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Designing for the Future of Safe Flight, Energy Generation and National Security
May 17, 2021 | Elizabeth Witherspoon
The sky's the limit for Duke engineers designing the safest, most cost-efficient aircraft and turbomachinery
Lives are at stake in the work of designing the engines and airframes of passenger airplanes, high-performance military jets and helicopters.
Engineers must perform exceptionally complicated mathematical modeling and develop computer simulations that factor in many dynamic forces interacting with one another–long before a test pilot enters the cockpit.
Thanks to the internationally recognized team of Aerodynamics & Aeroelasticity faculty in Duke University’s Thomas Lord Department of Mechanical Engineering & Materials Science (MEMS), designs are safer and more cost-efficient than ever before.
These veterans of industry, NASA, military research centers and elite academic institutions have deep expertise in a range of aerodynamic, aeroelasticity and aerospace issues. Earning the field’s top honors for their contributions over decades, the team continues to push into new domains--whether designing aircraft to fly safely at hypersonic speeds or harvesting energy from wind turbines for a more sustainable future here on the Earth.
Duke MEMS researchers lead in both computational and simulation development to improve testing, as well as experimental work with physical models of turbines, wings and frames using the department’s own low-speed wind tunnel and the high-speed wind tunnels of NASA, the U.S. Air Force Research Laboratory and Sandia National Laboratory. They work with elite institutions, industry partners and collaborators around the world to deliver designs for the future of flight, energy generation, commerce and national security.
Peers internationally have recognized the impact of this work by electing two faculty to membership in the National Academy of Engineering and bestowing the highest honors from the American Society of Mechanical Engineers and the American Institute of Aeronautics and Astronautics, among many others. Likewise, their students continue to soar to new heights–winning design and research awards and landing key positions at leading aerospace companies, military laboratories, universities and NASA.
Explore the Duke MEMS aero group’s work below.
Controlling Flutter for Safer Aircraft and Cleaner Energy
William Holland Hall Distinguished Professor of Mechanical Engineering
Recruited to Duke University in 1983 to serve as dean of engineering, Earl Dowell never stops reaching higher for new knowledge. Dowell is an established leader in computational modeling and experimentation for aeroelastic systems in which fluids (air or water) and structures interact, particularly in flutter. Flutter occurs when the dynamic forces of airflow interact with the elastic body of an airplane, such as the tail or wings, leading to unstable oscillations that can escalate to the point of catastrophic aircraft failure. Dowell’s contributions to understanding and preventing such instability include early seminal mathematical models and experimental data still used decades later by researchers.
Today, with support from the National Science Foundation, Dowell is exploring ways to create and control flutter to harvest electrical energy from aeroelastic systems, providing potential new sources of clean power.
Dowell also is developing computational models to help the Air Force design hypersonic aircraft with greater accuracy and confidence. He conducts low-speed wind tunnel experiments at Duke with computations to assess the models and helps design experiments for collaborators such as the Air Force Research Laboratory, Sandia National Laboratory and NASA in their hypersonic wind tunnels to perform.
“Flutter is critical in all flow ranges, but at hypersonic speeds the effects of high temperatures and rapidly changing aerodynamic forces add to the complexity.”
Peers have acknowledged the impact of Dowell’s lasting work with the highest honors in the field, including election to the National Academy of Engineering. In 2016 the American Institute of Aeronautics and Astronautics conferred upon him the Reed Aeronautics Award–the highest honor an individual can receive for notable achievement in aeronautics–for “pioneering contributions to aeroelasticity, structural dynamics, and unsteady aerodynamics, which have had a tremendous influence on aerospace technology.”
Designing Turbines for Optimal Safety and Efficiency
Professor of the Practice in Mechanical Engineering & Materials Science
Director, GUIde Consortium for Aeroelasticity
Director, THRUST Master's Degree in Engineering Program
Robert Kielb is committed to both turbines and his students. He has spent almost five decades contributing to new designs in gas turbines, understanding the intricacies of how turbine blade shape affects flutter and developing modeling and simulation tools to analyze the blades’ responses aeroelastic forces. His worldwide collaborators rely on these tools to design safe and efficient turbomachinery for aircraft propulsion and power generation.
Kielb directs the 30-year-old GUIde Consortium for Aeroelasticity, which consists of 12 companies, five universities and two government organizations that work together to solve practical challenges in controlling the vibration of bladed disks in gas turbine engines, which both threatens safety and increases costs.
As director of the 10-year-old THRUST MEng Program, a two-year Master of Engineering program during which students spend a year at Duke and one at the Swedish Royal Institute of Technology (KTH) or the University of Liège, Belgium, and advisor to many PhD students over his two decades at Duke, he prepares the next generation of aerospace engineers to aim even higher.
Kielb’s expertise in gas turbine engines, which he readily shares with students, industry and academia, comes from his experience working with the U.S. Air Force, NASA Lewis Research Center and GE Aircraft Engines as manager of aeromechanics technology before joining Duke University. The American Society of Mechanical Engineers recently recognized his outstanding contributions to the field internationally with its highest honor in gas turbines, the R. Thomas Sawyer Award–his fourth major ASME award.
“My students are not expected to memorize equations; the goal is full understanding of the fundamental physics of turbomachinery aeroelasticity. As Confucius said, ‘To know what you know and what you do not know, that is true knowledge.’”
Understanding Unsteady Air Flows
Julian Francis Abele Distinguished Professor of Mechanical Engineering & Materials Science
Kenneth Hall’s research focuses on understanding and modeling the behavior of unsteady air flows and their interaction with elastic aerospace structures, for example in the fan, compressor and turbine stages of gas turbine engines used in aircraft and for power generation. Such unsteady aerodynamic forces can cause unwanted and damaging blade vibrations in these engines.
Hall and his colleagues at Duke pioneered the computational fluid dynamic method known as Harmonic Balance, which works in the frequency, rather than time, domain to simulate complex aerodynamic flow fields. The method is useful both in understanding the underlying physical mechanisms and in designing more efficient, safer gas turbine engines and aerospace vehicles—while also making computations much faster than conventional methods.
He is also currently working on a new application of system identification to fill an important gap in the data analysis of the aeromechanic behavior of turbomachinery compressors for the GUIde 6 Consortium. System identification techniques use various computational techniques to estimate the important physical properties of a system from sometimes sparse and noisy data. Together with a colleague at the Massachusetts Institute of Technology, he is using a maximum likelihood system identification technique to deduce the structural dynamic properties of, and the unsteady aerodynamic air loads acting on, the blades of a compressor inferred from rotor blade vibration data collected by researchers at Purdue University.
In other work, Hall focuses on optimizing aerodynamics to understand the limits of birds in flight, and how to design helicopter rotors to minimize the power needed to operate them.
Peers have recognized Hall’s work with, among other honors, election to the National Academy of Engineering and the American Institute of Aeronautics and Astronautics Aerodynamics Award
“I've been very fortunate in my career to have some great colleagues in the department to collaborate with, and I've had exceptional Duke students who have contributed significantly to my research."
Building Lighter, More Slender Aircraft
Professor of Mechanical Engineering & Materials Science
Flight, with its many fluid forces of unpredictable wind gusts, changing air pressure, temperature, gravity and other forces in combination with the elastic reactions of aircraft structures, is a perfect example of a “highly complex nonlinear dynamical system.” Lawrie Virgin is an expert in these systems, in which reactions to forces may be exponentially greater than those of simple linear forces, such as straight-line gravitational pull on a stationary bridge.
Virgin develops computational models that account for these complexities to help design lighter, more slender aircraft and prevent the catastrophic threat of vibration fatigue. Using his background in structural engineering, he builds and tests precise 3D-printed models of the designs in his laboratory and conducts even more sophisticated experiments at the U.S. Air Force Research Laboratory.
His work also has included a project for NASA to design massive one-kilometer-square solar sails to deploy in space like an umbrella that the sun would propel via the light force of its photons rather than by converting energy through solar cell panels.
Virgin is currently working on an NSF-funded project to study loads and the unpredictable weak spots of cylinders that could lead to sudden failure, not unlike a crushed drink can. This is important to many applications in aerospace from the fuselage of aircraft to cylindrical solid rocket boosters for NASA.
Peers have recognized Virgin’s work by conferring upon him honors including the John P. Davis Award for a research paper with Robert Kielb by the International Gas Turbine Institute of the American Society of Mechanical Engineers and the M.M. Frocht Award for outstanding achievement as an educator from the Society for Experimental Mechanics. He is active in the classroom teaching in the undergraduate Aerospace Certificate program.
“What makes it interesting for me is the constant quest to make structures lighter weight in aerospace.”
Donald B. Bliss
Modeling Vortices and Wakes for Rotorcraft and Wind Turbines
Associate Professor of Mechanical Engineering & Materials Science
Donald Bliss conducts research in aerodynamics and acoustics, with an emphasis on modeling vortex wakes for flight vehicles–primarily rotorcraft–and horizontal- and vertical-axis wind turbines. His theoretical research seeks physical understanding and analytical formulations that lead to more computationally efficient solutions to real-world problems.
Vortical structures (regions of rotational motion created by the viscous shearing motion created by the interaction of the fluid with solid surfaces, such as wings and rotor or turbine blades) are very persistent and strongly influence the overall flow field. Wake structures substantially change performance and create power loss associated with the energy left behind in the wake. Coupled together, and since the wake also interacts with itself, vortical and wake structure problems are especially challenging analytically and computationally.
Helicopters experience coupled wake/blade influences, where the blades pass over their own wakes, and sometimes through them. Vortex wakes are also especially important to understand given their effect on the performance of wind turbines, both from their influence on the turbine that generated the wake and due to the “wake shielding” effect in wind farms, where downwind turbines operate less efficiently due to the wakes of upwind turbines.
Bliss’ other interests in aerodynamics include an improved analytical theory for low-aspect-ratio wings, the prediction of air loads in unsteady supersonic flow, and the acoustics of flight vehicle interior noise. For interior noise acoustic modeling, Bliss has developed an accurate and efficient analysis method that includes acoustic radiation, reflection and transmission associated with flexible structural surfaces.
A dedicated educator, Bliss also serves as the director of undergraduate studies in the Department of Mechanical Engineering & Materials Science.
“The creative development of new engineering methodologies, both in practice and in education, requires understanding the linkage between physical insight, mathematical analysis, and computational implementation. This understanding leads to accurate and efficient ways to reduce the complexity of engineering problems.”