Double diffusive instability with a constriction

Confined geometries have an effect on hydrodynamic instabilities, and this provides opportunities for controlling the rate of mixing in flows of engineering relevance. In multi-component fluids, differential diffusion allows for novel types of hydrodynamic instability that have finite amplitude manifestations even in millimeter-scale channels. We present numerical simulations that demonstrate that localized channel constrictions can serve to partially "catch " the manifestations of double diffusive instabilities. The fluid collects just above the narrowest point of the constriction and eventually undergoes a secondary instability. We study this secondary instability, focusing on its chaotic nature and on the way in which flow into the region below the constriction is controlled by the constriction amplitude and shape.