CAJ | 학술논문

在灌区渠道系统中，长喉槽是一种常用的测流设施，其测量精度依赖于水头损失的精确计算。为了检验其测流精度及有效性并加以改进，该文进行了一系列模型试验。试验结果表明对于进口有侧收缩的长喉槽，现有理论模型的计算流量皆大于实测值。该文认为这是由于现有的基于边界层理论的水头损失计算模型忽略了上游收缩段局部水头损失引起的。在试验数据和分析的基础上，该文引入了进口段局部水头损失?Hcon，对于侧收缩比分别为1∶2、1∶2.5、1∶3的长喉槽，建议局部水头损失系数分别取为0.317、0.263和0.203。在引入进口段局部水头损失后，计算误差从9.59%减少到了4.14%，研究结果为侧收缩长喉槽测流计算精度的提高提供了重要参考。
재관구거도계통중，장후조시일충상용적측류설시，기측량정도의뢰우수두손실적정학계산。위료검험기측류정도급유효성병가이개진，해문진행료일계렬모형시험。시험결과표명대우진구유측수축적장후조，현유이론모형적계산류량개대우실측치。해문인위저시유우현유적기우변계층이론적수두손실계산모형홀략료상유수축단국부수두손실인기적。재시험수거화분석적기출상，해문인입료진구단국부수두손실?Hcon，대우측수축비분별위1∶2、1∶2.5、1∶3적장후조，건의국부수두손실계수분별취위0.317、0.263화0.203。재인입진구단국부수두손실후，계산오차종9.59%감소도료4.14%，연구결과위측수축장후조측류계산정도적제고제공료중요삼고。
Long-throated flume has been widely used as a flow measurement structure in irrigation channel systems. The key element of the long-throated flume is a throat, the length of which is longer than one and one-half times of the upstream energy head referenced to the throat invert or crest. Measurement accuracy of this flume is based on an accurate prediction of head-loss. This long throat causes the streamlines to be straight enough to sufficiently approximate hydrostatic pressure distributions. Replogle proposed a mathematic model for discharge calculation of various kinds of measurement structures. It is claimed that the model can achieve an accuracy within 2% based on the results of laboratory studies (within the range 0/07 <H1/L < 0.7). However, the additional inaccuracy is associated with errors in measured water level, which can also be improved by proper design of stilling well. When the calibration error is combined with the measurement error, the resulting accuracy of the measurement can be lower than 5%. A series of experiments were carried out in China to verify the effectiveness of this structure. In this experiment, three sets of flume were used: 1) five trapezoid flumes with only side contraction; 2) five trapezoid flumes with only vertical contraction; 3) two flumes with both side and vertical contractions, one with a trapezoid cross section and one with a U-shaped cross section. For the five flumes with only side-contractions, four upstream converging rates (length to width: 2, 2.5, 3, and 4:1) were used. The downstream side expansion rate was the same for all fumes, namelyEM′=6. An extra set of models was created from the set with an upstream converging rate of 3:1 by cutting off the half-length of the downstream diverging section. For vertical contraction experiments, the sill height wasp1=p2=0.2 m. Four upstream vertical-converging rates (2, 2.5, 3, and 4:1, horizontal to vertical) were used, and the downstream vertical convergence rate remained fixed at EM=6. An extra set of model runs was created from the set with vertical-converging rate of 3:1 by cutting off the half-length of the downstream diverging section similar as with the side contracted flumes. For flumes with side and vertical converge, both trapezoid and U-shaped cross sections were tested. Laboratory experiments showed that the long-throated flume was an effective flow-measurement structure with simple shape, low head loss, high modular limit, high robustness and accuracy. The best side contraction rate and vertical rate was 1:3 for accurate field calibration. When designed properly, downstream water level did not influence measurement accuracy. This should be advocated in most irrigation canals. Based upon test results, a modification was made to the original head loss equation to further increase the calculation accuracy. A local head lossΔHcon was introduced to the original model to account for the head loss associated with a side contraction. This included a local head loss parameterci that was used to calculate this extra head loss at the entrance to the throat section. In this paper, the recommendedci value for side contraction rates of 1:2, 1:2.5 and 1:3 were 0.317, 0.263 and 0.203, respectively. For flumes with only one side contraction, the modified method reduced the calculation error from 9.59% to 4.14%. With this new energy loss, calculation accuracy was significantly improved for long-throated flumes with side contractions, this will be helpful for water management.