Development of an acoustic doppler flow profiler and flow testing

The flow test with acoustic Doppler current profiler (ADCP) is a new flow test method developed and applied only in the last decade or so. However, because it involves many disciplines such as hydroacoustic physics, ultrasonic transducer technology, electronics, and signal processing, most of the ADCPs used at the present stage are imported, and the domestically produced high-precision ADCPs have a lot of space in the domestic market. This paper introduces the measurement principle of ADCP. Based on this principle, we have overcome the weak signal processing circuit and successfully developed a domestic horizontal fixed acoustic Doppler flow profiler. The instrument is used for flow tests in Yixing (South) Hydrological Station, and the results of the comparison and error analysis meet the requirements of the national first-class precision hydrological station in the “Hydrological Data Compilation Specification” to satisfy the needs of the daily test in the station. The “Horizontal Fixed Acoustic Doppler Profile Flow Rate System” was selected as one of the 100 typical cases of popularizing and applying the achievements of the Ministry of Water Resources in 2020, and it can replace the imported instruments.


Introduction
The flow test with Acoustic Doppler Current Profilers (ADCP) is a new flow test method developed and applied in the last decade or so.Due to the superiority of its principle, it breaks through the traditional mechanical rotation-based flow rate measurement and has the characteristics of directly measuring the flow rate profile of the cross-section, while not disturbing the flow field, short test duration, and large range of speed measurement.
In the mid-1970s, scientists led by Prof. Pinkel of Scripps Institute of Oceanography in the U.S. first proposed to measure the flow rate by acoustic method and got the data of the flow rate of multi-layers in a cross-section.In the mid-1980s, the U.S. RDI developed a narrowband ADCP; in the late 1980s, RDI developed a broadband ADCP; and in the mid-1990s, RDI developed a phased-array ADCP [11][12] [15].
Since the 1990s, CSIC 715 carried out research on shipboard phased-array ADCP, successfully developed the principle prototype of shipboard 38 kHz phased-array ADCP in 2000, and developed the prototype of 150 kHz phased-array ADCP in 2005, which has been further developed and improved with the support of the state, and the Shipboard 38 kHz phased-array ADCP in 2014.The 38 kHz phased-array ADCP has passed the third-party test at sea in the South China Sea, and it has started to be installed and used on domestic ships in recent years [14].In 2012, the Institute of Acoustics of the Chinese Academy of Sciences (CAS) carried out the research and development of riverine-type ADCPs based on marine-type ADCPs.Besides, the RIV-600 ADCP passed the appraisal of the Ministry of Water Resources (MWR) as a new product in September 2016, which has achieved the hydrographic test in Acoustic Doppler flow profiler localization, filling the gap in this field in China.At present, RIV-type ADCP has been mass produced and sold by Wuxi Seahawk Gako Marine Technology Co. Since the 1990s, China Shipbuilding Industry Corporation (CSIC) 715 has carried out research on shipboard phased-array ADCP and successfully developed a shipboard 38 kHz phased-array ADCP prototype in 2000, and a 150 kHz phased-array ADCP prototype in 2005, which has been further developed and perfected under the support of the state [13].
With the national emphasis on water conservancy and the national emphasis on the localization of high-tech products, the demand for domestic high-precision ADCP has become increasingly strong [5][6][7][8].

Measurement principle [10]
According to the Doppler Principle, there are a large number of scatterers in a natural river such as sediment, tiny particles, plankton, air bubbles, etc.The ultrasonic transducer of the ADCP transmits ultrasonic waves into the water, which are scattered by the above-mentioned scatterers, and the frequency of the scattered return signals is correlated with the speed of these scatterers, which are received and processed by the ADCP.These scatterers flow with the water, and their velocity is the same as the velocity of the water flow, under this premise, the measured velocity of these scatterers is the flow velocity.It is known from Liu et al. [1].
where  is the water flow velocity, is the Doppler shift, c is the speed of sound in the water,  is the angle between the direction of water flow and the direction of ultrasonic transmission, and  is generally taken as 60° 70°.According to the difference in the ultrasonic transmission distance, the ADCP divides the cross-section into several measurement units and measures the average flow velocity in the number of measurement units respectively.The schematic diagram is shown in Figure 1.

Instrument development
Horizontal Fixed Acoustic Doppler Current Profiler (HADCP) is a branch of Acoustic Doppler Current Profiler (ADFP), which mainly consists of three ultrasonic transducers, one ultrasonic beam emitting upwards to measure the depth of the water, and two ultrasonic beams emitting horizontally to measure the two-dimensional flow velocity of the water at a certain section of the local water flow by using the acoustic Doppler principle [9][16].
The self-developed horizontal fixed acoustic Doppler flow velocity profiler mainly consists of three ultrasonic transducers, temperature sensors, water pressure sensors, inclination sensors, measurement and control circuits, and microprocessors.The system block diagram of the flow velocity measurement is shown in Figure 2, which consists of a power supply module, excitation module, weak signal processing system, and microprocessor, and the key is the weak signal processing system.The filter circuit adopts RC active filtering, automatic gain amplification is realized by using an op-amp and digital potentiometer, the digital potentiometer is used as the feedback resistor of the amplification circuit, the resistance value of the digital potentiometer is adjusted according to the amplitude of the amplified signal, and the segmented amplification is realized to achieve the gain control.Scattered return signal return signal filtered and amplified to 2Vp-p.The appearance of the instrument is shown in Figure 3, and its main technical specifications are as follows: •    The unit flow velocity measured by the analyzing instrument is plotted on a flow velocity distribution graph (Figure 6), in which four curves represent four measurements, the horizontal coordinates indicate the distance from the section to the shore, and the vertical coordinates indicate the flow velocity at the section.From Figure 6, it can be seen that the flow velocity data jumps frequently after the starting distance of 20 m.Within the starting distance of 10 m, there is a certain jump in the case of high flow velocity and low flow velocity, so the preliminary selection of flow velocity unit between the starting distance of 10-20 m is homogenized and processed as a representative of the flow velocity to participate in the rate of fixing.The flow test uses the average value of the data in the period from the beginning to the end of a complete manual test for comparison.The walk-around ADCP measurement data uses its total flow data, divided by the area of the cross-section of the instrument monitoring location in the same period to obtain the average flow rate of the cross-section.The flow data of the selected measurement returns meet the requirements of "Acoustic Doppler Flow Test Specification" (SL 337-2006) [2], and the error between the measured value of the flow rate and the calculated average flow rate in half of the same measurement return is no more than 5%, and the average flow rate is selected as the flow rate of this measurement return.
The artificial flow meter method tests data using its total flow data, divided by the same period that the instrument monitors the location section of the cross-section overwater area to get the cross-section average flow rate.The artificial flow meter method is in line with the "river flow test specification" (GB 50179-2015) [3], and requirements and comparison measurement data are shown in Table 1.In the table, V1 is the average flow rate of the section obtained by dividing the manually measured flow rate by the over-water area of the section at the installation location of the instrument; V2 is the average value of the instrument measured representative flow rate corresponding to the period of the manual test; as can be seen in Table 1.The amount of data meets the requirements of the "Specification for Hydrological Patrols" (SL 195-2015), and is more than 30 times than the sample data, and covers the flow rate corresponding to high, medium, and low water level levels.The linear fitting using V1 and V2 yields the following linear relationship, which can be seen in Figure 7: Based on this linear relationship, the calculated results after rate determination were obtained as in Table 2 The results of the error analysis show that the accuracy of the instrumental flow measurement reaches the requirements of the national first-class precision hydrological station, i.e., the systematic error of the first-class station with one-line method is not more than ±1%, and the random uncertainty is not more than 8%, and the random uncertainty of the flow measurement with ADCP can be increased by 2%.

Conclusion
The flow test data show that the horizontal fixed acoustic Doppler flow profiler installed at the test station meets the "Hydrological Data Compilation Specification" SL247-2012 in terms of comparison and error analysis, and can meet the daily test requirements of the national first-class precision hydrological stations.The engineering test verified the excellent performance of the instrument and the comprehensive system "Horizontal Fixed Acoustic Doppler Profiler Flow Rate System" was selected as a typical case of the Ministry of Water Resources to popularize the application of 100 achievements in 2020.It can replace imported similar products, and with the advantages of localization in price, technical support, and after-sales service, it lays a solid foundation for further commercialization and application.

Figure 4 .
Figure 4. Physical map of survey station.

4. 2
Product testThe instrument has been commissioned and installed since March 2020 and has been measuring the flow every 5 minutes, after 6 months of operation through high and low water levels.According to the requirements of the Hydrological Patrol Specification(SL 195-2015), the horizontal fixed acoustic Doppler flow profiler installed at the site applies the rotor-type flowmeter or walk-around acoustic Doppler flowmeter as the benchmark for comparative analysis.The locations of the monitoring sections for the three types of tests are shown in Figure5:

Figure 5 .
Figure 5. Location of monitoring section.

Table 2 .
[4]omparison data after calibration.According to the "hydrological data compilation specification" SL247-2012[4]requirements, the systematic error is estimated by the mean value of relative error, and the calculation results are shown in