[vof64]

Hello!

I'm relatively new to Fluent (using version 14.5) and am studying the topic of air bubble coalescence in a water column. To do this, my eventual goal is to model two 3D bubbles that merge together while rising in a column of water.

At the moment, I am starting with a single 2D bubble rising through a water column and would appreciate feedback concerning modelling this from start to finish (Geometry, Meshing, Setup, Solution and Results).


Geometry:
In DesignModeler, I have set up a simple 2D rectangular geometry about 4cm by 6cm to represent the column of water. The bubble will be about 2mm in diameter initially and located on the vertical centre axis, near the bottom of the water tank.


Meshing:
I would like to find a way to concentrate the mesh more finely in the centre vertical axis of the column since more is happening around that area with respect to the rising bubble, as opposed to the edges where the mesh could be coarser. Is there a way to accomplish this?

I have also heard that using an axisymmetric mesh is wise here in order to cut down on calculation time. To activate this, must I simply select the axisymmetric mesh option in the Setup stage? Or must I explicitly specify the axis of symmetry somewhere along the way?


Setup and Solution:
On the left-hand menu, for the "Solution Setup", I'd appreciate commentary on what needs to be done, especially in the Cell Zone Conditions step, as well as the Boundary Conditions step. Do I need to specify parameters such as wall adhesion, or are such factors irrelevant since the bubble will be staying essentially on the centre vertical axis of the water column as it rises, away from the water tank walls?

On the same left-hand menu, I'm also wondering about the "Solution" menu that includes "Solution Methods", "Solution Controls", "Monitors", "Solution Initialization", "Calculation Activities" and "Run Calculation". Is PISO the correct scheme to use? I also understand that this is the section where I will be specifying (patching?) the 2mm air bubble to be located somewhere near the bottom of the tank, before running the calcuation. Could you please clarify how this can be accomplished and how I can visually verify the location of the bubble before running the calculation?

I also would like to include gravity, as well as surface tension forces between the air (bubble) and water (surrounding) phases.


Results:
The final step is to view the results in animation form to see how the bubble moves through the water column over time. How can this be accomplished?

I know this is a long post and a rather thorough set of questions spanning start to finish of the modelling process, but I'd very much appreciate any feedback you might have on the five main points in the modelling process- Geometry, Meshing, Setup, Solution and Results. Many thanks in advance!