"Air vortex rings" . "toroidal" . . . . "separated vortex ring" . . . "doughnut-shaped air vortices" . . "Vortex ring" . . . . "ring vortices" . "vortex rings" . "vortex ring" . "vortex_ring#Vortex_ring_formation_and_structure" . . . "Vortex ring" .