Simulating liquid droplet deformation

mebinitap · June 13, 2013, 08:50

Dear all,

I want to see the deformations in a liquid droplet in presence of air flow. So I used a squared domain (in blockMeshDict file) and setFieldsDict for the droplet. I stated the boundaries as atmospheric and used the freeStream BC conditions for P and U. Problem is that droplet moves away leaving the domain if I put some value to velocity. Since I want to track the shape changes I have to move along with the droplet. Can anyone please give me a hint on that? Also are the BCs correct ?

I want to model the shape changes in a static 3D droplet placed in a convective flow field and then to see how it response in presence an acoustic wave (acoustic levitation).

michielm · June 14, 2013, 05:30

Hi mebinitap,
A note upfront: the deformation of a static and a dynamic droplet is not the same so you cannot use the deformation of the moving droplet to study your problem. Therefore, moving along with the droplet is not going to help you.

A droplet in an air flow can only remain static if the surface shows contact angle hysteresis. Experimentally, this is pretty much always the case. Numerically however, by default there will be no contact angle hysteresis which will cause the droplet to move already for the tiniest amount of air flow.
Implementing contact angle hysteresis is not that simple because you need to check whether the contact angle is in a certain range and if so, adapt the local contact angle instead of moving the contact line.

A trick you could give a try is to adjust your boundary condition with funkySetBoundaryField. You can set the contact angle to a certain value underneath the droplet and to a higher value outside of the droplet. This will cause the droplet to stick to the spot it is located and effectively generates contact angle hysteresis.

mebinitap · June 14, 2013, 08:45

Thank you so much for your suggestion. I will try to implement it and let you know.

mebinitap · June 27, 2013, 00:39

Hi again,

I am a bit confused about surface tension force implementation in interFoam. It seems it is handled as : fvc::interpolate(interface.sigmaK())*fvc::snGrad(a lpha1). But if a drop is sitting on vibrating surface and contact angle is not in equilibrium, then how are the diff sigmas at all surfaces (solid-liq, liq-gas, gas-solid) are taken care of, i mean there should be some kind of Young's type of eqn. But I dont realize how is that implemented. Should that be given as some dynamic contact angle boundary condition ? It will be great if I can get any kind of hint.

Thanks

June 13, 2013, 08:50	Simulating liquid droplet deformation	#1
mebinitap New Member Join Date: Apr 2013 Posts: 24 Rep Power: 13	Dear all, I want to see the deformations in a liquid droplet in presence of air flow. So I used a squared domain (in blockMeshDict file) and setFieldsDict for the droplet. I stated the boundaries as atmospheric and used the freeStream BC conditions for P and U. Problem is that droplet moves away leaving the domain if I put some value to velocity. Since I want to track the shape changes I have to move along with the droplet. Can anyone please give me a hint on that? Also are the BCs correct ? I want to model the shape changes in a static 3D droplet placed in a convective flow field and then to see how it response in presence an acoustic wave (acoustic levitation).

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June 14, 2013, 05:30		#2
michielm Member Michiel Join Date: Oct 2010 Location: Delft, Netherlands Posts: 97 Rep Power: 15	Hi mebinitap, A note upfront: the deformation of a static and a dynamic droplet is not the same so you cannot use the deformation of the moving droplet to study your problem. Therefore, moving along with the droplet is not going to help you. A droplet in an air flow can only remain static if the surface shows contact angle hysteresis. Experimentally, this is pretty much always the case. Numerically however, by default there will be no contact angle hysteresis which will cause the droplet to move already for the tiniest amount of air flow. Implementing contact angle hysteresis is not that simple because you need to check whether the contact angle is in a certain range and if so, adapt the local contact angle instead of moving the contact line. A trick you could give a try is to adjust your boundary condition with funkySetBoundaryField. You can set the contact angle to a certain value underneath the droplet and to a higher value outside of the droplet. This will cause the droplet to stick to the spot it is located and effectively generates contact angle hysteresis.

June 14, 2013, 08:45		#3
mebinitap New Member Join Date: Apr 2013 Posts: 24 Rep Power: 13	Thank you so much for your suggestion. I will try to implement it and let you know.

June 27, 2013, 00:39		#4
mebinitap New Member Join Date: Apr 2013 Posts: 24 Rep Power: 13	Hi again, I am a bit confused about surface tension force implementation in interFoam. It seems it is handled as : fvc::interpolate(interface.sigmaK())fvc::snGrad(a lpha1).* But if a drop is sitting on vibrating surface and contact angle is not in equilibrium, then how are the diff sigmas at all surfaces (solid-liq, liq-gas, gas-solid) are taken care of, i mean there should be some kind of Young's type of eqn. But I dont realize how is that implemented. Should that be given as some dynamic contact angle boundary condition ? It will be great if I can get any kind of hint. Thanks