| At present,as an important auxiliary means of sonar detection,the magnetic anomaly detection mainly relies on magnetic anomalies caused by the metal body of submarine.But with the application of low-magnetic materials and the development of demagnetization technology,the ferromagnetic field of submarine decreases continuously and the difficulty of detection increases continuously,it is urgent to seek and develop a new target magnetic source.Ion motion magnetic field is a kind of magnetic field generated by hydrodynamic behavior of submarine.Although this kind of magnetic field signal is relatively weak,it is a kind of magnetic field which has nothing to do with the material characteristics of submarine itself,so it has good application prospects in the future.In order to apply the ionic moving magnetic field to the detection and recognition of underwater submarines,the mechanism of the ionic moving magnetic field is deeply analyzed and the theoretical model of the ionic moving magnetic field in the ocean environment is established.Based on the numerical simulation method,the magnitude,signal characteristics,distribution and propagation law of the ionic moving magnetic field are evaluated,which provides the theory and method for the future application of the ionic moving magnetic field to the detection of underwater submarines in our army.The main work and innovations are as follows:1.The mechanism and characteristics of ion motion effect magnetic field are revealed,and the theoretical model of ion motion magnetic field in seawater environment is established.The influence of physical and chemical parameters of seawater/submarine motion parameters on ion motion effect magnetic field is expounded.On the basis of the traditional ion force equation,the new ion force equation under the ocean environment is established according to the physical and chemical parameters of ions in seawater.Through the qualitative analysis of the forces acting on the ion force equation,the formula of ionic polarization current density is deduced and simplified,and the final formula of ionic motion magnetic field theory is obtained through the electromagnetic field theory.Thus,the physical quantity transfer chain from the external excitation source velocity field to the micro current density and then to the radiation electromagnetic field is established.In addition,the effects of ion concentration,ion mass,ion viscosity coefficient and background geomagnetic field on ion motion magnetic field are discussed,which provides theoretical and methodological guidance for future research on signal enhancement methods of ion motion magnetic field.2.A numerical simulation platform for submarine hydrodynamic behavior is built,and a complete scheme for submarine hydrodynamic behavior simulation is proposed.Based on ANSYS FLUENT’s good parallel compatibility mode and powerful post-processing function,a software and hardware platform based on ANSYS FLUENT and Tianhe II is proposed,which provides conditions for the study of submarine hydrodynamic behavior.Based on DARPA SUBOFF model,the simulation method of submarine motion flow field is studied,and the effectiveness of the numerical calculation scheme adopted in this paper is verified from qualitative analysis and quantitative analysis.Compared with the experimental data of DARPA SUBOFF pool,the error between the total resistance calculated by simulation and the experimental measurement is about 2.8%,and the error between the calculated friction resistance and the calculated value by Sanghai formula is 3.2%.This verifies the accuracy and reliability of the proposed simulation scheme,and provides the precondition for the subsequent accurate calculation of ion motion magnetic field.3.The problem of solving the ion motion magnetic field based on water wave excitation is studied.An integrated magnetic field solution method based on parallel UDF is innovatively proposed,which solves the problem of coupling calculation of hydrodynamics and electromagnetics in the ion motion magnetic field,and realizes the joint calculation of the flow field and electromagnetic field in the same platform with high efficiency.Submarine is a large-scale target.In order to achieve high-precision simulation,it is necessary to have a sufficiently fine grid,resulting in a huge amount of data.If hydrodynamics,electromagnetics and other computations can not be integrated on the same platform,there will be a huge problem of data storage and transmission.Therefore,it is necessary to establish an integrated simulation platform for ion motion and magnetic field.Utilizing UDF(User Defined Functions,user-defined functions)for post-processing development,the integrated simulation calculation of flow field and magnetic field is finally realized.Using the integrated simulation method established in this paper,the magnitude,frequency characteristics and attenuation law of ion motion magnetic field under flow excitation at low speed(speed 3m/s,speed 1rps)and high speed(speed 10m/s,speed 2rps)are analyzed.The simulation results show that the magnitude of magnetic field is 0.26 nT and 0.75 nT at 1L length and 1.02 p T and 3.09 pT,at 10 L respectively.and the magnetic signal has obvious frequency characteristics.4.The conversion relationship of magnetic field between different scales based on similarity principle is deduced,and validated and evaluated based on integrated simulation platform,which solves the conversion problem from scaled model simulation results to actual model in the case of insufficient computing resources.Firstly,the necessity of researching the magnetic field conversion relationship between different scales is analyzed,and based on the similarity principle of fluid calculation,the similarity relationship satisfied by hull and propeller is obtained.Then,the magnetic field conversion relationship between different scales is deduced by combining the theoretical formula of ionic motion magnetic field.Finally,the numerical simulation under 1.5 and 3 times scales is carried out based on the integrated simulation platform.The results show that the errors of magnetic field conversion are 0.76% and 2.2% respectively.Therefore,within the allowable error range,the scaled model results can be converted to the actual model according to the derived magnetic field conversion relationship.5.The problem of solving the ion motion magnetic field based on acoustic wave excitation is studied,the solution scheme of equivalent sound source and space dimension reduction of ionic motion magnetic field is proposed,and realizes the solution of ionic motion magnetic field under the excitation of equivalent linear spectrum sound source.Compared with the area around submarine and wake,the acoustic wave attenuates slowly and propagates over a long distance in seawater.Firstly,based on FLUENT,the FW-H equation is applied to solve the submarine flow noise,and the equivalent sound source under the line spectrum component is obtained.Then,the pressure acoustics(acpr)module and magnetic field(mf)module in COMSOL are used to establish the coupling relationship between the two modules through the coupling equation,and the solution of the ionic moving magnetic field based on acoustic excitation is realized.Finally,the results of magnetic field calculation under three boundary conditions,namely,seabed absorption,seabed semi-reflection and seabed total reflection,are compared and analyzed,and the magnitude,signal characteristics,distribution and propagation law of ion motion magnetic field under these three boundary conditions are obtained.Under the seabed total reflection boundary with the strongest magnetic field signal,the magnitude of ion motion magnetic field is between sub-0.1fT and 1fT,and it has the characteristics of no obvious attenuation with distance in a certain range.6.Based on the numerical simulation results,the feasibility study of the new magnetic source for underwater target detection is carried out,and the detection range and efficiency under the maneuvering and fixed detection modes are evaluated.The maneuvering mode can give full play to the maneuverability of the platform and extract the significant spatial variation characteristics of the magnitude of the magnetic measurement signal.Fixed mode is a time series with no obvious change in amplitude.After removing the DC component,the characteristic frequency of the signal is significant.In view of the large variation of magnitude of magnetic measurement signal in maneuvering mode,a detection method based on Minimum Information Entropy(MIE)is proposed,and the detection performance of the detector is evaluated by simulating the generation of magnetic measurement signals and geomagnetic background noise.From the test results,it can be seen that when CPA is 200 meters,the detection probability of target signal reaches more than 88%,and when CPA is 400 meters,the detection probability of target signal is about 70%.Aiming at the frequency characteristics of magnetic measurement signal in fixed mode,a signal detection scheme based on frequency domain adaptive cancellation preprocessing method of dual magnetometer,signal enhancement method based on stochastic resonance and adaptive threshold decision based on power spectrum amplitude is proposed.The detection performance of this method is also evaluated by simulating the generation of magnetic measurement signals and geomagnetic background noise.The detection results show that when the monitoring distance is 1000 meters,the detection probability of target signal reaches more than 95%,and when the monitoring distance is 1500 meters,the detection probability of target signal reaches about 70%. |
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