础产蝉迟谤补肠迟:听Kelvin-Helmholtz (KH) instability is a primary route to turbulence in stratified shear flows, with mixing often associated with the large overturning cores. However, recent estuarine observations indicate that the strongest mixing may instead occur in the braid regions of KH billows. We study the onset of two-dimensional secondary shear instability (SSI) in the braids as the mechanism behind this braid-dominated mixing. While strain induced by the billows stabilises the braids, it also compresses tilted isopycnals, enhancing baroclinic shear that enables rapid perturbation growth. By modifying the classical analysis of Corcos & Sherman (J. Fluid Mech. 73, 241-264, 1976) in braid-aligned coordinates and adding an instability criterion based on the ratio of strain rate to shear, we develop an inviscid, time-dependent model for the braid evolution and the onset of SSI. We show that the instability criterion can be achieved significantly earlier than the saturation of the primary billow at sufficiently high initial Richardson number. Two-dimensional DNS up to Reynolds numbers Re=10^7 quantifies viscous effects; at high Re, we find that SSI indeed develops early in the braid, as predicted by the inviscid model, while the primary billow is still growing and before viscosity slows braid thinning. These results suggest that, at geophysically relevant Ri and Re, SSI can control the three-dimensional transition and ensuing mixing by preceding vortex pairing and secondary convective instabilities.
Short bio: Emma is a 4th-year PhD student at the University of Cambridge, primarily researching flow in soft porous media for industrial applications. Her recent work on the Kelvin-Helmholtz instability is a result of my geophysical fluid dynamics fellowship at Woods Hole Oceanographic Institution last year. Her work is unified by the goal of explaining observed phenomena through reduced theoretical models, allowing for greater physical insight.