Secrets from the Deep and the Shallow: Using Experimental Fluid Mechanics to Study Oil Spills and Seagrass Meadows

Tracy Mandel
Assistant Professor

Ocean Engineering and Mechanical Engineering

Friday, Sep. 11, 2020, 3:10pm
Chase 130

Many processes in our environment are governed by physics: the seasonal overturning of a lake, the breaking of waves over coral reefs, and even the waving of wheat in a farmer’s field. In this presentation I will focus on two problems in environmental fluid mechanics, and how we study them in an idealized laboratory environment. First, I will discuss the first step in remote characterization of seagrass beds by studying the overlying water surface. Flow through a seagrass bed can generate large overturning vortex structures, which cause small perturbations in the water surface slope. Using laboratory experiments, we are able to generate a parameterized model to reconstruct within-canopy velocity profiles solely from water surface measurements, suggesting that the subsurface hydrodynamics and geometry can be predicted by measuring the water surface behavior alone.

Second, I will address the role of the ocean’s stratification in the subsurface trapping of oil well leaks. As a droplet of oil rises in the ocean, it encounters fluid of varying density, and can become trapped as an intrusion layer in a region of strong density gradients. We find that for drop Froude numbers less than 1, fluid entrainment and drop retention are significant, as the buoyancy timescale (1/N) is less than the timescale of drop motion (U/d). For larger Froude numbers, retention is minimal. We expect this scaling to have applications in other small-scale ocean dynamics, such as natural oil and gas seeps in coastal waters, and in the behavior of particulates and biota in stratified benthic boundary layers.

Through these two case studies, I will demonstrate the laboratory tools we use, and how results obtained in a tank in the lab can be applied to their associated real-world setting. I hope the audience will gain an appreciation of the utility of idealized fluid mechanics experiments in isolating, studying, and furthering our understanding of the physics of our environment. I will close with a brief discussion of the new directions and projects my students and I are working on here at UNH.


Dr. Tracy Mandel joined UNH in January 2020 as an Assistant Professor in Ocean Engineering and Mechanical Engineering. She received her M.S. and Ph.D. in Environmental Fluid Mechanics and Hydrology from Stanford University in 2013 and 2018, and a B.S. in Environmental Engineering from Cornell University in 2012. Prior to coming to UNH, she was a postdoctoral scholar at University of California, Merced in Physics and Applied Math. Her broader research interests include coastal processes, canopy flow, experimental methods in fluid dynamics, and the water-surface signatures of aquatic ecosystems and other ocean processes.