An IntemationalJournal Available online at www.sciencedirectcom computers& .~,..~.@°,..~. mathematics with applications ELSEVIER Computers and Mathematics with Applications 49 (2005) 23-38 www.elsevier.com/locate/camwa Toward hp-Adaptive Solution of 3D Electromagnetic Scattering from Cavities A. ZDUNEK* Swedish Defence Research Agency FOI, Aeronautics Division (FFA) Computational Physics Department, SE-172 90 Stockholm, Sweden zka©foi, se W. RACHOWICZ t Institute for Computer Modelling, Section of Applied Mathematics Cracow University of Technology, P1 31-155 Cracow, Poland waldek©mlody, zms. pk. edu. pl N. SEHLSTEDT* Swedish Defence Research Agency FOI, Aeronautics Division (FFA) Computational Physics Department, SE-172 90 Stockholm, Sweden stn~foi.se (Received and accepted September 2003) Abstract--An effective hp-adaptive finite-element (FE) approach is presented for a reliable and accurate solution of 3D electromagnetic scattering problems. The far field is approximated with the infinite-element method. This allows one to reduce the external domain (discretised with finite elements) to a minimum preserving the possibility of arbitrary reduction of the error as the method does not introduce modelling error. The work is focused on scattering from cavity backed apertures recessed in a ground plane. Near optimal discretisations that can effectively resolve local rapid variations in the scattered field can be obtained adaptively by local mesh refinements (so called h- type refinements) blended with graded polynomial enrichments (p-enrichments). The discretisation error can be controlled by a self-adaptive process, which is driven by a posteriori error estimates in terms of the energy norm or in a quantity of interest. The radar cross section (RCS) is an example of the latter, h- and p-adaptively constructed solutions are compared to pure uniform p approximations. Numerical, highly accurate, and fairly converged solutions for generic cavities are given and compared to previously published results. (~) 2005 Elsevier Ltd. All rights reserved. Keywords--hp-version finite-element method (hp-FEM), Infinite elements, Electromagnetic scat- tering, Error control, Adaptivity, Radar cross section. *Research supported by, FMV:FoT-25,SAT1,2002. fResearch supported by, Polish Committee for Scientific Research under Grant 7 TllF 014 20. Financial support for Zdunek and Sehlstedt from the Swedish Defence Materiel Administration, FMV, under contract SAT1 FoT-25 01 is gratefully acknowledged. Financial support for Rachowicz by the Polish Committee for Scientific Research under Grant 7 TllF 014 20 is also gratefully acknowledged. Special thanks go to Prof. L. Demkowicz at ICES for his guidance and for providing us with his software platform. Thanks also to W. Cecot for providing the infinite element. 0898-1221/05/$ - see front matter (~ 2005 Elsevier Ltd. All rights reserved. Typeset by .4A4S-TEX doi: 10.1016/j.camwa.2005.01.003 24 A. ZDUNEKst al. 1. INTRODUCTION The finite-element method (FEM) is by now well established in computational electromagnetics (CEM) [1,2]. It is perhaps the method of choice when it comes to predicting the electromagnetic fields within materially inhomogeneous bodies of complicated shape. Special arrangements have to be made in order to handle open-region scattering problems though. The literature on vari- ous so-called nonrefleetive boundary conditions that approximate the proper decay condition in the far field is very rich, see e.g., [3]. In general, such boundary conditions introduce a certain modelling error which might be difficult to control. Controlling the discretisation error in electro- magnetic scattering problems with complex geometry and material inhomogeneities is of prime consideration. Therefore we look for a suitable so-called numerically exact method, by which we mean a technique free from modelling errors and whose approximation error can be arbitrarily reduced by an appropriate increase of the number of degrees-of-freedom. Various boundary in- tegral (BI) formulations and the method of moments (MoM) are popular methods that belong to this class of methods for exterior problems. The FE + BI method [4,5] is a powerful numeri- cally exact hybrid method for coupled interior and exterior problems. Merits and drawbacks of different formulations of this hybrid are discussed in [6,7]. Recently the combined finite/infinite-element approach was proposed by Demkowicz and Pal [8] as an alternative numerically exact method for solving electromagnetic scattering problems in unbounded domains. Their work presents the proof of convergence of the method which turns out to be exponential with the number of radial terms and with a fixed truncation radius. The implementation of infinite elements and tests of convergence were presented by Cecot et al. [9]. The exponential convergence of the combined FE + IE method was experimentally confirmed in our work [10] where we were able to reduce the global L2-error to the level of 0.05% for a smooth problem. We also mention that these developments for electromagnetics followed the former analogous results of' Demkowicz and Gerdes [11,12] and Demkowicz and Ihlenburg [13,14] in the closely related area of acoustics. As advantages of infinite elements, one can consider the fact that their stiffness matrices are sparse and their evaluation requires only integrals of smooth functions. Also, infinite elements can be very easily incorporated into a finite-element code as they are processed in exactly the same way as standard finite elements. In this work we use the FE + IE approach to solve the 3D-problem of scattering of electromagnetic waves from cavity backed apertures with or without interior obstacles. Our goal is to provide reliable radar cross section (RCS) data with a requested accuracy in a possibly efficient way. For linear elliptic problems the reliability and efficiency of the h-, p-, and hp-versions of the finite-element methods are well recognised and are backed by convergence proofs [15]. Robust a posteriori error estimation techniques are available that can be used to control the discretisation error [16]. Recently also a fully automatic hp-adaptive scheme has been proposed [17]. As opposed to this the mathematical foundations of three-dimensional hp elements for electro- magnetics are still incomplete (in two dimensions exponential convergence of hp-adaptive FEM has been recently shown by Demkowicz and Babu~ka [19]). A version of FEM which is stable for nonuniform distribution of element orders p was first proposed in [19], this method also turned out to satisfy commutativity of the de Rham diagram [20]. The three-dimensional hp-adaptive implementation of this method was presented in [21], and it is used in our computations. Among other authors experimenting with adaptivity in electromagnetics we mention Sun et al. [22], Lee [23], and Golias et al. [24] using h-adaptivity, and Andersen and Volakis [25] and Webb [26] working on p-method. Error estimation techniques similar to those for the elliptic case are also emerging [27]. In [10], we presented a promising pilot implementation of this residual method. In this work we take a step toward self-adaptivity and let the error estimator guide an adaptive mesh refinement process. Numerical results obtained with uniform p-enrichments are compared with results based Toward hp-Adaptive Solution 25 on h- and p-adaptive mesh refinements. The combined hp refinements are also available in the code but the strategy of such refinements has been developed thus far only for two dimensions [28]. We illustrate application of hp adaptivity with solutions on meshes with element sizes h being geometrically reduced towards the a priori known location of singularity and orders p increasing as one moves away from it. Such meshes are advocated for a class of singular solutions by Babu~ka [15] as resulting in exponential convergence. Numerical, highly accurate, and fairly converged solutions for generic cavities are presented and compared to previously published results in order to validate the FE ÷ IE approach. Fi- nally, we summarise the FE ÷ IE approach, discuss its merits and drawbacks and sketch future developments. 2. PROBLEM STATEMENT We consider scattering from a cavity-backed aperture recessed in a perfectly electrically con- ducting (PEC) infinite ground plane. The scattering problem under consideration is depicted in Figure 1. The cavity occupies the domain ~ in R 3. The exterior to the cavity is denoted ~+ = ~0 U ~0+ C R 3. It is here taken to be the half-space (z > 0) above the (PEC) infinite ground plane denoted Foo. The cavity is bounded by the surface F. The surface Fa := Foo N F forms the aperture in the infinite ground plane. The remainder of the bounding surface of the cavity, i.e., F \ F~, forms the cavity bottom and side walls. These are also assumed to be PEC. The cavity ~ may contain different dielectric materials. The conductivity ~(x), relative permittivity e~(x) and relative permeability #r(x) will thus, in general, be functions of the position x C ~, respectively. Material interfaces are denoted Yi. ~UDo: FE Do+ :IE Do F~ gro~ Figure 1. A cavity backed aperture recessed in an infinite ground-plane. The excitation is given in terms of an incident plane linearly polarised electric wave of amplitude E0 and circular frequency w E i"¢ (x, t) = EopeJ('~t-k°d'x) , (2.1) where d denotes the direction of propagation, k0 = w ex/~ is the wave number in free space, and where the unit vector p determines the polarisation. Here and throughout j denotes the imaginary unit. We decompose the exterior domain ~+ into a near field and a far field by introducing the hemispherical or a hemi-spheroidal shaped artificial boundary F0. Planar truncation boundaries 26 A. ZDUNEK et al. are not admissible using infinite elements. The domain enclosed by the infinite ground plane F~ and F0 is denoted ~0. The (unbounded) reduced exterior is denoted ~+. For the infinite-element modelling of scattering on a cavity it is necessary to decompose the total field E t°t into the (known) incident and reflected fields, E inc -~- E ref, and the remaining part which we refer as the scattered field E.