Research On The Detection Method Of Internal Solitary Waves In Ocean Based On Very Low Frequency Piezoelectric Sensor

Internal solitary waves（ISWs）of the ocean are a kind of fluctuation that occurs inside the stratified seawater with stable density.The sudden strong current induced during its propagation will pose a serious threat to marine engineering structures,submarine activities,and underwater weapon launches.At present,the observation of ISWs mainly focuses on detecting the changes of velocity,temperature,salinity and density of seawater induced by ISWs,or the characteristics of sea surface convergence and divergence.However,the propagation of ISWs will inevitably induce the change of dynamic pressure of flow field.A detection method of ISW based on dynamic pressure measurement is proposed in this paper by studying the characteristics of dynamic pressure induced by ISWs and its relationship with the amplitude of ISW.A digital very low frequency（VLF）piezoelectric sensor system is developed,and its feasibility is verified by internal wave flume experiments.Based on the applicability of three kinds of ISW theory,a numerical flume with accurate amplitude and waveform control is established.Based on this flume,the characteristics of ISW induced dynamic pressure under four density stratifications are studied.The results show that the change of ISW induced dynamic pressure is synchronous with the change of interface displacement,and the maximum dynamic pressure under the same stratification has a strong linear relationship with the corresponding amplitude,which can be described by Miyata-Choi-Camassa（MCC）theory.In addition,the lower frequency limit of the dynamic pressure induced by ISWs at the flume scale is as low as 0.02 Hz.According to the water depth range of the ISW occurrence and the low frequency characteristics of the dynamic pressure induced by ISWs,design index of the digital VHF piezoelectric sensor system is analyzed and designed,such as frequency response,sensitivity,working depth,etc.The mechanical structure,hardware and software of the system are designed in detail.In terms of hardware,the preamplifier that affects the low-frequency response characteristics and noise performance of the whole system is mainly designed.A low noise data acquisition and storage system is designed to facilitate the processing and storage of analog signals output by preamplifier.In terms of software,with FPGA as the logic control core,the corresponding driver for each hardware module is designed.Considering the communication between PC and VLF piezoelectric sensor system,PC software is designed.The main modules and the whole machine of the developed VLF piezoelectric sensor system are tested.The test results show that the sensitivity of the system is-168 d B（ref.1 V/μPa）in the frequency range of 0.01 Hz～10 Hz,and the RMS of the equivalent input noise in the band is 6.31μV_rms.Finally,the performance of VLF piezoelectric sensor system for detecting ISWs was tested in a large internal wave flume.The results show that it can accurately measure the maximum dynamic pressure induced by ISWs under different amplitudes.The maximum error between the amplitude retrieved by MCC theory and the measured amplitude is less than 8%,which verifies the feasibility of the proposed inversion method for the amplitude parameters of ISWs by using dynamic pressure measurement.