Gravity-driven flows in stratified fluids

University Of California-San Diego, La Jolla CA

Investigators

Abstract

CBET-0756396 Linden When two regions within a stratified fluid with different vertical stratifications come into contact, buoyancy forces associated with the horizontal density differences cause the fluids from the two regions to intermingle. This intermingling process occurs through the intrusion of one fluid into the another - the motion of one fluid along isopycnal surfaces of the other. Fluid can be advected large distances within an intrusion, with important consequences for the transport of mass, tracers, nutrients, pollutants and biological material, and the ultimate mixing of the two fluids. The dependence of the propagation speed and the size of the intrusion on the properties of the stratification is largely unknown. The goal of this proposal is to determine these relationships, to determine the interactions between intrusions and internal waves, and to obtain a deeper understanding of the dynamics of stratified fluids. Even the simplest case of an intrusion propagating along an interface between two well mixed layers is not well understood, despite theoretical and experimental work extending back over the past 25 years. Recently, the PI has made significant progress on this problem and shown that upstream deflection of the interface is a key feature of this flow. This result raises the possibility of gaining new insights into the more general case of a continuously stratified ambient fluid which is capable of supporting a spectrum of wave modes. It is planned to conduct a closely integrated experimental, theoretical and computational investigation into the dynamics of gravity-driven intrusions in stratified fluids. This project is aimed at exploring these issues with the goal of developing a comprehensive theory that couples the intrusion to the internal wave field that it generates. Intellectual merit: As an immediate scientific contribution, the planned work will identify and quantify the mechanisms that dominate the spatio-temporal evolution of intrusions in a stratified fluid. Specifically, the research will address such issues as the speed of propagation and its dependence on the properties of the intrusion and the ambient stratification, the generation of interfacial and internal waves by the intrusion and their subsequent effect on the dynamics of the intrusion. Laboratory experiments and numerical calculations will be used to develop an understanding of the dynamics and guide a theoretical development that will enable the results to be translated into a deeper understanding of stratified flows. Broader impact: The ability to predict the speed and other physical properties of intrusions is of importance to a variety of geophysical and environmental flows. On a more fundamental level this project addresses the question of how different fluids in a gravitational field intermingle. The PIs expect the results of this study to provide new insights on this question. It will also provide an example of the combined use of numerical, experimental and theoretical techniques on a complex fluid dynamical problem, leading to refinements in all these techniques. On the educational side, the planned investigation will allow for the training of graduate and undergraduate students, and the further development of foreign collaboration, in the concepts and methods of advanced experimental techniques, theoretical modeling, and numerical simulations.