Study Compares 2D and 3D Model Simulations of Oil Plume Behavior
– JUNE 4, 2019
Scientists assessed an economical 2D model simulation of deep-ocean oil plume dynamics against 3D model results using conditions similar to Deepwater Horizon to better understand point-source buoyant convection, which affects the oil’s spreading rate and environmental impact. The 2D model worked best for thermal plumes without bubbles. Although the 2D model successfully captured the transition from a constant volume flux jet to a thermal plume near the wellhead, it did not capture turbulence at the wellhead. Comparisons of both model simulations uncovered an unexpected influence of the Earth’s rotation on near-field swirl speeds, an effect not typically included in classical oil/gas plume models. Given this new insight, the authors suggested exploring other physics normally recognized as leading-order important, such as cross flow and time variable background fields, in the presence of rotation.
The researchers published their findings in Ocean Modelling: Rotating 2d point source plume models with application to Deepwater Horizon.
Historically, environmental modeling of a multiphase oil plume has used a Lagrangian framework, where particles representing oil droplets and gas bubbles with evolving physio-chemical properties are advected by an imposed background flow field. Earlier studies compared Lagrangian models applied to the Deepwater Horizon event (Marine Pollution Bulletin Socolofsky et al., 2015) and applied a turbulence-resolving model to plumes in a thermally-stratified environment (Journal of Geophysical Research: Oceans Fabregat et al., 2016, 2017) in rotating and nonrotating settings. These studies showed that rotating multiphase plumes differed dramatically from non-rotating cases and that buoyant plumes do not rise in a straight path from the source even when there is relatively slow planetary rotation. Rather, buoyant plumes twist sideways and precess about the vertical axis, significantly changing the turbulent mixing properties and the vertical distribution of plume effluent.
This study’s team sought to complement Lagrangian/integral multiphase plume models by using computational techniques not available 15 years ago. They developed a 3D Eulerian-based multiphase near-field plume model that includes turbulence representation and, though more computationally demanding, offered advantages to Lagrangian frameworks and fits relatively seamlessly within existing ocean general circulation models. They compared results from a more economical 2D model, which they developed, with their 3D model using differing rotations and fluxes (a thermal buoyancy flux and a hybrid gas/thermal buoyancy flux).
The 2D model compared reasonably well to the 3D model for non-rotating thermal plumes; however, the presence of a slipping gas phase degraded the model comparisons. The biggest distinctions in the rotating and nonrotating plume structures occurred in the velocity fields, where swirl velocities were absent from the non-rotating plumes and the rotating plumes were much broader in near-field diameter. The authors noted that it has proven surprisingly difficult to develop a 2D model that compares well to 3D model results in non-rotating and rotating settings, and that they were unable to produce an acceptable 2D model of a hybrid plume. Despite these quantitative shortcomings, the effects of rotation were sufficiently strong to appear in both models. Subsequent plume experiments on a rotating platform at the University of Cambridge’s Batchelor Lab confirmed the basic physics (Frank, D., Landel, J. R., Dalziel, S. B., and Linden, P. F. 2017, Anticyclonic precession of a plume in a rotating environment, Geophys. Res. Lett., 44, 9400– 9407, doi:10.1002/2017GL074191).
Regarding the influence of the Earth’s rotation on near-field swirl speeds, study author William Dewar explained, “Not surprisingly, flows at large scales, like the Gulf stream, are strongly influenced by the Earth’s rotation. What we found that was surprising is the plume from the Deepwater Horizon wellhead was also strongly affected by the Earth’s rotation, even though the plume was a very small feature.”
Possible explanations for this phenomenon are that 1) oil is very light relative to water, so the oil plume created a very strong disturbance in the water and 2) the plume lasted for 87 days, which gave rotation an opportunity to assert its presence. “This was a very strong effect, influencing all aspects of the plume, even down to the nature of the plume turbulence,” said Dewar.
Dewar noted the importance of accounting for the Earth’s rotation when tracing subsurface oil, where much of the Deepwater Horizon oil remained, as compounds in the oil can work their way through subsurface ecosystems over long time periods. “As a result of this work, we are in a much stronger position to forecast the fate of future deep oil spills in the Gulf of Mexico or elsewhere,” said Dewar.
The authors noted that they plan to pursue simulations that help distinguish crossflow and stratification factors in rotating buoyant point source releases, though this can be computationally demanding since the 3D model is needed to simulate multiphase behaviors over a larger area.
Data are publicly available through the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at DOI:10.7266/N72B8WF3.
By Nilde Maggie Dannreuther. Contact firstname.lastname@example.org with questions or comments.
This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment II (CARTHE II).
The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit https://gulfresearchinitiative.org/.
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