Research On Noise Wake Detection Method Based On Laser Doppler Technology

At present,the optical detection method of the ship’s wake is mainly to determining whether the ship’s wake exists due to the change of the detection light parameters caused by the bubbles and turbulence in the wake.The underwater vehicle’s radiated noise stimulates the water surface micro-amplitude wave at the water-air interface and the noise wake that drives the movement of suspended particles in the water,etc,all contain the characteristic information of the underwater vehicle.Based on the analysis of the detectability and radiation characteristics of underwater vehicle noise,this paper uses laser Doppler technology to carry out research on the ship’s noise wake optical coherent detection.This method is an indirect detection method that effectively combines the strong directivity of light waves and the strong penetration of sound waves.The spatial distribution of the scattered light field on the water surface,the theory of micro-amplitude wave on the water surface,the wake theory of the noise in the water and the optical coherent detection technology of the wake are studied.This provides a new idea for solving difficult problems such as underwater target tracking,underwater fluid medium characteristic parameter extraction and even underwater weapon guidance.The theoretical models of interferometric detection of micro-amplitude waves on the water surface and noise wakes in the water are established and their feasibility and correctness are experimentally verified.The main research work is as follows:(1)The theoretical model of the spatial distribution of the scattered light field on the water surface is established according to the Kirchoff’s diffraction theory.Under the Kirchhoff approximation,the expression of scattered light intensity at any observation point in the scattered light field on the water surface is obtained by using Green’s theorem.The relationship between the spatial distribution of the scattered light field intensity on the water surface and the geometric conditions of the incident light and scattered light is theoretically analyzed.The results show that the scattered light intensity has a maximum value when the incident angle is equal to the scattering angle(not 0°)under the condition that the water body parameters,light wave field parameters and detection distance are unchanged.When the incident angle is 0°,the scattered light intensity is the strongest.When the incident angle is in the range of 0°～53°(Brewster angle is about 53°).the scattered light intensity increases with the incident angle.When the incident angle is in the range of 53°～90°,the scattered light intensity increases first and then decreases as the incident angle increases(2)Theoretical modeling research is conducted on the problem that the radiated noise waves of underwater sound sources propagate to the water-air interface and produce micro-amplitude waves on the water surface.Under the approximate conditions of the far-field plane wave,the mathematical relationship between the amplitude and vibration velocity of the water surface micro-amplitude wave and the incident sound pressure and the vibration frequency of the underwater sound source is solved by the water surface micro-amplitude wave equation.Theoretical studies have shown that,on the order of nanometers to submicrometers,the amplitude of micro-amplitude waves on the water surface is proportional to the incident sound pressure and inversely proportional to the vibration frequency of the underwater sound source.The vibration velocity of the micro-amplitude waves on the water surface is only proportional to the incident sound pressure.(3)The main factors influencing the detection efficiency of underwater acoustic information were analyzed using geometric optics Snell’s law.The principle of optical coherent detection of water surface micro-amplitude waves and water wakes in water is studied.The expressions of the time-domain and frequency-domain interference signals of water-surface micro-amplitude waves are given.The time-domain and frequency-domain spectra of the interference signals are theoretically analyzed.Characteristics,and study the application of spectrum analysis techniques to extract the characteristic parameters of water surface micro-amplitude waves and underwater noise wakes.(4)The feasibility and correctness of optical coherent method to detect micro-amplitude waves on water surface and spectrum analysis method to extract characteristic parameters of micro-amplitude waves on water surface were experimentally studied.The results show that the optical coherent detection method realizes real-time detection of underwater sound sources with sound frequencies between 300 Hz and 1700 Hz;the actual sound frequency of the underwater sound sources demodulated by the spectrum analysis method is less than 1.27 Hz and the accuracy is better than 0.42%,the amplitude and vibration velocity of the demodulated micro-amplitude wave on the water surface are in good agreement with the theoretical calculation results.(5)In the experiment,the variation of the spectral characteristics of the backward detection of noise wakes in water varies with the frequency of the noise wave,the water environment and the type,number density,and size of the scatterers.The results show that when the area to be measured is the noise wake,the noise wake doped with sol impurities,the bubble wake and the noise wake,the backward coherent detection of the water noise wake in the range of 100 Hz to 3000 Hz is achieved.Although the difference in the actual noise wave frequency caused by the difference is different,the difference from the standard value is less than 1.7Hz and the accuracy is better than 0.6%.The movement speed of different scatterers in different water environments is consistent with the theoretical speed.Under the same noise wave frequency and sound intensity,the inconsistency of the characteristics of the peak amplitude and maximum frequency shift in the detection spectrum of different areas to be measured reflects that the difference in the water environment and the characteristics of the scatterers will cause the scatterers to have inconsistent motion states.