Computation of radar cross section of jet engine inlets

Abstract

COMPUTATION OF RADAR CROSS SECTION OF JET ENGINE INLETSJianKey words: radar cross sections; finite element method; numerical method INTRODUCTIONElectromagnetic scattering from jet engine inlets is an important topic of research due to the significant contribution that the inlet has to the radar cross section (RCS) of modern aircraft. Many researchers have studied this topic using an assortment of different methods. Some techniques applied to the problem are standard methods, such as mode matching (MM) [1, 2], shooting and bouncing ray (SBR) [3, 4], as well as the finite element method (FEM) and method of moment (MoM) techniques [5,6]. It is known, however, that these standard methods have some serious drawbacks that prevent them from being fully used to compute the RCS for practical jet engine inlets. In the case of MM, the resulting system demonstrates slow convergence and requires a large number of modes as well as a poorly conditioned large matrix to invert. Furthermore, rigorous modal methods are only useful for simple geometries that can have canonical terminations [2]. The problems with SBR techniques are that, while they are simple and efficient, they lack the required accuracy to handle arbitrary complex structures. Standard FEM and MoM have trouble handling the entire structure due to its large size.More recently, hybrid methods have been introduced to solve this important problem. Some of them combine high-frequency techniques in the empty part of the inlet with numerical methods in the loaded section [7][8][9][10][11][12]. Despite the advantages of these methods, they still require a large number of degrees of freedom, which makes them very difficult to apply to large structures. Some researchers reduce this domain for the case of simple periodic geometries, but for arbitrarily shaped inlets these methods are still limited by their memory and CPU time requirements.A particular method that is very well suited to the computation of RCS of jet engine inlets is the finite element-boundary integral (FE-BI) method [13,14]. Since jet engine inlets are complex geometries that may have complex material composition, this is well suited for a finite element approach. The domain size is truncated with a boundary integral, which reduces the overall simulation domain. Furthermore, the techniques described in [13] and [14] exploit the structure of the FE-BI equations to require minimal memory (only proportional to the maximum cross section of the inlet and independent of its depth) and computation time, which increases linearly with respect to the inlet depth.In both [13] and [14], the FE-BI method was applied to empty cavities. Although the calculated results were compared extensively with the results obtained using other techniques, little comparison was given with measured data. In this article, we extend the FE-BI method to inlets with complex internal structures and present results applying this technique to some special engine cases described in the following section and compare the numerical results with measu...

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