Experiment And Research On The Cylinder Measuring Water Device For U-shape Channel

Based on the achievements of Hager and Zohrab Samani et al., a cylinder measuring water device was put forward for U-shape channel with a circular arc-shape bottom and tangent straight sides in this paper. The principle of this measuring water device was that using a cylinder vertically installed on the axis of U-shape channel to contract and narrow the water flowing section, produces enough water surface falling and forming the critical flow state. It can be obtained the stable correlation between water level and discharge. So the discharge could be measured only by water level at the stagnation point. In the experiment study, using the method integrated the theoretical analysis with the model testing, the calculating formulae of critical water depth with the discharge has been derived out. The adjustment factor for the discharge and the experiential calculating formula for the discharge under the condition of free outflow have been determined. Moreover, the performance of the measuring water device has been analysed.When using the cylinder to measure the discharge in U-shape channel, due to the water velocity nonuniform distribution and the impact of crooked streamline at critical flow section, so the calculated discharge should be revised by an adjustment factor. For the U-shape channel with a circular arc-shape bottom and tangent straight sides, after the central angle (θ) and radius (r) of arc-shape bottom were given, there was unique correlation between the chord length (L) to the channel section. Therefore, the adjustment factor can be given by the function of relative energy H/L (ratio of upstream water head (H) to the chord length (L)). Through the analysis for the experiment data by the different diameters of the cylinder with the different discharges, have shown very good correlativity between the relative energy H/L and the relative discharge Qm/Qc (ratio of measured discharge (Qm) and the calculated theoretical discharge (Qc) ). Also using the experiment data, without dimension equations of relative energy H/L and relative discharge Qm/Qc have been established.Using the nine different diameters of cylinder to carry out the experiment in U-shape channel, the correlation between water level and discharge for the cylinder measuring water device, the cylinder diameter selecting range (which determines the shrinkage ratio), the measuring error and the boundary of critical submergence flow have been analyzed. According to the analysis for the experiment data, between the critical water depth, the upstream water depth and the stagnation-point water depth total have been shown the favorable linear correlation one another, also among the stagnation-point water depth, measured discharge and calculated discharge have shown the good linear relationship too. The diameter of cylinder was the important factor for affecting the measuring precision. If the diameter of the cylinder selected too big or too small, total will be caused the measuring error. However, only limited the diameter of the cylinder in a certain of range, i.e. the shrinkage ratio(ε) in a certain of range, well then the discharge measuring error could be acceptable. Through the analysis for the experiment data, have obtained when the shrinkage ratio(ε)was within 0.57 to 0.69, the measuring error will be rather small and the backwater level will be less than 0.1m. So the diameter of the cylinder should be selected within the shrinkage ratio from 0.57 to 0.69, the maximum absolute error was less than 3.78%, and the critical submergence degree was 0.73 to 0.84.The innovation points in this paper were:①Pointed to the U-shape channel with a circular arc-shape bottom and tangent straight sides to carry out the experiment and research on the cylinder water measuring device for the first time, has derived the calculation formula for the theoretical discharge and obtained the experienced calculation equation of discharge under the condition of free outflow;②Put forward the calculated discharge should be revised by an adjustment factor that could be determined by the function of relative energy H/L, Also using the experiment data, the without dimension equations of relative energy H/L and relative discharge Qm/Qc have been established;③The relationship between stagnation-point water depth and the critical water depth with the upstream water head have been analysed, between the three depths have shown rather good linear relations. Also the stagnation-point water depth with the measured discharge and calculated discharge have been shown good correlation too. Therefore, it was reliable that using the stagnation-point water depth to calculate theoretical discharge;④The selecting range of appropriate diameters of cylinder, the error of measuring discharge and the critical submergence degree have been analysed.The cylinder water measuring device has the advantages such as simple structure, easy construct and install with a low cost and high measuring precision, with a maximal absolute error of 3.78% to satisfy the water measuring requirement in irrigation district. It could be used from one place to other, and no sediment problem in the U-shape channel. The section shrink ratio of this device was from 0.57 to 0.69, therefore, the diameter range of cylinder could be chosen widely. The maximum submergence degree was 0.84, so with wide range of free outflow, and hardly to cause the submergence outflow and so on.