Flow over Weirs

OBJECTIVE

To determine the coefficient of discharge C for Rectangular and Triangular weirs.

INTRODUCTION

Weir is a device used for measuring discharge in free surface flows such as rivers and open channels. A weir can be of different shapes –Rectangular, Triangular, Trapezoidal, etc. Figure (4.7) shows two different shapes, Rectangular and Triangular (or V-shaped) weirs. Triangular weir is practically suited for the measurement of small discharges.

APPARATUS

Figure (4.8) shows the arrangement in which water from the bench supply valve is led through a flexible hose to a pipe distributing the water fairly evenly in the enlarged end of the tank. A contraction section leads the water to a short channel, into which either the Rectangular or Triangular weirs may be fitted. Water flowing over the weir is collected in the exit tank, the outlet of which leads to the weigh tanks of the Hydraulic Bench. The water level in the channel may be observed in the still tube which is connected to the side of the channel. In this tube, there is a point gauge, where the hook is secured to a vertical screwed rod. By turning a nut at the top of the tube, the hook may be raised or lowered, its elevation at any time being read off a scale and markings on the nut. The screw rod has 1 thread per millimeter and the nut is marked off into equal divisions where each division represents tenth of millimeter.

PROCEDURE

1. Connect the bench supply hose to the inlet pipe. Push the flexible outlet hose into the pipe leading to the bench weighing tank (see Figure 4.8).

2. Insert the weir into the hydraulic bench.

3. Open the water supply until the level reaches the crest of the weir. Close the supply valve

4. Set the point gauge dial to zero and slide the hook up or down until the point just coincides with the water surface. Subsequent readings of the water level will then be relative to the true datum at crest level.