Experimental and numerical Study of Hydraulic characteristics of flow over the sharp-crested weirs in the effect of increasing upstream bed level

Abstract
Background and objectives: Sharp crested weirs are used for the purpose of flow measurement, flow diversion and water level control in hydraulics, irrigation, and environmental projects. So exploring the features and characteristics of the hydraulic properties are an important issue in the design of these structures. Various studies have been done about sharp-crested weir. Few studies have been done about the impact of inequalityin the upstream and downstream bed level on hydraulic properties. The sharp-crested weirs like other weirs, unequal in the upstream and downstream bed level (such as the Check drop) cause changes on the hydraulic characteristics that must be studied.
Materials and Methods: Research conducted on Hydraulic laboratory which Situated in Research Institute of Soil Conservation and Watershed Management. The experiments were performed in the flume with 14 meters length, width of 60 cm and a height of 50 cm. Sharp crested weirs was built of Plexiglas with a thickness of 6 mm, edge thickness of 2 mm, a height of 20 cm and a length of 60 cm in the workshop and was placed within the flume. Upstream bed level increased with proper materials in three level 5, 10 and 15 cm from floor. At any stage, values of the weir crest level and upstream and downstream water level were recorded for different discharges. Computational Fluid Dynamics (CFD) was used to generalize the results. For this purpose, FLOW 3D software was used for modeling of Free-surface flow over weir. In This software, weir and it’s free surface are considered by using Fractional Area Volume Obstacle Representation and Volume Of Fluid methods respectively. The governing equations were Navier-Stokes and continuity equations for incompressible flows. For modeling turbulence, was used Re-Normalization Group (RNG) model.
Results: The results showed a good agreement Between experimental data and numerical simulation. changing procedure of discharge coefficient was the same in both methods. Maximum deference in the H, extracted from two methods, is 5% that is acceptable. The results showed that by increasing the upstream bed level, the upstream flow depth decreases, velocity and Froude number increase. But rising the upstream bed level to 0.75 (Z/P=0.75) does not affect on the discharge coefficient. In numerical method, Discharge coefficient values for H/P≥0.5 can be considered the average value of 0.73 for all cases. With increasing discharge Froude numbers are converging in different ratios of upstream bed level. In the special case where the upstream bed level is rised to Crest (vertical drop or Z/P=1), the discharge coefficient value will be 0.6. This value is the lowest between all cases and its magnitude is equal to discharge coefficient of the broad-crested weir. So in this case, the level of water is higher than the same rate of discharge in the other cases and this difference goes up by increasing discharge. In Z/P=1 he Froude number will be equal to a fixed value Fr = 0.94Cd.
Conclusion: In summary it can be concluded that by increasing the upstream bed level, the Froude number will increase and thus the nape becomes more horizontal. In the range of H / P≥0.5, except when the Z/P tend towards one, in other cases, the rising bed level and the increasing H/P have no significant impact on discharge coefficient. In the Z/P
Keywords: vertical drop, sharp-crested weirs, discharge coefficient, Froude number, Check Drop