New insights into flow over sharp-crested and pivot weirs using computational fluid dynamics

2021 Summer.Includes bibliographical references.Irrigation for agriculture is the highest use of fresh water in the world. Efficient and equitable access and distribution of this water is vital to survival of the Earth's population. Open channels are the most common means of conveying water for agricultural irrigation and hydraulic structures are often used in these open channels to regulate and measure flow to achieve desired conditions. Sharp-crested weirs are one of the most popular of these structures and pivot weirs are quickly becoming a more widely used hydraulic structure. The purpose of this study was to reexamine both types of weirs to better understand how they operate for flow regulation and measurement and to provide insights into the flow structure around the weir. Computational fluid dynamics, or CFD, was the primary tool used, with a commercial code called FLOW-3D being the specific software selected. Prior to investigating the weirs, preliminary studies were carried out to identify the best-practices in building an open-channel and hydraulic flow simulation in FLOW-3D. It was found that because FLOW-3D has no method of specifying developed flow prior to entering the model domain, additional care had to be taken to develop flow within the computational domain. The upstream length in the models was often extended to give the simulated flow more time and distance to develop. Additionally, the first-cell height had to be within a certain dimension to produce accurate velocity profiles due to the use of the logarithmic law of the wall boundary condition to solve for velocity in the first cell. Finally, a study analyzing the effects of the simulated downstream distance after a free-flowing sharp-crested weir revealed that the downstream distance has no effect on upstream flow. The sharp-crested weir parametric study analyzed velocity and pressure profiles over the crest, several calculated discharge coefficients, and turbulence flow structures upstream of the weir using high-resolution two-dimensional simulations. Three distinct operating regimes were identified based on the profiles over the crest as well as plots of the discharge coefficient against h/P where h is the upstream potentiometric head above the weir crest and P is the height of the crest above the channel bed. The first regime, the high-acceleration regime, occurs when h/P 2.0. The weir is often submerged here and the effect of the weir on the flow is diminished due to the high depth of flow. Turbulence patterns upstream of the weir appear to have a relationship to the Reynolds number, Re, of the flow with eddies reaching a minimum size at a Re = 70,000. The region of smallest eddy size correlated to the ideal-operating regime, again lending to the hypothesis that flow is more efficiently controlled within this regime. Six flow rates at five different gate angles (27°, 47°, 57°, 72°, and 90°) were tested for the pivot weir study. After analysis of the h/P values and discharge coefficients, it was found that the flow rates bounding the ideal-operating regime shift lower in magnitude as the gate angle decreases. Each angle also has an associated relatively constant discharge coefficient in its ideal-operating regime, meaning a single coefficient value may be used with minimal error. Comparison of the average discharge coefficient for each angle revealed a minimum value at 72° and a maximum value at 27°. The fraction of the total upstream mechanical energy head comprised of the velocity head was found to increase as gate angle decreased. Visual contours of velocity and pressure depicted how the flow changes as it approaches weirs of varying angles, with the recirculation zone moving from upstream of the weir to solely downstream of the weir for angles below 47°. Plots of the non-dimensional pressure and velocity profiles over the weir crest revealed that velocity over the crest increases as the inclination angle decreases. At the 47° weir, the flow acceleration created a region of negative relative pressure close to the weir. These results highlight how flow over both the sharp-crested weir and pivot weir varies considerably. Thus, caution must be exercised in using empirical discharge coefficients for a broad range of h/P value