Investigation Into The Forward Light Scattered By Ship Wakes And Its Detecting Technology

To improve the apperception for bubbles wakes by optical technique in wakes homing, a new technology for detection of bubbles wakes is proposed. This technology arrives at the apperception by detection of forward light scattered by wake bubbles, inversion of light beam transfer model in small angle approximation and obtainment of the bubbles' number density in wakes. According to the characteristics of detecting bubbles wakes by forward light scattering, the model of light transfer in small angle approximation is established based on light scattering properties of wake bubbles. Compared to traditional transfer equation in small angle approximation, this model has two improvements. One is that correctional linear combination of two Gauss function replaces old one Gauss function in phase function fit; the other is the correction of forward scattering to un-scattering intensity. Finally, the method is confirmed to be feasible by detecting simulated wake bubbles using the testing equipment developed in this paper.Firstly, the light scattering properties of wake bubbles are investigated, which include the scattering properties of single bubble, bubble populations and the extinction features of light beam transferring in bubbles wakes. The optical efficiency factor, polarization, intensity distribution and energy distribution of light scattered by single bubble are discussed. Based on single bubble's light scattering properties, the characteristics of light scattered by bubble populations are studied. The energy distribution function is introduced for analyzing the energy distribution of light scattering conveniently. According to the fact that bubbles in ocean are coated with organic film in their most part of lifetime, the searches for bubbles' scattering characteristics includes clean bubbles and dirty bubbles coated with different component and thickness organic film. The approach used to research into the bubbles scattering is Mie theory. According to the properties of intensity and energy distribution of bubbles' light scattering, traditional Gauss fit for scattering phase applied in traditional transfer equation in small angle approximation is improved, and the forward scattering error is also corrected. Based on the improved transfer equation, the multiple scattering, the correction to multiple scattering and the relationship between bubbles number density and light beam attenuation are analyzed when light beam propagates through bubbles wakes. These analyses give a theoretical support for design of testing equipment for detecting bubbles number density.Secondly, the approaches, which obtain bubbles' number density by detecting forward light scattered by wake bubbles, are discussed, on basis of transfer characteristics of light beam in wakes. An inversion method to transfer equation is presented, which is fit for multiple scattering. A series of points about bubbles number density and energy transmission ratio are obtained by numeric calculation utilizing the solution of improved light transfer equation in small angle approximation. Then function between bubbles number density and energy transmission ratio can be gotten by fit the points in the least squares sense. So in actual detection of wake bubbles, one can easily obtain the bubbles number density utilizing the function, after the energy transmission ratio is measured.Thirdly, wake bubbles are simulated by hydrogen bubbles produced by water electrolysis. To validate the method of obtaining bubbles number density presented in this paper, a wake bubbles having known size distribution function and number density is necessary to act as benchmark. Here, the simulated wake bubbles' size distribution function is gotten by micrography, digital image processing and statistic analysis; the number density is achieved by controlling the current in electrolysis in terms of current conservation.Lastly, testing equipment for validating the detecting method proposed in this paper is designed and manufactured, which consists of optical system, electronic system, PC software and mechanical structure. The simulated wake bubbles were detected by the testing equipment, and the number density measured by the testing equipment is fairly consistent with the number density controlled in electrochemistry, which confirm that the technology presented in this paper is feasible.