## 1 Introduction  
Increasing attention is being paid to measurement technologies designed for natural gas owing to their clean and efficient characteristics with extensive use in energy supply [1; 2; 3]. Further, it has received attention for its extreme facility of transportation, widespread and vast reserves, ease of use, and lack of solid residual after burning. Natural gas currently makes up over 24% of the world's energy and 6.2% of China's energy. As one of the most important energy networks in China, the West-East Natural Gas Transmission Project (WENGTP) was established and has been service since 2001, transports over 72 billion cubic meters of natural gas from western China to eastern Chinaeach year, and results in the alleviation of domestic energy tensions and improvement of the fuel structure [4; 5]. An inevitable challenge, however, for natural gas transmission and trade is how to measure the natural gas exactly and reduce measurement uncertainty. Aiming at this problem, high-precision ultrasonic flowmeters (UFM) with different diameters are usually used to measure the natural gas flow in the pipelines because of their high accuracy and repeatability as well as their low flow resistance and disturbance. As a result, the verification of natural gas flowmeters is not only increasingly required but also, from the perspective of accurate energy consumption and settlement of work, is even more important. Unfortunately, the current verification method (JJG1030-2007 Verification Regulation of Ultrasonic Flowmeters [6]; ISO 17089-1:2010(en) Measurement of fluid flow in closed conduits [7]) is time-consuming and has low efficiency because the installation, disassembly, and verification of each flowmeter take one to two hours or more, and only one flowmeter would be tested in a calibration process according to the above standard regulations. Moreover, in order to accurately reproduce the operating environment, the UFMs in WENGTP would be calibrated in several important natural gas measurement stations along the mainline of WENGTP, such as Nanjing natural gas measurement station (NJMS) and Chengdu (CDMS) natural gas measurement station of China National Petroleum Corporation, and the natural gas flow in the mainline should be switched to the bypass line for UFMs' verification. Therefore, this metrology method would result in an unusual natural gas supply and urge a more efficient and practical verification regulation to reduce the disturbance of UFMs' verification on the natural gas supply. Consequently, increasingly more attention has been paid to the necessity of improving the efficiency of multi-UFMs' verification. As proposed and practically applied by some related manufacturers and metrological services, the simultaneous verification of multi-UFMs in tandem arrangement (SVUFMST) is a highly efficient metrology method to address this problem. However, there are only a few published studies regarding SVUFMST. [PERSON], who works in CDMS, calibrated two UFMs in tandem arrangement and analyzed the error values to confirm the flowmeter measurement performance, and his results showed that two flowmeters in a tandem arrangement can be simultaneously calibrated [8]. However, there is still no flow analysis or defined applicable conditions for SVUFMST. The schematic of the SVUFMST is illustrated in Figure 1, in which the tested UFMs were tandemly arranged downstream of the master meters and checking meters successively, with upstream and downstream straight pipes of appropriate length. Although this method has been applied in practical verification, it is still in the exploration stage and there is no corresponding operation standard accessible. Therefore, it is urgent to carry out comprehensive research on this method to perfect it. In this paper, the effect of the global flow field characteristics on the performance of the SVUFMST was studied to determine whether the flow field has a significant impact on the SVUFMST, and, if so, to explain the mechanism of its action.