Investigations Of The Fast Hybrid Methods For Scattering Characteristics Of Three-Dimensional Electrically Large Complex Cavity

In modern war, the weapon platform can survive or not is determined chiefly by the its stealth characteristic. This situation is more serious for airplane. The inlet of modern aircraft has great contribution to radar cross section,so the electromagnetic scattering analysis for inlets has great importance in the Stealth(low-observable) technique,anti-Stealth technique and target character analysis of aircraft. In this dissertation we propose a high efficient fast solver for the scattering characteristics of electrically large complex deep cavities while keeping a reasonable accuracy .The general principle of the Iterative Physical Optics is introduced at the begining of the dissertation. Then, the Fast Multipole Method (FMM) is employed to accelerate the iterative process of IPO. The methods that the unknown current elements are spatially grouped is discussed here. once the unknown current elements is structured grouped using a cube or cuboid,the invariance characteristic of translation and the angle invariance characteristic of translator can be used to optimize calculation and memory.In the conventional Fast Multipole Method, k|^ at all directions of translation between two groups should be computed, it is expensive. The Ray-Propagation Fast Multipole Algorithm(RPFMA) is applied to simplify the translation when two groups are well separated. In fact, the amount of translators to be calculated should be decrease with the increase of distance between two groups, to take full advantage of the characteristics; a self-adaptive ray-propagation fast multipole algorithm which can modulate the bound of cone zone of RPFMA is adopted. When the distance between two groups is large enough, the Fast Far-Field Approximation (FaFFA) is used to further simplify the translation.In view of the problem that IPO combined with fast method can not guarantee the convergence of the solution, extensive study for Jacobi Minimal Residual(JMRES) iterative algorithm has been carried out, FMM is implemented to accelerate the computation of the integral operator in JMRES. Applying the propagation characteristics of electromagnetic wave in the cavity, the forward-backward(FB) iterative method is combined with the IPO and FMM to improve convergent.To improve computing speed more efficiently the cavity is subdivided into several sections, and each of them is analyzed independently using the IPO combined fast compute method from the rest of the cavity. Then a connection scheme based on generalized network principle is used to get the scattering field. the Generalized Reciprocity Integral(GRI) method is applied to treat the cavities with complex termination. The cavity is divide into two different parts by a surface S_? which islocated sufficiently close to the complex termination, the front section (typically simple and smooth) is analyzed by a hybrid method which IPO combined with FMM, the complex termination can be analyzed by more accurate methods,such as FDTD and FEM.For reducing the RCS of cavity,the thin radar absorbing materials maybe be coated on the inner surface of the cavity. The Equivalent Impedance Surface Boundary(EIBC) is used to characterize such imperfectly conducting cavity.At last, the Multilevel Fast Multipole Algorithm (MLFMA) is researched. The iterative formulations for the IPO+MLFMA hybrid algorithm is derived.All numerical results given in the disseration demonstrated that our research works are accurate enough and high efficient.