falling drop
hi,
I need to find the external and internal velocity fields of a falling drop (let's say water droplet falling through air) under gravity (i.e. free fall). How should i define the liquidgas interphase? which model i should use to solve this problem? (It seems that i should somehow define wall boundary condition at the interphase,since i couldn't get anything reasonable without that) I will appreciate any help even regarding constant velocity falling drop (the otherway is quite complicated since the drag force is dependent on the velocity fields on the drop surface) Thanks in advance! 
Re: falling drop
Hi!
In your case you should use the VoF multyphase model. I have some expiriences with "water drop faling in water simulation"  it works quite well in Fluent. If you haven't done any work with VoF I guess you should first go trough tutorial case, and then work your way up. The thing is you are probabilly gonig to have to simulate this unsteady, so yu can increasse the inlet velocity the same way the doplet gains speed  it's not so complicated to calculate this even with drag. You can also let your droplet fall in your domain... I hope I helped MATEUS 
!! FALLING DROP !!
Hi MATEUS,
First, thanks for your reply. I did not consider the multiphase model, since i have only one fluid particle at all. For the VoF model, it is said to be good for free surface flows ( I assume that i have a small enough drop so that it stays as spherical), stratified and slug flows, but i have something like droplet flow (as shown in 18.1 in Fluent manuel). Fluent recommends to use Discrete Phase Model for discrete phase vol.frac.<10, but i couldn't find a way out for my specific problem with that. So, in that sense, my problem seems a bit simpler, since i do not need to find out the trajectory of my drop, since i expect it to fall straight down. But i have to find out the terminal velocity (which depends on the fields on the liqidgas interphase) of the drop and the velocity fields inside and outside the drops. If a manage that, then i will hopefully switch to the modelling mass transfer of a third component between the phases. Do you have perhaps more suggestions? 
Re: !! FALLING DROP !!
Hi, Allow me to make some comment.
1 – I do not see where is the difficulty to get the terminal velocity of your drop knowing the approximation you are assuming. A force balance on the drop will give you the slip velocity between your particle and the environment depending on the (drop diameter and Drag coefficient). Then as you are assuming a constant diameter for your particle you have a drag coefficient only depending of the slip velocity. It should not be difficult to find some drag laws for drop falling in the air with constant diameter. The result of all that is 1 equation for 1 unknown. In your case the slip velocity is the drop terminal velocity. 2 – According to what you said, the aim of your simulation is to get the terminal velocity, the inside and outside velocity field of an isolated drop falling down in the air. If you want to continue your project doing mass transfer you got to use the VOF model (See Mateus message). Concerning the way to simulate that my point of view is to simulate a drop falling down in the middle of an enought large tube and initialized the drop velocity as the previously calculated terminal velocity in order to limitate the calculation time. Then concerning the mesh, I have never done that but I have already seen people using deforming mesh to follow your particle in the domain or dynamic mesh gradient adaptation. Maybe you do not any special mesh. But I am sure you will find tonnes of example on the web cause this subject seems to be a classical final year project in engineering school. Hope I gave further and clear informations. Let me know from what country are you from :). cheers Thomas 
Re: !! FALLING DROP !!
Thank you for your suggestions Thomas,
Analytical drag equations for flow around fluid spheres are unfortunately limited to Re<1 (creeping flow) which is not my case.But, i can probably find an approximate value for that. (Actually, waterair system is just for a simple starting case. I have to apply my simulation to systems under high pressure like supercritical CO2 instead of air. Then i will have more coupling of the internal and external phases). I will look deeper to VoF model, and hope it will works for my single falling drop problem. But I have a feel that, somehow I will need to write a UDF to define the coupling of the internal and external flows at the interphase. Finally, I am from Turkey, but study M.Sc. Process Eng. in Hamburg/ Germany at the moment. You wonder because of my strange name?:) Where are u from by the way? özgür 
can you pleaase tell me how to create a droplet. i have worked with vof but only in the cases with volume fraction at inlet ranging fro 0 to 1. i dont know how to create a water droplet . please help

You need to create a circular/spherical register for 2d/3d case respectively. Assign liquid volume fraction of 1.0 in that register after initializing the case.
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