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On the effects of vertical offset and core structure in streamwise-oriented vortex–wing interactions

Published online by Cambridge University Press:  21 June 2016

C. J. Barnes ,
M. R. Visbal  and
P. G. Huang
C. J. Barnes*
Affiliation:
Air Force Research Laboratory, Wright-Patterson AFB, OH 45433, USA
M. R. Visbal
Affiliation:
Air Force Research Laboratory, Wright-Patterson AFB, OH 45433, USA
P. G. Huang
Affiliation:
Department of Mechanical and Materials Engineering, Wright State University, Dayton, OH 45435, USA
*
†Email address for correspondence: [email protected]
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Abstract

This article explores the three-dimensional flow structure of a streamwise-oriented vortex incident on a finite aspect-ratio wing. The vertical positioning of the incident vortex relative to the wing is shown to have a significant impact on the unsteady flow structure. A direct impingement of the streamwise vortex produces a spiralling instability in the vortex just upstream of the leading edge, reminiscent of the helical instability modes of a Batchelor vortex. A small negative vertical offset develops a more pronounced instability while a positive vertical offset removes the instability altogether. These differences in vertical position are a consequence of the upstream influence of pressure gradients provided by the wing. Direct impingement or a negative vertical offset subject the vortex to an adverse pressure gradient that leads to a reduced axial velocity and diminished swirl conducive to hydrodynamic instability. Conversely, a positive vertical offset removes instability by placing the streamwise vortex in line with a favourable pressure gradient, thereby enhancing swirl and inhibiting the growth of unstable modes. In every case, the helical instability only occurs when the properties of the incident vortex fall within the instability threshold predicted by linear stability theory. The influence of pressure gradients associated with separation and stall downstream also have the potential to introduce suction-side instabilities for a positive vertical offset. The influence of the wing is more severe for larger vortices and diminishes with vortex size due to weaker interaction and increased viscous stability. Helical instability is not the only possible outcome in a direct impingement. Jet-like vortices and a higher swirl ratio in wake-like vortices can retain stability upon impact, resulting in the laminar vortex splitting over either side of the wing.

JFM classification

Vortex Flows: Vortex instability Vortex Flows: Vortex interactions
Type
Papers
Information
Journal of Fluid Mechanics , Volume 799 , 25 July 2016 , pp. 128 - 158
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Copyright
© Cambridge University Press 2016. This is a work of the U.S. Government and is not subject to copyright protection in the United States. 

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Barnes et al. supplementary movie

Top view of instantaneous flow structure using an iso-surface of Q-criterion (Q=15) for several vertical positions. A dashed line roughly locates the onset of instability.

Download Barnes et al. supplementary movie(Video)
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