Figure 11.

Conceptual diagrams of hydrodynamic forces acting on the hand during ‘S’ shaped (upper panel) and ‘I’ shaped (lower panel). By the end of insweep in ‘S’ shaped, a clockwise vortex has formed near the thumb. As the hand changes from insweep to upsweep, this vortex sheds, forming a counterclockwise-bound vortex around the hand. This circulatory flow combines with the thrust, producing lift on the hand. In ‘I’ shaped, near the middle of the motion at which the thrust is maximum, a Kármán vortex street has formed from which clockwise and counterclockwise vortices are alternatively shed from the hand. At this point, the pressure difference between the dorsal and palm sides of the hand are large, producing drag that contributes to the thrust (Takagi et al., 2014a).