[Siba11]

I want to vary the density of a bubble(air) rising in a tube(filled with water). But I have got no idea how to do that?
Can anyone help me regarding the same?

[vinerm]

It depends on the model being used for the bubble simulation. Is it Lagrangian or Eulerian?

[Siba11]

It's Eulerian.

[vinerm]

Materials for all phases are defined under Material Panel. For all fluid materials, there are various options available to define density, such as, temperature dependent, ideal gas, incompressible ideal gas, etc. However, if you want to implement something that is not in the list, you have to use UDF.

For the air bubble rising in water, you can use Ideal Gas. It will represent the density properly. Do note that you should enable gravity in correct direction and operating pressure should be set to 0. Provide proper pressure value in initialization panel while initializing; it should be a positive value, equal to 101325 Pa if system is at atmospheric pressure.

[Siba11]

After running my simulation how will I be able to measure the change in volume?
Also, do I have to specify anything in particular after selecting 'Ideal gas' , if I want to regulate the magnitude of expansion?

[vinerm]

You can plot the density contours and you will observe how it is changing from bottom to top. As far as bounding is concerned, you just need to ensure that the pressure and temperature values are correctly given. E.g., if the vessel is open from top, you should provide a pressure and temperature value as per the atmosphere. Since operating pressure and operating density must be set to 0, the value of the pressure at this top boundary must be set to absolute pressure. So, if it is open to atmosphere, pressure should be 101325 Pa and temperature around 300 K.

Do note that there are no real bubbles in Eulerian model. Therefore, interfacial area is fixed. Though the size of the bubbles should increase, it will not. User has to write a UDF for that. If you are using VOF, since you mentioned a bubble, then you don't need to worry about the interfacial area. It will automatically be taken care of.

[vinerm]

If you want density to be controlled arbitrarily or based on some equation that is not available as an in-built model for density, then you have to write a function or expression.