. Only when i restrict the max. time step to a small value, the amplitude of the peaks is smaller and the simulation can run untill the copper reach the end of the gap, otherwise a great peak appears at some time and the simulation could be terminated with error.

2)For the modells with bigger gap size. The capillary effect is not so significant. Due to the gravitation, the copper should flow along the lower side of the gap. But when i run it with modells with bigger aspect ratio inside the gap, it will flow along the upper side. When i change the aspect ratio to 1, the copper flows along the lower side. But the periodical peaks still exist. I think that there might be still something wrong with other settings, and i will try further.

Why the modell with better aspect ratio for the smaller gap is unstabler? Is'nt the aspect ratio of 1 the best aspect ratio?

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July 28, 2013, 07:39		#21
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I would not worry about it. I suspect it will disappear with tighter convergence. But as the interface is reconstructed every time step these minor wiggles should dissappear wuick enough. A much more important issue is that you will never achieve grid convergence with simulations involving gas/liquid/solid interfaces and moving contact lines. It is impossible with a Navier Stokes solver like CFX (or Fluent, Star-CD or any of the others to my knowledge).

August 5, 2013, 05:03		#22
rabbitmelon New Member Join Date: May 2013 Posts: 22 Rep Power: 12	hello, Mr. Horrocks. I've tried to solve the simulation with fine mesh. The results seem to be better. The copper flows to the end of the gap, when the gap is small. But there is always a small distance between the opening boundary at the end of the gap and the copper. I think that may because i have set the volume fraction at the opening for copper to 0. What does the "zero gradient" option for volume fraction at opening boundary mean?

August 5, 2013, 06:47		#23
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Can you post an image of the small gap? zero gradient means the boundary condition is a Neumann type.

August 5, 2013, 09:25		#24
rabbitmelon New Member Join Date: May 2013 Posts: 22 Rep Power: 12	in the attach is the image of the small gap between the end and the molten copper gap.jpg

August 5, 2013, 18:27		#25
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Yes, I suspect that is due to your specification of VF=0 at the boundary. I would not worry about it, as long as it is far away from the area of interest it should not affect things.

August 16, 2013, 03:05		#26
rabbitmelon New Member Join Date: May 2013 Posts: 22 Rep Power: 12	Hi, Mr. Horrocks. I've try various settings for the simulation and found out the things below. 1)For the modells with smaller gap size, where the capillary effect is significant, when the aspect ratio of the elements inside the gap bigger than 1 ist, for example 2.5 (the longer side of the element is in the flow direction of the copper), the molten copper will flow in the gap and form a symmetrical meniscus. The residual is stable and there are no periodical peaks. But when i run it with modells with the element aspect ratio of 1, there are peaks. There is no symmetrical meniscus, the copper flows either a litte more along the upper or the lower side of the gap at different time during the simulation. Only when i restrict the max. time step to a small value, the amplitude of the peaks is smaller and the simulation can run untill the copper reach the end of the gap, otherwise a great peak appears at some time and the simulation could be terminated with error. 2)For the modells with bigger gap size. The capillary effect is not so significant. Due to the gravitation, the copper should flow along the lower side of the gap. But when i run it with modells with bigger aspect ratio inside the gap, it will flow along the upper side. When i change the aspect ratio to 1, the copper flows along the lower side. But the periodical peaks still exist. I think that there might be still something wrong with other settings, and i will try further. Why the modell with better aspect ratio for the smaller gap is unstabler? Is'nt the aspect ratio of 1 the best aspect ratio?