Theory And Implementation Of Acoustic Doppler Flow Measurement With Environmental Adaptability

Acoustic Doppler flow measurement technology is a telemetry measurement technology that uses sound waves to measure the velocity of water flow.Due to the advantages of noninterference with the flow field,fast and high estimation accuracy,the acoustic Doppler flow measurement technology has been widely used in various measurement applications in ocean,rivers,lakes and ditches.The water flow field is diverse,time-varying and complex.The measurement environment refers to the water velocity range,depth,velocity distribution,scatterer characteristics and subjective measurement requirements.In order to achieve high estimation accuracy in a variety of environments,it is necessary to endow the measurement instrument with the ability to adapt to the environment.Focusing on improving the environmental adaptability of the acoustic Doppler flow measurement instrument,the influences of several typical environmental factors on the measurement results are analyzed.A variety of environmentally adaptable measurement methods have been proposed from the perspectives of the selection of flow measurement methods,the optimization of transmitted waveform,signal processing,data screening and measurement error correction.The main research contents and results are as follows:The influences of environmental factors such as the concentration and size of the scatterers,the distribution of water velocity,the movement of the bed load,and the measurement platform on the measurement result are analyzed.Based on the physical model of sound scattering,the influence of the size and concentration of the underwater scatterers on the sound propagation attenuation,the water scattering intensity and the echo signal-to-noise ratio(SNR)are analyzed.The sonar equation of echo SNR is derived based on the research of acoustic propagation loss and Rayleigh scattering of underwater particles.The analysis results show that the echo SNR increases first and then decreases with the increase of scatterer concentration or size.The estimation expressions of flow velocity and discharge under the condition of moving bed load are deduced,and the influences of moving bed load on the flow measurement result are analyzed,which provide a theoretical basis for the novel error correction methods in the chapter IV.Then the characteristics of low-speed flow,high-speed flow,turbulence,vertical stratified two-way flow and echo decorrelation effects and their influences on flow measurement result are analyzed,which provide a theoretical support for proposing improved methods in the chapter V.Finally,the influences of three common flow measurement platforms,such as manned ship,tethered ship and remote control ship,on flow measurement result are analyzed.A variety of optimized measurement methods for bottom depth and velocity are proposed.First,the factors which influence the measurement accuracy of bottom depth and velocity are analyzed,including strong water echo,water surface echo,bottom echo distortion,movement of the measurement platform and noise.In order to improve the measurement accuracy of the bottom depth,a multi-feature joint estimation method is proposed.Based on the principle of energy focusing,an accurate depth estimation is obtained in this method by jointly detecting the position,intensity and width of the energy pulses of echoes.Then,a waveform tracking method for bottom velocity estimation is proposed.The estimation accuracy of the bottom velocity is improved in this method by tracking the stability zone of the bottom echo.And thirdly,based on the pre-estimation of velocity estimation accuracy,an accuracy tracking method for bottom velocity estimation is proposed.This method pre-estimates the velocity estimation accuracy of the bottom echo segment and tracks the echo segment with the strongest SNR to achieve an accurate measurement of the bottom velocity.Finally,a correction method based on the principle of vector cancellation for the error caused by the moving bed is proposed.This method does not require satellite positioning and additional depth tracking equipment,and it can correct the flow velocity error of each micro-section after two inverse cross-section flow measurements.A variety of optimized methods of flow velocity measurement is proposed according to the characteristics of environment and measurement requirements.In order to obtain highprecision flow measurement under low-speed flow condition,an improved design method of the coherent pulse train is proposed.This method obtains the pulse interval and repetition that minimizes the velocity estimation error under the constraints of the range of velocity and distance,and improves the velocity estimation accuracy under low-speed flow condition.Aiming at the measurement applications that require high velocity estimation accuracy,a calculation method of waveform parameters for the broadband phase-encoded signal is proposed.This dynamic setting method of waveform parameters such as code length and number of repeats minimizes the velocity estimation error under the constraints of stratification accuracy and velocity measurement range.Aiming at the measurement applications that require high stratification accuracy,another calculation method of waveform parameters for the phaseencoded signal is proposed.This dynamic setting method of waveform parameters such as the code length and the number of repeats minimizes the layer thickness under the constraints of velocity estimation accuracy and velocity measurement range.The influences of the water scattering characteristics on the quality of the echo are analyzed,and an optimization method of the frequency of transmitted acoustic signal is proposed based on the water scattering characteristics.This method obtains the echo attenuation through theoretical calculations related to the concentration and particle size of the scatterers or on-site measurement of various frequency signals,and further estimates the optimal transmission frequency according to the sonar equation.The results of the numerical calculations show that this method can obtain a reasonable estimation of the optimal transmission frequency under several typical scatterer conditions.A flow measurement experimental system with the environmental adaptability is designed and constructed,several river and lake experiments are conducted and the experimental results are demonstrated.The overall hardware and software architectures of the environmental system are given,and the three main links of the signal flow and data flow are introduced.The advantages of the software design method are analyzed,and the framework designs of the main state machines of the system including the environment-adaptive flow measurement state machines are given.An echo quality criterion and a screening method of flow measurement data are proposed based on the principle of the velocity variance estimation using the autocorrelation function of echo segments.This method improves the judgment ability and processing efficiency of the flow measurement data.Finally,the velocity and depth estimation performance of the flow measurement experimental system and an imported instrument in the field experiments are analyzed and compared.The experimental results prove the effectiveness of the proposed methods in this dissertation.