Lowell hydraulic experiments, being a selection from experiments on hydraulic motors, on the flow of water over weirs by Francis James B

Author:Francis, James B[icheno], 1815-1892. [from old catalog]
Language: eng
Format: epub
Tags: Hydraulics, Turbines
Publisher: Boston, Little, Brown and company
Published: 1855-03-25T05:00:00+00:00

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EXPERIMENTS ON THE EFFECT PRODUCED ON THE FLOW OF WATER OVER WEIRS, BY THE HEIGHT OF THE WATER ON THE DOWNSTREAM SIDE.

137. These were made at the weir at the centre-vent wheel for moving the guard gates, with the apparatus used in the preceding experiments.

A singular phenomenon was here produced, namely: under particular circumstances, the flow of water over a weir may be increased by raising the height of the water on the downstream side of the weir. Ordinarily, when water flows over a weir having contraction on the bottom, the under side of the sheet near the weir, is elevated above the level of the top of the weir, taking a curved form; representations of this curve are given in several works, the most perfect of which are by M. M. Poncelet and Lesbros,* who ascertained with great care the forms for several depths upon the weir. In such cases, the space between the sheet of water and the plank or other material of which the weir is composed, is filled with air which communicates more or less freely with the external atmosphere.

Suppose the sheet, after passing the weir, to fall into a body of water of considerable depth, in which the natural level of the surface is not very much below the top of the weir, but sufficiently so, as not sensibly to affect the discharge. The weir having complete contraction, the air will remain under the sheet, even if the weir is of very considerable length in proportion to the depth flowing over. Suppose now, that the communication of the air under the sheet, with the external atmosphere, is entirely cut off by placing boards on the downstream side of the weir in contact with each side of the sheet, or by other means, the effect will ordinarily be, that the air under the sheet will be wholly or partially driven out by the lateral communication of motion in fluids, and a partial vacuum will be produced, unless water takes the place of the air that is driven out In either case, the equilibrium of the atmospheric pressure on the upper and lower sides of the sheet, will be destroyed, the pressure on the upper side preponderating, the effect will be to alter the form of the sheet, and to increase the discharge, by the operation of forces bearing some resemblance to the action in the well-known experiment with Venturis tube.

In the following experiments, this effect was produced by raising the level

* Experiences Bydraidiques sur les lots de Vecovlement, etc Paris: 1832. Plate 6.

of the water on the downstream side of the weir, to a height a little above the top of the weir, in consequence of which, by the lateral communication of motion, the air was driven out, and the flow over the weir facilitated.

During the following experiments, the apparatus was arranged in the same

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