基于悬垂平板偏转角的明渠流量估算模型及验证

针对平板量水设施缺乏适用性广泛的流量计算模型,该文从2个角度提出流量估算模型,首先分析绕轴自由旋转薄平板在水中的受力,根据升力简化为竖直方向静水压力设想,提出压力体计算假设,根据动量定理与力矩平衡公式得到了流量、角度、水深三者的理论关系式,通过U型和矩形渠道进行试验,验证假设合理性;根据闸孔出流流量公式针对矩形渠道建立闸孔出流半径验计算模型,拟合得出半径验流量公式。对于第1种模型,对于U型渠道,2种压力体假设均适用于流量计算,除流量小于10 L/s时,相对误差超过10%,其他均小于10%,流量大于17 L/s时误差均在5%左右;对于矩形渠道,仅假设1适用流量计算,假设2不成立,应用假设1计算压力体时,当流量较小(10 L/s左右)时的个别工况误差会偏大,大部分工况下计算误差均小于10%;对于闸孔出流计算模型,计算流量与实测流量之间最大误差不超过18%,大部分工况下计算误差在10%以下。当悬垂薄平板与明渠横断面等大时,来流量与偏转角度存在单值对应关系,角度随着来流量的增大而增大;同一流量下,板前后水深比、板前与下游水深比分别与偏转角度呈现出单独的函数关系,板前后水深比、板前与下游水深比随着平板偏转角度的增大而减小,但减小幅度变缓。对于不同流量,板前后水深比、板前与下游水深比随着角度增大而增大,但增大幅度变缓。研究可为灌区量水设施设计及应用提供新思路。

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.