Experiments on the interfacial mixing in a stratified exchange flow over a sill

Stratified exchange flows driven by a density difference are found in many natural water bodies and in the atmosphere. A typical case in nature is the strait of Gibraltar, where the water of the Mediterranean Sea flows under the less saline water of the Atlantic Ocean. At the interface between the layers shear is responsible for instabilities (Kelvin-Helmholtz), which results in turbulent mixing and entrainment. Gaining deeper understanding of these processes is the main motivation for this thesis. An experimental study into the development of a mixing layer of a twolayer stratified exchange flow is performed and an LIF measurement system was used to obtain the concentration fields. The main objectives of this study were to calibrate and understand the experimental set-up and to investigate the influence of four different parameters on the development of the mixing layer. The calibration of the experimental set-up has led to a simple calibration procedure, which was applied with success. To study the development of the mixing layer the influence of four different parameters was analyzed namely: the buoyant acceleration, the bottom friction, the water depth and the sill slope. After careful analysis of the experimental results it was observed that fluid from the upper layer was entrained into the lower layer in all the experiments. Analysis of the dye visualization showed that the large-scale structures, the Kelvin-Helmholtz instabilities, are mainly responsible for the mixing and entrainment. The overall conclusion was that the variation of geometric condition on the development of the mixing layer has a much stronger influence than the variation of the hydraulic conditions. An enhanced bottom friction affected the Kelvin-Helmholtz instabilities but the mechanism is not completely understood by the author.
Civil Engineering and Geosciences