  Flow in channels and ducts with adjoining cavities are common in natural and
engineered systems. Here we report numerical and experimental results of 3D
confined cavity flow, identifying critical conditions in the recirculating flow
formation and mass transport over a range of channel flow properties ($0.1\leq
Re \leq 300$) and cavity aspect ratio ($0.1 \leq H/X_{s} \leq1$). In contrast
to 2D systems, a mass flux boundary is not formed in 3D confined cavity-channel
flow. Streamlines directly enter a recirculating vortex in the cavity and exit
to the main channel leading to an exponential increase in the cavity mass flux
when the recirculating vortex fills the cavity volume. These findings extend
our understanding of flow entry and exit in cavities and suggest conditions
where convective mass transport into and out of cavities would be amplified and
vortex particle capture reduced.
