Research On Fast FDTD Algorithm For Long Distance Propagation Of Underwater Electromagnetic Wave At Low Frequency

Water is a kind of lossy dispersive medium,only low frequency or extremely low frequency electromagnetic waves can carry more information and have less attenuation when they spread long distance in underwater environment.However,the wavelength of low frequency electromagnetic wave is long.Then it inevitably leads to the calculation time become longer.In the Conventional FDTD method,the smallest size that can be simulated by a finite difference scheme is a grid.The space discretization must be meshed so small that can simulate subtle and complex structure radiation source accurately which contains the pores,thin plate and thin coating.That is to say,the entire computing space is divided into more and more grids.By the numerical stability,the finer the grid subdivision will result in the decrease of the iteration interval,which in turn resulting in time and memory consumption surge in the calculation process.Therefore,the physical size of the calculation area is restricted.This shortcoming limits the calculation speed of low frequency electromagnetic wave in the real underwater environment.For example,the calculation of electromagnetic scattering field of complex and fine target which exist in the vast underwater space,the simulation of various complex small structures in underwater environment.In this paper,in order to overcome those above problems and obtain an algorithm that has a higher computing speed and lower memory consumption,a novel 3D total scattering field source FDTD method is proposed based on the equivalence principle.The proposed method includes two step FDTD calculations: the first calculation is used to obtain near fields produced by an excited source;the second is to calculate far fields propagating over a long distance.These two calculations are related by setting a TSS boundary.The first calculation space,containing an excitation source,is meshed by a fine cell in order to model the source accurately;the second calculation space is meshed by a coarse cell in order to obtain far fields propagating in a long distance.In addition,The TSS boundary is set to connect these two step FDTD calculations.And the ratio N can be set very large depending on the calculation need,such as N=11 or N=13.Numerical results of four different examples prove the high accuracy of the proposed method: the maximum relative error is about 4.6%.The run time spent in the method and memory consumption reduce to 3.9% and 0.35% respectively of conventional FDTD method.In order to improve the computation efficiency,based on the message passing interface(MPI)parallel environment,subdomain division method,and the total-field scattered-field source FDTD method,a three dimensional parallel TSS-FDTD method is proposed in the thesis,which combine the parallel method and TSS-FDTD method.This method converts serial computation to parallel by computing multiple subdomains simultaneously.The accuracy and validity were verified by comparing examples with different calculation scale.Numerical results show that the parallel speedup efficiency of this method is 68.4% which is able to realize complex and huge computation problems with less computation consumption.