Gravity currents form when a denser fluid propagates into a lighter one in a predominantly horizontal direction. They are frequently encountered in environmental and engineering applications. Examples are cold river outflows into a warmer lake, or a cold air front propagating into a warmer air mass. Turbidity currents represent a special class of gravity currents in which the driving force results from differential particle loading, as is the case for a sediment-laden river outflow. Turbidity currents are difficult to analyze, as they may exchange particles with surrounding fluid and/or a sediment bed by deposition or resuspension.
In a geophysical context turbidity currents - essentially submarine avalanches - play a crucial role in the global sediment cycle, as the principal means of sediment transport across the continental shelves into the deep oceans. Ancient deposits of turbidite sand, deeply buried and compacted, also form an important class of hydrocarbon reservoirs, and the host rocks for a particular type of gold deposits. In an environmental engineering context, turbidity currents are responsible for much of the sedimentation in reservoirs, with consequent loss of water storage capacity.
Frequently, gravity and turbidity currents interact with effective interfaces between stratified fluid layers, or with free surfaces. The resulting current/wave interactions give rise to a host of interesting and complex phenomena. For example, when turbidity currents form in shallow water as a result of submarine landslides, their interaction with the surface of the ocean (or lake) can result in the generation of tsunamis.
To investigate the fundamental dynamics of the above types of flows, a detailed computational investigation of turbidity currents interacting with interfaces and free surfaces will be undertaken. The simulations and experiments will provide information about the various energy conversion mechanisms active in such flows, which in turn will allow us to develop simplified theoretical models for their analysis. The proposed research will develop such models for a wide range of particle-laden flows, from turbidity currents, river plumes, pyroclastic flows and powder snow avalanches, to tsunamis generated by submarine landslides. In this way, it will benefit such diverse areas as global sediment cycle research and environmental hazard assessment.
Gravity-driven flows such as gravity currents, turbidity currents, hydraulic jumps or internal bores are ubiquitous in the environment, appearing both in the atmosphere as well as in the ocean. They strongly influence the transport of mass, momentum, energy and dust or sediment in these environments, so that we need accurate models for their dynamics in large-scale environmental forecasting tools, such as weather or climate models. Taditional models for such currents have had to rely on empirical assumptions regarding the conservation of energy. These assumptions were frequently not based on solid physical arguments, so that they introduce significant inaccuracies into environmental forecasting tools. Under this project, we have developed a new class of models that relies on the conservation of mass and momentum alone, without requiring any assumptions about energy. Predictions by the new theory are shown to be in close agreement with results from high-resolution numerical simulations. The new models have the added advantage that they allow us to calculate the energy budget of gravity-driven flows, rather than making a priori assumptions about the energy budget. In addition, the present project has resulted in the development of a new computer simulation code that allows for the high-resolution simulation of gravity-driven flows and their interaction with complex seafloor shapes.
