AN EXPERIMENTAL-STUDY OF MIXING AND INSTABILITY-DEVELOPMENT IN VARIABLE-DENSITY SYSTEMS

This study presents results of flow-tank experiments on variable-densi ty flow in a layered system. Less dense water displaced more dense wat er in a system layered with a lower over a higher hydraulic conductivi ty unit. This configuration created a potentially unstable interface b etween the displaced water and the displacing water. The displacing wa ter wedge (i.e., less dense water) in the higher hydraulic conductivit y layer traveled downgradient faster than the displacing water wedge m oving in the layer above. Downward movement of more dense water from t he upper layer into the freshwater wedge in the lower layer caused the latter to become more saline. Flow rate and density difference betwee n displacing and displaced water, and hydraulic conductivity differenc e between layers, were each analyzed for their effect on mixing behavi or 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 concentra tion values to analyze the salinization of the displacing water. In ma ny cases, the unstable stratification and density gradient also promot ed the upward growth of finger-shaped instabilities into the less perm eable layer. An analytical stability analysis was able to reasonably p redict the wavelength of these fingers. Results from the experiments s uggest that density variations can promote complex flow and mixing pat terns in even the simple layered systems considered herein. This concl usion has important implications for both contaminant transport and fl uid displacement processes occurring during remediation. (C) 1998 Publ ished by Elsevier Science B.V. All rights reserved.