Wings which redirect air to waggle
sideways could cut airline fuel
bills by 20% according to research funded by the Engineering and
Physical Sciences Research Council (EPSRC) and Airbus in the UK.
The new approach, which promises to
dramatically reduce mid-flight
drag, uses tiny air powered jets which redirect the air, making it flow
sideways back and forth over the wing.
The jets work by the Helmholtz resonance
principle --- when air is
forced into a cavity the pressure increases, which forces air out and
sucks it back in again, causing an oscillation – the same phenomenon
that happen when blowing over a bottle.
Dr Duncan Lockerby, from the University
of Warwick, who is leading
the project, said: “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
Simon Crook, EPSRC Senior manager for
aerospace & defence, said:
"This could help drastically reduce the environmental cost of flying.
Research like this highlights the way UK scientists and engineers
continue to make significant contributions to our lives."
The UK aviation industry has announced
targets to reduce emissions per passenger km by 50% by 2020.
Part of these savings will be made from
lighter aircraft plus
improvements in engines and fuel efficiencies but drag friction is also
a major factor in fuel consumption during flights.
Engineers have known for some time that
tiny ridges known as
‘riblets’ - like those found on sharks bodies - can reduce
skin-friction drag, (a major portion of mid-flight 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, 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.
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
and postgraduate training, to help the nation handle the next
generation of technological change.
For further information contact:
aim to reduce fuel burn per passenger km by at least 50% by 2020 will
be difficult to achieve without a 30 to 50% reduction in skin-friction
drag / the drag arising from the friction generated on the aircraft's
surface by the direct action of the air flow. We propose, therefore, to
investigate novel, practical, effective flow-control techniques for
achieving this aim.
drag in turbulent boundary
layers is governed by the flow physics very close to the surface in a
region of the flow field known as the viscous sublayer. The generation
of wall friction is also known to be quasi-cyclic. An essential
characteristic of this cycle and the near-wall flow physics are streaks
of low- and high-speed flow and their strong interaction with wave-like
disturbances. The resulting evolution of the streaks and their
explosive growth are intimately connected with the generation of wall
friction and thereby drag.
researchers focus on these
sublayer streaks because they are very closest to the wall and amenable
to wall-based actuation and sensing. We estimate, however, that there
are O(109) sublayer streaks over the fuselage of an Airbus A340-300 at
any instant during cruise. Others have made similar estimates. This
enormous number makes it utterly impractical to implement an active
control strategy targeting streaks individually. But disrupting the
cycle in a global untargeted way is feasible. Riblets (minute peaks and
troughs running in the flow direction with crossflow spacing of about
1/3 of a human hair width) do this by disrupting streak growth, in
effect by regularizing and partially stabilizing them. But conventional
riblets only deliver less than 1.5% drag reduction in flight tests,
although 6% is achieved in idealized laboratory experiments. Unless
this poor performance can be greatly improved, riblets are of little
practical interest. Spanwise oscillations have been studied recently
and shown to be much more effective than riblets at reducing
skin-friction drag. Again these appear to work by forcing the streaks
into more stable orientations. But this technique requires substantial
power input. Given the cyclic process described above, another option
is to disrupt the interaction of the waves and streaks with randomized
perturbations. This was tried by Sirovich et al. who obtained 12% drag
reduction in experimental flow studies with randomized surface
roughness elements. This approach has not really been further
investigated, although disrupting the wave-streak interaction with
randomized perturbations is likely to be much more effective than
propose to investigate: (i) the use of randomized
distributions of small-scale Helmholtz resonators that create strong
microjets without any power input; thus are likely to be more effective
than roughness elements or riblets; (ii) conventional riblets localize
the streaks, thus combining them with resonators could be much more
effective than riblets alone; (iii) improving effectiveness with
unconventional riblets; e.g., wavy riblets mimicking spanwise
oscillations and other 3D patterns. Our study will be based on our
simplified theoretical model of the sublayer streaks which can be used
at flight Reynolds number.
resonators hold great
promise as passive control devices because: (i) the control disturbance
produced is proportionately much greater than for roughness elements,
including riblets; (ii) they require no power input; and (iii)
consisting simply of a cavity with a necked exit orifice, they are
straightforward to manufacture at MEMS (micro) scale.
Final Report Summary
is seeking to develop technologies that will enable the ACARE VISION
2020 targets to be met, allowing sustained air travel growth whilst
having zero additional environmental impact. Achieving this requires a
50% reduction in both fuel consumption and CO2 emissions. To meet the
target of reducing fuel burn per passenger km by at least 50% by 2020
will be difficult without a 30 to 50% reduction in skin-friction drag.
Warwick and Cardiff, we have developed efficient numerical methods to
explore a range of novel, but practical, passive control strategies
(i.e. those requiring no power input) capable of reducing skin-friction
drag on passenger jet aircraft. Importantly, these simulation tools are
capable of modelling basic configurations at flight-scale Reynolds
numbers (far beyond the reach of conventional computational methods).
Using these techniques, the characteristics of critical near-wall flow
structures ('streaks') at cruise conditions have been estimated, and a
very substantial drag reduction (~40%) has been demonstrated using
spanwise flow oscillations at the flight scale. Passive means for
creating spanwise oscillations have been explored, and the potential of
using Helmholtz resonators (small plenum chambers, sunk beneath the
aircraft surface) to generate oscillating jets has been identified.
Low-speed Direct Numerical Simulations (coupled with an actuator model)
have shown that, driven by boundary-layer pressure fluctuations, a
strong and coherent jet flow is generated through the orifice of the
resonator - such a response, in ensemble, could be harnessed to create
spanwise forcing without the need for electrical input. The key
achievements, which directly shape and support the follow-on
EPSRC/Airbus project "Scalable Wirelessly Interconnected Flow-Control
Technologies (SWIFT)" EP/G038686/1, are:
> Development of a reduced-order fluid
dynamics model for basic simulations at flight scale.
> Demonstration of major drag
reduction at cruise speeds using spanwise forcing.
> Discovery of a promising passive
strategy for the generation of spanwise forcing (using Helmholtz
Turbulence and Combustion,
Volume 78, Numbers 3-4 / June, 2007,
Helmholtz Resonance a Problem for
Micro-jet Actuators ?
Duncan A. Lockerby 1, Peter W.
Carpenter 1 Contact Information and Christopher Davies 2
(1) School of Engineering, University of
Warwick, Coventry, CV4 7AL, UK
(2) School of Mathematics, Cardiff University,
Cardiff, CF24 4AG, UK
-- A theoretical
analysis is described that determines the conditions for Helmholtz
resonance for a popular class of self-contained microjet actuator used
in both synthetic- and pressure-jump (pulse-jet) mode. It was
previously shown that the conditions for Helmholtz resonance are
identical to those for optimizing actuator performance for maximum mass
flux. The methodology is described for numerical-simulation studies on
how Helmholtz resonance affects the interaction of active and nominally
inactive micro-jet actuators with a laminar boundary layer. Two sets of
numerical simulations were carried out. The first set models the
interaction of an active actuator with the boundary layer. These
simulations confirm that our criterion for Helmholtz resonance is
broadly correct. When it is satisfied we find that the actuator cannot
be treated as a predetermined wall boundary condition because the
interaction with the boundary layer changes the pressure difference
across the exit orifice thereby affecting the outflow from the
actuator. We further show that strong inflow cannot be avoided even
when the actuator is used in pressure-jump mode. In the second set of
simulations two-dimensional Tollmien–Schlichting waves, with frequency
comparable with, but not particularly close to, the Helmholtz resonant
frequency, are incident on a nominally inactive micro-jet actuator. The
simulations show that under these circumstances the actuators act as
strong sources of 3D Tollmien–Schlichting waves. It is surmised that in
the real-life aeronautical applications with turbulent boundary layers
broadband disturbances of the pressure field, including acoustic waves,
would cause nominally inactive actuators, possibly including pulsed
jets, to act as strong disturbance sources. Should this be true it
would probably be disastrous for engineering applications of such
massless microjet actuators for flow control.
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:
Duncan Lockerby, Peter Carpenter, Christopher Davies (2005) Control
of Sublayer Streaks Using Microjet Actuators. AIAA Journal
vol.43 no.9 (1878-1886)
D.A. LOCKERBY & J.M. Reese (2008) On the modelling of
isothermal gas flows at the micro scale. Journal of Fluid Mechanics.
D.A. LOCKERBY, P.W. Carpenter & C. Davies (2007) Helmholtz
resonance and its effects on pulsed-jet actuators. Flow Turbul.
D.A. LOCKERBY, J.M. Reese & M.A. Gallis (2005) The usefulness
of higher-order constitutive relations for describing the Knudsen
layer. Phys. Fluids 17, 100609.
D.A. LOCKERBY, P.W. Carpenter & C. Davies (2005) Control of
sub-layer streaks using microjet actuators. AIAA J. 43(9) 1878-1887.
D.A. LOCKERBY, J.M. Reese, D.R. Emerson & R.W. Barber (2004)
The velocity boundary condition at solid walls in rarefied gas
calculations. Phys. Rev. E. 70 (017304).
D.A. LOCKERBY, J.M. Reese & M.A. Gallis (2005) Capturing the
Knudsen layer in continuum-fluid models of non-equilibrium gas flows.
AIAA J. 43(6) 1391-1393
D.A. LOCKERBY & P.W. Carpenter (2004) Modeling and design of
microjet actuators. AIAA J. 42(2) 220-227.
D.A. LOCKERBY & J.M. Reese (2003) High-resolution Burnett
simulations of micro Couette flow and heat transfer. J. Comp. Phys. 188
D.A. LOCKERBY, P.W. Carpenter & C. Davies (2002) Numerical
simulation of the interaction of MEMS actuators and boundary
AIAA J. 40(1), 67-73.
J.M Reese, Y. Zheng & D.A. LOCKERBY (2007) Computing the
Near-Wall Region in Gas Micro- and Nanofluidics: Critical Knudsen Layer
Phenomena. J. Comput. Theor. Nanoscience. 4(4), 807-813.
L. O’Hare, D.A. LOCKERBY, J.M. Reese & D.R. Emerson (2007)
Near-wall effects in rarefied gas micro-flows: some modern hydrodynamic
approaches. Int. J. Heat Fluid Flow, 28(1), 37-43.
R.S. Myong, D.A. LOCKERBY & J.M. Reese (2006) The effect of
gaseous slip on microscale heat transfer: An extended Graetz problem.
Int. J. Heat Mass Transfer, 49(15-16), 2502-2513
J.M. Reese, M.A. Gallis & D.A. LOCKERBY (2003) New directions
in fluid dynamics: non-equilibrium aerodynamic and microsystem flows.
Phil. Trans. Roy. Soc. 361 (1813): 2967-2988.
Refereed Conference Papers:
D.A. LOCKERBY &
J.M. Reese 2007 Near-wall scaling of the Navier-Stokes Constitutive
Relations for Accurate Micro Gas Flow Simulations. 5th International
Conference on Nanochannels, Microchannels and Minichannels, Puebla, 2007
D. Hayes-McCoy, X. Jiang & D.A. LOCKERBY Direct Computation of
Zero-Net-Mass-Flux Synthetic Jets. 5th IASME / WSEAS International
Conference on Fluid Mechanics and Aerodynamics, Athens, 2007
D.A. LOCKERBY, P.W Carpenter & C. Davies 2006, Is Helmholtz
resonance a problem for micro-jet actuators? IUTAM Symposium on Flow
control and MEMS, Imperial College, London, 2006.
Y. Zheng, J.M. Reese, T.J. Scanlon & D.A. LOCKERBY 2006,
Scaled Navier-Stokes-Fourier equations for rarefied gas flow and heat
transfer phenomena in micro- and nanosystems. 4th International
Conference on Nanochannels, Microchannels and Minichannels, Limerick,
C. Mares & D.A. LOCKERBY 2006, Developing professional skills
through group design projects and participation in student
competitions. International Conference on Innovation, Good Practice and
Research in Engineering Education, Liverpool, 2006
J.M. Reese, D.A. LOCKERBY & D.R. Emerson 2005, On hydrodynamic
predictions of near-wall effects in rarefied gases: some
phenomenological and modelling approaches. ECI International Conference
on Heat Transfer and Fluid Flow in Microscale, Castelvecchio Pascoli,
D.A. LOCKERBY, J.M. Reese & M.A. Gallis 2004, The constitutive
relations and boundary conditions for microflow modeling. Transport
Phenomena in Micro and Nanodevices, Island of Hawaii 2004.
D.A. LOCKERBY, J.M. Reese, D.R. Emerson, R.W. Barber 2004,
Geometry Curvature Dependence in the Solid-Wall Velocity Boundary
Condition for Rarefied Flows. 24th International Symposium on Rarefied
Gas Dynamics, Bari, Italy 2004.
D.A. LOCKERBY, J.M. Reese & M.A. Gallis 2004, A Wall-Function
Approach to Capturing the Knudsen Layer in Practical Gas Microfluidic
Geometries. 24th International Symposium on Rarefied Gas Dynamics,
Bari, Italy 2004.
C.L. Bailey, R.W. Barber, D.R. Emerson & D.A. LOCKERBY &
J.M. Reese, A critical review of the drag force on a sphere in the
transition flow regime. 24th International Symposium on Rarefied Gas
Dynamics, Bari, Italy 2004.
C. Davies, P.W. Carpenter, R. Ali & D.A. LOCKERBY 2004,
Disturbance Development in Boundary Layers over Compliant Surfaces
Laminar-Turbulent Transition, Proc. IUTAM Symp, Bangalore, India 13-17
December 2004. Proceedings published by Springer, pp. 225-230, 2006.
P.W. Carpenter, R. Ali, C. Davies & D.A. LOCKERBY 2003, A
simple computational model for studying the control of near-wall
structures in turbulent boundary layers. European Fluid Mechanics
Conference, Toulouse 2003.
K. Kudar, P.W. Carpenter, C. Davies & D..A. LOCKERBY 2003,
Numerical simulation of streak-like structures in swept boundary-layer
flows and their control. European Fluid Mechanics Conference, Toulouse
J.M. Reese & D.A. LOCKERBY 2002, A new design capability for
hypersonic flight vehicles and microscale devices? Computer-Based
Design: Engineering Design Conference 2002, Shahin TMS (ed).
Professional Engineering Publishing, Bury St Edmunds, UK.
C. Davies, P.W. Carpenter & D.A. LOCKERBY 2001, A novel
velocity-vorticity simulation method for boundary-layer disturbances.
Bull. American Phys. Soc. 46(6), p. 34.
P.W. Carpenter, C. Davies & D.A. LOCKERBY 2001, A novel
velocity-vorticity method with applications to flow control. ECCOMAS
Computational Fluid Dynamics Conference 2001, Swansea, 4-7 Sept., 20
pages on CD (published by IMA).
C. Davies, P.W. Carpenter & D.A. LOCKERBY 2001, A novel
velocity-vorticity method for simulating boundary-layer disturbance
evolution and control. Laminar-Turbulent Transition, Proc. IUTAM Symp.,
Sedona, Arizona, 13-17 September, 1999, (Eds H.F. Fasel and W.S.
Saric), Springer, pp. 313-318. (ISBN 3-540-67947-2).
P.W. Carpenter, D.A. LOCKERBY & C. Davies 2000, Is Helmholtz
resonance important for boundary-layer control by micro-jet actuators?
20th International Congr. of Theoretical & Applied Mechanics,
Abstract Book (ISSN 0073-5264), Univ. of Illinois at Urbana-Champaign,
P.W. Carpenter, D.A. LOCKERBY & C. Davies 2000, Numerical
simulation of the interaction of MEMS actuators and boundary layers.
AIAA Paper 2000-4330. Invited paper for Special Session on Flow
Control, 18th AIAA Applied Aerodynamics Conf.,14-17 August, Denver,
USA. In 18th AIAA Applied Aerodynamics Conference, Technical Papers,
P.W Carpenter, C. Davies & D.A. LOCKERBY 1998, A
novel-velocity-vorticity method for simulating the effects of MEMS
actuators on boundary layers. Proc. 3rd. Asian Computational Fluid
Dynamics Conference, Bangalore, India, 7-11 December, (Eds. T.S.
Prahlad et al.) Vol. 2, pp.44-49.