  We present a direct numerical simulation (DNS) study of buoyancy driven
bubbly flows in two-dimensions. We employ volume of fluid (VOF) method to track
the bubble interface. To investigate spectral properties of the flow, we derive
the scale-by-scale energy budget equation. We show that the Galilei number (Ga)
controls different scaling regimes in the energy spectrum. For high Galilei
numbers, we find the presence of an inverse energy cascade. Our study indicates
that the density ratio of the bubble with the ambient fluid or the presence of
coalescence between the bubbles does not alter the scaling behaviour.
