Abstract
An experimental study was conducted to examine the interfacial waves in two-layer exchange flows downslope of a bottom sill. The objective of the study was to understand the generation mechanisms of different interfacial waves and the generation conditions and development of Kelvin–Helmholtz (KH) instabilities in spatially varying flow field. During the flow regime of the steady maximal exchange, low-frequency oscillation of the density interface above the sill crest was observed, which resulted in periodic piling and releasing of the lower-layer fluid. This low-frequency oscillation led to the development of large-scale downslope waves that caused significant interfacial entrainment. The periodic time of those piling and releasing events was found to be inversely proportional to the square root of the reduced gravitational acceleration and to be related to the baroclinic forcing of the basin internal seiching. During late sub-maximal exchange, regular and frequent KH instabilities were generated at the interface down the slope at bulk Richardson number of approximately 0.07–0.21, which is considerably larger than previous theoretical predictions for parallel flows with a sharp density interface. Two different growth patterns of those KH instabilities were observed. Measurements of their initial growth rate and the wavelength were also obtained and compared to the theoretical predictions.