Effect of channel geometry on a density wave induced by gravity and chemoconvection in miscible reacting fluids

We study a two-layer system of initially separated aqueous solutions of an acid and a base placed in a vertically oriented Hele–Shaw cell. After the reactants, a second-order neutralization reaction begins, which is accompanied by the release of salt. Recently, we found that, contrary to expectations of the occurrence of fingering convection, a density wave pattern may occur in this system at some values of initial concentrations of acid and base. This wave has a perfectly planar front with a nearly discontinuous change in density across the front and propagates fast compared with the characteristic diffusion times. The shock wavefront separates the domains of a motionless fluid and intense convective mixing. In this work, we study, both theoretically and experimentally, the influence of the cavity geometry on the dynamics of the density wave. Specifically, we consider the following cases: when (a) the aspect ratio of the cavity changes, (b) the gap width of the Hele–Shaw cell is locally varied, and (c) the oblique obstacle is placed inside the cell. The set of governing equations includes the equation of motion based on Darcy’s law as well as the transfer equations for species. The experimental observations presented for the wave bending around the obstacle are in good agreement with the numerical simulation.