The aerodynamics ofManduca sexta: digital particle image velocimetry analysis of the leading-edge vortex

Abstract

SUMMARY Here we present the first digital particle image velocimetry (DPIV)analysis of the flow field around the wings of an insect (the tobacco hawkmoth Manduca sexta, tethered to a 6-component force-moment balance in a wind tunnel). A leading-edge vortex (LEV) is present above the wings towards the end of the downstroke, as the net upward force peaks. Our DPIV analyses and smoke visualisations match the results of previous flow visualisation experiments at midwing, and we extend the experiments to provide the first analysis of the flow field above the thorax. Detailed DPIV measurements show that towards the end of the downstroke, the LEV structure is consistent with that recently reported in free-flying butterflies and dragonflies: the LEV is continuous across the thorax and runs along each wing to the wingtip, where it inflects to form the wingtip trailing vortices. The LEV core is 2-3 mm in diameter (approximately 10% of local wing chord) both at the midwing position and over the centreline at 1.2 m s-1 and at 3.5 m s-1flight speeds. At 1.2 m s-1 the measured LEV circulation is 0.012±0.001 m2 s-1 (mean ± s.d.) at the centreline and 0.011±0.001 m2 s-1 halfway along the wing. At 3.5 m s-1LEV circulation is 0.011±0.001 m2 s-1 at the centreline and 0.020±0.004 m2 s-1 at midwing. The DPIV measurements suggest that if there is any spanwise flow in the LEV towards the end of the downstroke its velocity is less than 1 m s-1. Estimates of force production show that the LEV contributes significantly to supporting body weight during bouts of flight at both speeds(more than 10% of body weight at 1.2 m s-1 and 35-65% of body weight at 3.5 m s-1).

Aerodynamics of Flapping Wing at Low Reynolds Numbers: Force Measurement and Flow Visualization

“…The same can be realized by taking various cross-sections along the chord and span; it can be seen that there is a single vortex which connects the whole periphery of the wing. The same has been shown by many in the literature [2][3][4][5][6][7].…”

“…The same has been achieved by smoke flow visualization over flapping wing in a wind tunnel [3] for a chosen kinematics and by PIV [4] for the dynamics of these vortex dominated flows [5][6][7]. Significant observations have been made in identifying the important governing parameters like Reynolds number (Re) range, advance ratio (J) and phenomena like the leading edge vortex domination [4,8] of these flights hindering the flow separation, wake capture, clap and fling [8], and so on. Based on earlier studies flapping flight can be broadly divided into two categories: quasisteady for J > 1 and unsteady flight for J < 1 [9].…”

“…While flights of large birds can be approximately explained using quasisteady theory, the insects' and small bird's flight are highly unsteady. Further references of the subject can be obtained from [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].…”

Leading edge vortex in a slow-flying passerine

“…This is different from the spanwise LEV distribution observed in all previously studied animals, where the LEV strength consistently increases towards the wingtip [7,8,10,14,19]. The spanwise reduction in LEV strength at the hand wing parallels a spanwise reduction of a eff (figure 2b), which is achieved by a combination of two mechanisms.…”

“…LEVs, on the other hand, are typically produced by wings at relatively high angles-of-attack (a . 208), separation typically occurs at the leading edge, and LEV height is similar to its chordwise width [7][8][9]13,19]. As the flycatcher wing operates at a relatively high angle-of-attack during the downstroke (figure 2d ), the AVS is located at the leading edge and it is almost circular in shape (figure 1), we think the observed AVS should be Leading edge vortex in bird flight F. T. Muijres et al 555…”