Aircraft wings which redirect air to waggle sideways over their surfaces could significantly reduce drag and thus cut fuel consumption and emissions by 20%, according to researchers at the University of Warwick (UK). The new approach, which promises to dramatically reduce mid-flight drag, exploits Helmholtz resonance—the same phenomenon that happens when blowing over a bottle—to produce micro-scale jet flows in response to turbulent noise.
The main contributor to aerodynamic drag, and thus fuel consumption and emissions, is fine-scale turbulence that exists very near to the aircraft’s surface during cruise. The Turbulence Flow Control group at the University of Warwick has been studying non-powered (passive) flow control actuators as a means of reducing drag for some time, with the goal of developing flow-control technologies capable of major drag reductions on passenger jet aircraft.
Warwick is focusing on passive actuators because if an actuator is to be used on an aircraft, it must save more energy (in reduced drag) than it requires for its operation. Unfortunately, the researchers at Warwick note, many powered devices that have been developed for the drag-reduction application are unlikely to satisfy this fundamental net-saving requirement.
In a project funded jointly by the Engineering and Physical Sciences Research Council (EPSRC) and Airbus, Warwick and other universities are investigating the potential of using new types of passive devices for turbulent drag reduction. One of the ideas under investigation was the use of Helmholtz resonance.
Warwick is also investigating means by which the drag-reducing capabilities of riblets (surface ridges, such as on shark skin) can be amplified using these Helmholtz resonators.
This has come as a bit of a surprise to all of us in the aerodynamics community. It was discovered, essentially, by waggling a piece of wing from side to side in a wind tunnel.
The truth is we’re not exactly sure why this technology reduces drag but with the pressure of climate change we can’t afford to wait around to find out. So we are pushing ahead with prototypes and have a separate three year project to look more carefully at the physics behind it.—Dr. Duncan Lockerby, University of Warwick, project leader
Engineers have known for some time that riblets can reduce skin-friction drag by around 5%. But the new micro-jet system being developed by Dr. Lockerby and his colleagues could reduce skin friction drag by up to 40%.
The research, part of a three-year £1.1-million (US$1.75-million) project being carried out with scientists at Cardiff, Imperial, Sheffield, and Queen’s University Belfast, is still at concept stage, although it is hoped the new wings could be ready for trials as early as 2012.
If successful this technology could also have a major impact on the aerodynamic design and fuel consumptions of cars, boats and trains.
The UK aviation industry has announced targets to reduce emissions per passenger km by 50% by 2020.
The Engineering and Physical Sciences Research Council (EPSRC) is the UK’s main agency for funding research in engineering and the physical sciences. The EPSRC invests more than £740 million a year in research and postgraduate training, to help the nation handle the next generation of technological change.
University of Warwick Turbulence Flow Control Group
Duncan A. Lockerby, Peter W. Carpenter and Christopher Davies (2007) Is Helmholtz Resonance a Problem for Micro-jet Actuators? Flow, Turbulence and Combustion, Volume 78, Numbers 3-4 doi: 10.1007/s10494-006-9056-0
Duncan Lockerby, Peter Carpenter, Christopher Davies (2005) Control of Sublayer Streaks Using Microjet Actuators. AIAA Journal vol.43 no.9 (1878-1886) doi: 10.2514/1.14443