Investigating the Effect of Side Contraction on the Discharge Coefficient of Cylindrical Weirs

Cylindrical weirs are a kind of flow measurement structures in open channels whose crest length is affected by the channel cross section and economical expenses. The current experimental research investigates the effect of side contracting on flow characteristics such as discharge coefficient of cylindrical weirs. The experiments were done on a laboratory flume with dimensions of 7500*300*460 mm, using various diameters of cylindrical weir models in the range of 75≤P≤160 mm, contraction ratios of 0.2≤b/B≤1, and various combinations of discharge and flow depth for free flow. The results show that an increase in the ratio of upstream water depth to the weir height (H/P), leads to an increase in the discharge coefficient for all weirs, but when b/B decreases, the gradient of Cd vs. H/P diagram, will decrease too. On the other hand, for any given P and H, the discharge coefficient will increase by increasing the b/B ratio. The contraction coefficient (Cc) for such weirs, determined by using weir theory and experimental data, varies in the range of 0.71 to 0.98 for 1.2≤H/P≤2.4. These results also show when H/P increases, Cc will increase for all weirs too. Besides, for any given H/P ratio, the flow contraction coefficient over the weir crest will decrease with a decrease in the b/B ratio.