Aerospace Engineering at Illinois Assistant Prof. Phillip Ansell has won a Young Investigator Award from the Air Force Office of Scientific Research (AFOSR) for his work in understanding the unsteady flow associated with the onset of dynamic stall.
Dynamic stall is a problem that is relevant to a number of different aerodynamic applications, including rotorcraft, fixed-wing aircraft, wind turbines, and flapping wing vehicles, to name a few. Such stalls can be undesirable when they cause an aircraft to become uncontrollable. However, they also can also be useful in applications requiring extreme maneuverability: a bird uses a dynamic stall to decelerate rapidly to perch on a branch, for example.
“I’m looking at this from the perspective of making the flow behave in such a way that we can either avoid or obtain the desired effect from dynamic stall, while maintaining control of a vehicle,” Ansell said.
“The significant difficulty in this problem is that the understanding we currently have of the flow is centered on the large-scale features, not the subscale components that contribute.
“We have difficulty understanding this because various aspects of (the flow) are changing as the wing motion continues. Time scales change in relation to spatial scales,” Ansell continued. “A lot of methods and approaches to analyze the flow simply won’t work because of the changes in scales, and we have to use more advanced signal processing techniques to characterize (the phenomena).”
Ansell conducts experiments on airfoils (wings) undergoing unsteady motion in a wind tunnel in the Aerodynamic Research Laboratory on campus. He takes measurements on the wing’s surface as well as time-resolved particle image velocimetry (TR-PIV) measurements in the flow field. PIV is an optical method of flow measurement and visualization.“When dynamic stall occurs, there is a large vortex that is shed from the lead edge of the wing,” Ansell said. “The vortex swirls around over the top of the airfoil creating a suction region, a low pressure that can act to increase drag and lift. The vortex itself we understand fairly well, though there are various subscale features that lead up to the shedding of this vortex that we actually don’t know nearly as much about.
“We want to mitigate the problems of losing control authority when going into those kinds of maneuvers, and the key is to better understand how to influence the subscale features. By manipulating these subscale flow structures, we can avoid or delay the production of the dynamic stall vortex and the ensuing effects on the performance and vehicle controllability.
“Once we have the measurements, we use advanced signal processing techniques to determine the characteristics of the subscale structures. If you instrument a wing section and you measure the unsteady components, you can correlate the scales and structures observed in experiments to those being measured in real-time. This allows you to make predictions as to what the state of the flow is, and actuate the flow to prevent the vehicle from becoming uncontrollable.”
While studying this problem for the Air Force, Ansell also is working on two projects for the National Aeronautics and Space Administration (NASA).
The first focuses on designing airfoils with a new type of propulsion system. The design would reduce the skin friction drag of a wing by extending the stretch of laminar flow across the surface, thereby reducing the amount of turbulent flow across the wing. A laminar flow runs in parallel layers in the streamwise direction across the wing, while turbulent flow is much more chaotic, with layers of fluid mixing in all different directions. The skin friction produced by a turbulent flow is typically much larger than that produced by a laminar flow.With the new concept, Ansell said, “propulsion is built into the airfoil so that the intake sucks in the flow across the upper surface (of the wing), and exhausts the air out the trailing edge. This provides us new ways to tailor the way the pressure is distributed across the airfoil surface and also employ our propulsion system directly at the location of the wing wake. Both of these effects offer distinct benefits within an aircraft design.”
Ansell is working with Mike Kerho, an AE alumnus and employee of Rolling Hills Research Corporation, on this project. The company, based in El Segundo, California, is designing the new airfoil sections. Ansell will be performing experiments on this new airfoil design and the embedded propulsion system. Steve D’Urso, Coordinator of AE’s Aerospace Systems Engineering Program, also is involved in the project, examining the effects of the concept from an aircraft systems level perspective.
Ansell’s third project involves understanding how hybrid electric technology can be incorporated into aviation. Mentoring him on the work are Kiruba Sivasubramaniam Haran and Philip Krein, both of the Electrical and Computer Engineering Department.
“We’re looking at different types of hybrid systems and how to use them on an aircraft platform to decrease the fuel burn in the next generation of aircraft,” Ansell said. “There are a number of ways to lay out the hybrid electric system, and a different one might be appropriate for different sizes of aircraft. What could be beneficial for a regional transport might be drastically different for commercial transport.
“What we are trying to understand is how different hybrid electric aircraft components and system architectures can be most effectively utilized for a wide variety of flight profiles and missions.”