Study On Ultrasonic Flow Measurement Based On The Coupling Between Flow And Acoustic Field

With the development of digital signal processing,transducer material and the electronic technology,substantial progress has achieved in ultrasonic flow measurement technology.Ultrasonic flowmeters with different principles and various forms have appeared in every area of national economy to satisfy separate occasions.The transit-time one is the most commonly used ultrasonic flowmeter with the advantages of high precision,wide measurement range,no moving parts and complete communication function.The implementation of transit-time ultrasonic flowmeter lies in complex flow in pipe with the effect on transducer protrusion or recess and acoustic propagation under non-uniform velocity distribution for time difference of downstream and upstream transmission.In order to deal with the flow characteristics with different structure parameters of ultrasonic flowmeter,the stationary background.flow field is obtained with appropriate turbulence model by comparing the K factor or the meter coefficient of the numerical simulation and the real flow experiment.Then the influence of different structural parameters on the flow characteristics of ultrasonic flowmeter is analyzed.After correctly mapping the simulation solution onto the sound mesh,the effects of different velocity distributions on ultrasonic propagation is discussed by means of coupling between flow field and acoustic field.According to the calculation result of Ray Tracing Method,the traditional mathematical model under the hypothesis of wave propagation along the straight line is modified.The acoustic propagation and the interaction with the wall are presented in a graphical form after the acoustic propagation equation is solved by the Discontinuous Galerkin Method.The main works of the disseration are listed as following:1.The non-linear trajectory of downstream and upstream propagation under the fully developed turbulent flow is calculated by Ray Trace Method,which the velocity distribution in the circular tube is described by the empirical formula.Due to the large gap in sound speed and velocity of water,the medium has less influence on the wave propagation trajectory and direction,so the results still meet the linear propagation hypothesis within the measurement range of the DN50 caliber ultrasonic flowmeter.The RKDG(Runge-Kutta Discontinuous Galerkin)method that is the combination of Discontinuous Galerkin method for space discretization together with an explicit Runge-Kutta approach for time derivatives has been adopted to show the 3D simulation of a Z type ultrasonic flowmeter in water flow measurement field,which divided into flow field and sound field.Based on the stationary background meanflow obtained by solving the standard k-? model,the control equations are derived from continuity,momentum,and equation of state to describe the propagation of linear acoustic waves in an adiabatic way.To validate the correctness of mesh size and time steps settings,a comparison between analytical solutions from three aeroacoustic test cases and numerical solutions are made.It is supposed to simply as the normal velocity boundary condition for transient characteristics of piezoelectric ultrasonic transducer,excited by the square voltage.Then the acoustic characteristic is shown in the figure when it moves in the direction of the main flow with the presence of a background flow by defining the normal velocity on the surface of transmitter.Through the recalculation,the average sound pressure on corresponding surface of receiver under upstream propagation can be generated by simply switching the source and receiver boundary.The average sound pressure signal on corresponding surface of receiver for downstream and upstream propagation are processed by threshold detection technology,as the difference in travel times which is lower than experimental result.According to the linear propagation hypothesis,the tendency of K factor between simulation and experiment are consistent.This numerical methodology opens possibilities in simulating acoustic propagation in other complex configurations,as well as complicated flows.2.But the high speed flow cannot be ignored in gas measurement,mainly reflected by the non-linear propagation and the downstream drift of sound beam.After correctly mapping the simulation solution onto the acoustics domain,the S-shape trajectory has been investigated in fully developed turbulence.The relationship between offset ?d derived from Ray Trace Method and air speed,provided by the least square method can direct the installation positions of non-invasive UFM transducers to obtain strong signal.The measurement error has been reduced down to 2%after the correction,based on the systematic analysis on transit-time,the length of sound path and acoustical path angle.If the protrusion and recess effect of invasive UFM is taken into consideration,there is an obvious bending in propagation trajectory,which the shape looks more like U.Then the RKDG is introduced to show the downstream shift of sound beam.The ultrasonic beams are composed of an main part and the adjacent sphere wave.when air speed increasing,the ultrasonic beam is carried away from the receiver.So the reception is the combination of the important part and the extendings,rather than the well centered wave.The energy difference between two parts of ultrasonic beam results in low amplitude.The research results are beneficial for determining suitable threshold of signal processing.3.The Finite Element Method is introduced to study the reflection device of ultrasonic flowmeter,which connects the sound pressure in fluid domain and structural deformation in solid domain based on acoustic-structure coupling.The acoustics pressure solved by computing the Helmholtz equation in the water on the surface of the reflection device acts as a boundary load in the solid domain.The model calculates displacements and stresses in the solid domain,and it then uses the normal acceleration of the solid surface in the acoustics domain boundary.The reflection device of ultrasonic flowmeter immersed in the infinite space is truncated as a sphere which represents the computational domain.Ultrasonic transducer is simplified into an incident plane wave.The sound pressure level along three paths of ultrasonic propagation is analyzed by comparing the result between sound-hard boundary and aluminum,stainless steel.The result shows structural deformation has an impact on the distribution of sound pressure and stainless steel suits better for the material of the reflection device.4.The reflection device is fixed in ultrasonic flowmeter to extend sound path when the diameter of the measuring tube is small due to high measurement precision.Because of its great influence on the flow characteristics,two improvement approach,removing the part of the upper tip and adding deflector on the basis of above,have been proposed to optimize the structure of flowmeter and numerically simulated by SST k-? model to analyze the flow field distribution,velocity stability of sound path and pressure loss.The results show the latter one of two modifications can improve the distribution of flow field compared to the other.Meanwhile,both of two can increase the stability of velocity in sound path and reduce the loss of pressure but the latter effect is more significant.