Scattering analysis of a large body with deep cavities

Abstract

Accurate analysis of scattering by a modern jet aircraft is an important problem with a number of applications. This problem is, however, extremely difficult because it requires accurate simulation of scattering by a large body, possibly coated with inhomogeneous composite materials, and accurate simulation of scattering by jet engine inlets, which are usually large, deep, and contain complex internal structures. Both simulations have been long considered grand challenges in computational electromagnetics; fortunately, much progress has been made during the past few years. Since the scattering from the interior of a complex jet engine inlet contributes significantly to the overall radar cross section (RCS) of a jet aircraft, a variety of techniques have been developed to characterize the scattering by an arbitrarily shaped deep cavity. These techniques include several high-frequency (HF) asymptotic methods, the method of moments (MOM), the finite difference time domain (FDTD) method, the finite element method (FEM), and various hybrid methods. Although the HF methods are applicable to large and simple cavities, it has been found that numerical methods (such as FDTD, FEM, and MOM) are the only approaches that are capable of predicting accurately the RCS of arbitrarily shaped complex cavities. Unfortunately, the excessively long computation times and high memory requirements severely limit the size of the cavities to be simulated. Recently, a very efficient numerical technique has been developed for the analysis of scattering from a large, deep, and arbitrarily shaped cavity [l, 21. This technique employs the FEM to discretize the interior of the cavity and then eliminates each interior unknown as soon as its associated equation is completely assembled. The elimination process starts from the bottom of the cavity and moves toward its opening. Once all the interior unknowns are eliminated, a matrix is obtained that relates the electric and magnetic fields at the opening of the cavity. By assuming that the opening coincides with an aperture in an infinite ground plane, another matrix that relates the electric and magnetic fields at the aperture can be obtained by discretizing the boundary integral equation (BIE) obtained using the half-space Green’s function. The two matrices can be combined to solve for the electric and magnetic fields at the aperture, from which the RCS of the cavity can be determined. Clearly, this technique exploits the unique features of the FEM equations and, more importantly, the unique features of the problem of scattering by a large and deep cavity. As such, it uses minimal memory, which is proportional to the maximum cross section of the cavity and independent of the depth of the cavity, and its computation time increases only linearly with the depth of the cavity. Furthermore, it computes the scattered fields for all angles of incidence without repeating the elimination of the FEM matrix that consumes most (often more than 99%) of the computation times. The technique has been applied successfully to