Measurement locations from the current study are compared with measurement locations of (a) bird wakes and (b) bat wakes. The streamwise (x) and spanwise (z) locations have been normalized by the mean wing chord, c, and semi-span, b semi , of the respective study. Solid lines indicate that measurements were taken in a spanwise-normal plane. Dashed lines indicate that measurements were taken in a streamwise-normal plane. Dashed rectangles indicate the range from which streamwise-normal measurements were taken. Abbreviations used in the legend represent the thrush nightingale (TN), lesser short-nosed fruit bat (LSNFB), Pallas’ long-tongued bat (PLTB), southern long-nosed fruit bat (SLNB), and the lesser long-nosed bat (LLNB).
A schematic side view of the experimental setup. All dimensions are given in millimeters.
A comparison of the planform outline of the bird from Exp. 1 and Exp. 2.
Simultaneous realization of the bird and the wake. Streak lines originating from the wing root and wing tip have been added from a kinematic analysis after the experiment.
The mean position of the bird is shown in both (a) the x-y plane and (b) the z-y plane. In (a), the position of the bird's wing root relative to the PIV field of view, Δx and Δy, for various measurement sequences is shown. In (b), the position of the field of view relative to the bird is shown as well as the limits of the wings’ motion. The upper and lower angular extremes are indicated with USDS and DSUS, respectively. Uncertainty in these positions is indicated with error bars. The wing is shown both fully outstretched and flexed, representing mid downstroke and mid upstroke, respectively. The wing flexion during the upstroke is estimated from measurements of a thrush nightingale 35 and a house-martin. 13
The average velocity of the bird for various measurement sequences. Measurement sequences are described by the phase of the wing beat cycle and the spanwise offset of the bird from the PIV measurement plane.
An overview of the wake where the spanwise position at which the measurements were taken is indicated by the green line plotted over the planform outline of the bird. The wing tip and wing root traces are indicated by the dashed black lines. For scale, the length of the average chord is indicated.
The wake created by the downstroke to upstroke transition, as indicated by the wing tip trace (dashed black line). Measurements are taken at a spanwise location z/b semi ∼ 0.6. The vertical axis, y/c, has its origin at the bird's wing root. The streamwise axis, x c /c, has its origin arbitrarily set at the upstream edge of the wake composite. The downstream edge of each flow field measurement used to generate the composite is indicated with a short black line extending from the upper edge of the plot. For each of these measurements, the streamwise distance between the bird's wing root at the upstream edge of the PIV field of view, x/c, is indicated.
The wake generated by the downstroke to upstroke transition at a spanwise position slightly inboard of the wing tips, at z/b semi ∼ 0.45. Contour levels are the same as in Fig. 7 .
The wake generated by the wing at spanwise locations near the body at z/b semi ∼ 0.2. Vorticity at the downstroke to upstroke transition is still visible, but a greater disturbance is seen downstream of the vortex.
The wake sampled at spanwise locations near the center of the wake at z/b semi ∼ 0.1. Characteristics of a “double branch structure” are observed. Details of the axes and the color bar are included in Fig. 7 .
The wake generated by the downstroke to upstroke transition at approximately the centerplane. In (a) DSUS vorticity is observed underneath the wake of the body while in (b) the field of view is too high to show this feature.
The wake generated by the upstroke to downstroke transition relatively close to the wing tips, at z/b semi ∼ 0.35.
Circulation in vortices associated with the USDS transition and DSUS transition. In (a), an instantaneous contour map of swirling strength is shown. The contour level of ɛc/ U ∞ = 0.3 has been highlighted with a red dashed line. This contour is used to identify an area over which to integrate vorticity, (b), in order to calculate circulation of the vortex. In (c), the circulation of the strongest DSUS vortex in a given cycle is shown as a function of spanwise position. The wake from the USDS transition is not dominated by any particular vortical structure, therefore, the circulations of several individual vortices are plotted.
The wake generated by the upstroke to downstroke transition at a spanwise position of z/b semi ∼ 0.2. A double branch structure is observed.
The wake generated by the upstroke to downstroke transition at the approximate centerplane of the wake.
Morphological and experimental parameters.
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