Figure 5.

Vortex wakes at flight speed, U=4 and 8 m s⁻¹ for a house martin, as shown by composite colour coded spanwise vorticity, ω_y, with superimposed velocity field vectors. The individual panels originate from different series of data due to the phase-lock of laser and wingbeat frequency at 10 Hz. Data are from the vertical centreline in (a) and (b) and from mid-wing in (c). The bird silhouette is drawn to scale, but the data are taken about 22 chord lengths downstream of the bird. The boxes in (b) and (c) show the wake signature of the wingbeat pause in mid-upstroke. ω_y is symmetrically mapped onto a 10-step colour bar (ω_y is ±4U/c at 4 m s⁻¹ and ±U/2c at 8 m s⁻¹), and the resolution represents the uncertainty in the measurements. The wavelengths of the downstroke, λ_d, and upstroke, λ_u, and the mean chord, c, are shown for scale reference to (a) and (b).