Study On Efficient Time-domain Algorithm For Near-zone Coupling And Far-zone Scattering Of A Target Above Rough Surface

With the wide application of the high power pulse source,sensitive electronics,and technology for radar detecting target,the near-zone coupling and far-zone scattering from a target above rough surface have attracted has attracted more and more interest of researchers.In the methods of solving the electromagnetic problem,the finite difference time domain(FDTD)method is popular in the computaion of the transient radiation and coupling and scattering,due to its flexibility for handing the complicated model and complex media.When the traditional FDTD method is used to the simulation of the near-zone coupling and far-zone scattering from a target above rough surface,this method requires too much memory and long computation time because a large calculation domain need be constructed to enclose the rough surface and target.Especially for a greater distances between rough surface and target,the computaion burden will become heavier.Based on the idea that the composite model are separatly handled,the rough surface and target are located into two adjacnet calculation domain to improve the computional efficency and to keep the advatange of the FDTD in treating the structure with complex shape.The electromagnetic contribution between the two domain is considered through the calculation of the near-field radiation of the equivalent soures and through the introduction of the near-field scattering wave into the computaitonal domain.The main research contents are as follows1.For solving the key problem in the domain decomposition scheme,the hybrid algorithm combining the FDTD and the time-domain Huygens’ principle is proposed to simulate the transient near-field scattering from rough surface in the two-dimensional(2-D)mode.In this scheme,the task for solving the near-field scattering is partioned into the calculation of the equivalent source on the contour enclosing the rough surface using FDTD method and the calculation of the transient near-field radiation with the Huygens’ principle.The near-field radiation can be found as the time-domain convolution intergral of the equvalent soures and the 2-D Green function.Meanwhile,the convolution operation need be executed over the whole output boundary of the rough surface FDTD domain.For a lage scale rough surface,the direct intergration of the time-domain convolution is such cumbersome as to be hard to complete because of the huge computational cost.The smeianalytical recursive convolution(SARC)approach for the calculation of the near-field radiaiton is presented to gain the high efficiency.Fistly,the convolution kernel is approximated as a sum of exponential functions based on the idea of the signal prediction in the high-speed electronic ciruit.Then the updating formlas are deduced by applying the SARC scheme in the digital signal processing.The reliability and efficiency are revealed through comparison with the near-field scattering by the FDTD and by the SARC-FDTD.2.An improved-finite difference time domian(Improved-FDTD)is presented to reduce the consumption of the computation resource in calculating the coupling of a target above rough surface under high power electromagnetic pulse(HPEMP)incidence,in which the rough surface and the target are respectively allocated into two nonadjacent FDTD subdomains.The incidnet waves towad the target FDTD subdomain include the directly illuminating wave and the near-field scattering wave from the underlying rough surface,where the later is obtained by applying the FDTD with the three-dimensional(3-D)Huygens’ principle.Here a scheme for solving the 2-D near-field radiation with use of the3-D recusive approach is proposed to rise efficiency.The incident wave field of the target subdomain is obtained by superposing the directly incident field and the near field scattered by the rough surface,and the toal field/scattering field boundary source is installed by applying the equivalence principle.The transient electromagnetic field in the target subdomain and the coupling energy flux density at the dielectric-coated wire are computed by implementing the improved-FDTD method.The accuracy and efficiency of the proposed approach are shown by comparing the numerical results with the tradiational FDTD method.Finally,the influence of the parameters of the rough surface and coated medium and thin wire on the coupling energy flux is depicted in detial.3.A domain decompostion-finite difference time domian(DD-FDTD)approach is proposed to reduce the computation cost in the FDTD analysis of the composite scattering from a target above rough surface.In this approach,the two non-adjacent FDTD subomain respectively enclose only rough surface and only target.The electromagnetic interactions between the two non-adjacent FDTD subdomain are computed by applying the Huygens’ principle,and they are introduced into the corresponding subdomains through the use of the equivlance principle,solving the fundamental problem of the field continuity in the domain decomposition scheme.The composite scattering field is got by superposing the far-zone electromagnetic contributions from the rough surface and target subdomains.Compared with the conventional FDTD method,this DD-FDTD approach is more efficienct because a large amount of meshes are deleted.The important properties of the composite scattering from a target above rough surface are obtained by a detailedly analyzing the numerical results for the typical examples.4.The domain decomositon scheme combining the time-domain physical optics(TDPO)and FDTD is proposed to boost the computaion efficiency furthur in caluclating the composite scattering from a target and underlying slight rough surface,compared with the DD-FDTD above.As for this scheme,the electromagnetic problems of the rough surface and target are analyzed using the TDPO and FDTD method,respectively.The time-domain Huygens’ principle is executed to compute the electromagnetic contributions between rough surface and target,in which the time-domain convolution is calculated by applying the SARC with the linear difference approximations.For the monostatic scattering case,the simple scheme for computing the secondary scattering of the rough surface and target is presented by applying the Reciprocity thereom.The far-zone scattering field is obtained by superposing the directly scattering field from the rough surface and target to their secondary scattering field.The calculation results indicate that the proposed TDPO-FDTD can maintain the accuracy of FDTD and can reduce the memory usage and computation time.