region of cavitation in fact, whose pressure is zero. Spiral vortices of this type occur when a screw propeller gives rise to cavitation, the void being filled with water vapour. In other cases the core is constituted by a region filled with some other fluid. We again find an example in the motion produced by a screw propeller when the tips of the propeller emerge from the surface and carry down with them into the water a quantity of air—such vortices may frequently be seen astern of a vessel when steaming under a light load.
A vortex cylinder or filament may be regarded as a portion of a vortex ring of infinite diameter; it can only exist either if infinitely long or if its ends terminate on boundary surfaces.
Fig. 52.
A single straight filament in infinite space is theoretically stable without motion of translation; two such filaments in the neighbourhood of one another mutually interact, and are only stable with superposed motion.
The superposed motion proper to two filaments depends upon their relative position; parallel filaments of like rotation rotate round one another at a velocity proper, each to each, to the cyclic motion of its neighbour, as in Fig. 52 ; parallel filaments with counter-rotation are in equilibrium when possessed of motion of translation (Fig. 52 ); when two such vortices are equal to one another the combination is termed a vortex pair, and
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