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View of the SAFL building from the river

Lian Shen, an Associate Professor of Mechanical Engineering and the Associate Director for Research at the St. Anthony Falls Laboratory (SAFL), is leading the efforts to investigate oil movement in water systems. His group of students that he oversees is using computer simulations to study the problems that oil spills cause as they travel.

Oil spills pose a significant threat to aquatic ecosystems.  Spills on large scales, such as the BP Deepwater Horizon spill in 2010 that leaked more than 400,000 tons of oil, tend to dominate media coverage. But a comparable amount of oil is spilled in large numbers of small spills in aquatic environments annually. Statistics show that even without massive events, oil spills occur frequently at various scales due to tanker accidents, vessel operational discharge, pipeline ruptures, and natural seeps, with more than a million tons of oil discharged each year.

Even though the local spills in the Great Lakes are not large when compared to the Deepwater Horizon or Exxon Valdez incidents, these smaller spills happen in relatively confined systems with vulnerable ecosystems, and increased activities make it more likely that the Great Lakes community will face a catastrophic event at some point.

Oil not only floats at the water surface, but also appears in the form of clouds of droplets in the water below. Wind, waves, and currents mainly drive the slicks horizontally while turbulence mixes oil under the water surface and affects the dissolution of oil in water.

To develop improved models for the prediction of oil trajectory, SAFL researchers will use large-eddy simulation (LES) to understand the physical processes that occur in oil spills to improve future simulation technology. They will simulate the transport of oil droplets in lake waters under a variety of wind and wave conditions, with the dynamically evolving waves directly captured.

The objectives of this project are to perform process study using LES of wind-wave-current coupled motions to investigate the effects of Langmuir cells and wave breaking on the transport of oil droplets in water, and to use the physical insights obtained from the process study to seek improvement in large-scale simulation tools for oil spill modeling and prediction applications in lake water. As a result, the formation and evolution of Langmuir cells, the injection of momentum and energy from wave breaking to the water column, and the forcing by wind on the lake currents and waves can be elucidated and quantified in a wave-phase-resolved computational framework for the first time. 

Lian Shen says, "The importance of research like this is that, for the first time ever, we will be able to obtain detailed descriptions of the flow field and pollutant field, for example, with the waves explicitly resolved.  Such detailed information will be of tremendous value.  Based on the computer simulation data, we hope to obtain physical insights into the problem for the development of better prediction tools for the transport and fate of oil spills once such accidents happen."

The proposed simulation is the first of its kind.  Most of existing theories of wave-current interaction were formulated with the wave phases being averaged, and previous simulations were usually limited to flat-surface approximation with the wave effect modeled through the Stokes drift.  The study proposed here will resolve wave phases explicitly and will address wave breaking directly. If the objectives are met, this advance could improve the accuracy in the modeling oil droplet transport in water column to better understand future oil spills.

 

To learn more about this project, contact Lian Shen at shen@umn.edu