James Akkala

Vorticity Generation and Transport on a Plunging Wing

Two-component particle image velocimetry and surface pressure measurements are used to characterize the flow field over a plunging nominally two-dimensional flat-plate airfoil at zero geometric angle of attack, and a finite wing with rectangular planform and a semiaspect-ratio sAR = 2. Phase-averaged horizontal and vertical planes of PIV data are used to reconstruct a three-dimensional volume in which the evolution of the vortex structure is rendered, and vorticity transport is quantified within a chordwise planar control volume bounded by the flat plate surface, and containing the leading-edge vortex. It is shown that, for the two-dimensional airfoil, generation of secondary vorticity of opposite sign to the leading-edge vortex occurs at a rate of approximately half that of the leading-edge shear layer flux, suggesting that entrainment of this vorticity into the leading-edge vortex has a significant impact on the strength of the vortex. Also, spanwise convection of vorticity has a non-negligible impact on control-volume circulation during the second half of the stroke. In the case of the finite wing, the initial development of the leading-edge vortex is qualitatively similar to that of the nominally two-dimensional case; however, through the mid-portion of the stroke, the leading-edge vortex rapidly evolves into an arch structure as it convects along the chord, as seen in previous studies. In contrast to the case of the nominally two-dimensional airfoil, spanwise flow acts to significantly deplete the circulation within the leading-edge vortex. The difference between control-volume circulation and the sum of the integrated convective boundary fluxes suggests that the fraction of the total vorticity flux supplied by the finite wing surface beneath the leading-edge vortex is similar to that of the two-dimensional case.

The Role of Vorticity Transport in the Detachment of Unsteady Leading-Edge Vortices

Multi-plane particle image velocimetry data and surface pressure measurements are used to analyze the nominally two-dimensional flow field of a high-aspect-ratio flat-plate airfoil undergoing a pure-plunge motion, as well as the three-dimensional flow field produced by a plunging flat-plate wing with aspect-ratio 2. The sources and sinks of vorticity within these flows were quantified by means of a vorticity flux analysis, the results of which verified prior conclusions that the diffusive flux of vorticity from the surface of the airfoil acts primarily as a sink of leading-edge-vortex (LEV) vorticity, with a magnitude roughly half that of the flux of vorticity introduced by the leading-edge shear layer. Inspection of the chordwise distribution of the surface diffusive flux of vorticity within the 2D case showed it to correlate very well with the evolution of the flow field. Specifically, the diffusive flux experienced a major increase during the phase interval in which the LEV was still attached to the downstream boundary layer, which ultimately triggered the roll-up of the LEV and generation of the secondary vortex. It was also noted that the accumulation of secondary vorticity near the leading edge prevented the diffusive flux from continuing to increase after the roll-up of the LEV. This result was validated through analysis of the 3D case, which demonstrated that maintaining an LEV that stays attached to the downstream boundary layer produces a larger diffusive flux of vorticity–presumably consistent with an enhancement of both lift and thrust. Despite the increased diffusive flux, this state was maintained by a strong spanwise convective flux of vorticity. This observation provides a new interpretation of the role of spanwise flow on vortex evolution, and suggests a physical mechanism that may be leveraged to control the flow.