Sharif Digital Repository

The CLLC resonant converter is a promising candidate for high efficient, bidirectional power transfer applications, such as vehicle-to-grid on-board charger and hybrid vehicle dc-dc converter. Nevertheless, the analysis of CLLC still remains challenging because of its complex multiresonant nature and several storage elements. In this article, a circuit analysis method based on change of variables is presented that maps the state-space equations into two decoupled sets of equations. The analyses are carried out in two state-plane coordinate systems, then the results are mapped onto the original region. The proposed method is then used to thoroughly analyze the CLLC resonant converter... 

Excitation Controlled Synchronous Generator-based Wind Turbine (ECSG WT) is a recently proposed Wind Turbine (WT) scheme that has not been fully investigated in detail. This paper intends to analyze the performance of the ECSG WT scheme and compare it with those of two mainstream WT schemes based on electrically excited synchronous generator, i.e., VSC-based full converter WT and diode bridge rectifier-based WT equipped with boost converter on its Direct Current (DC) link. The objective of this comparison is to demonstrate great potentials of ECSG WT to be considered in the wind industry. To do so, two successful Wind Turbine (WT) schemes in the market that are structurally close to ECSG WT... 

Fracture process of fiber-reinforced concrete notched beams is investigated here. Polypropylene macrosynthetic fibers are utilized for reinforcing concrete specimens, and a high-strength mix design is used to produce strong bonds between the embossed polypropylene fibers and the cementitious matrix of beams. Considering different locations for the notch, this study focuses on bridging mechanism under different conditions using both experimental and numerical approaches. First mode of fracture occurs due to opening of crack faces. This mode of failure is simulated by imposing symmetric boundary conditions on middle-notched beams. Inducing the notch with an offset from the middle, mixed-mode... 

This paper aims to develop a deteriorated bridge maintenance strategy that ensures the safe operation of steel structures and minimizes the total risk. Five common failure modes are considered for the deteriorated bridge: flexure, shear, deflection, fatigue failure for girder, and chloride attack for the concrete deck. Time-dependent and system reliability analyses are carried out to find the probability of failure under these failure modes. Risk-cost optimization is then used to determine the maintenance strategy. This method was applied to a working example. It was found that the developed maintenance strategy can predict when, where, and what to maintain for a bridge to ensure its safe... 

In this paper, the application of Endurance Time (ET) method to the seismic analysis of bridges is elaborated. ET method is a novel seismic analysis method based on time history analysis in which a structure is subjected to a predefined intensifying acceleration function. First, six concrete bridges were modeled in this study. Three Endurance Time Acceleration Functions (ETAFs) were applied to the models, and the average of responses was calculated. Next, the time history analysis was conducted using seven real accelerograms that are scaled using the method recommended by Federal Highway Administration (FHWA) to be compatible with the design spectrum of American Association of State Highway... 

In this paper, the evolution of elastic–plastic damage in the composite laminates under fatigue conditions is modeled. Continuum damage mechanics (CDM) has been coupled with the bridge micromechanics model to estimate the fatigue damage and life for laminated composite structures. Based on the elastic–plastic bridging model, three damage variables are defined. These variables estimate the fiber, matrix, and fiber/matrix damage response at the ply scale. To model the beginning of plastic deformation, a yield function is utilized, and evolution equations of the damage variables are obtained. Then the developed deformation plastic model is calculated. The model parameters are calibrated by... 

In this research, the effect of different amounts of Cr2O3 (2.5, 5, 7.5, and 10 wt.%) and sintering temperature (1850, 1900, and 1950 °C) on the sinterability and mechanical properties of liquid-phase sintered SiC-matrix composites was studied. First, raw materials were ground for 3 h using a planetary mill whose rotational speed was 180 rpm. The process of pressing the samples was completed using uniaxial pressing with the applied pressure of 90 MPa. Finally, the samples were sintered under an argon atmosphere at various temperatures for 1.5 h. In the end, the best sintered sample was annealed at 2000°C for 2 h. The phases, microstructure, and chemical composition of the samples were... 

This paper develops a trend line-based algorithm for signal decomposition in which the adjusted Savitzky–Golay filter is utilized to initiate the decomposition process. In this line, the proposed algorithm determines some special trend lines, mainly composed of the natural frequency of a bridge. An easy-to-implement algorithm is then provided to formulate this process and to decompose the given signal into its components in a systematic way. Additionally, a residual signal is generated by the proposed algorithm to store the detected noise and to reconstruct the original signal. To verify the proposed algorithm in the field of bridge health monitoring, a set of numerical and experimental... 

Following the idea of using unbonded isolators to eliminate the tensile stress and providing a cost-effective isolation system, recent experimental research has shown ordinary unbonded steel reinforced elastomeric bearings (SREBs) as an attractive option for seismic isolation of highway bridges. The focus of current research work is on the evaluation of the seismic vulnerability of a highway bridge isolated by unbonded SREBs through developing fragility curves of the structure. This assessment considers parameters playing a key role in the isolation system behavior, namely the friction coefficient, as well as the isolator aging effects. In this regard, a typical three-span highway bridge is... 

Reduction of large tensile stresses and cost-efficiency has made unbonded isolators an attractive option as an isolation system. Following this idea and the results obtained by various researchers, it is shown that unbonded steel-reinforced elastomeric bearings (SREBs), which are used and designed for non-seismic actions, have inherently acceptable characteristics in tolerating large shear deformations for a cost-effective isolation system and can provide improved structural performance. However, this capacity is neglected in the design process. The current article presents an experimental program to analyze the mechanical properties of steel-reinforced natural rubber bearings without the... 

Nanofluids - dispersions of nanometer-sized particles in a liquid medium - have been proposed for a wide variety of thermal management applications. It is known that a solid-like nanolayer of liquid of typical thicknesses of 0.5-1 nm surrounding the colloidal nanoparticles can act as a thermal bridge between the nanoparticle and the bulk liquid. Yet, its effect on the nanofluid viscosity has not been elucidated so far. In this article, we compute the local viscosity of the nanolayer using equilibrium molecular dynamics based on the Green-Kubo formula. We first assess the validity of the method to predict the viscosity locally. We apply this methodology to the calculation of the local... 

The method for modeling switching converters plays a key role in real-time simulators. Associate discrete circuit (ADC) modeling technique is a commonly used method for modeling the switching converter. However, the optimal selection of the ADC-based switch model parameters has great importance in the accuracy of the real-time simulator. In this paper, the design of a real-time simulator for a switching power converter has been done, in which a novel method for detecting optimum values of the switch model parameters has been expressed. Particle swarm optimization (PSO) algorithm is used to find these optimum values using state-space analysis of the modeled circuit in the z-domain. The... 

The CLLC resonant converter is a promising candidate for high efficient, bidirectional power transfer applications such as vehicle-to-grid (V2G) on-board charger and hybrid vehicle DC-DC converter. Nevertheless, the analysis of CLLC still remains challenging because of its complex multi-resonant nature and several storage elements. In this paper, a circuit analysis method based on change of variables is presented that maps the state space equations into two decoupled sets of equations. The analyses are carried out in two state-plane coordinate systems, then the results are mapped onto the original region. The proposed method is then used to thoroughly analyze the CLLC resonant converter... 

In this paper, the whale optimization algorithm is proposed for harmonics elimination in a cascaded multilevel inverter. In selective harmonic elimination pulse width modulation, the selected low-order harmonics are eliminated by solving nonlinear equations, while the fundamental of output waveform is adjusted to a desired value. In this paper, whale optimization algorithm is applied to a 7-level cascaded H-bridge inverter to solve the equations. Also, it was validated by experimental results, since this algorithm has an ability to search in entire solution space, the probability of catching a global best solution is very high. This method has higher accuracy and probability of convergence... 

In this paper, the whale optimization algorithm is proposed for harmonics elimination in a cascaded multilevel inverter. In selective harmonic elimination pulse width modulation, the selected low-order harmonics are eliminated by solving nonlinear equations, while the fundamental of output waveform is adjusted to a desired value. In this paper, whale optimization algorithm is applied to a 7-level cascaded H-bridge inverter to solve the equations. Also, it was validated by experimental results, since this algorithm has an ability to search in entire solution space, the probability of catching a global best solution is very high. This method has higher accuracy and probability of convergence... 

In this study, a new LLC resonant converter for high-voltage high-power applications is introduced. The introduced power converter is a two-phase interleaved full-bridge based that uses a transformer with secondary and tertiary windings to obtain higher output voltage. Zero voltage switching (ZVS) at MOSFETs turn on and zero current switching (ZCS) for all the output diodes at turn off are achieved for a wide range of input voltage (100 V-200 V) and output power (200 W-1500 W) variations. Simulation results show a 95% peak efficiency. © 2021 IEEE  

The wet granular material creates networks in which interstitial liquid provides capillary bridges needed to hold grains together. There is an optimal fraction of the interstitial liquid in which the bridges are formed and the friction coefficient is minimal. We found that the size of the grains affects the friction of wet granular media. Our observations demonstrates that the optimum volume fraction, in which the friction coefficient is minimized, increases with grain size, but for sand immersed in water, this minimum friction coefficient itself increases with size and for glass beads immersed in silicone oil it decreases, indicating that the shape of the grains also has an effect on this... 

Shear lag phenomenon has long been taken into consideration in various structural codes; however, the AISC provisions have not proposed any specific equation to calculate the shear lag ratio in some cases such as fillet-welded connections of front-to-front double channel sections. Moreover, those equations and formulas proposed by structural codes are based on the studies that were conducted on riveted and bolted connections, and can be applied to single channel sections whilst using them for fillet-welded double channels would be extremely conservative due to the symmetrical shape and the fact that bending moments will not develop in the gusset plate, resulting in less stress concentration.... 

Capillary continuity between adjacent matrix blocks through formation of liquid bridge controls the recovery factor of gravity drainage process in fractured reservoirs. However, stability of liquid bridges as well as related capillary pressure in horizontal rough fractures is not well discussed in the available literature. In this work, new models of rough-walled fracture are developed and the role of roughness size and frequency on formation of liquid bridge and fracture capillary pressure are investigated. The Young-Laplace equation is numerically solved to characterize the liquid bridge formed in the proposed models of rough fractures. Critical fracture aperture for a range of liquid... 

The vertical stiffness of elastomeric bearing is a dominant parameter of the base isolation design. Several empirical relations have been used to calculate the vertical stiffness of the elastomeric bearing systems, although, in certain conditions such as the presence of rotation, these relations are not accurate enough. In this paper, by using a nonlinear finite element program, the effect of rotation on the vertical stiffness investigated. It was observed that the vertical stiffness of the isolator could be increased or decreased depending on the amount of rotation and the value of lateral displacement limit. © 2019 Elsevier Ltd