Investigation of Flow Side Contraction in Combined Structure of Cylindrical Weir-Gate

In most cases flow control and measuring structures on open channels, are built in a length smaller than the channel width due to the high expenses or for corresponding to the channel cross section. Hydraulics of flow in such structures is different from full width structures. In the current experimental research the effect of contracting 2 sides of a cylindrical weir-gate on the combined discharge coefficient has been investigated. The experiments were done in a laboratory flume with 7.5 m length, 30 cm width and 46 cm height, using cylindrical weir-gate models with diameters of 75, 110, 125 and 160 mm, constant gate opening of 3 cm, similar contraction ratio for the gate and weir in the range of (0.2≤b/B≤1), and different discharges and flow depths for the free flow. The results showed that the combined discharge coefficient was directly related to the ratio of upstream water depth to the weir-gate height (H/P), the ratio of water head over the weir to upstream water depth (Hw/H), and the ratio of structure length to the channel width (b/B). On the other hand, when the ratio of (b/B) decreased, the rate of Cd changes vs. (H/P) decreased, too. At the end, the multiple nonlinear regression analysis was used to correlate the discharge coefficient of such structures to both hydraulic and geometrical effective parameters.