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.

The main purpose of this study is to test the ability of competition fluid dynamic (CFD) model which (FLUENT) program to simulate the flow over broad crested weir with lower cost and lesser time. This purpose is done by operating an experimental model in the laboratory and then validating this model in the GAMBIT program and FLUENT (ANSYS R 15.0), by comparing the results. The FLUENT can solve Navier- Stokes equations of the flow numerically, the volume of fluid method (VOF) and Standard k– ε turbulence equation is depended to simulate water level. Generally, it can be established that the experimental results of flow have a good agreement with the numerical results. The percentage of error (RE %) between numerical and experimental discharge is 2.94%, while the maximum RE % for water level is 6.25%. The numerical results of flow over broad crested weir showed in counter and vector results, and the streamlines of flow was clear than that of the experimental model. Finally, the FLUENT proved that can it be relied upon in future by designing weirs without needing to work experimentally.

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The flow over broad-crested weirs was simulated by computational fluid dynamic model. The water surface profile over broad crested weir was measured in a laboratory model and validated using two and three dimensional Fluent programs. The Reynolds Averaged Navier-Stokes equations coupled with the turbulent standard (k-) model and volume of fluid method were applied to estimate the water surface profile. The results of numerical model were compared with experimental results to evaluate the ability of model in describing the behaviour of water surface profile over the weir. The results indicated that the 3D required more time in comparison with 2D results and the flow over weir changed from subcritical flow at the upstream (U/S) face of weir to critical flow over the crest and to supercritical flow at downstream (D/S). A reasonable agreement was noticed between numerical results and experimental observations with mean error less than 2 %.

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