Flow estimation model and verification based on deflection angle of dangling plate in open channel

Abstract: In view of lack of effective regional water measuring facilities for gentle slope canal with silt current in irrigation areas of northern China, a portable flat water measuring device was proposed as a flow water measuring equipment in the field. Many factors such as canal sizes, slopes and flat plate materials are related to the device, thus a widely applicable flow calculation model was needed. This study was to research its hydraulic performance of plate water measuring facilities based on prototype test carried out in Northwest A & F University in Yangling, Shannxi of China. In order to explore dynamic characteristics and deflection phenomenon of a draping thin plate under the impact of open canal flow rate, 2 calculation models were proposed. In the 1st model, we assumed that lift force was the vertical component of total static water pressure (pressure body) and based on the momentum theorem and the moment-equilibrium, the theoretical relationship between the deflection angle of the plate and the flow rate was deduced and tested in a U-shaped canal and a rectangular canal to verify the rationality of the theoretical formula. In the 2nd model, the flow pattern was analyzed and the formula for calculating the outlet flow of gate was applied to the flow relative to measuring device and the flow calculation model was established. The undetermined coefficients in the flow coefficient calculation model were estimated. The measuring device was installed at 5.0 m far from the inlet of upstream of U-shaped canal while 4.5 m far from the inlet in rectangular canal. The base slope of U-shaped canal was 1/2 000 while the zero slope in the rectangular canal . The triangular weir was installed at the end of the downstream of canals to measure current flow. Experiments and were performed for total 25 working conditions on the plate measuring devices with flow rate up to 44 L/s to verify calculation models. The results showed that both pressure body assumptions were applicable to flow calculation in the U-shaped canal. The relative error between calculated and measured flow was less than 10% except when the flow rate was less than 10 L/s. When the flow was greater than 17 L/s, the error was less than 5%. In the rectangular channel, only hypothesis 1 was applicable for flow calculation when the hypothesis 2 was invalid. Brake orifice discharge model developed for upstream depth versus discharge under different working conditions were satisfying with the relative error of 10% under most working conditions, which met the common requirements of flow measurement in irrigation areas. The flow measurement range was between 10 L/s and 44 L/s. When the size of draping thin plate was close to cross-sectional dimension of the canal, the flow rate and the deflection angle had a corresponding relationship. There were 2 water depth ratios, one of which was water depth ratio of upstream and downstream and the other was the ratio of water depth in front of the plate to downstream. Under the same flow rates, the 2 ratios decreased as an increase in the deflection angles. Under different flow rates, the 2 ratios increased as an increase in the deflection angles. The rationality of the model need further verification, when the canal size, water flow conditions, and plate shape changed. This study provided important information for flow measurement of terminal canals in irrigation areas.