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摘要
We employ experiments to study aspect effects on the vortex structure, circulation and lift force for flat-plate wings rotating from rest at 45 angle of attack, which represents a simplified hovering-wing half-stroke. We use the time-varying, volumetric AR = 2 data of Carr et ol. (Exp. Fluids, vol. 54, 2013, pp. 1-26), reconstructed from phase-locked, phase-averaged stereoscopic digital particle image velocimetry (S-DP1V), and an AR = 4 volumetric data set matching the span based Reynolds number (Re) of IH= 2. For.,41= 1-4 and Reva of 0(103-104), we directly measure the lift force. The total leading-edge-region circulation for = 2 and 4 compares best overall using a span-based normalization and for matching rotation angles. The total circulation increases across the span to the tip region, and is larger for 7R, = 2. After the startup, the total circulation for each M. has a similar slope and a slow growth. The first leading edge Voir te x (LEV) and the tip vortex (TV) for 4 move past the trailing edge, followed by substantial breakdown. For.,4e 2 the outboard, aft tilted LEV merges with the TV and resides over the tip, although breakdown also occurs. Where the LEV is 'stable' inboard, its circulation saturates for.,.47.= 2 and the growth slows for.R = 4. Aft LEV tilting reduces the spanwise LEV circulation for each JR. Both positive and negative axial flow are found in the first LEV for "R.= 2 and 4, with the positive component being somewhat larger. This yields a generally positive (outboard) average vorticity flux. The average lift coefficient is essentially constant with., AR, from 1 to 4 during the slow growth phase, although the large time behaviour shows a slight decrease in lift coefficient with increasing R. The S-DPIV data are used to obtain the lift impulse and the spanwise and streamwise components contributing to the lift coefficient. The spanwise contribution is similar for AR = 2 and 4, due to similar trailing-edge vortex interactions, LEV saturation behaviour and total circulation slopes. However, for AR = 2 the streamwise contribution is much larger, because of the stronger, coherent 'TV and aft-tilted LEV, which will create a relatively lower -pressure region over the tip.
- 出版日期2015-3
