Browsing by Subject "Finite difference method"

Now showing 1 - 20 of 23

Open Access
Accurate plane-wave excitation in the FDTD method
(IEEE, 1997) Gürel, Levent; Oğuz, Uğur; Arıkan, Orhan
Different techniques are developed to implement plane-wave excitation on the finite-difference time-domain (FDTD) method, such as the initial-condition, the hard-source, and the connecting-condition techniques, for the total-field/scattered field (TF/SF) formulation. In the TF/SF formulation, the incident field is computed and fed to the 3D FDTD grid on the boundary separating the total-field and the scattered-field regions. Since the incedent field is a known quantity, a closed-form expression can be evaluated on every point of this boundary. A more efficient way of computing the incedent field is by using an incedent-field array (IFA), which is a 1D FDTD grid set-up to numerically propagate the incedent field into the 3D FDTD.
Open Access
Animation of deformable models
(Pergamon Press, 1994) Güdükbay, Uğur; Özgüç, B.
Although kinematic modelling methods are adequate for describing the shapes of static objects, they are insufficient when it comes to producing realistic animation. Physically based modelling remedies this problem by including forces, masses, strain energies and other physical quantities. The paper describes a system for the animation of deformable models. The system uses physically based modelling methods and approaches from elasticity theory for animating the models. Two different formulations, namely the primal formulation and the hybrid formulation, are implemented so that the user can select the one most suitable for an animation depending on the rigidity of the models. Collision of the models with impenetrable obstacles and constraining of the model points to fixed positions in space are implemented for use in the animations. © 1994.
Open Access
Application of signal-processing techniques to dipole excitations in the finite-difference time-domain method
(Taylor & Francis, 2002) Oğuz, U.; Gürel, Levent
The applications of discrete-time signal-processing techniques, such as windowing and filtering for the purpose of implementing accurate excitation schemes in the finite-difference time-domain (FDTD) method are demonstrated. The effects of smoothing windows of various lengths and digital lowpass filters of various bandwidths and characteristics are investigated on finite-source excitations of the FDTD computational domain. Both single-frequency sinusoidal signals and multifrequency arbitrary signals are considered.
Open Access
Application of signal-processing techniques to reduce the errors related to the FDTD excitations
(IEEE, 2001) Gürel, Levent; Oğuz, Uğur
A study on the reduction of the errors related to the finite-difference time-domain (FDTD) excitations was performed by employing signal-processing techniques. Plane-wave scattering problems were simulated. The improvements in both plane-wave and finite-source excitation schemes were demonstrated. The result showed that a visible DC offset value was exhibited even after five periods of the incident wave.
Open Access
Band-dropping via coupled photonic crystal waveguides
(Optical Society of American (OSA), 2002) Bayındır, Mehmet; Özbay, Ekmel
We observe the dropping of electromagnetic waves having a specific frequency or a certain frequency band in two-dimensional dielectric photonic crystals. The single frequency is dropped via cavity-waveguide coupling. Tunability of the demultiplexing mode can be achieved by modifying the cavity properties. The band-dropping phenomenon is achieved by introducing interaction between an input planar, or coupled-cavity, waveguide and the output coupled-cavity waveguides (CCWs). The dropping band can be tuned by changing the coupling strength between the localized cavity modes of the output CCWs. We also calculate the transmission spectra and the field patterns by using the finite-difference-time-domain (FDTD) method. Calculated results agree well with the microwave measurements. © 2002 Optical Society of America.
Open Access
Comparative evaluation of absorbing boundary conditions using Green's functions for layered media
(IEEE, 1995) Aksun, M. İrşadi; Dural, G.
Absorbing boundary conditions are comparatively studied using the Green's functions of the vector and scalar potentials for multilayer geometries and general sources. The absorbing boundaries are introduced as additional layers with predefined reflection coefficients into the calculation of the Green's functions. The Green's functions are calculated using different reflection coefficients corresponding to different absorbing boundaries and compared to those obtained with no absorbing boundary. This approach provides an absolute measure of the effectiveness of different absorbing boundaries.
Open Access
Coupled-cavity structures in photonic crystals
(Materials Research Society, 2002) Bayındır, Mehmet; Özbay, Ekmel
We investigate the localized coupled-cavity modes in two-dimensional dielectric photonic crystals. The transmission, phase, and delay time characteristics of the various coupled-cavity structures are measured and calculated. We observed waveguiding through the coupled cavities, splitting of electromagnetic waves in waveguide ports, and switching effect in such structures. The corresponding field patterns and the transmission spectra are obtained from the finite-difference-time-domain (FDTD) simulations. We also develop a theory based on the classical wave analog of the tight-binding (TB) approximation in solid state physics. Experimental results are in good agreement with the FDTD simulations and predictions of the TB approximation.
Open Access
Enhanced transmission of microwave radiation in one-dimensional metallic gratings with subwavelength aperture
(American Institute of Physics, 2004) Akarca-Biyikli, S. S.; Bulu, I.; Özbay, Ekmel
We report a theoretical and experimental demonstration of enhanced microwave transmission through subwavelength apertures in metallic structures with double-sided gratings. Three different types of aluminum gratings (sinusoidal, symmetric rectangular, and asymmetric rectangular shaped) are designed and analyzed. Our samples have a periodicity of 16 mm, and a slit width of 2 mm. Transmission measurements are taken in the 10–37.5 GHz frequency spectrum, which corresponds to 8–30 mm wavelength region. All three structures display significantly enhanced transmission around surface plasmon resonance frequencies. The experimental results agree well with finite-difference-time-domain based theoretical simulations. Asymmetric rectangular grating structure exhibits the best results with ,50% transmission at 20.7 mm, enhancement factor of ,25, and ±4° angular divergence.
Open Access
FDTD simulations of multiple GPR systems
(IEEE, 2003-06) Oǧuz, Uğur; Gürel, Levent
A multiple-GPR detection system was simulated. The main advantage of such a system was that it saves time by detecting both the transverse and the longitudinal positions of the target by a B-scan measurement, whereas the same detection can be achieved by a C-scan with a single-GPR system. Finite-domain time-difference (FDTD) method was employed to perform the simulations, in which the ground was homogeneous and the target was perfectly conducting.
Open Access
Frequency responses of ground-penetrating radars operating over highly lossy grounds
(IEEE, 2002) Oğuz, U.; Gürel, Levent
The finite-difference time-domain (FDTD) method is used to investigate the effects of highly lossy grounds and the frequency-band selection on ground-penetrating-radar (GPR) signals. The ground is modeled as a heterogeneous half space with arbitrary background permittivity and conductivity. The heterogeneities encompass both embedded scatterers and surface holes, which model the surface roughness. The decay of the waves in relation to the conductivity of the ground is demonstrated. The detectability of the buried targets is investigated with respect to the operating frequency of the GPR, the background conductivity of the ground, the density of the conducting inhomogeneities in the ground, and the surface roughness. The GPR is modeled as transmitting and receiving antennas isolated by conducting shields, whose inner walls are coated with absorbers simulated by perfectly matched layers (PML). The feed of the transmitter is modeled by a single-cell dipole with constant current density in its volume. The time variation of the current density is selected as a smooth pulse with arbitrary center frequency, which is referred to as the operating frequency of the GPR.
Open Access
Highly directional resonant antennas built around photonic crystals
(IEEE, 1999) Özbay, Ekmel; Temelkuran, Burak; Bayındır, Mehmet; Biswas, R.; Sigalas, M. M.; Tuttle, G.; Ho, K. M.
We report a photonic crystal-based resonant antenna with a very high directivity and gain. The layer-by-layer dielectric photonic crystal we used in our experiments was designed to have a three dimensional band gap with a mid-gap frequency around 12 GHz. We used the output port of a microwave network analyzer and a monopole antenna to obtain EM waves. The input port of the network analyzer and a standard gain horn antenna were used to receive the radiated EM field from the monopole antenna. The receiver was kept free to rotate around the antenna. We investigated the radiation characteristics of this monopole antenna, which was inserted into the planar defect structures built around a photonic crystal that consisted of 20 layers. The planar defect was formed by separating the 8th and 9th layers of the structure. In order to suppress the radiation in the backward direction, we intentionally chose one of the mirrors of the cavity to have a higher reflectivity (/spl sim/18-20 dB) than the front mirror.
Open Access
Interpolation techniques to improve the accuracy of the plane wave excitations in the finite difference time domain method
(Wiley-Blackwell Publishing, Inc., 1997-11) Oğuz, U.; Gürel, Levent
The importance of matching the phase velocity of the incident plane wave to the numerical phase velocity imposed by the numerical dispersion of the three-dimensional (3-D) finite difference time domain (FDTD) grid is demonstrated. In separate-field formulation of the FDTD method, a plane wave may be introduced to the 3-D computational domain either by evaluating closed-form incident-field expressions or by interpolating from a 1-D incident-field array (IFA), which is a 1-D FDTD grid simulating the propagation of the plane wave. The relative accuracies and efficiencies of these two excitation schemes are compared, and it has been shown that higher-order interpolation techniques can be used to improve the accuracy of the IFA scheme, which is already quite efficient.
Open Access
Investigation of localized coupled-cavity modes in two-dimensional photonic bandgap structures
(IEEE, 2002) Özbay, Ekmel; Bayındır, Mehmet; Bulu, I.; Cubukcu, E.
We present a detailed study of the localized coupled-cavity modes in 2-D dielectric photonic crystals. The transmission, phase, and delay time characteristics of the various coupled-cavity structures are measured and calculated. We observed the eigenmode splitting, waveguiding through the coupled cavities, splitting of electromagnetic waves in waveguide ports, and switching effect in such structures. The corresponding field patterns and the transmission spectra are obtained from the finite-difference-time-domain (FDTD) simulations. We also develop a theory based on the classical wave analog of the tight-binding (TB) approximation in solid state physics. Experimental results are in good agreement with the FDTD simulations and predictions of the TB approximation.
Open Access
Modeling of ground-penetrating-radar antennas with shields and simulated absorbers
(IEEE, 2001) Oğuz, U.; Gürel, Levent
A three-dimensional (3-D) finite-difference time domain (FDTD) scheme is employed to simulate ground-penetrating radars. Conducting shield walls and absorbers are used to reduce the direct coupling to the receiver. Perfectly matched layer (PML) absorbing boundary conditions are used for matching the multi-layered media and simulating physical absorbers inside the FDTD computational domain. Targets are modeled by rectangular prisms of arbitrary permittivity and conductivity. The ground is modeled by homogeneous and lossless dielectric media.
Open Access
On the frequency-band selection for ground-penetrating radars operating over lossy and heterogeneous grounds
(IEEE, 2001-07) Oǧuz, Uğur; Gürel, Levent
The frequency-band selection for ground penetrating radars operating over lossy and heterogenous grounds was discussed. The simulations were carried out using perfectly matched layer (PML) absorbing boundary conditions (ABC). The results demonstrated that for conductivities below 0.5 S/m the change in centre frequency does not influence the energy scattered from the deeper target. The selection of the center frequency thus influences the GPR measurements for the measurements performed over a highly-conducting soil.
Open Access
Reducing the dispersion errors of the finite-difference time-domain method for multifrequency plane-wave excitations
(Taylor & Francis, 2003) Oğuz, U.; Gürel, Levent
We demonstrate the applications of discrete-time signal-processing (SP) techniques for the purpose of generating accurate plane waves in the finite-difference time-domain (FDTD) method. The SP techniques are used either to reduce the high-frequency content of the source excitation or to compute more precise incident-field values in the computational domain. The effects of smoothing windows of various lengths, digital lowpass filters of various bandwidths and characteristics, and polynomial interpolation schemes of various orders are investigated. Arbitrary signals with multifrequency content are considered.
Open Access
Simulation of TRT-configured ground-penetrating radars over heterogeneous grounds
(IEEE, 2001) Oǧuz, Uğur; Gürel, Levent
The simulation of transmitter-reciever-transmitter (TRT) configured ground-penetrating radars over heterogeneous grounds was discussed. The finite-difference time-domain (FDTD) methods along with the perfectly-matched layer (PML) absorbing boundary conditions (ABC) were used for the simulations. Scattered-field image demonstrated that the buried target was easily detected when buried in a homogenous ground. Results show that the TRT-configured GPR is sensitive to surface roughness and the main source of noise is the detoriorations in the ground-air interface.
Open Access
Simulations of ground-penetrating radars over lossy and heterogeneous grounds
(IEEE, 2001) Gürel, Levent; Oğuz, U.
The versatility of the three-dimensional (3-D) finite-difference time-domain (FDTD) method to model arbitrarily inhomogeneous geometries is exploited to simulate realistic groundpenetrating radar (GPR) scenarios for the purpose of assisting the subsequent designs of high-performance GPR hardware and software. The buried targets are modeled by conducting and dielectric prisms and disks. The ground model is implemented as lossy with surface roughness, and containing numerous inhomogeneities of arbitrary permittivities, conductivities, sizes, and locations. The impact of such an inhomogeneous ground model on the GPR signal is demonstrated. A simple detection algorithm is introduced and used to process these GPR signals. In addition to the transmitting and receiving antennas, the GPR unit is modeled with conducting and absorbing shield walls, which are employed to reduce the direct coupling to the receiver. Perfectly matched layer absorbing boundary condition is used for both simulating the physical absorbers inside the FDTD computational domain and terminating the lossy and layered background medium at the borders.
Open Access
Subsurface-scattering calculations via the 3D FDTD method employing PML ABC for layered media
(IEEE, 1997) Oğuz, Uğur; Gürel, Levent
A three-dimensional finite-difference time-domain method that employs pure scattered-field formulation and perfectly matched layers (PML) as the absorbing boundary condition is developed for solving subsurface-scattering. A subsurface radar is modeled and the fields scattered from various buried objects with different parameters such as the size, depth, and number are observed and distinguished. The `derivative' signal, which can easily be obtained in practical systems, is useful in identifying the buried objects.
Open Access
Three-dimensional FDTD modeling of a GPR
(IEEE, 2000) Oğuz, Uğur; Gürel, Levent
The power and flexibility of the Finite-Difference Time-Domain (FDTD) method are combined with the accuracy of the perfectly-matched layer (PML) absorbing boundary conditions to simulate realistic ground-penetrating radar (GPR) scenarios. Three-dimensional geometries containing modes of radar units, buried objects and surrounding environments are simulated. Simulation results are analyzed in detail.