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Evolution of the streamwise vortices generated between leading edge tubercles

Published online by Cambridge University Press:  12 January 2016

Kristy L. Hansen ,
Nikan Rostamzadeh ,
Richard M. Kelso  and
Bassam B. Dally
Kristy L. Hansen*
Affiliation:
School of Computer Science, Engineering and Mathematics, Flinders University, Tonsley Campus, Adelaide, South Australia 5042, Australia
Nikan Rostamzadeh
Affiliation:
School of Mechanical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia
Richard M. Kelso
Affiliation:
School of Mechanical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia
Bassam B. Dally
Affiliation:
School of Mechanical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia
*
†Email address for correspondence: [email protected]
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Abstract

Sinusoidal modifications to the leading edge of a foil, or tubercles, have been shown to improve aerodynamic performance under certain flow conditions. One of the mechanisms of performance enhancement is believed to be the generation of streamwise vortices, which improve the momentum exchange in the boundary layer. This experimental and numerical study investigates the formation and evolution of these streamwise vortices at a low Reynolds number of $Re=2230$, providing insight into both the averaged and time-dependent flow patterns. Furthermore, the strength of the vortices is quantified through calculation of the vorticity and circulation, and it is found that the circulation increases in the downstream direction. There is strong agreement between the experimental and numerical observations, and this allows close examination of the flow structure. The results demonstrate that the presence of strong pressure gradients near the leading edge gives rise to a significant surface flux of vorticity in this region. As soon as this vorticity is created, it is stretched, tilted and diffused in a highly three-dimensional manner. These processes lead to the generation of a pair of streamwise vortices between the tubercle peaks. A horseshoe-shaped separation zone is shown to initiate behind a tubercle trough, and this region of separation is bounded by a canopy of boundary-layer vorticity. Along the sides of this shear layer canopy, a continued influx of boundary-layer vorticity occurs, resulting in an increase in circulation of the primary streamwise vortices in the downstream direction. Flow visualisation and particle image velocimetry studies support these observations and demonstrate that the flow characteristics vary with time, particularly near the trailing edge and at a higher angle of attack. Numerical evaluation of the lift and drag coefficients reveals that, for this particular flow regime, the performance of a foil with tubercles is slightly better than that of an unmodified foil.

JFM classification

Aerodynamics: Aerodynamics Flow Control: Flow Control Flow Control: Mixing enhancement
Type
Papers
Information
Journal of Fluid Mechanics , Volume 788 , 10 February 2016 , pp. 730 - 766
Copyright
© 2016 Cambridge University Press 

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