With the recent Deepwater Horizon oil spill in the Gulf of Mexico, the behavior of oil plumes in the natural environment has come become a central question. Experiments performed in the University of North Carolina (UNC) Fluids Laboratory have shown that buoyant plumes can be trapped underwater at regions of strong density stratification. This project will explore the mechanisms leading to underwater trapping of multi-phase plumes like those forming in the ongoing Gulf spill. Specifically, the project will focus on fluid dynamics experiments in the density-stratified flow facility in the UNC Fluids Lab to better quantify the combined roles of turbulent mixing, strong ambient stratifications, plume buoyancy, and use of surfactants (dispersants) in creating under water trapped plumes. Theoretical closure models will be developed to interpret the experiments and guide field studies. Further research will include direct numerical simulations to explore plume behavior under the effects of high temperature, high pressure, internal waves, and shear associated with this current spill. The intellectual merit of this theoretical and experimental research includes developing a benchmarked model for actual oil plume distribution, residence time, and surface versus sub-surface oil fractions. Broader impacts of the research will involve disseminating results with other groups directly working on the Gulf Oil Spill, including those collecting data on the subsurface plume distribution. Additionally, the PIs will make results available to the petroleum industry and regulatory agencies such as EPA to assist in the understanding of current and future spills. The research project will also involve the education of two undergraduate researchers and create new outreach opportunities for high school students to participate in the UNC Fluids Lab.
Research performed under this NSF RAPID grant developed experimental and theoretical methods for predicting the behavior of multiphase buoyant plumes rising in stratified waters, directly pertaining to the gulf oil spill. Two major findings resulted from this work. First, it was discovered that oil plumes which form underwater in laboratory experiments may undergo instabilities in which after a period of time, the oil may suddenly rise from the trapped layer to the surface. This has profound implications for the fate of deepwater oil spills in that oil trapped may eventually rise to the surface far from the well head. The timescales for this destabilization have been further shown to depend upon the mixing of the emulsions, and depends upon the mixing ratios between oil, water, and dispersant. These timescales may vary from minutes to weeks depending upon the ratios. Secondly, an exact mathematical formula was developed which predicts the critical height an oil plume must rise before impinging on a sharp stratification, where for plumes rising less than this critical height, the oil will escape to the surface, while for plumes rising more than this critical height, the plume will be trapped. This formula depends non-trivially upon the emulsion density, volume, speed of the injections, and the strength of the ocean stratification. The intellectual merit of this work has been in discovering this remarkably concise formula, as well as demonstrating the success of the formula, and in documenting the possibility for plume destabilization. The broader impacts of this work involved the training of undergraduate to perform laboratory fluid dynamic experiments pertinent to the project, as well as in making the general public aware of the complex processes involved with deep water oil spill.
