An experimental study of mixing and instability development in variable-density systems

This study presents results of flow-tank experiments on variable-density flow in a layered system. Less dense water displaced more dense water in a system layered with a lower over a higher hydraulic conductivity unit. This configuration created a potentially unstable interface between the displaced water and the displacing water. The displacing water wedge (i.e., less dense water) in the higher hydraulic conductivity layer traveled downgradient faster than the displacing water wedge moving in the layer above. Downward movement of more dense water from the upper layer into the freshwater wedge in the lower layer caused the latter to become more saline. Flow rate and density difference between displacing and displaced water, and hydraulic conductivity difference between layers, were each analyzed for their effect on mixing behavior in this system. Fluid movement and mixing processes were monitored using time sequence photography. Digital processing of black and white negatives provided a large and semi-continuous data base of concentration values to analyze the salinization of the displacing water. In many cases, the unstable stratification and density gradient also promoted the upward growth of finger-shaped instabilities into the less permeable layer. An analytical stability analysis was able to reasonably predict the wavelength of these fingers. Results from the experiments suggest that density variations can promote complex flow and mixing patterns in even the simple layered systems considered herein. This conclusion has important implications for both contaminant transport and fluid displacement processes occurring during remediation.