Project Overview

Summary:

Dr. Andrés Tejada-Martínez at the University of South Florida was awarded an RFP-VI grant at $999,185 to conduct the RFP-VI project titled, “Turbulent Vertical Mixing and the Formation of Oil Particle Aggregates: LES, Measurements and Modeling”. The project consisted of 2 other institutions (the New Jersey Institute of Technology and the University of Florida), 1 principal investigator (Tejada-Martinez), 3 co-PIs (Drs. Boufadel, Murphy, Valle-Levinson), 3 graduate students (Li, Perez, Zeidi), and 2 undergraduate students (Patel, Perez).

As oil approaches the shorelines and encounters shallow depths, it is affected by bottom processes through upward diffusion of turbulence. It is commonly assumed that the oil does not interact with sediment until the surf zone. However, field measurements have shown that during strong winds, Langmuir supercells (i.e. full-depth Langmuir cells) can form at shallow depths ranging between 10 m and 30 m, bringing sediments upward allowing them to interact with the oil on the water surface and to form oil particle aggregates (OPAs). Some of the OPAs will settle to the bed while others will not. The Langmuir cells can create Stommel retention zones serving to provide a submerged pathway for OPAs to reach the surf zone. We also hypothesize that this premature formation of OPAs causes the settling of oil in the seaward part of the surf zone, which was never considered in oil spill research, which considered that “oil mats” are located within a few hundred meters from the shoreline. The project uses recently developed models by the PIs along with a coastal circulation model coupled with wave and sediment models to provide a holistic description for the physical behavior of oil from the inner shelf through the surf zone. In particular, an OPA model developed by one of the PIs will be integrated with the coastal circulation model within a Lagrangian-Eulerian framework serving to provide estimates of OPA formation throughout these zones.

     The expected framework will be equipped with turbulence parameterizations accounting for Langmuir turbulence and associated Langmuir supercells in the surf-shelf transition zone. Highly resolved large-eddy simulations (LES) validated by and in conjunction with acoustic Doppler current profiler (ADCP) measurements at different shallow shelf regions throughout the Gulf of Mexico, will be used to develop the parameterizations of the Langmuir turbulence. Turbulence closures in coastal circulation models currently do not consider the effects of Langmuir turbulence and associated Langmuir cells. Meanwhile, the intense vertical mixing provided by the Langmuir supercells through non-local transport impacting the bottom boundary layer is expected to strongly impact sediment concentration predictions of the coastal circulation model and ultimately the prediction of OPA formation.

Laboratory studies in a Langmuir circulation facility to be built will explore Langmuir supercell dynamics inducing the suspension of sediments and entrainment of oil droplets promoting the formation of OPAs. These studies will (1) identify zones within the Langmuir supercells where mixing between oil droplets and sediments can occur and establish residence times of sediments and oil droplets within these zones, (2) characterize the mixing energy (i.e. the turbulent kinetic energy) within these zones, and (3) measure the coagulation efficiency of interactions between oil and sediment particles. The laboratory studies will serve to provide a case with spatially non-uniform OPA formation by which the OPA model used in the proposed Lagrangian-Eulerian modeling framework may be validated. Overall, these experiments will allow extension of the OPA model to the surf-shelf transition zone in inner shelves where Langmuir supercells are expected to occur.

Research Highlights

Dr. Tejada-Martinez’s research resulted in to date 1 peer-reviewed publication, 10 scientific presentations, and 5 datasets submitted to the GoMRI Information and Data Cooperative (GRIIDC), which are available to the public.  Significant outcomes of their research (all related to GoMRI Research Theme 1) are highlighted below.

Our project centers about Langmuir cells in the coastal ocean. Langmuir cells consist of parallel counter rotating vortices roughly aligned in the direction of the wind. These cells are well-known to be generated via interaction between the wind-driven shear current and the surface gravity waves.

In the coastal ocean, specifically in the inner-shelf region known as the surf-shelf transition (SST) zone, Langmuir cells can reach the bottom of the water column and promote the suspension of sediment particles during the passage of storms. The cells can simultaneously entrain oil droplets into the water column from the surface.

The overarching goal of the project has been to understand the capability of full-depth Langmuir cells in the SST zone to mix suspended sediments with entrained oil droplets leading to the formation of oil-particle aggregates or OPAs. Oil trapped in OPAs is likely to deposit at the seabed leading to what is often referred to as “submerged oil mats” submerged by sediments. Based on this understanding, we have developed a hydrodynamic model of coastal ocean flows that accounts for the formation of OPAs induced by Langmuir cells in order to predict how much oil is transported on-shore versus how much oil becomes trapped in OPAs off-shore.

Key accomplishments of the project to date are as follows:

(1)  An unstructured-grid finite volume-based hydrodynamic model capable of representing flows characterized by Langmuir cells in the SST zone has been developed by the PI Tejada-Martinez at USF and co-PI Michel Boufadel at NJIT. The governing fluid flow equations in the model are the Reynolds-averaged Navier-Stokes (RANS) equations augmented with the well-known Craik-Leibovich vortex force representing the Langmuir cell generation process.

(2)  Recently the hydrodynamic model has been extended to the nearshore via coupling with a wave model allowing for the calculation of the depth-dependent wavenumber, wave direction, amplitude and frequency required for the Stokes drift velocity present in the Craik-Leibovich vortex force.

(3)  The model has also been coupled with an OPA formation model and on-going simulations are aiming to answer the question “how much oil reaches the nearshore versus how much oil sinks to the bottom trapped in OPAs in the SST zone?”

(4)  A Ph.D. student has helped develop the hydrodynamic model above and is currently in his second year of studies. He will continue with this topic for his Ph.D. dissertation under T.A. support from Civil and Environmental Engineering at USF and support from other projects.

(5)  A Langmuir circulation tank facility has been designed and constructed by co-PI David Murphy at USF recreating the downwelling velocities characterizing Langmuir cells in the field. Meanwhile, PI Tejada-Martinez has developed a computational model of this tank showing good agreement with the laboratory measurements in terms of downwelling velocities and turbulent kinetic energy. The tank and model are currently being used to investigate accumulation of oil droplets within the dowelling limbs of the laboratory cells, i.e. the so-called Stommel retention zones and the potential for the development of OPAs within this zone. A Ph.D. student has helped develop the laboratory tank above and is currently in his second year of studies. He will continue working with the tank for his Ph.D. dissertation under T.A. support from Mechanical Engineering at USF and support from other projects.

(6)  UF Co-PI Arnoldo Valle-Levinson performed two field campaigns measuring Langmuir cells on the Gulf of Mexico's shelf, off the Yucatan peninsula, at depths between 8 and 12 m. Each campaign lasted approximately four weeks, the first campaign performed in Fall 2018 and the second campaign performed in Spring 2020. Measurements were made using acoustic Doppler current profilers with sampling rates at 2 Hz. Valle-Levinson is currently analyzing the data from these campaigns to understand the wind and wave forcing conditions that lead to the formation of Langmuir cells and in particular the conditions that lead to the cells reaching the bottom of the water column causing sediment resuspension.  

 Proposal Abstract - RFP-VI PI Andrés Tejada-Martínez