• The Journal of Korean Institute of Communications and Information Sciences
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• v.23 no.6
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• pp.1419-1424
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• 1998

In phased array antennas that scan elevation, the number of radiator rows is one of the important factors to minimizing both cost and weitht. Therefore, the antenna tilt angles having relation with element spacing are among the improtant design parameters. The exact optimum tilt angles for several types of uniformly and nonuniformly excited arrays are obtained theoretically. Four types of uniform linear arrays and Chebyshev array factors as a nonuniformly excited arrav are considered.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.05a
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• pp.749-754
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• 2003

In this paper, we introduce the measurement of the underwater noise with 32channel hydrophone array of Samsung CAvitation Tunnel (SCAT) and the detection technique of noise sources by using the beam-forming method. Measurement and way signal Processing under fluid flow are essential works for the underwater acoustics, especially for the detection of noise sources. As the acoustic impedance of the water is relatively high and the tunnel is an enclosed system, we have to consider the interaction between tunnel and water together with the reflection of noise in the beam-forming technique. Also, for a hydrophone array system that is fixed on one side of tunnel wall as done in SCAT is liable to suffer from some limitations in the detection of the noise sources with the array, we discuss these limitations particularly on the frequency range and spacing of noise sources.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2408-2413
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• 2007

Since the heat generation in a chip increases as the components are miniaturized and the computing speed becomes faster, suitable heat dissipation has become one of the primary limiting factors to ensure the reliable operation of the electronic devices. A pin-fin array could be used as an alterative cooling system of the electronic equipment. In this study, convective heat transfer through the pin-fin array is analyzed experimentally based on porous medium approach. The influence of the structure of the pin-fin array including the pin-fin spacing, the pin diameter and plate length on heat transfer characteristics is investigated and compared with the previous analytical results and existing correlation equations. Nowadays, electronic and mechanical devices become smaller and smaller. In this sense, the main purpose of this study is to decide the optimum pin-fin arrangement to get similar heat transfer performance when the length of the existing cooling system is reduced as a half.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.18 no.7
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• pp.701-710
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• 2008

Including a TV set and a monitor, personal audio system is raising a great interest. In this study, we applied a method to make a good bright zone around the user and dark zone to other region by maximizing the ratio of sound energy between the bright and dark zone. It has been well known as acoustic contrast control. We have attempted to use a line loudspeaker array system to localize the sound in our listening zone. It depends on the size of the zone and array parameters, for example, array size, loudspeaker unit spacing, wave length of sound. We have considered these parameters as spatial variables and studied the effects. And we have found that each spatial variable has its own characteristic and shows very different effect. Genetic algorithms are introduced to find out the optimum value of spatial variables. As a result, we can improve the result of the acoustic contrast control by optimum value of spatial variables.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.19 no.5
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• pp.418-425
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• 2007

The power consumption and heat generation in a chip increase as the components are miniaturized and the computing speed becomes faster. Therefore, suitable heat dissipation has become one of the primary limiting factors to ensure the guaranteed performance and reliable operation of the electronic devices. A pin-fin array which may be considered as a porous medium could be used as an alterative cooling system of the electronic equipment. The aim of the present study is to investigate the forced-convective heat transfer characteristics of pin-fin heat exchangers. Convective heat transfer through the pin~fin array is analyzed experimentally based on porous medium approach. The influence of the structure of the pin-fin array including the pin-fin spacing, the pin diameter and plate length on heat transfer characteristic is investigated and compared with the Previous analytical results and existing correlation equations. Nowadays, electronic and mechanical devices become smaller and smaller. In this sense, the main purpose of this study is to decide the optimum pin-fin arrangement to get similar heat transfer performance when the length of the existing cooling system is reduced as a half.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.1
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• pp.140-148
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• 1999

Experiments on flame spread in an one-dimensional droplet array up to supercritical pressures of fuel droplet have been conducted In normal gravity and microgravity. Evaporating process around unburnt droplet is observed through high-speed Schlieren and direct visualizations in detail, and flame spread rate is measured using high speed chemiluminescence images of OH radical. Flame spread behaviors are categorized into three: flame spread is continuous at low pressures and is regularly intermittent up to the critical pressure of fuel. flame spread is irregularly intermittent and zig-zag at supercritical pressures of fuel. At atmospheric pressure, the limit droplet spacing and the droplet spacing of maximum flame spread rate in microgravity are larger than those in normal gravity. In microgravity, the flame spread rate with the increase of ambient pressure decreases initially, takes a minimum, and then decreases after taking maximum. This is so because the flame spread time is determined by competing effects between the increased transfer time of thermal boundary layer due to reduced flame diameter and the reduced ignition delay time in terms of the increase of ambient pressure. Consequently, it is found that flame spread behaviors in microgravity are considerably different from those in normal gravity due to the absence of natural convection.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.1
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• pp.131-139
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• 1999

Experimental investigations on flame spread in droplet arrays have been conducted under supercritical ambient pressures of fuel droplet. Flame spread rates are measured for n-Decane droplet of diameters of 0.75 and 1.0mm, using high speed images of OH chemiluminescence up to 3.0MPa. The pattern of flame spread is categorized into two: a continuous mode and an intermittent one. There exists a limit droplet spacing, above which flame spread does not occur. Flame spread rate with the decrease of droplet spacing increases and then decreases after takin& a maximum. It is also seen that there exists a limit ambient pressure, above which flame spread does not occur. Flame spread rate decreases monotonically with the increase of ambient pressure. Exceptionally, In the case of a small droplet spacing, flame spread with the increase of ambient pressure is extended to supercritical pressures of fuel droplet. This is caused by enhanced vaporization with the increase of ambient pressure. Consequently, in flame spread with droplet droplet spacing, the relative position of flame to droplet spacing plays an important role. The monotonic decrease with ambient pressure is mainly related to the reduction of flame radius in subcritical pressures and the extension to supercritical pressures of flame spread is caused by the reduction of ignition time of unburnt droplet due to the enhanced vaporization at supercritical pressures.

• Journal of the Institute of Electronics and Information Engineers
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• v.50 no.6
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• pp.43-55
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• 2013

In this paper, we propose a cylindrical active phased array antenna of Beam Steering Characteristics in the horizontal plane(H-plane) and vertical plane(E-Plane) on the cylinder form array structure. We design the bent dipole antenna of the cylindrical array structure adapted excellent mutual-coupling characteristics, designed and manufactured the cylindrical array antennas and power combiner/divider unit for power dividing and combining on the antenna. The radiating elements array spacing of Cylindrical array antenna were determined to avoid grating lobes at half power beam steering. Beam steering of the antenna was implemented with 6-bit phase shifter in the transceiver and have been designed based on the characteristics the antenna beam steering at -24 degrees to 24 degrees horizontal, vertical 0 degrees to 36 degrees beam steering. A cylindrical active phased array antenna that produced for verification the performance of the antenna are measured radiation characteristics in accordance with beam steering at L-Band.

• The Journal of the Acoustical Society of Korea
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• v.35 no.6
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• pp.427-435
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• 2016

Array Gain (AG) is a metric to measure the performance of an array of acoustic sensors. AG is affected by the configuration of array, frequency and array element spacing, and the directivity of the ambient noise. In this paper, an algorithm to calculate AG based on the spatial coherence is used, and the results are verified through sea-going experiment. The method using the spatial coherence can be used to consider the arbitrary shape of an array and directionality of ambient noise. In the sea-going experiment, the towed source was used to transmit the Continuous Wave (CW), and was received at the horizontal line array on the seabed. The ambient noise was measured between the source transmission. The experimental AG was calculated from the SNR (Signal to Noise Ratio) of single sensor and an array of sensors. Finally, the predicted AG is shown to agree with the experimental value of AG.

• The Journal of the Acoustical Society of Korea
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• v.39 no.3
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• pp.170-178
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• 2020

Cymbal transducers are often used as an array rather than single because they have a high quality factor and low energy conversion efficiency. When used as an array, there occurs a big change in the frequency characteristics of the array due to the interaction between constituent transducers. In this study, we designed the structure of a cymbal transducer array to have ultra-wideband characteristics using this property. First, cymbal transducers with specific center frequencies were designed. Then, a 2×2 planar array was constructed with the designed transducers, where the cymbal transducers were arranged to have same or opposite polarization directions. For this structure, we analyzed the effect of the difference in the center frequency of and the spacing between the constituent transducers on the acoustical characteristics of the array. Based on the analysis, we designed the structure of the cymbal transducer array to have the widest possible bandwidth.