Experiments in Fluids

, Volume 53, Issue 3, pp 637–653 | Cite as

The flow over a thin airfoil subjected to elevated levels of freestream turbulence at low Reynolds numbers

  • Sridhar Ravi
  • Simon Watkins
  • Jon Watmuff
  • Kevin Massey
  • Phred Petersen
  • Matthew Marino
  • Anuradha Ravi
Research Article

Abstract

Micro Air Vehicles (MAVs) can be difficult to control in the outdoor environment as they fly at relatively low speeds and are of low mass, yet exposed to high levels of freestream turbulence present within the Atmospheric Boundary Layer. In order to examine transient flow phenomena, two turbulence conditions of nominally the same longitudinal integral length scale (Lxx/c = 1) but with significantly different intensities (Ti = 7.2 % and 12.3 %) were generated within a wind tunnel; time-varying surface pressure measurements, smoke flow visualization, and wake velocity measurements were made on a thin flat plate airfoil. Rapid changes in oncoming flow pitch angle resulted in the shear layer to separate from the leading edge of the airfoil even at lower geometric angles of attack. At higher geometric angles of attack, massive flow separation occurred at the leading edge followed by enhanced roll up of the shear layer. This lead to the formation of large Leading Edge Vortices (LEVs) that advected at a rate much lower than the mean flow speed while imparting high pressure fluctuations over the airfoil. The rate of LEV formation was dependent on the angle of attack until 10° and it was independent of the turbulence properties tested. The fluctuations in surface pressures and consequently aerodynamic loads were considerably limited on the airfoil bottom surface due to the favorable pressure gradient.

Abbreviations

c

Chord

Cl

Lift coefficient

Cm

Pitching moment coefficient

CP

Pressure coefficient

F

Frequency

LE

Leading edge

LEV

Leading edge vortex

Lxx

Longitudinal integral length scale

S

Spectral density

St

Strouhal number (\( St = f.c.{ \sin }(\alpha )/v \))

TE

Trailing edge

Ti

Turbulence intensity

α

Angle of attack

σ

Standard deviation of time-varying surface pressure

μ

Wave number (\( \mu = f \times c/v \))

References

  1. Bergh H, Tijdeman H (1965) Theoretical and experimental results for the dynamic response of pressure measuring systems. Technical Report NLR-TR F238, National Aerospace LaboratoriesGoogle Scholar
  2. Carr LW (1988) Progress in analysis and prediction of dynamic stall. J Aircr 25(1):6–17CrossRefGoogle Scholar
  3. Chen JM, Fang YC (1996) Strouhal numbers of inclined flat plates. J Wind Eng Ind Aerodyn 61:99–112CrossRefGoogle Scholar
  4. Cherry NJ, Hillier R, Latour MEMP (1983) The unsteady structure of two-dimensional separated and reattaching flows. J Wind Eng Ind Aerodyn 11(1983):95–105CrossRefGoogle Scholar
  5. Comte-Bellot GA (1966) The use of contraction to improve the isotropy of grid generated turbulence. J Fluid Mech 25(4):657–682CrossRefGoogle Scholar
  6. Cooper RK, Watkins S (2007) The unsteady wind environment of road vehicles, part one: a review of the on-road environment. SAE International Transactions Journal of Passenger Cars—Mechanical Systems 2007-01-1236, April 16–19. Detroit, MichiganGoogle Scholar
  7. Cruz E, Watkins S, Loxton B (2007) A study of the effects of turbulence on airfoils at low reynolds numbers. Interim Report AOARD-06-4037. RMIT University, MelbourneGoogle Scholar
  8. Devinant P, Laverne T, Hureau J (2002) Experimental study of wind-turbine airfoil aerodynamics in high turbulence. J Wind Eng Ind Aerodyn 90:689–707CrossRefGoogle Scholar
  9. Flay RGJ (1978) Structure of a rural atmospheric boundary layer near the ground. PhD. thesis, Department of Mechanical Engineering, University of Canterbury, New ZealandGoogle Scholar
  10. Gartshore IS (1973) The effects of freestream turbulence on the drag of rectangular two dimensional prism. BLWT, University of Western Ontario, Canada, pp 4–73Google Scholar
  11. Hillier R, Cherry NJ (1981) The effects of stream turbulence on separation bubbles. J Wind Eng Ind Aerodyn 8:49–58CrossRefGoogle Scholar
  12. Hoarau Y, Braza M, Ventikos Y, Faghani D, Tzabiras G (2003) Organized modes and the three-dimensional transition to turbulence in the incompressible flow around a NACA0012 wing. J Fluid Mech 496:63–72MATHCrossRefGoogle Scholar
  13. Jancauskas ED (1983) The cross-wind excitation of bluff structures and the incident turbulence mechanism. Doctoral thesis, Monash University, ClaytonGoogle Scholar
  14. Li QS, Melbourne WH (1999) The effect of large-scale turbulence on pressure fluctuations in separated and reattaching flows. J Wind Eng Ind Aerodyn 83:159–169CrossRefGoogle Scholar
  15. Loxton B, Watkins S, Watmuff J, Trivalio P, Cruz E, Ravi S (2009) The Influence of atmospheric turbulence on the aerodynamics of a flat plate micro air vehicle wing. 3rd Australasian unmanned air vehicles conference. Melbourne, AustraliaGoogle Scholar
  16. Melbourne WH (1993) Turbulence and the leading edge phenomenon. J Wind Eng Ind Aerodyn 49:45–64CrossRefGoogle Scholar
  17. Milbank J, Loxton B, Watkins S, Melbourne WH (2005) Replication of atmospheric conditions for the purpose of testing MAVs MAV flight environment project: final report. AFOSR Final Report (USAF Project No: AOARD 05-4075), RMITGoogle Scholar
  18. Mousley PD, Watkins S, Hooper JD (1998) Use of a hot-wire anemometer to examine the pressure signal of a high-frequency pressure probe. 13th Australasian Fluid Mechanics Conference. Monash University, Melbourne, AustraliaGoogle Scholar
  19. Mueller TJ (1999) Aerodynamic measurements at low reynolds numbers for fixed wing micro-air vehicles. In: Proceedings of the development and operation of UAVs for military and civil applications short course, Belgium (published in Rto-EN-9 by NATO, April 2000)Google Scholar
  20. Mueller TJ, Pohlen LJ, Conigliaro PE, Jansen BJ Jr (1983) The influence of free-stream disturbances on low Reynolds number airfoil experiments. Exp Fluids 1:3–14CrossRefGoogle Scholar
  21. Ol MV, Bernal L, Kang CK, Shyy W (2009) Shallow and deep dynamic stall for flapping low Reynolds number airfoils. Exp Fluids 46(5): 883–901. doi: 10.1007/s00348-009-0660-3 Google Scholar
  22. Pagliarella RM (2009) On the aerodynamic performance of automotive vehicle platoons featuring pre and post-critical leading forms. PhD thesis, RMIT UniversityGoogle Scholar
  23. Pines DJ, Bohorquez F (2006) Challenges facing future micro-air-vehicle development. J Aircr 23(2):290–305CrossRefGoogle Scholar
  24. Ravi S (2011) The influence of turbulence on a flat plate airfoil at Reynolds numbers relevant to MAVs. PhD thesis, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT UniversityGoogle Scholar
  25. Ravi S, Watkins S, Watmuff J, Petersen P (2012) Influence of turbulence on airfoil performance at low Reynolds number. AIAA J. Accepted (in press)Google Scholar
  26. Rival D, Tropea C (2010) Characteristics of pitching and plunging airfoils under dynamic-stall conditions. J Aircr 47(1):80–87CrossRefGoogle Scholar
  27. Roshko A (1961) Experiments on the flow past a circular cylinder at very high Reynolds numbers. J Fluid Mech 10:315–356CrossRefGoogle Scholar
  28. Saathoff PJ (1988) Effects of free-stream turbulence on surface pressure fluctuations in separated and reattaching flow. Ph.D. thesis, Monash UniversityGoogle Scholar
  29. Saathoff PJ, Melbourne WH (1997) Effects of free-stream turbulence on surface pressure fluctuations in a separation bubble. J Fluid Mech 337:1–24CrossRefGoogle Scholar
  30. Sicot C, Aubrun S, Loyer S, Devinant P (2006) Unsteady characteristics of the static stall of an airfoil subjected to freestream turbulence level up to 16 %. Exp Fluids 41:641–648CrossRefGoogle Scholar
  31. Stack J (1931) Tests in the variable density wind tunnel to investigate the effects of scale and turbulence on airfoil characteristics. NACA-TN-364, NACA Langley Memorial Aeronautical LaboratoryGoogle Scholar
  32. Swalwell K (2005) The effect of turbulence on stall of horizontal axis wind turbines. PhD thesis, Department of Mechanical Engineering, Monash UniversityGoogle Scholar
  33. Thompson M, Watkins S (2010) Gust inputs relevant to bees, birds and MAVs. 25th Bristol international unmanned aerial vehicle systems (UAVS) conference. Bristol, UKGoogle Scholar
  34. Vickery BJ (1966) Fluctuating lift and drag on a long cylinder of square cross-section in a smooth and in a turbulent stream. J Fluid Mech 25:481–494CrossRefGoogle Scholar
  35. Vino G (2005) An experimental investigation into the time-averaged and unsteady aerodynamics of the simplified passenger vehicle in isolation and in convoys. PhD thesis, RMIT UniversityGoogle Scholar
  36. Watkins S, Cooper KR (2007) The unsteady wind environment of road vehicles, part two: effects on vehicle development and simulation of turbulence. SAE International Transactions Journal of Passenger Cars—Mechanical Systems 2007-01-1237, 16–19 April, Detroit, Michigan, USA. Detroit, MichiganGoogle Scholar
  37. Watkins S, Mousley P, Hooper J (2002) Measurement of fluctuating flows using multi-hole probes. In: Proceedings of the 9th international congress of sound and vibration, Orlando, Florida, USA, 8–11 JulyGoogle Scholar
  38. Watkins S, Milbank J, Loxton BJ, Melbourne WH (2006) Atmospheric winds and their effects on micro air vehicles. AIAA J 44(11):2591–2600CrossRefGoogle Scholar
  39. Watkins S, Cruz E, Loxton BJ (2008) A study of the effects of turbulence on airfoils at low Reynolds numbers. AFOSR Final Report (USAF Project No: AOARD 06-4037), RMIT University. RMIT University, AustraliaGoogle Scholar
  40. Watkins S, Thompson M, Loxton B, Abdulrahim M (2010) On low altitude flight through the atmospheric boundary layer. Int J Micro Air Vehic 2:55–68CrossRefGoogle Scholar

Copyright information

© Springer-Verlag 2012

Authors and Affiliations

  • Sridhar Ravi
    • 1
  • Simon Watkins
    • 2
  • Jon Watmuff
    • 2
  • Kevin Massey
    • 2
  • Phred Petersen
    • 2
  • Matthew Marino
    • 2
  • Anuradha Ravi
    • 3
  1. 1.University of TuebingenTuebingenGermany
  2. 2.RMIT UniversityMelbourneAustralia
  3. 3.Vellore Institute of TechnologyVelloreIndia

Personalised recommendations