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Novel Passive Techniques for Reducing Skin-Friction Drag

Dr D.A. Lockerby, Dr. Y.M. Chung, & Dr C Davies

Airbus's 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.

Skin-friction 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.

Most 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 riblets.

We 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.

Helmholtz 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

Airbus 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.

At 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 resonators).

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http://www.springerlink.com/content/u966rk46202262r9/
Flow, Turbulence and Combustion, Volume 78, Numbers 3-4 / June, 2007, Pages 205-222


(1) School of Engineering, University of Warwick, Coventry, CV4 7AL, UK
(2) School of Mathematics, Cardiff University, Cardiff, CF24 4AG, UK

Abstract  -- 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.

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Resources

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

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http://www2.warwick.ac.uk/fac/sci/eng/staff/dal/publications/

Journal publications:

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. Combust. doi:10.1007/s10494-006-9056-0.

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 (2) 333-347.

D.A. LOCKERBY, P.W. Carpenter & C. Davies (2002) Numerical simulation of the interaction of MEMS actuators and boundary layers.  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, 2006

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, 2005

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 2003.

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. 13

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, pp. 596-605

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.