free surface model

sjtusyc · September 5, 2012, 01:07

1)When we do multi-phase simulation, in the Fluid Model Tab,we have to specify whether it is a standard free surface model or not.
I am a little confused about this setting,when should we chose the standard free surface model option.What will it determine,say,the governing equation?Does it equal free surface flow?
2)For problems that the interphase between the two phases is well-defined,i will chose the homogeneous model and free surface model as recommended by the help.If we chose the homogeneous model,the one bulk N-S equations is determined,why we need use free surface model.

These questions seems silly,but i hope someone would shed light on it .
Thanks.

ghorrocks · September 5, 2012, 07:25

1) This determines whether the free surface sharpening stuff is turned on, and other options such as surface tension. You should choose the free surface model when you intend to resolve the free surface in your simulation. If you want the interface to diffuse then leave it off.
2) See above answer.

sjtusyc · September 5, 2012, 07:44

Thanks for your reply.
So, the free surface model is used to turn on the free surface sharpening stuff, like the VOF method,to get reasonable interface.
But , as i know,a complete physics question consists of the governing equations and the boundary condition. So why need free surface sharpening stuff? Why we need the reynolds
transport equation of F(volume of fluid).

ghorrocks · September 5, 2012, 19:33

We need free surface sharpening because otherwise it will diffuse. And you need a volume fraction equation as the volume fraction convects with the flow.

There other approaches of modelling free surfaces, but the volume fraction with free surface sharpening is the one implemented in CFX.

September 5, 2012, 01:07	free surface model	#1
sjtusyc Member leo Join Date: Jun 2012 Posts: 98 Rep Power: 14	1)When we do multi-phase simulation, in the Fluid Model Tab,we have to specify whether it is a standard free surface model or not. I am a little confused about this setting,when should we chose the standard free surface model option.What will it determine,say,the governing equation?Does it equal free surface flow? 2)For problems that the interphase between the two phases is well-defined,i will chose the homogeneous model and free surface model as recommended by the help.If we chose the homogeneous model,the one bulk N-S equations is determined,why we need use free surface model. These questions seems silly,but i hope someone would shed light on it . Thanks.

September 5, 2012, 07:25		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	1) This determines whether the free surface sharpening stuff is turned on, and other options such as surface tension. You should choose the free surface model when you intend to resolve the free surface in your simulation. If you want the interface to diffuse then leave it off. 2) See above answer. Danial Q likes this.

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September 5, 2012, 07:44		#3
sjtusyc Member leo Join Date: Jun 2012 Posts: 98 Rep Power: 14	Thanks for your reply. So, the free surface model is used to turn on the free surface sharpening stuff, like the VOF method,to get reasonable interface. But , as i know,a complete physics question consists of the governing equations and the boundary condition. So why need free surface sharpening stuff? Why we need the reynolds transport equation of F(volume of fluid).

September 5, 2012, 19:33		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	We need free surface sharpening because otherwise it will diffuse. And you need a volume fraction equation as the volume fraction convects with the flow. There other approaches of modelling free surfaces, but the volume fraction with free surface sharpening is the one implemented in CFX.