  Traditional approaches to mathematically describe spontaneous imbibition are
usually based on either macro-scale models, such as Richards equation, or
simplified pore-scale models, such as the bundle of capillary tubes (BCTM) or
pore-network modeling (PNM). It is well known that such models cannot provide
full microscopic details of the multiphase flow processes and that many
pore-scale mechanisms still lack proper mathematical descriptions. To improve
the predictive capabilities of traditional models, a fundamental understanding
of pore-scale dynamics is needed. The focus of this paper is obtaining detailed
insight and consistent explanation of particular processes during
capillary-controlled water imbibition into dry porous media. We use
two-dimensional model geometries and perform fully dynamic volume-of-fluid
based direct numerical simulations of air-water multiphase flow at the
pore-scale, to study processes that generally are not considered in traditional
models. More specifically, we investigate differences between converging and
diverging geometries, dynamic pressure and meniscus reconfiguration during
pore-filling events, and the influence of inertia and pore size on imbibition
dynamics and the occurrence of capillary barriers. Furthermore, we perform a
detailed comparison between non-interacting and interacting BCTM and study the
impact of the narrow contractions on imbibition dynamics and the trapping of
the non-wetting phase. Obtained knowledge can be used to improve predictive
models, which are broadly relevant considering the importance of spontaneous
imbibition in many different natural and industrial processes.
