Oil plumes and dispersion in Langmuir, upper-ocean turbulence: Large-eddy simulations and K-profile parameterization

Abstract

Once oil plumes such as those originating from underwater blowouts reach the ocean mixed layer (OML), their near-surface dispersion is influenced heavily by wind and wave-generated Langmuir turbulence. In this study, the complex oil spill dispersion process is modeled using large-eddy simulation (LES). The mean plume dispersion is characterized by performing statistical analysis of the resulting fields from the LES data. Although the instantaneous oil concentration exhibits high intermittency with complex spatial patterns such as Langmuir-induced striations, it is found that the time-averaged oil distribution can still be described quite well by smooth Gaussian-type plumes. LES results show that the competition between droplet rise velocity and vertical turbulent diffusion due to Langmuir turbulence is crucial in determining both the dilution rate and overall direction of transport of oil plumes in the OML. The smoothness of the mean plume makes it feasible to aim at modeling the oil dispersion using Reynolds-averaged type formulations, such as the K-profile parameterization (KPP) with sufficient vertical resolution to capture vertical profiles in the OML. Using LES data, we evaluate the eddy viscosity and eddy diffusivity following the KPP framework. We assess the performance of previous KPP models for pure shear turbulence and Langmuir turbulence by comparing them with the LES data. Based on the assessment a modified KPP model is proposed, which shows 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.

Large-eddy simulation and parameterization of buoyant plume dynamics in stratified flow

“…Using various types of SGS closures, these LES models were able to capture instantaneous fine-scale flow structures which were missing in RANS models. Very recently, Yang, Chamecki & Meneveau (2014a) and Yang et al (2015) developed a hybrid LES model for simulating hydrocarbon plume dispersion in ocean turbulence. Using their LES model, Yang et al (2014aYang et al ( , 2015 studied the complex dispersion phenomena of oil plumes released into the ocean mixed layer, and investigated the effects of various environmental mixing mechanisms such as shear turbulence, waves and Langmuir circulations.…”

“…In this study, the hybrid LES model developed by Yang et al (2014aYang et al ( , 2015) is adopted and modified to simulate bubble-driven buoyant plumes in vertically stratified ambient fluid. To validate the model and investigate the essential plume characteristics in a controlled environment, the key simulation parameters are chosen to be similar to those of the laboratory measurements of Seol et al (2009).…”

How Do Oil, Gas, and Water Interact Near a Subsea Blowout?

“…Using LES, Yang et al (2014Yang et al ( , 2015 and Chen et al (2016b) studied oil dispersion in ocean Langmuir turbulence, a flow system that combines the aforementioned fine-scale flow phenomena in the OML. Their LES model solved the filtered Craik-Leibovich equations, which comprise a wave-averaged version of the Navier-Stokes equations but differ from the regular Navier-Stokes equations by inclusion of an extra vortex force term that accounts for the cumulative effects of surface waves on the shear turbulence responsible for generating Langmuir circulations.…”

“…Using their LES model, Yang et al (2014Yang et al ( , 2015 studied the complex dispersion of oil plumes in the OML, capturing simultaneously the effects of Langmuir circulation, turbulence, Stokes drift, and oil droplet buoyancy (Figure 4b-d). Their results reveal that instantaneous oil plume patterns as well as the averaged plume transport direction are affected by two competing mechanisms, downward mixing induced by Langmuir turbulence, characterized by the velocity U S , and the droplet rise, given by w r , summarized by the ratio Db = U S /w r .…”

“…Based on an extensive set of LES runs, Yang et al (2015) studied the behavior of eddy viscosity and eddy diffusivity for oil dispersion, and proposed a modified K-profile parameterization (KPP) that incorporates the effects of Langmuir circulations and oil droplet buoyancy into classical KPP. Subsequently, Chen et al (2016a) also applied the LES model to study the effect of arbitrarily oriented ocean swell waves on the transport and dilution of oil in the OML.…”