Research On Forward Scattered Light Characteristics Of Wake Bubbles

The cavitation of moving targets in the water,such as ships,during navigation due to the churning of propellers and the breaking up of waves by the hull,creates a large number of bubbles in the wake of the ship.Due to the difference in the composition,density,and compression rate of the bubble gas,the transmission of light waves in the wake is very different from that in the water column,so the study of its optical properties can be used to achieve the detection and identification of ships.This thesis focuses on the spatial distribution and spatial-spectral properties of the forward scattered light field from wake bubbles.The first step is to calculate the scattering coefficients and the scattered light intensity distribution of individual bubbles based on Mie scattering theory.The Mie scattering coefficient is calculated by the upward recurrence method,and the influence of the change of individual bubble radius on the scattered light intensity distribution is investigated by the Mie scattered light intensity calculation formula,and the scattered light intensity distribution is calculated for single bubbles with the radius of 0.2 μm-0.9 μm and bubbles with the radius of 10 μm – 100 μm.The results of the calculations show that the forward scattered light from the wake bubbles becomes stronger as the bubble size increases.The scattering and extinction coefficients of the bubbles are calculated and the corresponding relationships between the scattering and extinction coefficients of the bubbles are investigated when the relative refractive indices are real and imaginary numbers respectively.The study shows that the extinction coefficient of the bubble and the distribution curve of the scattering coefficient follows the same trend,and the amplitude of the scattering coefficient is lower than that of the extinction coefficient;when the relative refractive index of the bubble increases the distribution curve of the scattering coefficient of the bubble shows an oscillating change and the amplitude increases.Secondly,based on the single bubble Mie scattering theory,a Monte Carlo method is used to model the underwater transmission of the laser beam,tracking the transmission of each photon in the wake bubble and recording the coordinates of the photon position and the total number of photons reaching the receiving surface.The distribution of photons arriving at the receiving surface is analyzed by varying the transmission distance,bubble number density,and bubble size.The calculation results show that as the transmission distance,bubble number density,and bubble size increase,the light intensity at the receiving surface weakens,and the spot size increases.At the same time,the spatial half-height and width of the bubble forward scattered light spectrum decrease as the transmission distance,bubble number density,and bubble size increase.Finally,experiments are designed to validate the simulation results,and a bubble forward scattered light detection system is set up in a laboratory environment to study the light intensity variation and spot distribution characteristics of the bubble forward scattered light under different pump pressure conditions.As the pressure increases,i.e.the bubble density and bubble size increase,the forward scattered light spot of the wake bubble becomes larger,the light intensity decreases,and the spatial spectrum half-height and width decreases,which is consistent with the theoretical calculation results.The paper discusses the effect of the change of bubble number density and bubble size on the scattered light intensity and the half-height and width of the spatial spectrum of the scattered light from the theoretical analysis to the further study of the optical properties of the wake bubble through simulation and experimental verification.