The main purpose of broad crested weir used in open channels is to raise and control upstream (U/S) water level. In this study, a new performance was added to this weir, by making a step at downstream (D/S) of weir. The energy dissipation, the height of the weir/the upstream water height ratio and Froude number relationships (E% − P/h-Fr) for three range of flume slop S = 0.0, 0.002 and 0.004 were simulated. The experiments were performed in a laboratory horizontal channel of 4.6 m length, 0.3 m width and 0.3 m depth for a wide range of discharge. The D/S step height of the weir was 7.5 cm. FLUENT software was used as numerical model which represent a type of Computational Fluid Dynamics (CFD) model in order to simulate flow over weirs. The Volume of Fluid (VOF) method with the Standard k − ε turbulence model was used to estimate the free surface profile and the structured mesh with high concentration near the wall regions. The experimental results of the water surface profile gave a high agreement with the results of the numerical models. The maximum value 28.78 of E% was obtained in single step broad crested weir in the experimental result and 27.35 in numerical result at S = 0.004. Finally, the range of the relative error of the energy dissipation between experimental and numerical results was achieved and the maximum was 6.76 in all runs.

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To predict flow pattern of shallow obstructed open channel flows, laboratory experiments of flow around a vertical emergent sidewall abutment (lateral constriction) in an open channel flow is simulated numerically and results of water surface and vertical profile of velocity are compared with the numerical results. The turbulence kinetic energy at the separation zone and bed shear stresses are also investigated. A computational fluid dynamic (CFD) numerical tool and volume of fluid (VOF) were used to predict water surface profiles. Results showed a very good agreement between the numerical and the experimental results of the water surface profiles. The numerical findings highlighted an increase in the velocity around the structure of two times the average flow velocity. The maximum pressure was observed in front of the constriction increases with increasing discharges. The vertical velocity profiles upstream the sidewall abutment at the three selected locations showed reasonable results. The maximum turbulence kinetic energy is shown at the zone of separation when the flow passes around the nose with high velocity. In this research, numerically it was found that the estimated bed shear stresses are 2-3 times the mean bed-shear stress of incoming flow. While, previous researchers finding showed a maximum value of bed shear stress near the leading edge 3.63 to 5 times the bed shear stress of the incoming flow. Because of the wide rang variation in calculating bed shear stresses by the previous researchers, so our research aims to calculate it using CFD and compare it with others. Also, dynamics of the flow around the obstacle was considered in this research under uniform flow conditions for an aspect ratio ranged 5 to 10.

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