ON MIXING AND TRANSPORT AT A SHEARED DENSITY INTERFACE

Mixing and transport of a stratifying scalar are investigated at a den sity interface imbedded in a turbulent shear flow. Steady-state interf acial shear flows are generated in a laboratory water channel for laye r Richardson numbers, Ri, between about 1 and 10. The flow field is ma de optically homogeneous, enabling the use of laser-induced fluorescen ce with photodiode array imaging to measure the concentration field at high resolution. False-colour images of the concentration field provi de valuable insight into interfacial dynamics: when the local mean she ar Richardson number, Ri(s), is less than about 0.40-0.45, interfacial mixing appears to be dominated by Kelvin-Helmholtz (K-H) instabilitie s; when Ri(s) is somewhat larger than this, interfacial mixing appears to be dominated by shear-driven wave breaking. In both cases, vertica l transport of mixed fluid from the interfacial region into adjacent t urbulent layers is accomplished by large-scale turbulent eddies which impinge on the interface and scour fluid from its outer edges. Motivat ed by the experimental findings, a model for interfacial mixing and en trainment is developed. A local equilibrium is assumed in which the ra te of loss of interfacial fluid by eddy scouring is balanced by the ra te of production (local mixing) by interfacial instabilities and molec ular diffusion. When a single layer is turbulent and entraining, the m odel results are as follows: in the molecular-diffusion-dominated regi me, delta/h approximately Pe-1/2 and E approximately Ri-1 Pe-1/2; in t he wave-breaking-dominated regime, delta/h approximately Ri-1/2 and E approximately Ri-3/2; and in the K-H-dominated regime, delta/h approxi mately Ri-1 and E approximately Ri-2, where delta is the interface thi ckness, h is the boundary-layer thickness, Pe is the Peclet number, an d E is the normalized entrainment velocity. In all three regimes, the maximum concentration anomaly, GAMMA(m) approximately Ri-1. When both layers are turbulent and entraining, E and delta depend on combination s of parameters from both layers.