By Marlene Cimons, National Science Foundation
To the average person, the word "turbulence'" usually means a bumpy airplane ride. To a scientist, however, it represents one of the last great unsolved mysteries of classical physics.
"The bottom line is that we don't understand turbulence," says Lin Ma, associate professor in the department of aerospace and ocean engineering at Virginia Polytechnic Institute and State University (Virginia Tech). "Turbulence is everywhere, but it is one of our last unresolved scientific questions."
Turbulence typically refers to a specific condition when the flow speed is high, and the degree of turbulence is measured using a formula that factors in speed, vessel size (such as the diameter of a pipe) and the viscosity of the substance (water, for example, is more viscous than air.) Because turbulence is random and chaotic, it is difficult to study. It becomes even more challenging when it is coupled with chemical reactions, which occurs repeatedly in modern energy-generating devices like car engines and aero-propulsion engines.
Ma is trying to better understand the interactions between chemical reactions and turbulence. "Turbulence and chemical reactions happen simultaneously all the time," he says. "When a jet engine sucks air in, mixes it with fuel and burns it, you have a chemical reaction under very high speed. These two things happen at the same time. Turbulence by itself is hard enough to understand, and we are adding chemical reactions to it, which makes the problem even more challenging."
He believes that deciphering the fundamental interactions between chemical reactions and turbulence will provide insights into designing more efficient energy devices with reduced pollutant emissions and at a lower cost. This could mean better and cleaner engines and power plants, he says.
"If you make a car engine more efficient, then you can cover the same distance with less fuel," he says. "When you burn less fuel, you emit less pollutants, and you minimize your carbon footprint."
Ma is studying turbulence and chemical reactions under a $400,000 National Science Foundation Faculty Early Career Development (CAREER) award over five years, which began in 2009. These NSF grants support the research of junior faculty who exemplify the role of teacher-scholars through research and education.
Ma, who joined Virginia Tech faculty last year after five years at Clemson University, focuses on the study of propulsion systems, with a special interest in the development and application of advanced optical diagnostics. In addition to his work in the area of fundamental turbulence-chemistry interactions, his research also includes aerospace propulsion devices, internal combustion engines, fuel spray physics and chemistry, and nano-scale energetic materials.
"Both turbulence and chemical reactions are profound scientific problems," Ma says. "A large part of our energy problem involves both turbulence and chemical reactions. When we burn fuel, the energy conversion process is a series of chemical reactions that convert fuel into the bad stuff, such as soot and carbon dioxide, and release heat in the fuel. In practice, such chemical reactions occur almost always under turbulent conditions. And that is why it is important to understand the interactions between turbulence and chemical reactions."
Ma uses laser diagnostics to study such interactions, with the goal of understanding the process at a fundamental level.
"Right now, we don't fully understand how chemical reactions couple with turbulence," he says. "Once we understand it, then we can optimize it. The philosophy of what we are doing is this: if we understand things at a fundamental level, then we can design new energy devices that will take advantage of this new knowledge without trial-and-error, which is a notoriously expensive and time-consuming process in the engine industry."
He sends laser beams--photons--into flames (a flame is a manifestation of the interaction between turbulence and chemical reactions) to see how the photons interact with the hot molecules.
"We monitor the photons that are transmitted and/or scattered out of the flames," he says. "Based on those, we can figure out what the flame temperature is, what chemical species are present and the velocity. All of these will help us paint a picture of what actually is happening in the flame and what it is made of."
The scientists plan to use such information to validate models that simulate chemical reactions that occur under turbulent conditions, he says. "Besides validating existing models, the data can also help people better visualize the physics and build more accurate models," he says. "Once we build the correct model, we can design and optimize our next generation energy devices on computers, speeding up the current trial-and-error process tremendously."