Browsing by Subject "Numerical models"

Now showing 1 - 6 of 6

Open Access
Analysis of an arbitrary-profile, cylindrical, impedance reflector surface illuminated by an E-polarized complex line source beam
(VSP BV, 2014) Kuyucuoglu, F.; Oǧuzer, T.; Avgin, I.; Altintas, A.
Electromagnetic scattering from a cylindrical reflector surface having an arbitrary conic section profile is studied. We assumed an electrically thin layer antenna illuminated by a complex line source in E-polarization mode. Our boundary value formulation, without loss of generality, involves an integral equation approach having impedance-type thin-layer boundary conditions. For simplicity, we also considered both faces of the reflector of the same uniform impedance value. Our computation employs the Method of Analytical Regularization (MAR) technique: the integral equations are converted into the discrete Fourier transform domain yielding two coupled dual series equations, which are then solved by the Fourier inversion and Riemann Hilbert Problem techniques. We demonstrate the accuracy and the convergence behaviors of our numerically solved MAR results that can serve as an accurate benchmark for comparison with widely used results obtained by approximate boundary conditions. © 2013 Taylor and Francis.
Open Access
Modeling AoII in push- and pull-based sampling of continuous time Markov chains
(IEEE, 2024) Cosandal, Ismail; Akar, Nail; Ulukus, Sennur
Age of incorrect information (AoII) has recently been proposed as an alternative to existing information freshness metrics for real-time sampling and estimation problems involving information sources that are tracked by remote monitors. Different from existing metrics, AoII penalizes the incorrect information by increasing linearly with time as long as the source and the monitor are de-synchronized, and is reset when they are synchronized back. While AoII has generally been investigated for discrete time information sources, we develop a novel analytical model in this paper for push- and pull-based sampling and transmission of a continuous time Markov chain (CTMC) process. In the pull-based model, the sensor starts transmitting information on the observed CTMC only when a pull request from the monitor is received. On the other hand, in the push-based scenario, the sensor, being aware of the AoII process, samples and transmits when the AoII process exceeds a random threshold. The proposed analytical model for both scenarios is based on the construction of a discrete time MC (DTMC) making state transitions at the embedded epochs of synchronization points, using the theory of absorbing CTMCs, and in particular phase-type distributions. For a given sampling policy, analytical models to obtain the mean AoII and the average sampling rate are developed. Numerical results are presented to validate the analytical models as well as to provide insight on optimal sampling policies under sampling rate constraints.
Open Access
Numerical modeling of ultrasonic particle manipulation for microfluidic applications
(Springer Verlag, 2014) Büyükkoçak S.; Özer, M. B.; Çetin B.
A numerical simulation methodology for ultrasonic particle/cell separation and cell washing processes is introduced and validated by comparing with the results from the literature. In this study, a finite element approach is used for modeling fluid flow in a microchannel and analytical relations are utilized for the calculation of the ultrasonic radiation forces. The solutions in acoustic and fluidic domains are coupled, and the particle separation under the influence of ultrasonic waves is numerically simulated. In order to simulate the cell washing process, diffusion and fluid dynamics solutions are coupled and solved. A Monte Carlo approach is chosen where statistical distributions are implemented in the simulations. Uniform distributions for the starting locations of particles/cells in the microchannel and normal distributions for the size of the particles are used in numerical simulations. In each case, 750 particles are used for the simulation, and the performance of separation process is evaluated by checking how many microparticles resulted in the targeted outlet channels. Channel geometries for the numerical simulations are adapted from the experimental studies in literature, and comparison between the reported experimental results and the numerical estimations is performed. It has been observed that the numerical estimations and experimental results from the literature are in good agreement, and the proposed methodology may be implemented as a design tool for ultrasonic particle manipulation for microfluidic applications. © 2014, Springer-Verlag Berlin Heidelberg.
Open Access
RF heating of deep brain stimulation implants during MRI in 1.2 T vertical scanners versus 1.5 T horizontal systems: a simulation study with realistic lead configurations
(Institute of Electrical and Electronics Engineers, 2020) Kazemivalipour, Ehsan; Vu, J.; Lin, S.; Bhusal, B.; Nguyen, B. T.; Kirsch, J.; Elahi, B.; Rosenow, J.; Atalar, Ergin; Golestanirad, L.
Patients with deep brain stimulation (DBS) implants are often denied access to magnetic resonance imaging (MRI) due to safety concerns associated with RF heating of implants. Although MR-conditional DBS devices are available, complying with manufacturer guidelines has proved to be difficult as pulse sequences that optimally visualize DBS target structures tend to have much higher specific absorption rate (SAR) of radiofrequency energy than current guidelines allow. The MR-labeling of DBS devices, as well as the majority of studies on RF heating of conductive implants have been limited to horizontal close-bore MRI scanners. Vertical MRI scanners, originally introduced as open low-field MRI systems, are now available at 1.2 T field strength, capable of high-resolution structural and functional imaging. No literature exists on DBS SAR in this class of scanners which have a 90° rotated transmit coil and thus, generate a fundamentally different electric and magnetic field distributions. Here we present a simulation study of RF heating in a cohort of forty patient-derived DBS lead models during MRI in a commercially available vertical openbore MRI system (1.2 T OASIS, Hitachi) and a standard horizontal 1.5 T birdcage coil. Simulations were performed at two major imaging landmarks representing head and chest imaging. We calculated the maximum of 0.1g-averaged SAR (0.1g-SAR Max ) around DBS lead tips when a B 1 + = 4 μT was generated on an axial plane passing through patients body. For head landmark, 0.1g-SAR Max reached 220±188 W/kg in the 1.5 T birdcage coil, but only 14±11 W/kg in the OASIS coil. For chest landmark, 0.1g-SAR Max was 24±17 W/kg in the 1.5 T birdcage coil and 3±2 W/kg in the OASIS coil. A paired two-tail t-test revealed a significant reduction in SAR with a large effect-size during head MRI (p <; 1.5×10 -8 , Cohen's d = 1.5) as well as chest MRI (p <; 6.5×10 -10 , Cohen's d = 1.7) in 1.2 T Hitachi OASIS coil compared to a standard 1.5 T birdcage transmitter. Our findings suggest that open-bore vertical scanners may offer an untapped opportunity for MRI of patients with DBS implants.
Open Access
Single and coupled metasurfaces for tunable polarization-sensitive terahertz filters
(IEEE, 2016) Serebryannikov A.E.; Lakhtakia A.; Özbay, Ekmel
We simulated the transmission of terahertz waves through a single metasurface and two coupled metasurfaces that comprise H-shaped subwavelength resonators made of InAs, a magnetically tunable material. The magnetostatic field was varied from 0 to 1 T. The obtained results demonstrate that the substrate permittivity and the coupling of metasurfaces can significantly affect filtering performance as well as the possibility of tuning for different orientations of the magnetostatic field. ï¿½ 2016 IEEE.
Open Access
Timely monitoring of Markov chains under sampling rate constraints
(IEEE, 2024) Akar, Nail; Ulukus, Sennur
We study a pull-based monitoring system in which a common remote monitor queries the states of a collection of heterogeneous finite-state irreducible continuous time Markov chain (CTMC) based information sources, according to a Poisson process with different per-source sampling rates, in order to maintain remote estimates of the states. Three information freshness models are considered to quantify the accuracy of the remote estimates: fresh when equal (FWE), fresh when sampled (FWS) and fresh when close (FWC). For each of these freshness models, closed-form expressions are derived for mean information freshness for each source, as a function of the sampling rate. Using these expressions, optimum sampling rates for all sources are obtained using water-filling based optimization for maximizing the weighted sum freshness of the monitoring system, under an overall sampling rate constraint. Numerical examples are presented to validate the effectiveness of the proposed method by comparing it to several baseline sampling policies.