Influence Of Transducer Protrusion On The Accuracy Of Flow Measurement In Open Channels By Ultrasonic Transit-time Method

In China,the strategic position of water is particularly prominent in the context of uneven spatial and temporal distribution of water resources,serious water pollution and low utilization efficiency.As an important process quantity,flowrate is the basis of water resources management and distribution.High-accuracy flow data is an essential prerequisite for fine management of water resources under China's “most stringent water resources management system”.It is of great significance to increase the accuracy of flow measurement to achieve the strategic goal of delicate water management in China as well as to solve practical problems in the actual projects such as water fee disputes.However,traditional open channel flow measurement methods can no longer meet the requirements of measurement accuracy.Ultrasonic flowmeters,which have gained rapid development in recent decades has become an effective method for measuring open channel flow due to its strong mobility and good stability.Although ultrasonic flowmeters have been successfully used in open channel flow measurement,few systematic studies have been conducted on the influencing factors of its measurement accuracy.In order to study the influence of the probe installation method(protruding/retracting)on the accuracy of the flow measurement of open channel flow by time-difference ultrasonic flowmeter,this paper synthetically used the method of indoor physical model test and numerical calculation to study the probe's disturbance of flow field during the time-difference ultrasonic flowmeter measurement of open channel flow.In the laboratory,an open channel model and a stable water supply system are set up to avoid the influence of external factors such as irregularities in the construction of the canal or site installation.By adjusting different flow rates and water levels,changing the installation method of the probe,comparing the measured value of the channel flow rate with the standard flow rate,measuring the measurement error of the ultrasonic flowmeter under different working conditions by the time difference method.The CFD software Fluent was used to simulate the three-dimensional flow field of the open channel under different water level-flow relationships.The local flow field under the protruding and retracted state of the probe was obtained,and the influence range of the probe installation method on the flow field and the influence on the measurement result were determined.This paper mainly studies the range,numerical value and variation law of the flow field disturbance caused by different probe installation methods.All tests and data analysis suggest that the installation of the probe has a significant impact on the accuracy of the measurement.Model tests and numerical calculations have shown that both the probe protrusion and indentation can cause positive errors,and the probe mounting method from indentation to protrusion can lead to an increase in measurement error.For the protruding probe,the shape of the local flow field is affected by the geometric characteristics of the inflow plane,the backflow plane,and the average flow velocity of the line.The influence range of the protruding probe is not significantly related to the average flow velocity of the line,and the effect on the mean flow velocity of the line increases with the average flow velocity of the line.For the case of probe indentation,the local flow field morphology is affected by the characteristics of the inflow and backflow surfaces and is insensitive to changes in the average flow rate of the line.When the probe is indented,the local recirculation area is closer to the rough wall surface;the influence scope of the flow field of the open channel main stream decreases with the increase of the line average flow velocity,and the effect on the line average flow velocity is independent of the average flow velocity of the line.