• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.1
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• pp.95-102
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• 2006

The PPT being currently developed for the flight model represents a significant leap in techniques and technology compared to the previous flight ones. The electrical energy to be charged in the pulsed plasma thruster (PPT) is a very important aspect to provide an uniform impulse bit ,, and a specific impulse ,, for satellite attitude control. In this paper, we propose a nonlinear control technique and a stability analysis based on the Lyapunov function for the pulsed plasma thruster. Specifically, the proposed control law guarantees to charge and discharge the electrical energy generated from the power processing unit (PPU) within the specified time.

• Structural Engineering and Mechanics
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• v.66 no.2
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• pp.249-262
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• 2018

In this paper, reaction of functionally graded (FG) thick nanoplates resting on a viscoelastic foundation to a moving nanoparticle/load is investigated. Nanoplate is assumed to be thick by using second order shear deformation theory and small-scale effects are taken into account in the framework of Eringen's nonlocal theory. Material properties are varied through the thickness using FG models by having power-law, sigmoid and exponential functions for material changes. FG nanoplate is assumed to be on a viscoelastic medium which is modeled using Kelvin-Voight viscoelastic model. Galerkin, state space and fourth-order Runge-Kutta methods are employed to solve the governing equations. A comprehensive parametric study is presetned to show the influence of different parameters on mechanical behavior of the system. It is shown that material variation in conjunction with nonlocal term have a significant effect on the dynamic deformation of nanoplate which could be used in comprehending and designing more efficient nanostructures. Moreover, it is shown that having a viscoelastic medium could play an important role in decreasing these dynamic deformations. With respect to the fresh studies on moving atoms, molecules, cells, nanocars, nanotrims and point loads on different nanosctructures using scanning tunneling microscopes (STM) and atomic force microscopes (AFM), this study could be a step forward in understanding, predicting and controlling such kind of behaviors by showing the influence of the moving path, velocity etc. on dynamic reaction of the plate.

• Structural Engineering and Mechanics
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• v.66 no.2
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• pp.237-248
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• 2018

This study presents the investigation of wave dispersion characteristics of a magneto-electro-elastic functionally graded (MEE-FG) nanosize beam utilizing nonlocal strain gradient theory (NSGT). In this theory, a material length scale parameter is propounded to show the influence of strain gradient stress field, and likewise, a nonlocal parameter is nominated to emphasize on the importance of elastic stress field effects. The material properties of heterogeneous nanobeam are supposed to vary smoothly through the thickness direction based on power-law form. Applying Hamilton's principle, the nonlocal governing equations of MEE-FG nanobeam are derived. Furthermore, to derive the wave frequency, phase velocity and escape frequency of MEE-FG nanobeam, an analytical solution is employed. The validation procedure is performed by comparing the results of present model with results exhibited by previous papers. Results are rendered in the framework of an exact parametric study by changing various parameters such as wave number, nonlocal parameter, length scale parameter, gradient index, magnetic potential and electric voltage to show their influence on the wave frequency, phase velocity and escape frequency of MEE-FG nanobeams.

• Transactions of Materials Processing
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• v.18 no.1
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• pp.26-38
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• 2009

It was reported that the semi-solid forming process has many advantages over the conventional forming process, such as a long die life, good mechanical properties and energy savings. It is very important, however, to control liquid segregation to gain mechanical property improvement of materials. During forming process, rheology material has complex characteristics, thixotropic behavior. Also, difference of velocity between solid and liquid in the semi-solid state material makes a liquid segregation and specific stress variation. Therefore, it is difficult for a numerical simulation of the rheology process to be performed. General plastic or fluid dynamic analysis is not suitable for the behavior of rheology material. The behavior and stress of solid particle in the rheology material during forging process is affected by viscosity, temperature and solid fraction. In this study, compression experiments of aluminum alloy were performed under each other tool shape which is rectangle shape(square array), rectangle shape(hexagonal array), and free shape tool. In addition, the dynamics behavior compare with Okano equation to power law model which is viscosity equation.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.55.1-55.1
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• 2013

We analyze the behaviors of reddening vectors in the Spitzer/IRAC photometric system for young stellar objects (YSOs) of different evolutionary stages, masses, and inclinations using the model spectral energy distributions (SED) by Robitaille et al. As reported in visible and near-infrared photometric systems, the magnitudes and colors of YSOs show strong SED dependence and non-linearity. In the [8.0] band where the 9.7 ${\mu}m$ interstellar silicate feature plays a significant role in extinction, the effective wavelength shifts "bluewards", not "redwards" as in most, if not all, optical and infrared bands including the other three IRAC bands, as the extinction in Ks increases up to ~2 mag, and then asymptotically reaches a constant value as the extinction further increases. This "bluening" is seen when the YSO is in later evolutionary stage and/or has a stellar mass of ~2 $M_{\odot}$ or greater. In many cases, the reddening vectors in the IRAC color-color diagrams are prominently curved, and in some extreme cases, the colors involving the [8.0] band becomes bluer in the beginning and then becomes redder later as the amount of extinction increases. We also present our "suggested" extinction laws employing the combination of a broken-power law and the 9.7 ${\mu}m$ silicate feature, which well reproduce the extinction behaviors observed in the IRAC bands.

• Journal of the Korean Society of Propulsion Engineers
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• v.13 no.4
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• pp.45-54
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• 2009

An exergy analysis is carried out for the regenerative steam-injection gas turbine systems which has a potential of enhanced thermal efficiency and specific power. Using the analysis model in the view of the second law of thermodynamics, the effects of pressure ratio, steam injection ratio, ambient temperature and turbine inlet temperature are investigated on the performance of the system such as exergetic efficiency, heat recovery ratio of heat exchangers, exergy destruction, loss ratios, and on the optimal conditions for maximum exergy efficiency. The results of computation show that the regenerative steam-injection gas turbine system can make a notable enhancement of exergy efficiency and reduce irreversibilities of the system.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.11a
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• pp.300-303
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• 2010

The present study investigates the flow characteristics of simulant gel propellant(carbopol 0.5%wt) in a variety of injectors. Rheological data for gel propellant has been measured and injector flow characteristics for plain-orifice, chamfered-orifice and venturi type injector have been numerically analyzed. The apparent viscosity of plain-orifice and chamfered-orifice have tendency to increase along axial direction, whereas for venturi type injector, low viscosity has been achieved in the injector flow. This phenomenon was clearly pronounced as Reynolds number is increased.

• Steel and Composite Structures
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• v.20 no.1
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• pp.205-225
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• 2016

In this study the finite element method is utilized to predict the deflection and vibration characteristics of rectangular plates made of saturated porous functionally graded materials (PFGM) within the framework of the third order shear deformation plate theory. Material properties of PFGM plate are supposed to vary continuously along the thickness direction according to the power-law form and the porous plate is assumed of the form where pores are saturated with fluid. Various edge conditions of the plate are analyzed. The governing equations of motion are derived through energy method, using calculus of variations while the finite element model is derived based on the constitutive equation of the porous material. According to the numerical results, it is revealed that the proposed modeling and finite element approach can provide accurate deflection and frequency results of the PFGM plates as compared to the previously published results in literature. The detailed mathematical derivations are presented and numerical investigations are performed while the emphasis is placed on investigating the effect of the several parameters such as porosity volume fraction, material distribution profile, mode number and boundary conditions on the natural frequencies and deflection of the PFGM plates in detail. It is explicitly shown that the deflection and vibration behaviour of porous FGM plates are significantly influenced by these effects. Numerical results are presented to serve as benchmarks for future analyses of FGM plates with porosity phases.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• v.2
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• pp.325-328
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• 2006

This study analyzed the fluid state around a container crane according to a wind direction when a wind load was applied to a container crane. The container crane for this research is a model of a 50-ton class used broadly in the current ports. The dimension of an external fluid field is $500m{\times}200m$. This study considered the change of a wind velocity according to an altitude in a criterion of a wind velocity, 50m/s, applying a power series law. An incident angle applied to an interval of 30 degrees in $0^{\circ}C$ ${\sim}$ $180^{\circ}C$ and this study carried out a computation fluid dynamics using a CFX 10. In this study, we indicate the wind pressure and coefficient according to the height and section figure of each member. In addition, we suggest the wind load according to a wind direction.

• Proceedings of the KIEE Conference
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• 2005.10c
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• pp.308-310
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• 2005

In recent years, the research and development for the photovoltaic(PV) energy system are making rapidly progress around the world and specially this country, too due the deregulation law for the renewable energy system seems to be born sooner or later. In PV generation system, the partially shaded PV array is the one of the worst case which reduces the efficiency of the total PV generation system. The partial shaded condition is the result of shadowing by cloud and dust building of on the surface of the panel. Some structural elements, such as antennas, booms etc. is also the reason of the shadowing. Even if only a small part of PV is shaded, the overall generation power of PV is significantly decreased. Therefore, several researchers who are focusing on the PV generation system take a time for the research related with the shadowing problem of PV array. In this paper, authors have developed the method which users can achieved the modeling of partially shaded PV array with. With several papers authors have already announced the availability of the EMTDC/PSCAD PV panel model component. This research result is the developed version of the previous papers.