Flow over oblique weirs

This report is the conclusion of a comprehensive set of experiments, which were performed on weirs placed obliquely in an open channel. Its purpose is to report on laboratory investigation on the flow over different types of oblique weirs, including behavior and hydraulic characteristic of the flow, different phenomena in the neighborhood of the weir, and hydraulic parameters and physical laws that govern the process. The report also aims at presenting a quantitative view on the energy loss and the discharge coefficient for oblique weirs. To that end, many experiments were performed in a shallow flume under various flow conditions. Three different types of impermeable weirs are tested, namely a rectangular sharp-crested weir, a rectangular broad-crested weir (both placed 45 degrees obliquely to the flow direction) and a dike-form weir with both upstream and downstream slopes of 1:4. The last type was tested with several oblique angles of 0, 45 and 60 degrees with the incident flow. By adjusting the flow discharge and the downstream water level, different flow regimes and states reveal the complex three dimensional structure of the flow with various phenomena like hydraulic jump, undulation, flow concentration, flow divergence, gyre formation, etc. In case of emerged flow condition, the flow behind weir becomes highly turbulent and very complex, which make it more difficult to perform accurate measurements. This flow regime also accounts for the higher head loss and energy dissipation than in case of submerged flow. Generally speaking, the hydraulic phenomena that happen in the neighborhood of an oblique weir are equivalent for different weir form, although there are some remarkable differences such as the size of the recirculation zone behind weir, and the amplitude of the undulation waves. Experimental data were obtained by many instruments and techniques; most of them had been carefully calibrated and were highly accurate and reliable. The data collected from acoustic and electro-magnetic single point velocimeters and depth measurements were used to investigate the hydraulic process and the phenomena of interest. Meanwhile the whole surface flow velocity field was measured using particle tracking velocimetry technique, which helps obtaining instantaneous whole field velocity maps. Combine with mathematic tools we can interpret the data and gain necessary statistical information. When the oblique angle of the weir is altered, both the flow direction and the flow rate change. The flow always tends to keep its direction to nearly perpendicular to the weir crest when it reaches and passes the weir. This leads to the difference in water levels at two ends of the weir, the flow concentration at on one side of the flume behind weir, the variation in flow velocity distribution and other asymmetries across the flow. Increasing the oblique angle, the effective length of the weir increases significantly, whereas the discharge coefficient Cd slightly decreases. Together they make the discharge capacity of the oblique weir increases. Finally, the discharge coefficient and its relations to other flow and geometry parameters obtained from this research were compared to the available knowledge on oblique weirs, including the published researches from De Vries (1959), Borghei et al. (2003) and the numerical models simulations from Wols (2005). The common findings between researches enhanced each other reliability; whereas the differences are a motivation for further studies.Civil Engineering and Geoscience