Numerical Analysis of Free Flow over Rectangular Sharp-Crested Compound Weirs

Compound weirs are used as flow diversion structures by adding additional flow resistance to the anticipated regions at the rivers. Furthermore, they are used for measuring and regulating flow rates accurately over a wide range of flow depths. When they are used for this purpose, the cross sections are generally selected as symmetrical having the lowest level of the weir at the middle to constrain large flow contractions. However, when used as a flow diversion structure, the cross sections should be adjusted for different degrees of flow contractions to satisfy the anticipated amount of flow resistance. The literature includes several studies in which compound weirs are modelled as flow measurement structures. Modelling them with the aim of flow diversion received little interest in literature. The adaptation of previously proposed analytical and experimental weir models has problems especially in cases with high flow contractions. In this study, the free flow over the compound weirs is modelled numerically with the application of flow diversion in mind. The results are validated by comparing them with the surface velocity measurements obtained using Surface Particle Image Velocimetry (SPIV). The study aims at understanding how far upstream flow redistribution takes place, how big the transverse mass fluxes are and how this affects the flow at the weir openings, at various degree of flow contractions. Three compound weir configurations were used: One with high flow contraction and the other two with moderate to low flow contractions. The numerical model is constructed by using a three-dimensional mesh using OpenFOAM CFD solver. A multiphase flow analysis was conducted by using Volume-of-Fluid (VOF) approach. We have applied RANS modelling with k-ω SST turbulence closure. SPIV experiments were conducted in a 3-meter-wide, 20-m-long rectangular horizontal flume at the Water Lab of Delft University of Technology. A camera was mounted over the flume to record the floating particle positions during flow. The results gave the possibility to quantify transverse distribution of mass transfer among the openings at various degrees of horizontal contractions. The initiation of streamline curvature locations at the upstream were labelled such that a comparison was achieved among the configurations. The numerical model results gave the possibility to complement experimental data regarding the effective flow sections at the weir openings. In summary, the numerical model validated by the SPIV measurements helped understanding the behaviour of flow under high horizontal contraction. However, to develop a correction methodology for high contraction for the simplified 1D weir discharge prediction models, the numerical runs should be extended for various configurations and covering the submerged weir flow as well.