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.
