Wednesday, February 1, 2017

Student Presenter Award Winners

Student Presenter Award Winners
Supported by the Gulf Research Program of the National Academies
and the Gulf of Mexico University Research Collaborative
2017 Gulf of Mexico Oil Spill and Ecosystem Science Conference

Congrats DROPPS students! DROPPS had a number of students who received the “Student Presenter Award” for their upcoming presentations that will take place at the 2017 Gulf of Mexico Oil Spill and Ecosystem Science Conference. Below summaries of their presentations. Be sure to check out them out at the conference!

Shigan Chu
Johns Hopkins University
Department of Mechanical Engineering

Title:  Effects of bubble and drop dissolution on the plume dynamics in a stratified ambient ocean
The present study uses a horizontally integrated model to study the dynamics of oceanic plumes rising under the action of bubbles and drops injected at the ocean floor in a stratified ambient. Since the bubbles and drops are partly soluble, the plume density changes as they rise due to the addition of dissolved material. The project focuses on various factors affecting the plume dynamics and, in particular, the effect of dissolving hydrocarbons and their mixing with the plume water. It is found that this mixing has a very significant effect on the plume rise and the so-called peel height, namely the maximum height reached by the plume. A parametric study of the sensitivity of the plume to the size of bubbles and drops, to the rate at which they are injected at the base of the plume and to the ambient stratification is also conducted. It is found that the plume is very sensitive to many of these parameters, particularly when the ambient is weakly stratified. In practice, therefore, the plume evolution will depend on the specifics of the situation under which it is generated. For the same reason, it is very difficult to make quantitative predictions of general validity except in extreme cases.

Chi Hung “Charles” Tang
University of Texas Marine Science Institute

Title: Crude oil and dispersant impair the grazing impact of heterotrophic dinoflagellates on phytoplankton.
Microzooplankton grazing is very important when considering the loss of primary production in oceanic surface water. The influence of oil pollution on the grazing impact of heterotrophic dinoflagellates on phytoplankton was studied in the laboratory. Results showed that the grazing rate of the dinoflagellate Oxyhhris marina and growth rate of autotrophic dinoflagellate Isochrysis galbana were most severely impaired when exposed to a combination of crude oil and dispersant. Meanwhile, dispersant-alone treatment did not have any obvious negative effect on both grazers and prey.

Feng Gao
New Jersey Institute of Technology
Department of Civil and Environmental Engineering

Title: Hydrodynamics of Subsurface Oil Release without and with Dispersant:  An Experimental and Numerical Study
 Dispersant has been widely used to mitigate the impact of the Deepwater Horizon oil spill. Dispersants reduce the interfacial tension, allowing larger droplets to breakup into smaller droplets and disperse in water. Not much is known about how dispersants affects submerged oil jet. We conducted two underwater oil releases: one without dispersant and one with oil premixed with dispersant. A high resolution camera and Vectrino Profiler were used to measure the hydrodynamic properties of the jet.  In parallel, we numerically modeled oil jet (with/without dispersant) by Computational Fluid Dynamics (CFD). We find out that the temperature of pre-heated oil has negligible influence on the oil jet hydrodynamics and the spread angle of oil jet premixed with dispersant is closer to that of miscible jet, compared to pure oil jet. The turbulence dissipation rate is increased as dispersant facilitates the generation of smaller droplets. Other jet characteristics (e.g., jet profile, velocity, turbulent kinetic energy, etc.) are also compared and discussed.
By combining experimental measurement and CFD, the results show a robust method to evaluate the effect of dispersant and the hydrodynamics of underwater oil jet, which would provide valuable information for decision makers and researchers.
   (a)    (b)
The instantaneous jet profile of (a) pure crude oil (b) crude oil premixed with dispersant

Xinzhi Xue
Johns Hopkins University
Department of Mechanical Engineering

Title: Refractive Index-matched Turbulent Immiscible Buoyant Oil Jet Breakup in Water
This study is about near field oil jet/plume fragmentation. It is traditionally difficult to visualize and quantify this process due to the opaque plume and dense oil droplets, so an index matched experiment was designed to overcome the challenge. Silicone oil and sugar water solution pair is used to surrogate crude oil and seawater. During the experiment, they were able to maintain the same density, viscosity and interfacial tension ratio while having the two phases of fluid refractive index matched. The oil is fluorescently dyed and both phases are seeded with PIV particles. This enables the use of a plane laser sheet to dissect the near field jet/plume, having simultaneous results of oil water interface distribution and the entire flow field. Results demonstrated the jet entrain ambient water, deformed into long stretching ligaments and then breakup to droplets. The droplets often contain water or smaller oil droplets inside forming nesting, "Russian Doll" like phenomenon. While this alone is fascinating, this phenomenon actually indicates that the inertia and effective buoyancy would be quite different than those pure oil droplets of the same size; therefore, affected their consequent far field transport. PIV results show that the resulting droplets size correlates with initial shear stress and droplets size based buoyancy affects the plume spreading rate.

Cheng Li
Johns Hopkins University
Department of Mechanical Engineering

Title: Measurements of Droplet Size Distribution Generated by Breaking Waves Acting on an Oil Slick

The droplet size distributions generated by the impact of breaking waves on oil slicks are of fundamental importance to predictions of the fate of oil spills. An extensive series of experiments examine the physical mechanisms involved, and the time evolution of size distribution over several hours, starting from the initial breakup phase. Investigated parameters include wave energy, oil viscosity and density, as well as oil-seawater interfacial tension. The experiments are performed in a transparent wave tank, with waves ranging from spilling to plunging breakers. The oil properties are varied by using crude, fish, and motor oils, with viscosities in the 9.4-306.5 cSt range. Interfacial tensions varying from 0.28 to 19mN/m are achieved by premixing the oil with Corexit 9500A at dispersant to oil ratios of 0.0, 1:25, 1:100, and 1:500. The size distributions are measured in-situ using high speed digital holography at resolutions of 11.1 and 1.1μm/pixel. Collected samples are also examined microscopically. Results show that decreasing the interfacial tension increases the generation of 2-10μm droplets by orders of magnitude and steepens the slope of the number size distribution. For DOR 1:25, the volumetric size distribution already peaks around 10μm. Being smaller than turbulence scales, these droplets are generated by micro-threading. A fraction of them remains suspended well over a day, and submicron droplets persist for longer periods.  Droplets larger than 100μm are generated by turbulent shearing; hence, their initial concentration increases with wave energy. Within this range, the size distribution slope steepens rather abruptly at a certain size, which depends on oil properties and time. The transition occurs for crude at a diameter smaller than that of more viscous oils. Due to buoyancy, the concentration of large droplets decreases with time, while smaller droplets remain suspended. Trends are consistent with those expected based on the droplet rise velocity.

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