While most people think of turbulence as the source of unsettling bouts of chaotic airflow during a flight on an airplane, physicists have a much deeper concept of its significance in the world around us. In fact, Nobel Laureate and theoretical physicist Richard Feynman once said, “Turbulence is the most important unsolved problem of classical physics.”
Probing the puzzling nature of turbulence is essential because of its impact on the flow physics of liquids and gases, and, by extension, the influence of those fundamental states of matter flowing outside and inside things. For example, turbulence must be considered in designing vehicles and in understanding how particles — such as pollution and volcanic ashes — disperse in the atmosphere. Turbulence also affects the flow inside a jet engine, a combustor or a nuclear reactor.
To convey the essence of the ubiquitous influence of turbulence, researcher Antonino Ferrante of the William E. Boeing Department of Aeronautics and Astronautics of the University of Washington, Seattle, quotes Greek philosopher Heraclitus: “’Everything flows and nothing abides; everything gives way and nothing stays fixed.”
“As I sit and look around me, I notice several examples of turbulent flows: the air moving in and out of my lungs, the air moving around me in my office, the smoke flowing out of a chimney, the wind moving between the leaves and branches of trees, massive clouds moving in the atmosphere, the air surrounding a flying bird and an airplane,” Ferrante says. “No matter how big or small, or how close or far you look, you will see fluids in motion. As the ratio of the inertial forces with the viscous forces in the flow (that is, what’s known as the Reynolds number) increases above a certain threshold, the flow transitions from laminar to turbulent, or from smooth to random. Most flows in nature and engineering applications are turbulent. Thus, understanding turbulent flows is very important for human progress and for a sustainable planet Earth.”
Ferrante is heading the Computational Fluid Mechanics group, a research team engaged in the study of turbulence simulations and modeling, one of the most challenging areas of fluid dynamics — the natural science of fluids, both gases and liquids, in motion. Turbulence simulations are particularly difficult because of the wide-range scales of motion involved. Resolving all of those scales requires fine computational grids with billions of points that include the tiniest of scales, where viscous dissipation — the heating of the fluid due to resistance — occurs.
Ferrante explains that numerical simulations of turbulent flows require high-performance computing (HPC), especially if the aim of the research is to simulate high Reynolds number flows or simulate turbulence coupled with other phenomena — including chemically reactive and multiphase turbulent flows — while expecting results in a reasonable time.
“As the computational resources become larger and larger, we can tackle more and more complex turbulent flow problems that will allow, for example, reducing fuel consumption and CO2 emissions,” Ferrante says. “Without HPC, turbulence simulation would just not be feasible, and our understanding of turbulent flows would not progress much.”
That’s why Ferrante and his research team decided to take advantage of the Extended Collaborative Support Services (ECSS) offered by XSEDE, the Extreme Science and Engineering Discovery Environment. XSEDE’s ECSS program provided an entryway for Ferrante’s team to access the HPC resources and expertise offered by the University of Tennessee’s National Institute for Computational Sciences (NICS) and by the National Center for Supercomputing Applications at the University of Illinois, Urbana–Champaign.
XSEDE is the most advanced, powerful and robust collection of integrated advanced digital resources and services in the world. It is a single virtual system that scientists can use to interactively share computing resources, data and expertise.
While in consultation with NICS staff, Ferrante’s team ran a direct numerical simulation (DNS) on Kraken, a supercomputer managed by NICS for the National Science Foundation. Ferrante explains that DNS modeling is particularly HPC enabled because it requires that several flow variables, such as velocity and pressure, be advanced in time in billions of grid points. The problem the researchers were investigating required a minimum resolution of 10,243 data points.
“Using Kraken, we were able to solve the problem involving billions of unknowns accurately,” Ferrante says. “For the first time, we are simulating the effects of droplets on turbulence by performing fully resolved DNS. In practice, we are running a real-life experiment on a supercomputer rather than a wind-tunnel.”
The Results and Future Research
Ferrante says the outcome of his team’s research was an accurate and robust numerical methodology that couples the droplets and the turbulent flow to better understand droplet-laden turbulent flows, and he says research in turbulence simulations will continue on larger scales and solve more complex flows.
Ferrante is recipient of the NSF Career Award. More information about his research can be found here, and the fluid dynamics videos educed from his team’s simulations are available at this Web address.
About NICS: The National Institute for Computational Sciences (NICS) operates the University of Tennessee supercomputing center, funded in part by the National Science Foundation. NICS is a major partner in NSF’s Extreme Science and Engineering Discovery Environment, known as XSEDE. The Remote Data Analysis and Visualization Center (RDAV) is a part of NICS.