Research On Underwater Turbulence Detection Based On Rotating Doppler Effect

With the development of marine resources and the increasing emphasis on maritime defense,the detection of turbulence in seawater and the detection of turbulence in the wake of ships has become particularly important.Turbulence is a complex fluid phenomenon caused by the irregular fluctuation of fluid density and the different refractive indices of the fluid,the temperature,and salinity distribution in the water body,the dissipation of its turbulent kinetic energy will produce different distributions of turbulent fields,in addition to various kinds of ships and boats in the process of moving will also generate a variety of different intensity of turbulent fields.The detection of underwater turbulence field information not only enables the tracking of vessels but also increases the safety of underwater resource extraction.In fluid mechanics,vorticity is an important physical quantity to describe turbulence,and accurate measurement of vorticity allows us to obtain better information about turbulence.In this thesis,the rotational Doppler effect of vortex light is used for underwater turbulence field detection,and the local vorticity and velocity information of the turbulence field is detected by a non-embedded optical detection method.Study of vortex light properties.The intensity,phase,and angular momentum properties of vortex light and its common generation methods are investigated;two typical monomorphic vortex light are introduced,and the superimposed vortex light carrying special orbital angular momentum and the grafted perfect vortex light are simulated and analyzed,and the corresponding beams are generated by a spatial light modulator.Study of the rotational Doppler effect of vortex light.Based on the classical Doppler effect theory,the expression of the rotational Doppler effect is derived,and the rotating rigid body model is established to simulate the rotational Doppler effect using single-state vortex light and superimposed vortex light as the detection light respectively;the spreading of the spectrum when the center of the incident beam does not coincide with the center of the rotating object is discussed,and the relationship between the width of the spectrum and the offset is analyzed.The rotational Doppler effect is used for the detection of turbulence information.Theoretically,the feasibility of Laguerre-Gaussian beam detection of fluid local vorticity is analyzed,and the relationship between fluid local vorticity and incident vortex light topological charge and rotational Doppler frequency shift is derived from the relationship between circulation and vorticity and the rotational Doppler effect.The results show that the vortex distribution curves can be correctly obtained for different velocity fields;On this basis,in view of the shortcoming that the traditional vortex light cannot measure the direction of the flow field velocity,the grafted perfect vortex light is introduced,and simulation proves that the grafted perfect vortex light can be used as the probe light to identify the direction of the fluid velocity;finally,a three-dimensional fluid model is established,respectively Traditional vortex light and grafted perfect vortex light are detected by incident light.The results show that combined with the classical Doppler effect,the traditional vortex light detection can calculate the longitudinal and lateral components of the fluid velocity,but the direction of the lateral velocity component cannot be judged.By grafting the perfect vortex light detection,the longitudinal velocity can be determined.The magnitude and direction of the component and the lateral component.