Experiments in Fluids

, Volume 53, Issue 3, pp 637–653

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

DOI: 10.1007/s00348-012-1316-2

Cite this article as:
Ravi, S., Watkins, S., Watmuff, J. et al. Exp Fluids (2012) 53: 637. doi:10.1007/s00348-012-1316-2

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

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

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