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First published online February 15, 2008
Journal of Experimental Biology 211, 717-730 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.012146
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Vortex wake and flight kinematics of a swift in cruising flight in a wind tunnel

P. Henningsson1,*, G. R. Spedding2 and A. Hedenström1

1 Department of Theoretical Ecology, Lund University, SE-223 62 Lund, Sweden
2 Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1191, USA


Figure 1
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  		Fig. 1. Rear view of the flying swift showing the method of digitizing from high-speed films. The shoulder joint (p1) and wingtip (p2) are located and with the measurements of distance in the horizontal ({Delta}y) and vertical ({Delta}z) direction between these, the stroke angle ({theta}) can be calculated. The peak-to-peak amplitude, 2A, is the distance between the wingtip at the upper and lower turning point.

 

Figure 2
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  		Fig. 2. The discrete classification system and notation. The LR position corresponds to the body, X is the arm section of the wing, and Y and Z are the inner and outer parts of the hand wing, respectively.

 

Figure 3
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  		Fig. 3. (A) The wingbeat frequency, f, decreases with increasing flight speed, U, from 9.1 Hz at 8.0 m s–1 to 8.3 Hz at 9.2 m s–1. (B) Upper (u), lower (d) and total (tot) angular excursion, {theta}, of the wing in the vertical plane as a function of flight speed, U. The increase in {theta}tot(U) comes only from the increase in {theta}u(U). (C) Amplitude, A, normalized by mean chord length, c. (D) The downstroke duration, Td, increases with flight speed, U. The downstroke fraction, {tau}, does not vary with flight speed, U. (E) The wing angular velocity at mid-upstroke and -downstroke, Formula, does not vary greatly with flight speed, U. (F) Wing semispans, bu,d at mid-upstroke and -downstroke normalized by mean chord length (c) vs flight speed, U. Wing semispan does not vary with flightspeed. The span ratio, R, does not change with flight speed, U. Error bars represent ±s.e.m.

 

Figure 4
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  		Fig. 4. Reconstruction of the vortex wake from a complete wingbeat at different spanwise locations: (A) arm wing (X in Fig. 2), (B) mid-hand (Y in Fig. 2), (C) outer-hand (Z in Fig. 2), and (D) body (LR in Fig. 2). The mean flow has been subtracted so the reference frame is in still air, as though the bird has passed from right to left. The spanwise vorticity, {omega}y(x,z), is colour coded on a constant scale, symmetric about 0 s–1. Velocity vectors are drawn with 1/3 of the actual density.

 

Figure 5
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  		Fig. 5. {omega}y(x,z) for outboard cuts through the edge of wingtip vortices on the upstroke (A) and downstroke (B).

 

Figure 6
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  		Fig. 6. {omega}y(x,z) for wakes generated during short gliding phases: (A) body (LR in Fig. 2) and (B) mid-hand (Y in Fig. 2).

 

Figure 7
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  		Fig. 7. Normalized total circulation in each phase of the wingbeat, where phase 5 at the right of the abscissa is the beginning of the downstroke and phase 1 at the left marks the end of the upstroke: (A) inner wing (X in Fig. 2), (B) mid-hand (Y in Fig. 2), (C) outer-hand (Z in Fig. 2) and (D) directly behind the body (LR in Fig. 2). Error bars represent ±s.e.m.

 

Figure 8
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  		Fig. 8. Maximum observed circulation of any coherent structure in the swift wake relative to two reference circulations, {Gamma}0 and {Gamma}1, for the power-glider and pulsed ring generator, respectively. On the abscissa, LR to Z are positions given in Fig. 2.

 

Figure 9
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  		Fig. 9. The variation in normalized circulation over the five wingbeat phases for the swift and for the thrush nightingale (TN) at two flight speeds, U=7 and 10 m s–1, that describe medium- and high-speed flight, respectively, in wind tunnel conditions. The swift data come from Fig. 7A and the thrush nightingale data from the database described in Spedding et al. (Spedding et al., 2003bGo). The data from both species come from the inner wing.

 

Figure 10
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  		Fig. 10. Three-dimensional wake structure of the swift in cruising flight. The image frame is as if the bird flew obliquely from right to left and slightly into the paper, leaving the trace of one wingbeat in still air, starting at the upper turning point. Green tubes show the wingtip vortices, cylinders in shades of red are spanwise vortices with positive circulation and cylinders in shades of blue have negative circulation. The colour intensities and tube diameters are proportional to the strengths of each component. The geometry is deduced from a combination of wingbeat kinematics and streamwise plane section data.

 

Figure 11
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  		Fig. 11. Approximating the wake shape by ellipses. Flight direction is from right to left, and wingtip traces are viewed from the side (A) and above (B). Heavy lines denote ellipses fitted to the down- and upstroke wake geometry. The ellipses are seen from above in B and obliquely from the side in C, and their projection onto the horizontal plane is shown in D.

 

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© The Company of Biologists Ltd 2008