Learning Forum

[Swong26]

I am currently developing a model to demonstrate the impact of a liquid droplet impacting a different liquid surface. For example an oil droplet hitting a water surface. The droplet should float on the surface of the water however the model says the droplet sinks. nDo I need to use the surface tension values or contact angles found under wall adhesion?.

[Rob]

Which models are you using? How quickly does the oil droplet hit the water? Surface tension may effect the droplet shape, wall adhesion is for the walls, so shouldn't come into it. n

[Swong26]

Using the VOF Model, Explicit with Implicit Body Forces. The droplet is a microdroplet with radius 200e-6 m dropped from a height of 1e-4 m so the velocity is minimal. The surface tension between the oil droplet and water layer is greater than the sum of the Oil/Air and Water/Air surface tension so in theory the droplet should float according to Neumann's Construction. n

[Rob]

How many cells are there across the droplet? That should work, but mesh resolution is going to mean you have a big model. Double check the density settings too, and that you got the phases the right way around: labelling phase-oil etc is recommended. n

[Swong26]

The droplet has 316 cells and I will increase mesh resolution. Regarding variable density parameters in operating conditions should it remain 'minimum phase average ' or have a 'user input ' and finally is there a way to specify the surface tension of the solid and liquids. So that if a droplet breaks through the water layer and touches the wall the three phase contact pinning would occur. I am assuming that would require wall contact angles?nThank you in advance n

[Rob]

The operating density won't do much in this case, it's the individual phase densities that want checking. The model works on surface tension of each phase, and wall contact. If the droplet does hit the wall without contact angle being set the forces holding it down (or repelling) won't be calculated as they would in reality. n

[Swong26]

Sorry just so I understand then. If no contact angle is set then the forces wont be calculated and this is more realistic?n

[Rob]

Less realistic, if there is no contact angle the fluid won't be attracted/repelled by the surface. Flow and buoyancy effects still occur but the effect of the wall finish isn't. nIt's all covered in some detail in the theory guide, which I've not read for several releases so that'll be more precise than my memory! n

[Swong26]

Okay thank you very much for your help today n

[Abir Malakar]

I am doing a similer thing, where I am trying to simulate droplet hiting a substrate. I did it for droplet hiitng a solid surface using VOF but can't do it for a substrate. The droplet bounces off everytime, i.e still treating it as a wall. Can you help me with the steps you did