| Computational fluid mechanics is an interdisciplinary subject between mathematics,fluid mechanics and computer.It plays an important role in the fields of aerodynamic engineering,weather prediction,heat dissipation design,engine development,flowmeter development and basic science.In the field of civil and industrial flow measurement,ultrasonic flowmeter with time difference method has gradually become one of the most important high-precision flow measurement equipment because of its outstanding advantages such as non-blocking parts,no moving parts,almost no pressure loss,bidirectional measurement,and high range ratio.However,its measurement accuracy is easily affected by probe local distortion,pipe installation effect,fluid flow state and other factors,which greatly limits its wide application.Based on computational fluid dynamics method,the governing equation describing the fluid movement in the working process of the ultrasonic flowmeter with time difference method is solved,so as to reveal the flow mechanism and flow law.Combined computational fluid dynamics method with theoretical analysis and experimental measurement,it provides a more comprehensive method and basis for us to design the ultrasonic flowmeter with high measurement accuracy.Based on the simplified turbulence model of flow separation at the wedge probe,the applicability of Reynolds stress closed equation and its parameters are studied in this paper.The parameter a1,which is related to the eddy viscosity coefficient,and stress-kinetic energy ratio （β∞*）1/2 in the turbulence model are correlated with the physical mechanism of separation flow on the surface of the ultrasonic probe,and their influence on the simulation results is analyzed.On this basis,the influence of mixed layer flow separation caused by ultrasonic probe on the measurement results of gas ultrasonic flowmeter is studied.The average flow field statistical results are obtained by simulation,and the time-averaged flow information near the probe are analyzed,such as the position of the backflow vortex center,the length of the backflow area and the height of the backflow vortex,to reveal the internal flow mechanism of the flow field.Then the corresponding primary velocity correction models are proposed for different probe installation modes.The flow experimental systems are built,and a secondary flow correction model based on Reynolds number is proposed according to the experimental results,and the general applicability of the modified model is verified.The influences of the evolution of non-ideal flow field caused by pipe installation effect on the measurement results and characteristic parameters of ultrasonic flowmeter are studied,and the flow field adjustment effect of flow conditioner is fully understood.Two kinds of universal flow conditioner are designed and developed,one of which was a sponge metal flow conditioner.The flow field adjustment effect is verified and the real flow pressure loss experiment is carried out.The adjustment effect of another combined flow conditioner is verified by simulation,and an evaluation method of flow field adjustment effect based on computational fluid dynamics is proposed.Then three key parameters of the combined flow conditioner are optimized.By means of theoretical analysis,numerical simulation and real flow testing,the method of determining the path distribution positions and the weight coefficient of each path of multi-path gas ultrasonic flowmeter with chordwise path arrangement are studied.And the uncertainty evaluation model is established by analyzing the variation of calibration velocity diagnosis parameters on each path of±0.809R/±0.309R four-path ultrasonic flowmeter based on Gaussian orthogonal integral during the first laboratory calibration to industrial field application,to estimate the calibration transfer uncertainty that may be introduced by inconsistent pipeline conditions.The change of calibration transfer uncertainty of ultrasonic flowmeter under different installation conditions is also evaluated. |
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