[ocyee]

Hello guys, would like to observe the droplet internal flow, velocity contour and streamline when the base of the water droplet is applied with a vibrating condition, a simple example would be a sine function. However, I could not figure out the boundary conditions for the FLUENT setup. Since there is no inflow or outflow from the droplet, what boundary conditions should I be applying to the droplet?

[vinerm]

Are you talking about a single droplet? If it is so, and there is no inlet and outlet, then the free-surface of the droplet behaves like a wall; a wall with some velocity and shear stress. The vibration will impose exactly that. However, if there are vibrations normal to the surface of the droplet, then you have to apply source terms adjacent to the outer surface because wall cannot have a normal force applied. Other option is to model it using free surface model, i.e., VOF and then you can apply any kind of force at the free-surface.

[ocyee]

Originally Posted by vinerm

Are you talking about a single droplet? If it is so, and there is no inlet and outlet, then the free-surface of the droplet behaves like a wall; a wall with some velocity and shear stress. The vibration will impose exactly that. However, if there are vibrations normal to the surface of the droplet, then you have to apply source terms adjacent to the outer surface because wall cannot have a normal force applied. Other option is to model it using free surface model, i.e., VOF and then you can apply any kind of force at the free-surface.

Yes. My idea is to observe droplet dynamics on a glass slide. But for simplicity, I want to just ignore the glass slide for now, and just focus on the droplet instead. Therefore, the vibrating condition underneath the droplet base itself, normal to the base of the droplet. Regarding the VOF model, i am currently using that in FLUENT, but isnt that just a setting that we just toggle it on and off in the application before calculation?

[ocyee]

Originally Posted by vinerm

Are you talking about a single droplet? If it is so, and there is no inlet and outlet, then the free-surface of the droplet behaves like a wall; a wall with some velocity and shear stress. The vibration will impose exactly that. However, if there are vibrations normal to the surface of the droplet, then you have to apply source terms adjacent to the outer surface because wall cannot have a normal force applied. Other option is to model it using free surface model, i.e., VOF and then you can apply any kind of force at the free-surface.

Just to be clear, regarding the boundary conditions, do I just set the water droplet base as wall and apply a UDF velocity, and the water droplet-air interface as wall with no slip boundary conditon? Thanks in advance!

[vinerm]

Volume of Fluid model is not just a setting. It's a model that allows to solve for two things - volume fraction of the secondary phase, such as, liquid droplet in the gas, and determination of the interface. Now, due to the vibrations, the interface shape and size might change. So, VOF will be able to predict that. If you assume that the shape and the size of the droplets do not change much and your interest lies solely in the prediction of flow within the droplet, then you do not require VOF and can work with single phase model.