oil & water - dynamic contact angle

manxu · October 16, 2013, 03:33

Hi,

I am trying to model the movement of water droplets on a layer of oil coated on an inclined plane (so that there is a body force on oil as well as water droplets) First of all, there is no option in fluent for imposing a contact angle between two phases (oil and water) other than at wall. .

Next, I couldn't find a proper way through UDF to impose a dynamic contact angle on the water droplets moving over the oil ! Although I can impose the dynamic contact angle on water droplets moving over a wall, is it possible to apply the contact angle at the oil surface rather than at wall ?

Or is fluent capable of imposing the dynamic contact angle between oil and water by itself ? (which I highly doubt so but still I can see some convincing results after the simulations so far without a UDF!)

Any kind of help would be really appreciated, coz I am stuck here for quite a bit of time.

Manxu

ghorrocks · October 17, 2013, 06:47

This sounds like a tricky simulation and I am not surprised you are having troubles with it.

I will have to think about the physics of this, but do you need to specify a contact angle? Does the three difference surface tensions (phase A-B, phase A-C and phase B-C) define the contact angle?

Finally - don't forget that the Navier Stokes equations are incompatible with the no slip condition and moving contact lines (http://croucher.math.ust.hk/proj_mcl.html or http://www3.imperial.ac.uk/newsandev...1-2013-9-14-13). Most CFD codes (Fluent and CFX included) just ignore this and solve it anyway. The result is that you have a singularity at the contact point and the numerics cannot achieve a mesh independant solution. Therefore you can never expect too much accuracy from a general CFD code on a moving contact line simulation.

manxu · October 17, 2013, 07:14

Thanks for your reply. Yes, you are right, there is no need to specify "static" contact angle since it can be obtained from the surface tension forces.

On the other hand, in order to impose the dynamic contact angle in fluent, first of all, there should be a parameter which defines the contact angle between two phases (like the contact angle between wall and a phase in the Wall adhesion option of fluent). Since there is no option of a contact angle specification in fluent between two liquid phases, there can't be a UDF to define the same. Hence, the usability of fluent is in itself a big question here.

Moreover, the problem of Navier-Stokes eqn with moving contact line is only with a solid-fluid-fluid junction or is it also for a fluid-fluid-fluid contact line ? Because there is a definite slip at the fluid-fluid interface which is not the case in a fluid-solid interface.

ghorrocks · October 17, 2013, 07:51

For the dynamic contact angle, rather than specifying the contact angle, isn't this just the 3 phase to phase surface tensions not being constant? So then you need to specify the surface tension force for each phase locally as a function of whatever you like and the dynamic contact angle will naturally come out of that.

Working out what those change in forces are does not sound trivial, and implementing it does not sound trivial either....

Good point about the moving contact line - you are probably OK here. And it looks like you are aware of the problem, and that is more than most people.

October 16, 2013, 03:33	oil & water - dynamic contact angle	#1
manxu New Member Rams Manu Join Date: Oct 2013 Posts: 21 Rep Power: 13	Hi, I am trying to model the movement of water droplets on a layer of oil coated on an inclined plane (so that there is a body force on oil as well as water droplets) First of all, there is no option in fluent for imposing a contact angle between two phases (oil and water) other than at wall. . Next, I couldn't find a proper way through UDF to impose a dynamic contact angle on the water droplets moving over the oil ! Although I can impose the dynamic contact angle on water droplets moving over a wall, is it possible to apply the contact angle at the oil surface rather than at wall ? Or is fluent capable of imposing the dynamic contact angle between oil and water by itself ? (which I highly doubt so but still I can see some convincing results after the simulations so far without a UDF!) Any kind of help would be really appreciated, coz I am stuck here for quite a bit of time. Manxu

October 17, 2013, 06:47		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	This sounds like a tricky simulation and I am not surprised you are having troubles with it. I will have to think about the physics of this, but do you need to specify a contact angle? Does the three difference surface tensions (phase A-B, phase A-C and phase B-C) define the contact angle? Finally - don't forget that the Navier Stokes equations are incompatible with the no slip condition and moving contact lines (http://croucher.math.ust.hk/proj_mcl.html or http://www3.imperial.ac.uk/newsandev...1-2013-9-14-13). Most CFD codes (Fluent and CFX included) just ignore this and solve it anyway. The result is that you have a singularity at the contact point and the numerics cannot achieve a mesh independant solution. Therefore you can never expect too much accuracy from a general CFD code on a moving contact line simulation. manxu likes this.

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October 17, 2013, 07:14		#3
manxu New Member Rams Manu Join Date: Oct 2013 Posts: 21 Rep Power: 13	Thanks for your reply. Yes, you are right, there is no need to specify "static" contact angle since it can be obtained from the surface tension forces. On the other hand, in order to impose the dynamic contact angle in fluent, first of all, there should be a parameter which defines the contact angle between two phases (like the contact angle between wall and a phase in the Wall adhesion option of fluent). Since there is no option of a contact angle specification in fluent between two liquid phases, there can't be a UDF to define the same. Hence, the usability of fluent is in itself a big question here. Moreover, the problem of Navier-Stokes eqn with moving contact line is only with a solid-fluid-fluid junction or is it also for a fluid-fluid-fluid contact line ? Because there is a definite slip at the fluid-fluid interface which is not the case in a fluid-solid interface.

October 17, 2013, 07:51		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	For the dynamic contact angle, rather than specifying the contact angle, isn't this just the 3 phase to phase surface tensions not being constant? So then you need to specify the surface tension force for each phase locally as a function of whatever you like and the dynamic contact angle will naturally come out of that. Working out what those change in forces are does not sound trivial, and implementing it does not sound trivial either.... Good point about the moving contact line - you are probably OK here. And it looks like you are aware of the problem, and that is more than most people.