Void fraction
= Void fraction
{title2=$\phi=V_{\rm gas}/V_{\rm mixture}$}
The <void fraction> is the fraction of a mixture's local <volume> occupied by gas. For a horizontally uniform bubbly liquid it is the transported <concentration> in a one-dimensional gas-volume <continuity equation>. It is distinct from gas <mass> fraction because gas and liquid have different <mass densities>.