Figure 1.
Illustrative scheme of the avian wind tunnel and the experimental setup system.
Figure 2.
The large image shows the kinematic camera field of view and the small window marked “PIV” is the PIV camera field of view.
Figure 3.
Variation of the average streamwise velocity in the wake with time.
Figure 4.
Different positions of the starling during the four wingbeats.
The labels of the photographs correspond to the wingbeat number; ‘a’ marks the beginning of the downstroke, and ‘b’ marks the beginning of the upstroke. Each image center embedded with a red dashed cross lines for a visualization of the starling’s position during each wingbeat.
Table 1.
The wingbeat frequency, Strouhal number and the wingtip angle of attack for the four wingbeat cycles.
Figure 5.
An estimation of the wing’s maximum angle of attack from its instantaneous pitch and velocity relative to the air.
In (a), the left wing of the bird points directly at the camera, and the pitch of the chord line at approximately (2/3)bsemi is represented by the blue line. In (b), the pitch of the chord line and the velocity of the wing section at (2/3)bsemi are used to estimate the angle of attack, α, as approximately 15°.
Figure 6.
A comparison of the peak normalized spanwise vorticity, ωc/U∞, measured in the wake of flapping animals from several studies.
Superscripts refer to the work of: 1 [20], 2 [22], 3 [45], 4 [4] and 5 [9]. Abbreviations used in the figure represent the Downstroke to upstroke transition (DSUS), upstroke to downstroke transition (USDS), the thrush nightingale (TN), house-martin (HM), and Pallas’ long-tongued bat (PLTB). Measurements from the Pallas’ long-tongued bat have been taken from the inner wing (z/bsemi<0.4) and outer wing (z/bsemi>0.75).
Figure 7.
Two consecutive spanwise vorticity fields (t2 = t1+2 msec). The air flows from left to right and each frame size is 9×9 cm2.
Figure 8.
Reconstruction of the starling’s wake consisting of four wingbeats as though the starling flew from right to left.
The average spatial flow has been subtracted, thus the vectors displayed are the velocity fluctuations. The contours represent the spanwise vorticity in each wingbeat and the half-wavelengths for the downstroke (λd) and the upstroke (λu) are noted for each wingbeat. Wingbeat numbers go from top to bottom: (a) 1, (b) 2, (c) 3, and (d) 4.
Figure 9.
The location of the laser sheet with respect to the right wing during wingbeats 1, 2, 3 and 4.
Figure 10.
Control volume around a two-dimensional bird wing section in a uniform free stream.
Figure 11.
Steady drag per unit span versus time, as computed according to Eq. (10), for wingbeats no. 1 (a), 2 (b), 3 (c) and 4 (d).
Figure 12.
Unsteady drag per unit span versus time, as computed according to Eq. (11), for wingbeats no. 1 (a), 2 (b), 3 (c) and 4 (d).
Figure 13.
Averaged steady and unsteady drag per unit span versus time, as computed according to Eq. (10) and (11).
Figure 14.
Schematic examples of a drag wake (a), momentumless wake (b) and a jet wake (c).