Steve de Bruyn Kops and Team Publish Breakthrough Study on Ocean Turbulence in Journal of Fluid Mechanics

October 28, 2024
Professor Steve de Bruyn Kops of the Mechanical and Industrial Engineering (MIE) Department leads an international research team which, in the last year, produced four papers related to ocean turbulence, the most recent of which was a cover article in the Journal of Fluid Mechanics, Volume 992 , on August 10, 2024. The papers are motivated by their 2023 paper in the Journal of Turbulence (study) that revealed surprising insights into how heat mixes in the ocean.

The research is based on simulations using up to 14-trillion grid points enabled by Summit, which had been the largest computer in the world, and Frontier, which is currently the most powerful machine for research www.top500.org. Both computers are at Oak Ridge National Laboratory. The research project, part of the Department of Energy (DoE) INCITE program, promises new avenues for projecting climate change, studying the dispersion of pollutants, and understanding fluid turbulence.

This cutting-edge research used Summit and Frontier to model the dynamics of ocean water and has generated one the most detailed simulations to date of how turbulence disperses heat through seawater under realistic conditions. Not only that, but the lessons learned can apply to other factors beyond the turbulence of seawater, such as pollution spreading through water or air.

de Bruyn Kops collaborated on the Journal of Fluid Mechanics paper – titled “Understanding the effect of Prandtl number on momentum and scalar mixing rates in neutral and stably stratified flows using gradient field dynamics” – with Andrew Bragg at Duke University. Other members of the research team are Miles Couchman of York University (Canada), James Riley at the University of Washington, and Colm-Cille Caulfield at the University of Cambridge (UK), as well as their students.

The breakthrough simulations enabled by Summit were the subject of the 2023 Journal of Turbulence paper – “The effect of Prandtl number on decaying stratified turbulence” – published by de Bruyn Kops with Riley and Couchman.

In a 2023 article posted by Oak Ridge National Laboratory about the basis of the trailblazing research by this team (Summit study fathoms troubled waters of ocean turbulence (ornl.gov)), it explained that “Scientists model the properties of turbulent mixing by using mathematical formulas called Navier-Stokes equations. These models depend heavily on a fluid’s Prandtl number, named for German physicist Ludwig Prandtl, which describes the ratio of how quickly momentum dissipates relative to heat in a fluid.”

As de Bruyn Kops said in that article about the models being prepared by his research team using the Navier-Stokes equations, “These are snapshots in time. You can think of each simulation as a cube of water made up of points on a digital grid. Each cube has almost 4-trillion total grid points, so we’re able to replicate conditions down to the centimeter level. We couldn’t have done this in a water tank because there’s no water tank big enough to accommodate 3D measurements at centimeter resolution.”

No researchers have ever been able to do this type of meticulous analysis before without a machine such as the Summit supercomputer, which allows scientists and engineers to observe such details across the vast range of relevant scales. The Frontier supercomputer is even bigger and has enabled 14-trillion-grid-points simulations that allow the team to study even more complex flows.

In the story about the Summit simulations, author Matt N. Lakin wrote that “Turbulent processes might sound simple, like stirring a cold splash of cream into a hot cup of coffee. They’re not simple. Turbulence occurs at scales that can vary widely — from a huge ocean wave to a tiny ripple, for example — as motion within cascading motion works its way through layer after layer of water, mixing heat and other substances along the way.”

As one of de Bruyn Kops’ collaborators, Couchman, explained in Lakin’s story, “In this case, you have colder fluid sitting on the ocean floor and warmer fluid above. One of the big uncertainties for climate modeling and other such applications arises from a lack of understanding of how heat mixes across these layers. The surface waters are being heated from above by the sun, but how does that heat get dispersed?

“It turns out that turbulent processes play a key role, which is what we’re now trying to understand. What we’re discovering can be applied not just to the mixing of heat in water but to pollutants mixing in the atmosphere and to a variety of other questions.”

The unprecedented level of detail offered by these state-of-the-art simulations has also uncovered features that might contradict long-held theories.

“Let’s go back to the example of the coffee and cream,” de Bruyn Kops explained. “A basic assumption of turbulence theory has been [that] the cold cream and the hot coffee should mix at the same rate, as you stir the coffee and it goes from black to brown. But we’re finding from these simulations that’s not the case. The heat’s mixing at a slower rate than the momentum from the turbulence. That’s a whole new avenue to explore.”

As the research team has concluded, “Through an analysis of the equations governing the fluctuating velocity and density gradients, we provide a mechanistic explanation for this surprising behavior and test the predictions using direct numerical simulations.”

Support for this research came from grants garnered by the various collaborators, including awards from the National Science Foundation CAREER Program, the DoE Office of Science’s Advanced Scientific Computing Research Program, the U.S. Office of Naval Research, United Kingdom Research and Innovation, and the European Research Institute. (October 2024)

www.umass.edu/engineering/news/de-bruyn-kops-cover-article