The effects of topography on mixing rates in controlled exchange flows

Abstract

We examine irreversible mixing in buoyancy-driven baroclinic exchange flows past bottom topography. In all cases the shear between the counter-flowing layers is unstable in the vicinity of a hydraulic control situated above the topography. The overall amount of irreversible turbulent mixing produced by overturning billows during a given time is determined from measurements of vertical density profiles after the exchange. The potential energy changes are used to evaluate the global mixing efficiency, defined as the fraction of the available potential energy released into kinetic energy that is converted to potential energy through irreversible mixing. In all cases examined, the mixing efficiency is no more than 11%±1%, which corresponds to the maximum mixing efficiency found in previous work for exchange flows through a lateral contraction. The mixing efficiency is found to reduce as the fractional depth of the topography increases and as the topographic slope decreases.