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Fig. 3. Spanwise vorticity,
y(x,z) for a mid-wing slice
taken at x=17c for the thrush nightingale in the Lund wind
tunnel. The reference frame moves with the mean flow, as for flight from right
to left through still air. A sequence of four frames from consecutive wing
beats is collected to cover just over one wake wavelength (
). + signs
show patches positive vorticity shed at the beginning of the downstroke; there
are two in this sequence. The equivalent patch of negative vorticity, noted by
the – sign, is much more diffuse. From the same dataset (but a different
sample) as Spedding et al. (Spedding et
al., 2003b).