The Analysis Methods Of Low-frequency Electromagnetic Characteristics Of Targets In Seawater Environment

This dissertation aims to calculate the low-frequency radiated electromagnetic(EM)fields from targets in seawater with surface integral equation(SIE)method.There are following three problems to be solved in the calculation.First,for multiscale underwater targets,the discretized equation convergences slowly when solved with a whole domain manner.Then the electric field integral operator(EFIO)in SIE has the low-frequency breakdown(LFB)problem due to the decoupling of electric field and mangetic field.The equation with EFIO can not deliver correct results when solved without special care.Finally,the conductivity and relative permeability of the shell target is much larger than those of seawater and air,result in high constrast ratio of constitutive parameters of different materials.The low-frequency radiated EM fields in the seawater from the shell can not be evaluated correctly if equations widely used to solve perfect electrically conducting(PEC)targets and homogeneous dielectrics are adopted.The dissertation focuses on solving problems metioned above.The main contents of the dissertation are as follows:1.The target in real life is large and multiscale,and it is slow to solve its EM problem with traditional method in a whole domain manner.A domain decomposition method(DDM)with a new way to connect subdomains is proposed to fast solve the EM wave problems of complex and multiscale targets.Different from conventional DDM where half Rao-Wilton-Glisson(h RWG)function is used,the two adjacent subdomains are connected with a basis function proposed by us,multibranch RaoWilton-Glisson(MB-RWG)function,in the proposed DDM.The average sizes of triangles of two parts in an MB-RWG function are different.Furthermore,the normal component of MB-RWG basis function is continuous.Therefore MB-RWG function can be used to connect two surfaces with different mesh resolutions.Because there is no current continuity problem near the boundary between two adjacent subdomains,the calculation of entries of impedance matrix is simple in the proposed DDM,without introducing interior penalty term,double contour integral term,and line surface integral term.Numerical examples are presented to validate the accuracy of MB-RWG function in solving SIE of PEC objects and homogeneous dielectrics,as well as the efficiency of DDM with MB-RWG function.2.In the low-frequency analysis with SIE method,due to the decoupling of electric field and magnetic field,the EFIO in SIE has LFB problem.A new loop-star decomposition method under a kind of nonconformal mesh is proposed to cure the LFB problem of EFIO.The loop and star functions are first constructed with RWG and MBRWG functions near the discontinuous mesh,including the local loop functions at the valid inner nodes and global loop functions associated with apertures and handles.Then loop and star functions are used to discretize the electric field integral equation(EFIE)of PEC objects.Because the equation after loop-star decomposition converges very slowly when the total number of unknowns is large,a DDM preconditioner is built to accelerate its convergence rate.The accuracy of the proposed method is verified with simply-connected/non-simply connected open and closed surfaces.It is demonstrated that the accuracy and efficiency of the proposed loop-star decomposition is close to that of conventional method but with better flexibility.The proposed loop-star decomposition is also applied to hybrid field integral equation(HFIE)of imperfect electrically conducting objects.3.A way to calculate the transmitted low-frequency EM waves from a magnetic shell with a source inside in the sea water,is proposed with SIE method.The shell has large but finite conductivity.The shell is considered as a homogeneous dielectric.SIEs can be built on each interface with equivalent principle.The magnetic field integral equation(MFIE)and EFIE are selected to describe the internal problem and external problem of the shell,respectively.In this way,the situation where the materials at two sides of interface have high constrast ratio of constitutive parameters can be calculated.Loop-star decomposition is applied to each operator to cure the LFB problem at low frequencies.The asymptotic behavior of each sub-block of the decomposed coefficient matrix is analyzed with Taylor expansion of Green’s function and its gradient.A scaling scheme is proposed to improve the conditioning of coefficient matrix at low frequencies.The loop function and star function constructed with MB-RWG function are used to connect two adjacent subdomains.The DDM preconditioner is constructed to accelerate the iteration process when solving equation.Adaptive cross approximation(ACA)method is adopted to compress the mutual-coupling matrices between two different subdomains and accelerate the matrix vector products in the iteration process.Some numerical results are presented to validate the accuracy of the proposed method.