## 5 Conclusions  
It is clear that SVUFMST has effectively solved the requirement of improving the efficiency of multi-UFMs' verification. As such, we recommend that the natural gas measurement station can use it in daily verification to reduce the disturbance of UFMs' verification on the natural gas supply. In this paper, a combined experimental and numerical analysis was performed to determine whether the flow field has a significant impact on the SVUFMST aiming at flowrate measurement of pressurized natural gas and, if so, to explain the mechanism of its action and the practicability of the SVUFMST. Compared with experimental methods, the simulation results using CFD can convincingly exclude the disturbance of industry manufactures and highly focuses on the aerodynamic factor. We pay particular attention to three test parameters, including the flowrate \(Q\), the diameter \(D\), and the number of the tested UFMs \(n\), each of which significantly influence the characteristics of the internal flow fields and, in turn, the performance of SVUFMST.  
Next, we will summarize each of the two models of tested UFMs. As for Daniel 3400(DN250), the actual flow experiments of \(n\leq 3\) were carried out in NJMS, according to the method of SVUFMST. Data obtained from these actual-flow experiments were first used to verify the simulated results and then the verified numerical method was extended to simulate the internal flow of UFMs for \(n=4\) and \(n=5\), which would lead to excessive experimental cost. As for Daniel 3400(DN150), we compared the experimental and numerical simulation results of 2 tandem arrangement, and then extended the numerical method to \(n=3\). The velocity profiles along the paths and the \(E\)-\(Q\) curves were extracted from the numerical results to reveal relationships between them and \(n\) and \(Q\). Comparison analysis shows that all the numerical results agree remarkably well with the available experimental data, and especially the overall \(E\)-\(Q\) negative power-law distribution is in good accordance with the previous results [12; 16; 17]. For both UFMs, the increased in \(Q\) can hardly change the shape characteristics of U-Profiles A and D, but that can significantly flatten U-Profiles B and C for any specified tested UFM. The negative power-law \(E\)-\(Q\) relationships can be partly attributed to this result. It is of interest to note that more than two UFMs under test can induce an overall shift in an \(E\)-\(Q\) curve and the two concentrated striped regions are consequently built on the abrupt shift in \(E\)-\(Q\) curves associated with \(n\), which also can be found for \(n=4\) and \(n=5\). However, different diameters of the tested UFMs can allow different values of n. A smaller diameter means a larger value of Re, which means a more violent turbulent environment at the same \(Q\). Thus, DN150 can easier reach its tandem arrangement limit by more obvious secondary flows due to the accumulation of large-scale roughness elements. Our numerical simulation results show that the DN250 for \(n\leq 4\) and the DN150 for \(n\leq 3\) are proved to be appropriate in SVUFMST.  
Conceptualization, M.Y. and C.T.; methodology, T.H., Q.H., C.T., X.G., and Z.Y.; software, T.H. and X.G.; validation, T.H., C.T., and F.B.; formal analysis, C.T., F.B., T.H., and Q.H.; investigation, M.Y., Q.H. and T.H.; resources, M.Y.; data curation, T.H. and M.Y.; writing--original draft preparation, C.T. and T.H.  
This research was supported by the National Natural Science Foundation of China (Grant No. 11602266, No. 11672284 and No. 11632016), S-T Projects of Petrochina West-East Natural Gas Pipeline Company (No. 201705) and Zhejiang Key Discipline of Instrument Science and technology.  
We express our thanks to Nanjing natural gas measurement station (NJMS), which is affiliated with Petrochina West-East Natural Gas Pipeline Company, for providing us the WSF to perform the actual flow measurement and other necessary supports to carry out this research.  
The authors declare no conflict of interest.