A paper written by Colorado State University researchers has been unanimously selected by the American Physical Society as the top Physics of Fluid article by young investigators in 2013.
Karan Venayagamoorthy, a CSU professor of civil and environmental engineering, and two of his graduate students, Benjamin Mater and Simon Schaad, recently received the Francois Frenkiel Award the society’s Division of Fluid Dynamics for their paper: “Relevance of Thorpe length scale in stably stratified turbulence.”
The paper was selected “For new and significant insights into scaling laws for stratified turbulence with potentially broad impact in the area of environmental flows,” according to the citation for the award. The winning paper is selected by an independent committee consisting of leading experts in fluid mechanics.
The award was presented to Venayagamoorthy and his students at a recent Division of Fluid Dynamics meeting in San Francisco. The annual meeting brings together approximately 3,000 fluid mechanics researchers from around the world.
“It’s a big honor for us to receive this national award from the leading society in fluid mechanics,” Venayagamoorthy said. “It’s only given to investigators under the age of 40.”
The Thorpe length scale
The paper is based on a study Venayagamoorthy and his students conducted to evaluate situations and establish parameters for using the Thorpe length scale, a technique developed in 1977 to measure the amount of turbulence and mixing in density stratified water bodies such as estuaries, lakes and the ocean.
The Thorpe scale provides a simple, objective measure of the size of overturning length scales in such water bodies. Over the years, it has been widely used in a broad range of flow conditions to infer dissipation rates of turbulent kinetic energy – a key indicator of the amount of mixing going in the ocean and a factor required for improved prediction of global climate and weather.
“There have been some uses and abuses of the Thorpe scale,” he said. “We wanted to know when it works and when it doesn’t so it can be used accurately and also universally.”
Their study shows new scaling laws that provide unique relationships for the range of applicability of the Thorpe scale for varying oceanic flow conditions.
For example, they discovered (based on further studies) that the Thorpe scale can be used to infer mixing in the ocean where the turbulence is generated locally due to shear instabilities such as in the oceanic thermocline in the North Atlantic Ocean. There are other situations such as in the South China Sea where the turbulence is generated by convective events caused by breaking nonlinear internal waves and hydraulic jumps where it does not simply work contrary to current praxis.
When used correctly, the Thorpe length scale is an important tool for measuring and calculating the turbulent kinetic energy swirling through the ocean, Venayagamoorthy said.
“There is a big quest to understand and quantify mixing in the oceans on a global scale due to the effect on climate,” he said. “The measurements can be done but it is important to ensure they are accurate. The Thorpe scale is simple and relatively inexpensive to measure and that’s why we wanted to evaluate its relevance.”
Venayagamoorthy is an associate professor of Civil and Environmental Engineering and Borland Professor at Colorado State. He received his bachelor’s and master’s degrees in civil engineering from the University of Kwazulu-Natal and his doctorate in Civil and Environmental Engineering from Stanford University.
Venayagamoorthy has received numerous honors, including a National Science Foundation CAREER Award, the Office of Naval Research Young Investigator Award and CSU’s Best Teacher Award.