Experimental assesment of discharge coefficient in curved compound weirs

Weir structure is widely used as a flow measurement, grade control structure and water surface control in sewer systems, open channels and stilling basins. Having a simple stage-discharge relationship is the most important reason for using these structures as measuring instruments. Curved weir is part of a circle with the non-linear crest axis. To increase the discharge coefficient, one can used curved weir.

 In this study, hydraulic performance of curved weir and system of compound curved weir with central angles of 90, 120 and 150 degree and with 18 centimeter height were used to investigate and obtain equations for discharge coefficient and flow discharge of this structures.

The experiments were conducted in a rectangular channel with length of 9 meter, height and width of 40 centimeter. The results showed that the increase of parameters h/p < /em> (water head of the weir to the height of the weir), H/L (water depth upstream of the weir to the length of the weir), h/B (water head of the weir to the width of the channel) and Froude number led to increase discharge coefficient in simple curved weir and compound curves weir. Comparing the weir discharge coefficient showed that the curvature and compounding of weir has a great influence on the discharge coefficient of this structures. So that in simple curved weir, angle of 90 degree has a greater discharge coefficient, but in compound curved weir, angle of 150 degree has a greater discharge coefficient. Also checking of results showed that compound simple compound weir, led to increase value of discharge coefficient around 17 percent.

Checking of discharge coefficient on compound curved weir showed that, increasing dimension of second step in compound curved weir, led to increasing discharge coefficient. So that in discharge equals 12 (lit/sec) discharge coefficient of compound curved weir that height of second step to the width of second step (b2/b1) in that equals 0.26) towards compound curved weir that height of second step to the width of second step (h2/b1) in that equals 0.4 in vertex angles of 90, 120 and 150 respectively increasing 6, 1.5 and 7 percent .