Experiments on the flow of water from a reservoir through an open horizontal channel I. The production of a uniform stream

If experiments on ship models held stationary on a stream of water are to be successful, it is clearly important that the undisturbed stream should be uniform in depth and velocity. With this aim an investigation was made of the flow through two horizontal channels 6 × 3 in. and 14 × 7 in. in cross-section fed from a reservoir through a rectangular contraction. Above a stream Froude number F = 1·3 in the smaller channel and 1·4 in the larger, the free surface was flat apart from ripples originating at the walls and disturbances due to the wake from the top of the contraction. At these limits a large wave, fixed in space, spontaneously appeared; and at lower velocities also, waves made the unobstructed channel useless for the purpose in view. With the top of the contraction removed from the larger channel and a weir used at the channel outlet, an acceptable stream with no measurable waves could be produced as long as F did not exceed about 0·5. Provided care was taken with the stilling arrangements in the reservoir, the velocity distribution in the channel was good and the stream was free from turbulence. Explanations are suggested of the existence of the waves and of the limiting value of F.