1. Fiery vortex rings are also a commonly produced trick by fire eaters. 2. The phenomenon, known as vortex ring state, can occur with any helicopter. 3. The formation, motion and interaction of vortex rings have been extensively studied. 4. This applies, in particular, to the formation and motion of vortex rings . 5. In the 1960s, Rayfield and Reif established the existence of quantized vortex rings . 6. If altitude permits, autorotations may also be used to recover from vortex ring state. 7. He is also ordering further tests into vortex ring state. 8. And a Pentagon report released last month recommended further testing to diagnose vortex ring state. 9. Devices have been invented which generate bubble vortex rings . 10. A vortex ring can also be created by a spinning propeller, as in a blender.
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