Vof in CFX ?

bohluly · April 11, 2009, 08:03

Hi All
is there any VOF module for sharp interface modeling in CFX ?
if yes, witch method it use (PLIC or SLIC ....)
"my mean is not only two phase model but exactly VOF and sharp interface modeling"

ghorrocks · April 13, 2009, 21:26

Hi,

CFX has a compressive differencing scheme for free surface simulations, but despite the rantings in the CFX advertising material it is far behind the free surface schemes in Fluent and other codes. You have to expect the interface to be smeared over several cells in CFX at best.

CFX does not have SLIC or PLIC.

Glenn Horrocks

latslosh · April 14, 2009, 08:34

Hi,

Depending on what the flow is doing, you're looking at a free surface 'thickness' of usually 1.5-2 mesh diagonals with CFX. So the only interface sharpening option available is refining the mesh...

latslosh.

ghorrocks · April 14, 2009, 20:43

Hi,

A free surface of 1.5-2 mesh diagonals in my experience is only possible for a tightly converged simulation with fine time steps. If you loosen the convergence or use bigger timesteps to accelerate the simulation you will blur the surface more.

Note surface tension makes free surface modelling significantly more complicated.

Glenn Horrocks

bohluly · April 15, 2009, 07:12

Tanks,

I saw some results of CFX that it can not save the sharp interface (for example in Slug flow modeling).
In some manuals of CFX presented that CFX has VOF module, but I think it is not correct, VOF basically must save the sharp interface by spacially methods such as Youngs method.

ghorrocks · April 15, 2009, 18:38

Hi,

You should not generalise Ashgar. There is nothing "wrong" with the free surface modelling in CFX, it is just that it does not capture the interface as sharply as some other approaches. Whether this is a problem or not depends on the application. CFX has been successfully used on many free surface applications so you definitely would not say it is "wrong".

Glenn Horrocks

bohluly · April 16, 2009, 01:55

That is ok !
My mean was that the free surface model of CFX is not VOF and it make some mass diffusion that it may damage physically on results for some application, for example in wave modeling we don't have any mass diffusion between air and water, when CFX make a tin or tick layer of fluid with density of 500 between air with density of 1 and water with density of 1000 it is harmful for momentum eq. But when we have phisicaly some mixed condition in free surface it is so usfull.

April 11, 2009, 08:03	Vof in CFX ?	#1
bohluly New Member Asghar Join Date: Apr 2009 Posts: 16 Rep Power: 17	Hi All is there any VOF module for sharp interface modeling in CFX ? if yes, witch method it use (PLIC or SLIC ....) "my mean is not only two phase model but exactly VOF and sharp interface modeling"

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April 13, 2009, 21:26		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Hi, CFX has a compressive differencing scheme for free surface simulations, but despite the rantings in the CFX advertising material it is far behind the free surface schemes in Fluent and other codes. You have to expect the interface to be smeared over several cells in CFX at best. CFX does not have SLIC or PLIC. Glenn Horrocks

April 14, 2009, 08:34		#3
latslosh New Member Join Date: Mar 2009 Posts: 4 Rep Power: 17	Hi, Depending on what the flow is doing, you're looking at a free surface 'thickness' of usually 1.5-2 mesh diagonals with CFX. So the only interface sharpening option available is refining the mesh... latslosh.

April 14, 2009, 20:43		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Hi, A free surface of 1.5-2 mesh diagonals in my experience is only possible for a tightly converged simulation with fine time steps. If you loosen the convergence or use bigger timesteps to accelerate the simulation you will blur the surface more. Note surface tension makes free surface modelling significantly more complicated. Glenn Horrocks

April 15, 2009, 07:12		#5
bohluly New Member Asghar Join Date: Apr 2009 Posts: 16 Rep Power: 17	Tanks, I saw some results of CFX that it can not save the sharp interface (for example in Slug flow modeling). In some manuals of CFX presented that CFX has VOF module, but I think it is not correct, VOF basically must save the sharp interface by spacially methods such as Youngs method.

April 15, 2009, 18:38		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Hi, You should not generalise Ashgar. There is nothing "wrong" with the free surface modelling in CFX, it is just that it does not capture the interface as sharply as some other approaches. Whether this is a problem or not depends on the application. CFX has been successfully used on many free surface applications so you definitely would not say it is "wrong". Glenn Horrocks

April 16, 2009, 01:55		#7
bohluly New Member Asghar Join Date: Apr 2009 Posts: 16 Rep Power: 17	That is ok ! My mean was that the free surface model of CFX is not VOF and it make some mass diffusion that it may damage physically on results for some application, for example in wave modeling we don't have any mass diffusion between air and water, when CFX make a tin or tick layer of fluid with density of 500 between air with density of 1 and water with density of 1000 it is harmful for momentum eq. But when we have phisicaly some mixed condition in free surface it is so usfull.