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m question is unable to penetrate into what should be the region of higher pressure beyond.
It would be a difficult matter to satisfy the necessary conditions for the walls of an expanding channel, even in two dimensions. The travelling bands of which the walls would be constituted should assume different velocities at different parts of their course. But it is quite possible that a very rough approximation to theoretical requirements would throw interesting light upon the subject, and I write in the hope of persuading some one with the necessary facilities, such as are to be found in some hydraulic laboratories, to undertake a comparatively simple experiment,
What I propose is the observation of the flow of liquid between two cylinders A, B (probably brass tubes), revolving about their axes in opposite directions. The diagram will sufficiently explain the idea. The circum-
ferential velocity of the cylinders should not be less than that of irrotational fluid in contact with the walls at the narrowest place. The simple motion may be unstable; but, as I have had occasion to remark before*, the critical situation would be so quickly traversed that perhaps the instability may be of little consequence. If no marked difference in the character of the flow could be detected by colour streaks, whether the cylinders were turning or not, the inference would be that Froude's explanation is inadequate. In the contrary event the question would arise whether practical advantage could be taken by specially stimulating the motion of fluid near the walls of expanding channels, e.g. with the aid of steam jets.
* Phil. Mag. Yol. xxvi. p. 776 (1913).    [This volume. Art. 376.]On the other hand, if in order to effect the conversion of velocity into pressure more rapidly, the expansion be made too violently, the fluid refuses to follow the walls, eddies result, and mechanical energy is lost by fluid friction. According to W. Froude's generally accepted view, the explanation is to be sought in the loss of velocity near the walls in consequence of fluid friction, which is such that the fluid