rise to surfaces of discontinuity (Fig. 19). Then the resistance will be increased, and the layer of fluid next the body will be violently stirred up; but if we examine the fluid some distance away we shall still find it comparatively unaffected. If we now suppose the body to consist of a few large blocks, the depth of fluid affected by turbulence will be greater, but at a sufficient distance away we may still expect to find lines of flow of characteristic streamline form. We may therefore generalise and say, All bodies passing through a fluid are surrounded by a streamline system of flow of a greater or less degree of perfection depending upon the conformability or otherwise of the surface or surfaces of the body.
This proposition, if not sufficiently obvious from the
Fig. 19.
considerations above given, may easily be demonstrated experimentally.
In the experiment described in § 17, the orbital motion of the particles of the fluid is demonstrated by the motion of an ichthyoid body in air irregularly charged with smoke. This orbital motion, with its consequent displacement, is quite characteristic, and if other shapes of body be substituted for the streamline form, the motion of the fluid a short distance away is not sensibly affected. In the case of a body of streamline form, the motion can be observed much closer to the axis of flight than is the case for a sphere or other bluff form; also when the movement is complete nothing further happens. In the case of a sphere, the looked-for movement duly takes place; but immediately after the whole of the fluid under observation is involved in
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