Abstract: Due to streamline curvature, non-hydrostatic pressure distribution, and nappe adherence on the weir wall, the discharge coefficient of weirs is less than 1.0. However, in cylindrical weirs, the discharge coefficient extends to values more than 1.0 due to the different pressures and velocity distributions. In this study, pressure and velocity distributions of different circular weirs are simulated by computational fluid dynamics (CFD) and Fluent software and compared with experimental results. The numerical results of CFD show a significant correlation with measured data from manometers and a laser Doppler velocimeter (LDV). Experimental observations show that the critical flow depth forms before and after the crest for greater and smaller discharge coefficients, respectively. Nappe separation depends on overflow discharge and will shift to the downstream face of the cylinder in high discharges. To recognize the location of critical flow and nappe separation, analytical formulations are proposed depending on weir size and inflow conditions. The results show good agreement between the analytical predictions and experimental observations.

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