Hello
I am new to Houdini too and I am also bearing the cross of total lack of knowledge BUT I am currently working on a raising bubbles/fluid simulation as well and it works.
The approach that worked for me was pretty straightforward and is as follow:
Create 1 simple Flip simulation (Flip fluid in a container of your choice)
Create 1 separate Pop particle system (without gravity) to emit particles from a geo such as a curve circle located inside the actual fluid. Attach ('copy to points' node) a simple sphere to your particles. Set your birth rate and sphere size according to something realistic. Set the initial Y axis velocity of the spheres so they raise up to the surface of the liquid at a reasonable pace and balance their life expectancy so they die right at the fluid surface. Turn off their visibility (display off at obj level).
Source the fluid container as a collider for the particles inside the Pop network (static object).
Back to the Flip sim Dop network. Create a 'deforming' collider object, that are the SPHERES. The sheres will deform naturally in the fluid (won't stay perfeclty round).
Play it
Depending on the scale of your scene, your particle resolution and your pop particle velocity it will likely explode at first. The first fix is to dampen the velocity at Flip level by lowering the 'velocity scale' param in the collision tab of the flipsolver.
Let me know if this turned out ok for you and if anyone else has any tips as to this matter please feel free.
V
