## 1 Introduction
Although there are few studies that especially focus on the mechanism of the action of the flow field on SVUFMST, there are several trials concerning how the unusual flow fields can affect the UFMs measurement errors. Despite having low flow resistance and disturbance with extremely high precision, it is inevitable to absolutely exclude the local flow distortion, mainly in form of negative velocity, near the transducer recess (or protrusion)  
Figure 1: Schematic of the simultaneous verification of multi-ultrasonic flowmeters (UFMs) in tandem arrangement: FF: Forcing Function Generator; PP: Pressure Transmitter; TT: Temperature transmitter. in the ultrasonic flowmeter [9] in which an ultrasonic transducer is installed. Further, the negative velocities generated at the protrusion and recess locations as a whole are the key factors for negative measurement errors of the ultrasonic flowmeter. This result therefore confirms that the transducer recess in the ultrasonic flowmeter can affect the measurement accuracy. In addition to transducer recess and protrusion, the flow profiles [10; 11], UFMs internal wall roughness [12], and pulsating flow [13] may all result in pronounced measurement errors of UFMs. It has been experimentally reported that the flow (velocity) profiles, such as the parabolic velocity distributions usually used for laminar and power law for turbulent flow, or a flattened airflow profile generated from a specially shaped convergent nozzle, also have a significant effect on the measurement error of ultrasonic meters [11], as does the complex flow profiles caused by the upstream elbow pipes as proposed by [PERSON] et al. [10]. Some of the above findings [9; 10] suggest that computational fluid dynamics (CFD) can be an effective method to reveal the mechanism of action of the flow field on UFMs' measurement accuracy, and thus, in this study, CFD is adopted to numerically simulate the internal flow field in the tandem multi-UFMs and help quantify the influence of the flow field on the SVUFMST performance. Further, in terms of SVUFMST, the influence of the detailed field characteristics of the flow on the ultrasonic flowrate signals, such as the upstream flow distortion near the transducer recess affecting the downstream flowmeter measurement, cannot be determined only from experimental results, because there always exists uncertainty resulting from the product differences in large-quantity production. Compared with experimental methods, however, the simulation results using CFD can convincingly exclude the disturbance of industry manufactures and are highly focused on the aerodynamic factor. By comparing the preliminary simulation results to the available experimental data, the grids, boundary conditions, and key model parameters were carefully adjusted to meet the numerical accuracy and maximum permissible error of UFMs, and then the optimized numerical model was extended to simulate the internal flow of UFMs in tandem arrangement in other severe conditions. This probably leads to the inability or excessive cost of executing the experiments for three or more UFMs tested in tandem arrangement or different types of UFMs in SVUFMST. Our study provides some effective guidance for the SVUFMST of Nanjing natural gas measurement station, which have greatly reduced the verification time and economic cost in practical work.