Whales make waves, of course, but so do the krill they eat. In fact, swarms of small swimmers could possibly generate enough kinetic energy to stir up nutrients in the ocean and create large-scale flow structures.
Yet for all their potential to punch above their weight and even alter currents, these tiny swimmers and the dynamics of their movement have been largely overlooked in ocean and climate modeling. Lei Fang, assistant professor in the Department of Civil and Environmental Engineering at the University of Pittsburgh Swanson School of Engineering, is helping to change that.
Fang has received a $325,089 National Science Foundation (NSF) grant, his second in just over a year, to investigate the mechanics of swarms of small swimmers and their ability to generate kinetic energy. The grant, “Aggregation Scale Eddy Generation During Collective Active Particle Migration: Spectral Energy Flux, Mechanisms, and Impact on Mixing Efficiency,” seeks to improve predictions of ocean behavior and increase understanding of marine and atmospheric dynamics.
“Animals in the ocean contain a lot of energy that can mix up nutrients and move colder water to the surface,” said Fang. “Based on observational data, if only one percent of their chemical energy is converted to kinetic energy, it creates a force comparable to wind or ocean currents.”
It’s easy to see how larger fish generate energy. However, the organisms that many of these fish feed on, like some plankton or krill, which can be between one millimeter and one centimeter in size, are too small to seemingly make a difference.
“While a few small swimmers alone can’t generate enough energy, a ball of them moving together could produce a group-scale eddy of up to a few meters. They could potentially create as much mixing as larger animals.”
For organisms that have a diurnal cycle, moving from the bottom of the ocean to the surface each day, a swarm that produces group-scale eddies could affect ocean temperatures by bringing colder water and nutrients closer to the surface.
“If some animals can mix up the ocean while others can’t, that changes the global climate model,” said Fang. “And the extent to which animals large and small can mix up nutrients and alter currents affects these models. Understanding how small swimmers generate energy and how that energy fluxes, or flows, allows us to improve climate modeling.”
Fang, who recently rewired the energy pathways of turbulence, will analyze the energy flux that swarms of small swimmers can produce. At the three-dimensional scale like in the ocean or the atmosphere, energy generically fluxes from large eddies to smaller ones until the energy dissipates. However, Fang has validated a novel framework that challenges long-held predictions about flow structures.
He will continue to research flow structures while investigating the mechanics and the mixing efficiency of small swimmers at the 3D scale. He will conduct tests of these organisms in a vertical migration tank using multi-camera stereo imaging to image a location and reconstruct the 3D velocity fields around them. He will then use a filter space technique to identify the direction that energy fluxes.
While helping improve long-term ocean forecasting models and shedding new light on systems with interactions involving many bodies and fluid structures, the research could potentially have far-reaching effects in areas as varied as aerodynamics and wastewater treatment.
And it might just give little swimmers their due for doing their part in stirring up nutrients and altering ocean currents.
