[Mircro_fluidics_]

Hi,
I am trying to model Liquid Liquid droplet flow in micro capillary tubes, ID: 580um, I have my model set up as as
Axisymmetric
Segregated flow
Lamiar
VOF
Implicit unsteady
Surface tension
Where I am coming into problems is with the surface tension, when I enable this in the Physics model selection it only allows me set three values the surface tension for each phase and the contact angle with my wall,

In relation to the surface tension, what surface tension value is this looking for, value of surface tension between tube wall and Liquid? surface tension between liquids? surface tension between liquid and air?

Also the contact angle, it only allows me set one contact angle, I have two phase so there should be at least two contact angles that I should be allowed to set.

Any help on this matter will be greatly appreciated as my droplets look nothing like droplets.

[Josh]

From the Help file: In the current implementation, each phase interaction is assigned its own surface tension coefficient and this is used to calculate the surface tension force between each of the defined phases in the phase interaction.

I'm not sure about the contact angles. I've never done liquid-liquid droplets. However, even with 2 liquids, it doesn't make sense to me to have 2 contact angles - the angle would be consistent around the tube, no?

[Mircro_fluidics_]

Thanks for the reply Josh,
I have seen this about the surface tension in the help file, what exact surface tension value is this looking for? Liquid to air, Liquid to Liquid or Liquid to solid?
It also says that the surface tension coefficients assigned to each phase should be equal, to me this sounds like a the opposite to what it states first, I have being setting this surface tension value for both phases to a value we calculated experimentally for the inter facial tension between the liquids, do you think this is correct?

In relation to the contact angle, when the simulation is started both phases are touching the wall, in reality both these phases have a different contact angle with the wall, the oil wets the wall and therefore has a contact angle of less than 90, while the water is completely hydrophobic with the wall therefore a contact angle of 180, In Star I am only allowed to set one contact angle which (I think) relates to the first phase, This will, no matter what way I set up the model lead to inaccurate results as the film thickness that eventually forms around the droplet is largely dependent on the contact angle of the oil with the wall,

Any more help on this matter would be greatly appreciated,

[Josh]

In your case, you have to define three multiphase interactions in the physics continua: water-oil, water-air, and oil-air. You can then specify three surface tensions, one for each interaction, using the Primary Phase and Secondary Phase in the VOF Phase Interaction Properties. So... the surface tension refers to the tensions between the two defined phases, e.g., for water-oil, it would be the surface tension between water and oil.

Now your region will have three phase interactions, each of which you can specify a contact angle for.

[Mircro_fluidics_]

I am running two computers here, one that has 32 bit Star-CCM+ 6.04.014 and one that has 64 bit STAR-CCM+ (6.02.007),
I am able to model Multi Phase interaction in the 32 bit, but am unable to do this in the 64 bit one when using VOF model, if I change to a Segregated multiphase I am able to use this, but I need to use the VOF for my models. Does anyone know the reason for this? or know a way around it?

[Mircro_fluidics_]

I have managed to install the 6.04 64 bit and this has solved my problem of modelling surface tension and contact angle, thanks for the help.

[prashant810]

Hi,

I would like to the study of implact of water droplet on solid surface,

I am doing experiment also. I have the experimental results and would like to validate it by using CFD.


How to model the case mean mesh and physics setup for impact of droplet on solid surface.

I have created case in star ccm+ using VoF but I think I am wrong somewhere for setup required Physics and also mesh.

I have created one cylinder and meshed it, and setup the case with the STAR CCM+ help VoF, its not working.

is there nedd to model the droplet separately?

Can you please share .sim file?
please help me for meshing and physics setup.

Thank you
Prashant

[Nartasan]

Hi all,

I'm trying to simulate an oil droplet in a microscale water tube without touching the walls in the meantime. My sim is based on Multiphase/VoF model and need to use csf model in solver. I'm using AVL Fire and now have some issues and questions:

1. when I active the csf option and put the correct values of "contact angle" and "Surface tension coefficient" my runs diverge. while the simulation goes pretty well when I don't activate csf option.

2. I know that contact angle define when there is a wall with a normal vector on the surface of the oil(in this case), but what if I don't have any contact with wall right now....should I assume only an imaginary parallel wall with real wall and find the contact angle based on that? or I should only use two fluids model and not vof?

3. if I want to define the surface tension as an output variable for checking its values over time, should I use csf method and assume surface tension as a body force and then right e new subroutines?

Could you please help me I'm new in this field.