Hydraulic performance of portable flow measuring flume of imitating aerofoil in the trapezoidal channel

The flow facility of aerofoil has presented the excellent hydraulic conditions and high flow measurement accuracy. However, the complex curve of aerofoil can restrict to promote the water measuring facility. This study is based on the portable imitating aerofoil measuring flume with simple structure to explore its applicability in the final trapezoidal channel. Among them, the 1/4 elliptic curve coupled with arc curve was used to approximate the complex aerofoil curve. The hydraulic trials were carried out in the Laboratory of Hydraulic Engineering and Hydraulics of Northwest A&F University in Shaanxi Province, China in 2022. A concrete trapezoidal channel was prepared (the flat slope, bottom width = 30 cm, channel depth = 30 cm, top width=90 cm, and side slope coefficient m = 1). The portable water measuring flumes of imitating aerofoil were all made of hollow wooden materials. A total of 35 trial schemes were designed, including one set of aerofoil chord length, five sets of contraction ratios, and seven sets of flow discharges. The lab experiment was used to only measure some specific hydraulic parameters, whereas, the computational fluid dynamics (CFD) platform was utilized to simulate the internal flow fields. Furthermore, the hydraulic performance was compared between the aerofoil flume and the imitating via the FLOW-3D software after the hydraulic trials. A systematic analysis was made to clarify the effects of contraction ratio and flow rate on the hydraulic performance of imitating aerofoil flume. The results showed that the numerical simulation was better agreed with the experimental, with the error of less than 4.91%, indicating the accurate and reliable modelling. More importantly, the aerofoil measuring flume remained the smooth flow and the small depth of backwater after simplification. The flow pattern was also obtained after numerical simulation. The water flow was stable at the upstream of the flume, whereas, the water level dropped faster at the downstream of the throat, where the water surface strongly fluctuated with the high flow velocity. Part of the water flow was generated some vortices near the side wall under the action of the transverse velocity. The upstream Froude numbers were less than 0.5 under all working conditions, and the backwater depth was less than 7.6 cm, fully meeting the requirements of flow measurement and channel safety. The best hydraulic performance was achieved in the contraction ratio ranging from 0.60 to 0.64. The high accuracy of flow formula was also observed using energy equation and critical flow principle, indicating the average flow measurement error of 2.75%. As such, the imitating aerofoil maintained the excellent hydraulic performance of the original aerofoil. Consequently, the high accuracy of flow measurement and the more simplified curve can be expected to easily promote the portable water measuring flume in the last small cross-section trapezoidal channel for the irrigation districts.