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Solid/Fluid Surface Tension Model

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Old   April 5, 2012, 01:44
Default Solid/Fluid Surface Tension Model
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Hi,

I am trying to look at the surface tension effects between a fluid and a solid in a container. I have modeled the container and used the fill by cavity option in order to create a domain for both the solid and fluid. In CFX-Pre, I have given each domain their properties (phase, material), however I cannot find a way to link the two in order to achieve a pairing and input a surface tension coefficient. I am only able to pair a fluid/fluid, which is not what I want to simulate.

Could anyone please give me help/instructions on how to solve a transient solid/fluid surface tension simulation?

Thanks for your help.
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Old   April 12, 2012, 00:05
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Bump.

Still have had no luck. I have tried modelling it as a 2 way FSI with no luck. I am still unable to find any option which allows me to couple the fluid and solid so as to input a surface tension coefficient, or even give each material a surface energy.

What i'm trying to do is partially submerge a solid material in a fluid and analyse the motion of the fluid over the solid. There is no fluid velocity.
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Old   April 15, 2012, 18:15
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What is the solid material? Can you post an image of what you are modelling?
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Old   April 16, 2012, 00:15
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Thanks for the reply Ghorrocks.

The solid material is aluminium, and the fluid is water. It is in a microgravity environment where surface tension is the dominant force. For an initial test I have simplified the geometry to an aluminium block that is partially submerged within a container with water. I have attached a picture of it.

Cheers.
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File Type: jpg SurfaceTension.jpg (76.7 KB, 35 views)
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Old   April 17, 2012, 01:13
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Sounds like you have an unusual application - you better explain what you are doing to get useful comments.

Is the aluminum a solid block or particles? What drives the flow? Where is the air/water, water/Al, air/Al interfaces? What time scale is this over (surface tension will take a while to move a block of aluminium)? How is the surface tension on the Al unbalanced to create a net force?

A sketch of the important features of the simulation would help.
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Old   April 17, 2012, 21:26
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The aluminium block is solid material, and both the aluminium block and container are in fixed positions (i.e. they will not move). According to theory, under a microgravity environment the water *should* flow up along the surface of the aluminium block as it has a higher free surface energy than that of the air, thus the free surface of the water will be attracted to the aluminium over the air.

The container will be 100% filled with water, and air is surrounding the whole system (i.e. at the free surface of the water as well as the free surface of the aluminium block).

The time scale is not known as that is one of the observations I would like to make (how long it takes the water to flow up the block), so I have set the time scale initially as 30s to see how much flow occurs in that time.

"Another way to view surface tension is in terms of energy. A molecule in contact with a neighbor is in a lower state of energy than if it were alone (not in contact with a neighbor). The interior molecules have as many neighbors as they can possibly have, but the boundary molecules are missing neighbors (compared to interior molecules) and therefore have a higher energy. For the liquid to minimize its energy state, the number of higher energy boundary molecules must be minimized. The minimized quantity of boundary molecules results in a minimized surface area." - taken from wikipedia if that helps understand in more detail. The free surface of any material has a higher energy than the internal layers, so if the free surface of the water comes into contact with the free surface of the aluminium, they should be attracted and flow up (this doesn't happen under gravity as the force of gravity far outweighs that of surface tension).

The flow is therefore driven by surface tension.

The problem is very unusual and I appreciate any help you can give.

Cheers.
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Old   April 18, 2012, 07:04
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I am well aware of surface tension and have many years experience in CFD modelling with surface tension.

The modelling of your application seems straight forward. Set it up as a normal air/water free surface multiphase simulation, and define appropriate wall contact angles for the interface on aluminium and anything else the surface contacts.

You do not require FSI, modelling of the solid or anything like that.

But be careful that getting this sort of model accurate is very difficult. You will need to do lots of verification as you proceed to ensure accuracy.

And you get bonus points if you identify the moving contact line/no slip wall issue in your analysis
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Old   April 18, 2012, 09:41
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Excellent, I'll give that a try. Do I have individual domains for the air, water and aluminium or have them all in the one domain?

Cheers.
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Old   April 18, 2012, 20:28
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Sorry, having some problems with the set up.

If I don't model the solid, then how do I define an interface between the aluminium block and the water? I am only able to define wall adhesion angles for fluid pairs, and the air/water fluid pairing is not of much significance except to show that the path of higher surface tension will be chosen (i.e. the water will be attracted to the aluminium).

How do I go about inputting a wall adhesion angle for the water/aluminium interface? (They will be the walls of the aluminium block)

I have created an enclosure around the model posted previously to model the air surrounding it, and it was simple enough to create the interface between the air and water and input a wall adhesion angle.
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Old   April 19, 2012, 09:04
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I do not understand what you are asking. There is no adhesion angle between the water/Al interface. The adhesion angle is of the air/water interface where it meets the Al - this is the angle you define, so it is defined per fluid pair on the wall boundary.
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Old   April 20, 2012, 01:00
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Hmm ok. I fail to see how ANSYS will recognise that the aluminium block has a higher surface tension pairing with the water than the water/air and so will chose that and "flow" up the block.

I'll give it a go - fingers crossed!
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Old   April 20, 2012, 07:41
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Either I do not understand what you are saying or you do not know how contact angles are simulated in CFX.

CFX does not model interface energy at all. CFX applies the surface tension force as a body force on the elements at the interface, and imposes the interface to have the defined angle with the wall to generate wall contact angle (and the resultant capilliary flows and so on).

And once you have got this model working make sure you do a mesh sensitivity study.
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