Passive Control of Wing-Body Junction Flows

When a boundary layer (either laminar or turbulent) encounters an obstacle protruding from the surface, some distance upstream of the obstacle, it separates as a result of the adverse pressure gradient and rolls-up creating... [ view full abstract ]

When a boundary layer (either laminar or turbulent) encounters an obstacle protruding from the surface, some distance upstream of the obstacle, it separates as a result of the adverse pressure gradient and rolls-up creating complex vortices, known as horseshoe vortices. These vortices often have large effects on the flow properties in the junction region, and can change the local heat transfer rates, increase skin friction and produce noise. The horseshoe vortices are encountered in the junction regions of, for example, airplane wings, pin fins found within channels or nozzles, guide vanes in gas turbine engines, turbomachinery blades, the bases of bridge piers, computer chips, ship appendages and hulls. The present study, experimentally investigates a passive control methodology for wing-body junction flows. This control method is based on the use of a flat plate at the wing-body junction, which starts at the leading edge of the wing and extends upstream. A range of plate lengths in the upstream direction, plate heights from the body, plate thicknesses as well as shapes (rectangular/triangular) are investigated with the aim of defining the control plate parameters that decrease the magnitude of the primary horseshoe vortex ahead of the wing and thereby lead to an increase in its aerodynamic performance. The wing model that is used is that of a NACA 0020 airfoil with a 10” chord length and a 22” wing span. The effects of the angle of attack on the control method are also investigated at incremental angles, with a maximum angle of 15°. The model is tested in a recirculating water tunnel located at the University of Toronto Institute for Aerospace Studies. A range of Reynolds numbers (based on the chord) with a maximum of Rec=50,000 is considered. The experiments characterize the effects of the passive control methodology on the formation and magnitude of the horseshoe vortices, for each plate configuration considered in this study. To this end, the tests begin with qualitative flow visualizations using the hydrogen bubble technique, and continue with Particle Image Velocimetry (PIV) measurements. The horseshoe vortices are identified and compared for different plate designs using the Δ-criterion defined by Chong et al. (1990), which utilizes the complex eigenvalues of the velocity gradient tensor. All design iterations of the proposed control method are reviewed in terms of their effects on the flow.


References
Chong, M. S., Perry, A. E. & Cantwell, B. J., 1990. A general classification of threedimensional flow fields. AIP Physics of Fluids A: Fluid Dynamics, Volume 765.