An experimental study of interfacial waves and instabilities in exchange flows over a smooth sill

An experimental study examining two-layer exchange flows over a sill in a channel connecting two reservoirs at slightly different densities is presented. The experiments were designed to investigate: the generation of Kelvin-Helmholtz (K-H) instabilities in downslope currents, the reservoir-channel interaction, and mixing mechanisms in the lee of the sill.; For the downslope flow, a new entrainment and mixing mechanism was observed during the maximal exchange. In that mechanism, a low frequency oscillation of the density interface and a shear-generated K-H instability above the sill crest resulted in frequent piling and release of chunks of lower layer fluids which led to the development of large-scale breaking waves. Those waves caused significant interfacial entrainment with the entrainment coefficient about 0.1, the latter notably bigger than in turbulent wall jets. During the sub-maximal exchange, K-H instabilities were generated at bulk Richardson number, J, of about 0.05--0.12, which is considerably larger than theoretical predictions for parallel flows. Observations and detailed measurements of growths of those instabilities revealed two growth patterns: pattern I in which the instability was entrained into the lower layer, and pattern II where it pinched off into the upper layer.; The channel outflow revealed the existence of large-amplitude interfacial K-H instabilities that were frequently generated outside of the channel and propagated into the reservoir. The amplitude of those instabilities reached almost ¼ of the total flow depth. The internal hydraulic theory illustrated that the flow within the channel was basin-controlled. The outflow behaved like a surface plume whose densimetric Froude number, Fo, was about 0.24. Hilbert-Huang transform was used to investigate the origin of the aforementioned low frequency oscillation at the sill crest.; In the lee of the sill, a dynamically passive recirculation zone at medium density was observed between the upper and lower flowing layers. Analysis of the lower layer interface indicated the existence of lee waves downstream of the sill base at one single frequency everywhere. The time-averaged concentration field, obtained through laser-induced fluorescence, revealed that 75% and up of the maximum concentration in the lower layer were limited only to the shallowest one centimeter.