Modelling of Composite Broad Crested Weir

Abstract

The current study deals with a composite broad crested (CBC) weir which is specially designed with the unique idea for a “constant coefficient of discharge (C_d) of 0.6”. It is investigated experimentally as well as numerically. The available designs of weir are unable to give constant C_d over a wide range of discharge as ‘C_d’ itself is relative of head over weir crest. Therefore, an attempt is made to restrict the C_d value to 0.6 irrespective of variable head on weir crest. This is achieved by controlling the head over the weir by adjusting the widths of weir. In case of rectangular broad crested weirs, width ‘b’ is constant and the only variable is head over weir crest ‘h’. Even though C_d is treated as constant, practically it also varies within some range. With the novel objective, C_d is frozen to a constant value of 0.6 and instead of it, ‘b’ is allowed to vary. The weir so designed is capable of producing constant C_d over a wide range of discharge and hence will be helpful from the viewpoint of field applications. Under existing laboratory conditions, the research is done for emissions varying from 100% to 20% of the design discharge. The numerical performance of CBC weir through FLOW 3D is experimentally validated to examine the crest width effect and head over weir crest. In the experiments conducted, C_d is found to vary proportionally with discharge in the range from 0.518 to 0.648. The coefficient of correlation is 0.999, with a mean error in discharge measurement being much less.