Sayed, Sadeed Bin; Ulku, Huseyin Arda; Bagci, Hakan(2014 IEEE Antennas and Propagation Society International Symposium (APSURSI), Institute of Electrical & Electronics Engineers (IEEE), 2014-07)[Conference Paper]
A marching on-in-time (MOT)-based time domain volume electric field integral equation (TD-VEFIE) solver is proposed for accurate and stable analysis of electromagnetic wave interactions on high-contrast scatterers. The stability is achieved using band-limited but two-sided (non-causal) temporal interpolation functions and an extrapolation scheme to cast the time marching into a causal form. The extrapolation scheme is designed to be highly accurate for oscillating and exponentially decaying fields, hence it accurately captures the physical behavior of the resonant modes that are excited inside the dielectric scatterer. Numerical results demonstrate that the resulting MOT scheme maintains its stability as the number of resonant modes increases with the contrast of the scatterer.
Sirenko, Kostyantyn; Asirim, Ozum Emre; Bagci, Hakan(2014 USNC-URSI Radio Science Meeting (Joint with AP-S Symposium), Institute of Electrical and Electronics Engineers (IEEE), 2014-07)[Conference Paper]
Discontinuous Galerkin time-domain method (DGTD) has been used extensively in computational electromagnetics for analyzing transient electromagnetic wave interactions on structures described with linear constitutive relations. DGTD expands unknown fields independently on disconnected mesh elements and uses numerical flux to realize information exchange between fields on different elements (J. S. Hesthaven and T. Warburton, Nodal Discontinuous Galerkin Method, 2008). The numerical flux of choice for 'linear' Maxwell equations is the upwind flux, which mimics accurately the physical behavior of electromagnetic waves on discontinuous boundaries. It is obtained from the analytical solution of the Riemann problem defined on the boundary of two neighboring mesh elements.
Desmal, Abdulla; Bagci, Hakan(2014 IEEE Antennas and Propagation Society International Symposium (APSURSI), Institute of Electrical & Electronics Engineers (IEEE), 2014-07)[Conference Paper]
A contrast-source inversion scheme is proposed for microwave imaging of domains with sparse content. The scheme uses inexact Newton and linear shrinkage methods to account for the nonlinearity and ill-posedness of the electromagnetic inverse scattering problem, respectively. Thresholded shrinkage iterations are accelerated using a preconditioning technique. Additionally, during Newton iterations, the weight of the penalty term is reduced consistently with the quadratic convergence of the Newton method to increase accuracy and efficiency. Numerical results demonstrate the applicability of the proposed method.
Desmal, Abdulla; Bagci, Hakan(2014 USNC-URSI Radio Science Meeting (Joint with AP-S Symposium), Institute of Electrical and Electronics Engineers (IEEE), 2014-07)[Conference Paper]
Development of microwave imaging methods applicable in sparse investigation domains is becoming a research focus in computational electromagnetics (D.W. Winters and S.C. Hagness, IEEE Trans. Antennas Propag., 58(1), 145-154, 2010). This is simply due to the fact that sparse/sparsified domains naturally exist in many applications including remote sensing, medical imaging, crack detection, hydrocarbon reservoir exploration, and see-through-the-wall imaging.
Yücel, Abdulkadir C.; Liu, Yang; Bagci, Hakan; Michielssen, Eric(2014 USNC-URSI Radio Science Meeting (Joint with AP-S Symposium), Institute of Electrical and Electronics Engineers (IEEE), 2014-07)[Conference Paper]
Reliable wireless communication and tracking systems in underground mines are of paramount importance to increase miners' productivity while monitoring the environmental conditions and increasing the effectiveness of rescue operations. Key to the design and optimization of such systems are electromagnetic (EM) simulation tools capable of analyzing wave propagation in electromagnetically large mine tunnels and galleries loaded with conducting cables (power, telephone) and mining equipment (trolleys, rails, carts), and potentially partially obstructed by debris from a cave-in. Current tools for simulating EM propagation in mine environments leverage (multi-) modal decompositions (Emslie et. al., IEEE Trans. Antennas Propag., 23, 192-205, 1975; Sun and Akyildiz, IEEE Trans. Commun., 58, 1758-1768, 2010), ray-tracing techniques (Zhang, IEEE Tran. Vehic. Tech., 5, 1308-1314, 2003), or full wave methods. Modal approaches and ray-tracing techniques cannot accurately account for the presence of conductors, intricate details of transmitters/receivers, wall roughness, or unstructured debris from a cave-in. Classical full-wave methods do not suffer from such restrictions. However, they require prohibitively large computational resources when applied to the analysis of electromagnetically large tunnels loaded with conductors. Recently, an efficient hybrid method of moment and transmission line solver has been developed to analyze the EM wave propagation inside tunnels loaded with conductors (Brocker et. al., in Proc IEEE AP-S Symp, pp.1,2, 2012). However, the applicability of the solver is limited to the characterization of EM wave propagation at medium frequency band.
Yücel, Abdulkadir C.; Gomez, Luis J.; Liu, Yang; Bagci, Hakan; Michielssen, Eric(2014 USNC-URSI Radio Science Meeting (Joint with AP-S Symposium), Institute of Electrical and Electronics Engineers (IEEE), 2014-07)[Conference Paper]
Space vehicles that re-enter the atmosphere often experience communication blackout. The blackout occurs when the vehicle becomes engulfed in plasma produced by interactions between the vehicle surface and the atmosphere. The plasma often is concentrated in a relatively thin shell around the vehicle, with higher densities near its nose than rear. A less structured, sometimes turbulent plasma wake often trails the vehicle. The plasma shell severely affects the performance of side-mounted antennas as it alters their characteristics (frequency response, gain patterns, axial ratio, and impedance) away from nominal, free-space values, sometimes entirely shielding the antenna from the outside world. The plasma plume/turbulent wake similarly affect the performance of antennas mounted at the back of the vehicle. The electromagnetic characteristics of the thin plasma shell and plume/turbulent wake heavily depend on the type of re-entry trajectory, the vehicle's speed, angles of attack, and chemical composition, as well as environmental conditions. To analyze the antennas' performance during blackout and to design robust communication antennas, efficient and accurate simulation tools for charactering the antennas' performance along the trajectory are called for.
Liu, Yang; Yücel, Abdulkadir C.; Bagci, Hakan; Michielssen, Eric(2014 USNC-URSI Radio Science Meeting (Joint with AP-S Symposium), Institute of Electrical and Electronics Engineers (IEEE), 2014-07)[Conference Paper]
The computational complexity and memory requirements of classically formulated marching-on-in-time (MOT)-based surface integral equation (SIE) solvers scale as O(Nt Ns 2) and O(Ns 2), respectively; here Nt and Ns denote the number of temporal and spatial degrees of freedom of the current density. The multilevel plane wave time domain (PWTD) algorithm, viz., the time domain counterpart of the multilevel fast multipole method, reduces these costs to O(Nt Nslog2 Ns) and O(Ns 1.5) (Ergin et al., IEEE Trans. Antennas Mag., 41, 39-52, 1999). Previously, PWTD-accelerated MOT-SIE solvers have been used to analyze transient scattering from perfect electrically conducting (PEC) and homogeneous dielectric objects discretized in terms of a million spatial unknowns (Shanker et al., IEEE Trans. Antennas Propag., 51, 628-641, 2003). More recently, an efficient parallelized solver that employs an advanced hierarchical and provably scalable spatial, angular, and temporal load partitioning strategy has been developed to analyze transient scattering problems that involve ten million spatial unknowns (Liu et. al., in URSI Digest, 2013).
Li, Ping; Jiang, Lijun; Shi, Yifei; Bagci, Hakan(2014 IEEE Antennas and Propagation Society International Symposium (APSURSI), Institute of Electrical & Electronics Engineers (IEEE), 2014-07)[Conference Paper]
This paper presents an algorithm hybridizing discontinuous Galerkin time domain (DGTD) method and time domain boundary integral (BI) algorithm for 3-D open region electromagnetic scattering analysis. The computational domain of DGTD is rigorously truncated by analytically evaluating the incoming numerical flux from the outside of the truncation boundary through BI method based on the Huygens' principle. The advantages of the proposed method are that it allows the truncation boundary to be conformal to arbitrary (convex/ concave) scattering objects, well-separated scatters can be truncated by their local meshes without losing the physics (such as coupling/multiple scattering) of the problem, thus reducing the total mesh elements. Furthermore, low frequency waves can be efficiently absorbed, and the field outside the truncation domain can be conveniently calculated using the same BI formulation. Numerical examples are benchmarked to demonstrate the accuracy and versatility of the proposed method.
Li, Ping; Jiang, Lijun; Bagci, Hakan(2014 IEEE Antennas and Propagation Society International Symposium (APSURSI), Institute of Electrical & Electronics Engineers (IEEE), 2014-07)[Conference Paper]
A discontinuous Galerkin time-domain (DGTD) method is proposed for analyzing electromagnetic field interactions on graphene from microwave to terahertz frequencies. An impedance boundary condition (IBC) is utilized to model the graphene within the DGTD framework. The numerical flux is reformulated to take into account the IBC. Highly dispersive surface conductivity of graphene present in the resulting flux expression is approximated in terms of rational functions using the fast-relaxation vector-fitting technique. Via inverse Laplace transform, this facilitates the time domain matrix equations into an integral form for time variable t, finite integral technique (FIT) with recursive convolution method is employed to discrete and solve the matrix equations. The accuracy and applicability of the proposed IBC-DGTD is verified by numerical experiments.
Shi, Yifei; Bagci, Hakan; Lu, Mingyu(2014 IEEE Antennas and Propagation Society International Symposium (APSURSI), Institute of Electrical & Electronics Engineers (IEEE), 2014-07)[Conference Paper]
When marching-on-in-time (MOT) method is applied to solve the time domain electric field integral equation (TD-EFIE), DC loop modes are always observed in the solution. In theory these modes should not be observed since they do not satisfy the relaxed initial conditions. Their appearance is attributed to numerical errors. It is shown here that when Rao-Wilton-Glisson basis and Lagrange interpolation functions are used to discretize the TD-EFIE, errors due to this space-time discretization have zero impact on the DC loop modes. Numerical experiments demonstrate that the numerical errors due to approximate solution of the MOT matrix system have more dominant impact on DC loop modes in the MOT solution.
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