Vorticity transport is analysed within the leading-edge vortex generated on a rectangular flat plate of aspect ratio 4 undergoing a starting rotation motion in a quiescent fluid. Two analyses are conducted on the inboard half of the blade to better understand the vorticity transport mechanisms responsible for maintaining the quasi-equilibrium state of the leading-edge vortex. An initial global analysis between the 25 and 50 % spanwise positions suggests that, although spanwise velocity is significant, spanwise convection of vorticity is insufficient to balance the flux of vorticity from the leading-edge shear layer. Subsequent detailed analyses of vorticity transport in planar control volumes at the 25 and 50 % spanwise positions verify this conclusion and demonstrate that vorticity annihilation due to interaction between the leading-edge vortex and the opposite-sign layer on the plate surface is an important, often dominant, mechanism for regulation of leading-edge-vortex circulation. Thus, it provides an important condition for maintenance of an attached leading-edge vortex on the inboard portion of the blade.

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The vortex dynamics during the initial rotation of a rotating blade were investigated using two-component and stereo digital particle image velocimetry. The structure and dynamics of spanwise vorticity in the leading-edge vortex (LEV) was investigated on two rotating flat plates of aspect ratios (AR), 2 and 4. Reynolds numbers of 4,000, 8,000, and 16,000, based on tip velocity, and angles of attack of 25 • , 35 • , and 45 • were investigated at five azimuthal locations (ψ = 90 • , 180 • , 235 • , 270 • , and 320 • ), and two span locations (25% and 50%). Circulation measurements of the leading-edge vortex structure exhibit trends of increasing circulation with increasing Reynolds number and angle of attack, independently, for both aspect ratios. In the particular case of the aspect ratio 4 plate at the 25% spanwise position and the aspect ratio 2 plate at the 50% spanwise position, measurements demonstrate that the LEV of the smaller aspect ratio plate has lower circulation for a given angle of attack and Reynolds number (based on the local velocity). A strong region of counter-rotating vorticity was observed on the surface of the plate due to the interactions of the LEV and the body. It is proposed that the interactions between the LEV and counterrotating surface vorticity play an important role in governing the dynamics and strength of the LEV which may ultimately determine whether the LEV remains attached. An analysis of vorticity transport in the LEV was conducted to estimate the relative contributions of spanwise flux, tilting, the shear layer, and annihilation to the rate of change of circulation of the LEV. Stereoscopic particle image velocimetry (SPIV) was implemented at three parallel chordwise planes to obtain the three components of velocity and vorticity vectors needed for the analysis. Results of the vorticity transport equation indicate that annihilation of the LEV due to entrainment of the surface vorticity is an important factor governing the dynamics of the LEV.