Figure 3
From: The strange flight behaviour of slowly spinning soccer balls
Unsteady forces on soccer balls, ball shift magnitudes and flow patterns visualised using the smoke method.
(a), Unsteady aerodynamic forces (CL, red line) and (Cs, blue line) on the type-A ball during a 10 s period (Re = 3.3 × 105, U = 22.0 m/s, 50 Hz sampling frequency). (b), Ten second ball shift in the Y-Z plane (U = 22.0 m/s). (c), Magnification of the Y-Z plane ball shift between 6 and 8 s (U = 22.0 m/s). (d), Images for each 0.128 s time step. During these periods, the vortices primarily existed in the lower right position with small variations. The two longitudinal vortices were in close proximity. Between 0.512 and 0.640 s, one vortex separated as it passed two vortices. (e–g), The mode change in the twin vortex images (e) to one vortex (f), followed by the two vortices or twin vortex pattern (g).
