Vortex ring

From Free net encyclopedia

A vortex ring is a mass of moving fluid moving through the same or different fluid where the flow pattern takes on a donut shape. The movement of the fluid is about the poloidal or circular axis of the donut, in a twisting vortex motion.

1 Vortex ring formation and structure
2 Vortex ring effect in helicopters
3 See also
4 External links
5 Reference

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Vortex ring formation and structure

One way a vortex ring may be formed is by pushing a spherical mass of fast moving fluid (A) into a mass of stationary fluid (B). A and B may chemically be the same fluid. As B hits the ball of A it pushes the outer layers of A with it. The inner layers are less affected. The main mass of A forms a 'shadow' of lower pressure behind it, and the layer peeled off by B begins to curve round back into the main mass of A. This inward curving flow initiates the vortex, and splits it into a donut shape. Now B flows past both the inner and outer circumferences of the donut. The greater outer perimeter causes a net rolling the donut of A.

The leading edge of a plume, sometimes called the 'starting-plume', usually has a vortex-ring structure, as does a smoke ring. The motion of an isolated vortex ring and the interaction of two or more vortices are discussed in eg Batchelor's text book (ref 1)

For many purposes a ring vortex may be approximated as having a vortex-core of small cross-section. However a simple theoretical solution, called Hill's spherical vortex, is known in which the vorticity is distributed within a sphere (the internal symmetry of the flow is however still annular). Such a structure or an electromagnetic equivalent has been suggested as an explanation for the internal structure of ball lightning.

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Vortex ring effect in helicopters

Image:Vringrah66.jpg In typical flight, the rotor disc directs the airflow downwards, creating lift. A vortex ring state though involves a toroid-shaped path of airflow circumscribing the blade disc, as the airflow moves down through the disc, then outward, and then down through the top again. This circulation can negate much of the lifting force and cause a catastrophic loss of altitude.

A helicopter typically induces a vortex ring state by descending into its own downwash. This requires low airspeed and a moderate rate of descent with power applied, and can lead to an undesirable phase of flight known as settling with power. This condition can be corrected by lowering the collective, which controls the pitch angle of the rotor blade, slightly pitching nose down, and establishing forward flight. The aircraft will fly into "clean air", and will be able to regain lift.

A clear understanding of this condition is essential for helicopter pilots to avoid danger.

On a fast descent, no vortex will form because the vertical airspeed is faster than the recirculation speed - although rapid descent through one's own downwash is itself a highly dangerous manoeuvre.
With high airspeed, no vortex will form because the translational airflow is faster than the recirculation speed.

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