Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/45

Rh that, according to the Eulerian theory, the lines of flow should carry the smoke along a symmetrical path to the opposite pole of the sphere, as in Fig. 45 (Chap. III.), the conclusion is plain.

The author has succeeded in photographing the flow round a cylinder in motion in a smoke-laden atmosphere (Fig. 9). In this example it may be noticed that the surface or stratum of discontinuity arises from a line some distance in front of the plane of maximum section; the difference in the behaviour of a cylinder and sphere in this respect is due to the fact that in the former case the lines of flow are cramped laterally, the motion being confined to two dimensions, whereas in the latter case, the motion being in three dimensions, the fluid can "get away" with greater facility. This difference is reflected in the lower coefficient of resistance found experimentally for the sphere than that ascertained for the cylinder. Thus in the experiments of Dines (§ 226) the pressure per square foot of maximum section on a ⅝-in, cylindrical rod was found to be more than double that on a 6-in, sphere, though doubtless the difference in size in the bodies compared may contribute something to the disparity.

The theory of discontinuity also receives support of the most convincing description from the experiments of Hutton, 1788, and Dines, 1889, by which it is shown that the pressure on a solid hemisphere, or a hemispherical cup (such as used on the Robinson anemometer), both in spherical presentation, does not differ from that on a complete sphere to an extent that experiment will disclose. This not only disposes of the streamline sphere of mathematical conception, but proves at the same time the approximate constancy of wake pressure under variation of rear body form. The same lesson is to be gleaned from experiments in the ease of the hemisphere, cone, and circular plate (all in base presentation), whose resistance is found to be approximately equal (Fig. 17).

§ 22. Wake and Counterwake Currents.—Reference has already been made to the frictional wake current to which a streamline