Capillary reorientation and interfacial transitions under geometric confinement in a bipropellant tank

Capillary-driven redistribution in a closed, geometrically confined container involves coupled wetting, interface-curvature evolution, and gas-pocket topology changes. Here, a three-dimensional volume-of-fluid model is used to examine these mechanisms in a common-bulkhead container under 10−6 g0 for fill fractions of 20%–95%. The Bond and Ohnesorge numbers indicate that the global response is dominated by capillary-inertial dynamics, with residual gravity providing only a weak axial preference. The interface evolves from an initially open surface through wall-wetting spreading to a closed central gas pocket. Fill fraction produces a non-monotonic response: low-fill cases are limited by insufficient liquid inventory and high-fill cases are limited by restricted gas space, whereas intermediate fill fractions of 40%–60% permit the greatest interface deformation and axial mass redistribution. The upper and lower compartments exhibit different center-of-mass responses because their boundaries, initial interfaces, and fluid properties set different capillary pathways and inertial time scales. These results identify the physical mechanism by which finite gas volume and compartment geometry regulate capillary-driven interfacial transitions in closed containers, with implications for spacecraft propellant management.