Browsing by Subject "Coupling"
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Item Open Access Investigation of hourly and daily patterns for lithosphere-ionosphere coupling before strong earthquakes(IEEE, 2009-06) Karatay, S.; Arıkan F.; Arıkan, OrhanThe ionosphere can be characterized with its electron density distribution which is a complex function of spatial and temporal variations, geomagnetic, solar and seismic activity. An important measurable quantity about the electron density is the Total Electron Content (TEC) which is proportional to the total number of electrons on a line crossing the atmosphere. TEC measurements enable monitoring variations in the space weather. Global Positioning System (GPS) and the network of world-wide receivers provide a cost-effective solution in estimating TEC over a significant proportion of global land mass. In this study, five earthquakes between 2003-2008 that occurred in Japan with different seismic properties, and the China earthquake in May 2008 are investigated. The TEC data set is investigated by using the Kullback-Leibler Divergence (KLI), Kullback-Leibler Distance (KLD) and L2-Norm (L2N) which are used for the first time in the literature in this context and Cross Correlation Function (CCF) which is used in the literature before for quiet day period (QDP), disturbed day period (DDP), periods of 15 days before a strong earthquake (BE) and after the earthquake (AE). In summary, it is observed that the CCF, KLD and L2N between the neighbouring GPS stations cannot be used as a definitive earthquake precursor due to the complicated nature of earthquakes and various uncontrolled parameters that effect the behavior of TEC such as distance to the earthquake epicenter, distance between the stations, depth of the earthquake, strength of the earthquake and tectonic structure of the earthquake. KLD, KLI and L2N are used for the first time in literature for the investigation of earthquake precursor for the first time in literature and the extensive study results indicate that for more reliable estimates further space-time TEC analysis is necessary over a denser GPS network in the earthquake zones. ©2009 IEEE.Item Open Access A method of two-scale chemo-thermal-mechanical coupling for concrete(CIMNE, 2011) Wu, T.; Temizer, İlker; Wriggers, P.The Alkali Silica Reaction(ASR) is one of the most important reasons to cause damage in cementitious constructions, which can be attributed to the expansion of hydrophilic gel produced in the reaction. In this contribution, the chemical extent is described depending on the temperature and it has influences on damage parameters. Expansions of the gel are assumed to only happen in the micropores of Hardened Cement Paste. Afterwards, the homogenization of damage in the microscale is initialized and the effective damage can be applied in the mesoscale directly. Moreover, parameter identification is implemented to extract the effective inelastic consititutive equation. In all, 3D multiscale chemo-thermo-mechanical coupled model is set up to describe the damage in the concrete due to ASR.Item Open Access A multiscale method to analyze the deterioration due to alkali silica reaction considering the effects of temperature and relative humidity(International Center for Numerical Methods in Engineering, 2013) Wu, T.; Temizer I.; Wriggers P.This work presents a three-dimensional multiscale framework to investigate the deterioration resulting from alkali silica reaction (ASR) in the concrete. In this contribution, 3D micro-CT scan of hardened cement paste (HCP) and aggregates with a random distribution embedded in a homogenized cement paste matrix represent the microscale and mesoscale of the concrete respectively. A 3D hydro-chemo-thermo-mechanical model based on staggered method is developed at the mesoscale of the concrete, yet taking into account the deterioration at the microscale due to ASR.Item Open Access Optimization of coupled Class-E RF power amplifiers for a transmit array system in 1.5T MRI(2020-01) Aldemir, Muhammed SaidRadio Frequency (RF) field is generated mostly by linear RF power amplifiers in Magnetic Resonance Imaging (MRI). These amplifiers have relatively low efficiency and are placed far from the transmit coil in MRI system room. Additional cooling systems and long transmission cables increase the cost of MRI hardware. Coupled Class-E on-coil RF amplifiers for a Transmit Array System is proposed instead of linear RF amplifiers. Class-E RF amplifier is a nonlinear switching amplifier which has 100% drain efficiency ideally. State-space models for single Class-E amplifier and coupled Class-E amplifiers are derived and steady-state operation of coupled Class-E amplifiers is investigated. The state-space models are verified with the simulation results of the Class-E amplifier. Effect of coupling between channels on the output characteristics of the transmit system is observed. Instead of implementing decoupling methods, coupling is maintained and optimization of circuit parameters for coupled Class-E amplifiers is proposed to achieve high drain efficiency and high output RF power. Output power of 600 W and overall drain efficiency higher than 90% are achieved by two coupled Class-E amplifiers even with high coupling levels in the state-space model. Power and efficiency measurements are done with coupled amplifiers and MRI experiments are performed in 1.5T Scimedix MRI Scanner. In conclusion, coupled Class-E RF power amplifiers can be operated as on-coil amplifiers in a Transmit Array system with high output characteristics by optimization of circuit parameters.