• The Transactions of The Korean Institute of Electrical Engineers
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• v.61 no.12
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• pp.1820-1827
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• 2012

This paper is to present a new method of cogging torque reduction for axial flux PM machines of multiple rotor surface mounted magnets. In order to start softly and to run a power generator even the case of weak wind power, reduction of cogging torque is one of the most important issues for a small wind turbine, Cogging torque is an inherent characteristic of PM machines and is caused by the geometry shape of the machine. Several methods have been already applied for reducing the cogging torque of conventional radial flux PM machines. Even though some of these techniques can be also applied to axial flux machines, manufacturing cost is especially higher due to the unique construction of the axial flux machine stator. Consequently, a simpler and low cost method is proposed to apply on axial flux PM machines. This new method is actually applied to a generator of 1.0kW, 16-poles axial flux surface magnet disc type machine with double-rotor-single-stator for small wind turbine. Design optimization of the adjacent magnet pole-arc which results in minimum cogging torque as well as assessment of the effect on the maximum available torque using 3D Finite Element Analysis (FEA) is investigated in this design. Although the design improvement is intended for small wind turbines, it is also applicable to larger wind turbines.

• Electrical & Electronic Materials
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• v.9 no.1
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• pp.67-75
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• 1996

This paper has presented the characteristics of thermoelectric devices and the plots of thermoelectric cooling and heating as a function of currents for different temperatures. The maximum cooling and heating(.DELTA.T) for (BiSb)$\_$2/Te$\_$3/ and Bi$\_$2/(TeSe)$\_$3/ as a function of currents is about 75.deg. C, A solderable ceramic insulated thermoelectric module. Each module contains 31 thermoelectric devices. Thermoelectric material is a quaternary alloy of bismuth, tellurium, selenium, and antimony with small amounts of suitable dopants, carefully processed to produce an oriented polycrystalline ingot with superior anisotropic thermoelectric properties. Metallized ceramic plates afford maximum electrical insulation and thermal conduction. Operating temperature range is from -156.deg. C to +104.deg. C. The amount of Peltier cooling is directly proportional to the current through the sample, and the temperature gradient at the thermoelectric materials junctions will depend on the system geometry.

• Structural Engineering and Mechanics
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• v.54 no.5
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• pp.829-853
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• 2015

This is an experimental study to investigate the behaviour of piled raft system in different types of sandy soil. A small scale "prototype" model was tested in a sand box with load applied to the foundation through a compression jack and measured by means of load cell. The settlement was measured at the raft by means of dial gauges, three strain gauges were attached on piles to measure the strains and calculate the load carried by each pile in the group. Nine configurations of group ($1{\times}2$, $1{\times}3$, $1{\times}4$, $2{\times}2$, $2{\times}3$, $2{\times}4$, $3{\times}3$, $3{\times}4$ and $4{\times}4$) were tested in the laboratory as a free standing pile group (the raft not in contact with the soil) and as a piled raft (the raft in contact with the soil), in addition to tests for raft (unpiled) with different sizes. It is found that when the number of piles within the group is small (less than 4), there is no evident contribution of the raft to the load carrying capacity. The failure load for a piled raft consisting of 9 piles is approximately 100% greater than free standing pile group containing the same number of piles. This difference increases to about 4 times for 16 pile group. The piles work as settlement reducers effectively when the number of piles is greater than 6 than when the number of piles is less than 6. The settlement can be increased by about 8 times in ($1{\times}2$) free standing pile group compared to the piled raft of the same size. The effect of piled raft in reducing the settlement vanishes when the number of piles exceeds 6.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.05a
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• pp.147-150
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• 2003

The purpose of this research was to develope components of micro solid propellant thruster. Micro solid propellant thruster had four basic components: combustion chamber, nozzle, solid propellant and micro heater for ignition. A performance of micro heater and characteristic of solid propellant was investigated. Micro heater was fabricated by conventional MEMS process and Platinum layer was used for heating element. Effect of geometry parameters on micro heater was tested. The temperature responses of heater with respect to each parameters was compared for a given electrical power. The characteristic of solid propellant(HTPB/AP) was investigated to obtain burning velocity in small chamber. Additionally, a capacity of filling propellant with high viscosity in small chamber was checked to guarantee for the micro fabrication.

• Wind and Structures
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• v.26 no.6
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• pp.397-413
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• 2018

Excitation mechanism of interference effect between two tall buildings is investigated with wind tunnel experiments. Synchronized building surface pressure and flow field measurements by particle image velocimetry (PIV) are conducted to explore the relationship between the disturbed wind flow field and the consequent wind load modification for twin buildings in tandem. This reveals evident excitation mechanisms for the fluctuating across-wind loads on the buildings. For small distance (X/D < 3) between two buildings, the disturbed flow pattern of impaired vortex shedding is observed and the fluctuating across-wind load on the downstream building decreases. For larger distance ($X/D{\geq}3$), strong correlation between the across-wind load of the downstream building and the oscillation of the wake of the upstream building is found. By further analysis with conditional sampling and phase-averaged techniques, the coherent flow structures in the building gap are clearly observed and the wake oscillation of the upstream building is confirmed to be the reason of the magnified across-wind force on the downstream building. For efficient PIV measurement, the experiments use a square-section high-rise building model with geometry scale smaller than the usual value. Interference factors for all three components of wind loads on the building models being surrounded by another identical building with various configurations are measured and compared with those from previous studies made at large geometry scale. The results support that for interference effect between buildings with sharp corners, the length scale effect plays a minor role provided that the minimum Reynolds number requirement is met.

• Journal of the Society of Naval Architects of Korea
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• v.61 no.2
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• pp.68-76
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• 2024

The rotor sail is one of the representative devices in eco-friendly wind-assisted propulsion systems that have been practically applied to commercial ships. The present study proposes an asymmetric vertical folding rotor sail (AFRS) designed for small ships, featuring asymmetric geometry along the vertical direction and the function of vertical folding. To evaluate the aerodynamic performance of rotor sail, the drag, lift and lift-to-drag ratio were derived using computational fluid dynamics. The aerodynamic performance of AFRS was compared with that of normal rotor sail with different aspect ratios and spin ratios. The effect of geometric parameters on the aerodynamic performance of AFRS was assessed by varying the asymmetric diameter ratio. The maximum improvement in lift-to-drag ratio for AFRS was approximately 12% in the considered case. Additionally, the resistance is decreased when AFRS is vertically folded without rotating. Throughout the present study, improved aerodynamic and resistance performances for AFRS were confirmed, which will successfully provide additional propulsion to small ships.

• Journal of Electrical Engineering and Technology
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• v.13 no.1
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• pp.400-405
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• 2018

This paper presents a compact ultra-wideband (UWB) flexible monopole antenna design on a paper substrate. The proposed antenna is made of iterations of a circular slot inside an octagonal metallic patch. This fractal-based geometry has been deployed to achieve compactness along with improved bandwidth, measured reflection coefficient -10 dB bandwidth ranging from 2.7 to 15.8 GHz. The overall size of the antenna is $26mm{\times}19mm{\times}0.5mm$, which makes it a compact one. The substrate used is paper and the main features like environment friendly, flexibility, green electronics applications and low cost of fabrication are the key factors for the proposed antenna. The aforementioned UWB prototype is suitable for many wireless communication systems such as WiMAX, WiFi, RFID and WSN applications. Antenna has been tested for the effect of bending by placing it over a curved surface of a very small radius of 10 mm.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2004.11a
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• pp.916-919
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• 2004

The monopole theory has long been used to model air-pumped effect from the elastic cavities in car tire. This approach models the change of an air as a piston moving backward and forward on a spring and equates local air movements exactly with the volume changes of the system. Thus, the monopole theory has a restricted domain of applicability due to the usual assumption of a small amplitude acoustic wave equation and acoustic monopole theory. This paper describes an approach to predict the air-pumping noise of a car ave with CFD/Kirchhoff integral method. The type groove is simply modeled as piston-cavity-sliding door geometry and with the aid of CFD technique flow properties in the groove of rolling car tyre are acquired. And these unsteady flow data are used as a air-pumping source in the next Cm calculation of full tyre-road geometry. Acoustic far field is predicted from Kirchhoff integral method by using unsteady flow data in space and time, which is provided by the CFD calculation of full tyre-road domain. This approach can cover the non-linearity of acoustic monopole theory with the aid of using Non-linear governing equation in CFD calculation. The method proposed in this paper is applied to the prediction of air-pumping noise of modeled car tyre and the predicted results are qualitatively compared with the experimental data.

• Steel and Composite Structures
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• v.48 no.4
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• pp.443-459
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• 2023

This paper investigates the stability of a bi-directional functionally graded (BD-FG) cylindrical beam made of imperfect concrete, taking into account size-dependency and the effect of geometry on its stability behavior. Both buckling and dynamic behavior are analyzed using the modified coupled stress theory and the classical beam theory. The BD-FG structure is created by using porosity-dependent FG concrete, with changing porosity voids and material distributions along the pipe radius, as well as uniform and nonuniform radius functions that vary along the beam length. Energy principles are used to generate partial differential equations (PDE) for stability analysis, which are then solved numerically. This study sheds light on the complex behavior of BD-FG structures, and the results can be useful for the design of stable cylindrical microstructures.

• Journal of Korean Society for Atmospheric Environment
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• v.34 no.3
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• pp.486-492
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• 2018

Black carbon concentrations were measured along the altitude at various locations using a drone coupled with a small black carbon detector. The measurement locations are Eunseok Mountain, downtown, four places in KOREATECH campus, Byeongcheon, Cheonan, Chungcheongnam-do, and Chungbu Expressway in Ochang-eup, Cheongju, Chungcheongbuk-do. The average concentration of black carbon measured in Eunseok Mountain was $1.64{\mu}g/m^3$ and the average concentration near the Chungbu Expressway was measured to be $3.86{\mu}g/m^3$. The average concentrations of four places inside campus ranged from 1.37 to $2.67{\mu}g/m^3$. The concentration of black carbon at all places tended to be slightly decreased according to the altitude, but the influence of pollution source, geometry, wind speed, and wind direction are thought to be larger than the effect of altitude. Effect of air flow caused by drone flight on the measurement of black carbon were investigated and it resulted in that the measurement of BC concentration was affected by less than 5%.