Free Surface not exactly at mean water level

FluidH · March 12, 2011, 05:41

Hi!

I am simulating a free surface problem and have some problems with the initial results.

The problem appears when I have several sections in the domain.
The initial free surface then get a small disturbance when crossing a section.
Do someone know how to avoid this disturbance ?

Regards

ghorrocks · March 13, 2011, 06:39

What is the graph showing? I guess you are extracting the free surface location somehow - so who are you extracting it?

Also, is this initial conditions or the final result of the simulation?

FluidH · March 13, 2011, 07:27

The figures shows the isovalue of the water.volume fraction at 0.5, Wich will be the location of the free surface in a 2 phase flow between air and water.
The lowest one shows a closer view at x=3m where I have a section and the free surface get a small jump.

The figures shows the initial result with t=0.0s. So it should be a straight line at z=0 [m].

ghorrocks · March 13, 2011, 18:56

If the surface location lines up closely with a element edge, then when you define the initial conditions one side of the interface could have the element edge just above the level and one just below. When you graph the free surface level this can appear as a jump of one element edge length.

If you don't like this then use a blurring function to blurr the transition over a few elements. Then small differences in node location do not affect the free surface location.

FluidH · March 18, 2011, 11:14

Thanks for your answer, but maybe I don't understand this blurring function.
The free surface will in general always be evaluated over some grid cells, as my figure shows.

Do You mean that there is a additional blurring function which smooths the surface?

I use the initial volume fraction smoothing in the solver control, but it gives no changes for the location of the free surface.

ghorrocks · March 20, 2011, 22:14

No, you have the write the blurring function. Hopefully a CEL expression will do it.

But do you see my point? If you have a line of nodes at z=1m and set the free surface at 1m, then with numerical accuracy the nodes at 0.9999999m will be set as water and the nodes at 1.00000001 will be set as air. This will result in the free surface not being as you intended as it jumps up or down with the mesh numerical accuracy.

Possibly an easier fix is to move the free surface up or down by a small amount. Small enough that it does not matter, but big enough that the mesh numerical accuracy does not affect things.

FluidH · March 21, 2011, 02:55

Thanks! I get the point. The free surface will always lie between some grid cells, so the free surface accuracy will be +-the smallest element size.
As you say one can move the initial free surface condition a small amount from the exact value to avoid numerical jumps of one grid cell.

I will try to set ZWL= +-0.00001 away from the actually zero water level.

Regards

March 12, 2011, 05:41	Free Surface not exactly at mean water level	#1
FluidH New Member Join Date: Mar 2011 Posts: 10 Rep Power: 15	Hi! I am simulating a free surface problem and have some problems with the initial results. The problem appears when I have several sections in the domain. The initial free surface then get a small disturbance when crossing a section. Do someone know how to avoid this disturbance ? Regards

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March 13, 2011, 06:39		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	What is the graph showing? I guess you are extracting the free surface location somehow - so who are you extracting it? Also, is this initial conditions or the final result of the simulation?

March 13, 2011, 07:27		#3
FluidH New Member Join Date: Mar 2011 Posts: 10 Rep Power: 15	The figures shows the isovalue of the water.volume fraction at 0.5, Wich will be the location of the free surface in a 2 phase flow between air and water. The lowest one shows a closer view at x=3m where I have a section and the free surface get a small jump. The figures shows the initial result with t=0.0s. So it should be a straight line at z=0 [m].

March 13, 2011, 18:56		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	If the surface location lines up closely with a element edge, then when you define the initial conditions one side of the interface could have the element edge just above the level and one just below. When you graph the free surface level this can appear as a jump of one element edge length. If you don't like this then use a blurring function to blurr the transition over a few elements. Then small differences in node location do not affect the free surface location.

March 18, 2011, 11:14		#5
FluidH New Member Join Date: Mar 2011 Posts: 10 Rep Power: 15	Thanks for your answer, but maybe I don't understand this blurring function. The free surface will in general always be evaluated over some grid cells, as my figure shows. Do You mean that there is a additional blurring function which smooths the surface? I use the initial volume fraction smoothing in the solver control, but it gives no changes for the location of the free surface.

March 20, 2011, 22:14		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	No, you have the write the blurring function. Hopefully a CEL expression will do it. But do you see my point? If you have a line of nodes at z=1m and set the free surface at 1m, then with numerical accuracy the nodes at 0.9999999m will be set as water and the nodes at 1.00000001 will be set as air. This will result in the free surface not being as you intended as it jumps up or down with the mesh numerical accuracy. Possibly an easier fix is to move the free surface up or down by a small amount. Small enough that it does not matter, but big enough that the mesh numerical accuracy does not affect things.

March 21, 2011, 02:55		#7
FluidH New Member Join Date: Mar 2011 Posts: 10 Rep Power: 15	Thanks! I get the point. The free surface will always lie between some grid cells, so the free surface accuracy will be +-the smallest element size. As you say one can move the initial free surface condition a small amount from the exact value to avoid numerical jumps of one grid cell. I will try to set ZWL= +-0.00001 away from the actually zero water level. Regards