Abstract
In this paper, results from an experimental study of the natural and forced evolution of a pair of counter rotating wing-tip vortices are reported. The vortices were generated using a pair of opposed wing-tips in a wind tunnel and measurements made up to 77 tip-spacings downstream of the models at a chord Reynolds number of 1.3 × 105. The wake was interrogated using 2D particle image velocimetry and the long-wave Crow instability observed. Velocity data were recorded throughout the lifetime of the instability from initial growth through linking, formation of vortex rings and their subsequent decay. Forcing was achieved using pulsed air jets blowing in the span-wise direction from the wing tip and imparting spatially periodic perturbations to the vortices. Forcing at a frequency within the range amplified by the Crow instability was found to enhance the instability growth rate whereas forcing at a frequency outside the amplified range was found to inhibit instability growth. In the latter case the imparted wavelength was observed to die out with a preferred wavelength growing in its place.
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Abbreviations
- A :
-
amplitude of the instability
- b 0 :
-
spacing between wing tips
- b :
-
separation between vortices
- b 2.3 :
-
separation between vortices at the first measurement plane (x = 2.3b0)
- \( C_{\dot{m}} \) :
-
mass flow coefficient \( {\left( {{\ifmmode\expandafter\dot\else\expandafter\.\fi{m}_{j} } \mathord{\left/ {\vphantom {{\ifmmode\expandafter\dot\else\expandafter\.\fi{m}_{j} } {\rho U_{\infty } S}}} \right. \kern-\nulldelimiterspace} {\rho U_{\infty } S}} \right)} \)
- \( \ifmmode\expandafter\dot\else\expandafter\.\fi{m}_{{\text{j}}} \) :
-
jet slot mass flow rate
- S :
-
planform area
- U ∞ :
-
freestream velocity
- α s :
-
dimensionless amplification rate
- Γ :
-
circulation
- λ :
-
wavelength
- ρ :
-
density
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Acknowledgments
The financial support of EPSRC and Qinetiq are gratefully acknowledged. The authors would also like to express their gratitude to the late Professor JH Whitelaw for his support and guidance.
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Bearman, P., Heyes, A., Lear, C. et al. Evolution of a forced counter rotating vortex pair for two selected forcing frequencies. Exp Fluids 43, 501–507 (2007). https://doi.org/10.1007/s00348-007-0312-4
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DOI: https://doi.org/10.1007/s00348-007-0312-4