The dynamics of steady, partial‐depth intrusive gravity currents

Abstract

Experiments of intrusive gravity currents generated by lock exchange offer insights into atmospheric and oceanic flows. However, whereas many previous investigations have considered the full‐depth’ lock exchange problem, in which the intermediate density fluid initially spans the entire channel depth, less is known about ‘partial‐depth’ releases, which represent a more appropriate analogue to environmental flows where the inceptive, localized interfacial mixing is relatively weak and/or the upper and lower ambient layers are of significant vertical expanse. Here, we consider this circumstance using a combination of experimental, (two‐dimensional) numerical and analytical techniques with a particular focus on equilibrium flow for which there is no auxiliary concentration or dilution of the active scalar, and the interface ahead of the intrusion remains approximately horizontal. In this case, the initial (steady) speed of propagation, U, can be well‐predicted by adapting a shallow‐water model for gravity currents that employs as its front condition the relationship of Benjamin (1968). When, in the initial state, the upper and lower halves of the density field are (stretched) mirror images of one another, the front travels at constant speed beyond 10 lock lengths, as was noted in the case of full‐depth lock releases by Sutherland et al. (2004) and Sutherland and Nault (2007). However, when this symmetry is broken either by altering the relative depth of either ambient layer or by changing the intrusion density, the flow begins to decelerate after travelling as few as three lock lengths.