Summary:
Overview
Dr. Charles Meneveau at Johns Hopkins University and Dr. Marcelo Chamecki at Penn State University was awarded an RFP-II grant at $708,851 to conduct the RFP-II project entitled “Large Eddy Simulation (LES) of Turbulent Dispersion of Oil in the Ocean Surface Layers: Development, Testing and Applications of Subgrid-Scale Parameterizations”. The project consisted mostly of 2 institutions and 4 research team members. At later stages of the project, a third institution (University of Houston) was included via a subaward.
Through this work, Dr. Meneveau and his team aimed to: 1) development and test improved subgrid-scale models to represent small-scale physics associated with interactions between turbulence and oil droplet and gas bubbles within the context of the Eulerian description of droplet and bubble concentration fields, and (ii) use a suite of properly tested and validated LES to examine dispersion of plumes of different characteristics (size, droplet size distribution, wind/wave forcing, etc.), and develop useful parameters and correlations based on the LES results.
Research Highlights
Dr. Meneveau and Dr. Chamecki’s research, which included 10 outreach products and activities, resulted in 7 peer-reviewed publications and 5 datasets being submitting to the GoMRI Information and Data Cooperative (GRIIDC), which are/will be available to the public. Dr. Meneveau and Dr. Chamecki engaged 1 doctoral student and 1 postdoctoral fellow over the award period. At late stages of the project the postdoc moved to fill a faculty position at University of Houston and continued as part of the project team. Significant outcomes of their research according to GoMRI Research Theme are highlighted below.
Theme 1: Development and use of improved Large Eddy Simulation models to study physical distribution, dispersion, and dilution of petroleum (oil and gas) under the action of physical oceanographic processes
- The team developed a Large Eddy Simulation (LES) framework to study oil plume evolution in a turbulent ocean, at scales ranging from meters to kilometers. Particular attention was placed on the flow and transport phenomena once the oil reaches the upper ocean’s mixed layer and ocean surface. At early stages of the project, it was found that an Eulerian description of the oil as a buoyant concentration field was sufficiently accurate at the desired scales. There was a need to include turbulent subgrid-scale fluxes of oil concentration fields for which a dynamic Smagorinsky model with prescribed Schmidt number was used, while there was no need for additional subgrid-scale terms.
- The simulations confirmed observations that oil spills from deep-water blowouts rise through and interact with the ocean mixed layer and Langmuir turbulence, leading to considerable diversity of oil slick dilution patterns observed on the ocean surface. Certain conditions can drive oil droplet plumes to organize into distinct bands called windrows, inhibiting oil dilution. Observations of blurred or even diffused plumes are also common, but conditions under which these various dilution regimes emerge were not well understood prior to our study. The team used the LES tool to explain and quantify the dilution patterns and their dependence on relevant physical parameters. Two mechanisms, the downwelling and dilution due to Langmuir cells and the inhibition of dilution due to buoyancy of oil droplets, were found to compete. This competition can be characterized by the ratio of Stokes drift to droplet rise velocity, the drift-to-buoyancy parameter.
- A follow-on study examined the effects of ocean swell conditions on the modulation of turbulence and its impact on oil transport. Swell conditions can occur when surface waves originate from distant regions and travel in a different direction than the prevailing direction of the local wind forcing at the surface. Such conditions were emulated in LES. Results showed that when the misalignment between the wind and the swell propagation direction is small, Langmuir cells develop and significantly enhance the vertical dilution of the oil plume. Conversely, when the misalignment is large, vertical dilution is suppressed when compared to the no-swell case. Due to the strong directional shear of the mean flow within the ocean mixed layer, plume depth significantly impacts mean transport direction. The size of oil droplets in the plume also plays an important role in vertical dilution and mean transport direction.
- One of the main objectives of the project was to develop parameterizations for larger-scale models based on the results of LES. Statistical analysis of the fields resulting from the LES was performed for different wind and wave (Stokes drift) conditions and oil droplet sizes. Although the instantaneous oil concentration exhibited high intermittency with complex spatial patterns such as Langmuir- induced striations, it was found that the time-averaged oil distribution could still be described quite well by smooth Gaussian-type plumes. The eddy viscosity and eddy diffusivity following the so-called “KPP” framework were evaluated from the LES. Based on the assessment a modified KPP model was proposed, which showed improved overall agreement with the LES results for both the eddy viscosity and the eddy diffusivity of the oil dispersion under a variety of flow conditions and droplet sizes.
- In a similar vein, the project examined models for vertical plumes, corresponding to the near-field stage of underwater blowouts. Thus, bubble-driven buoyant plumes in a stably stratified quiescent fluid were studied using LES. As a bubble plume entrains stratified ambient water, its net buoyancy decreases due to the increasing density difference between the entrained and ambient fluids. A large fraction of the entrained fluid eventually detrains and falls along an annular outer plume from a height of maximum rise (peel height) to a neutral buoyancy level (trap height), during which less buoyant scalars (e.g. small droplets) are trapped and dispersed horizontally, forming quasi-horizontal intrusion layers. The LES results were averaged to generate distributions of mean velocity and turbulent fluxes. These distributions provided data for assessing the performance of previously developed closures used in one-dimensional integral plume models, and to develop improved ones. This work was done in collaboration with Prof. S.A. Socolofsky at Texas A&M University.
- Initial development of a new method called ENDLESS was carried out as part of this project. The method aims to significantly extend the applicability of LES to simulations of oil dispersion in the ocean surface layer. Using a small-domain LES (e.g. 500m x 500m x 300m) with periodic horizontal boundary conditions for velocity and replicating it many times in horizontal directions one could cover a domain of, say 4km x 4km. In this extended domain, a scalar plume that can thus reach much larger horizontal extents than in the original, limited LES domain. Furthermore, an adaptive-domain is used to track the motion of the plume, and thus ENDLESS leads to huge computational savings over traditional LES approach for very large plumes.
