• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.2
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• pp.1500-1506
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• 2015

The headcut erosion at the confluence section of a mainstream and tributary can migrate up the tributary streams, and rapid degradation can threaten the stability of hydraulic structures installed in the channel. Therefore, quantitative analysis for the development and mechanism of headcut erosion is needed to prevent damage due to the headcut. In this study, hydraulic experiments for headcut erosion in the channel with noncohesive materials were performed and the knickpoint movement and final bed slope change were analyzed based on the different hydraulic conditions. As a result, the knickpoint movement was 1.5 times faster when the difference in velocity between the upstream and downstream sections was 2.5 times greater and the central part of the cross-section was eroded and collapsed earlier than the left and right sides. The movement length of headcut erosion was longer and the final bed slope was milder as the velocity difference between the upstream and downstream sections was increased. This study showed that a correlation between the knickpoint movement and bed slope change by headcut erosion and the water level difference of upstream and downstream sections was not constant compared to the velocity difference.

• Ecology and Resilient Infrastructure
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• v.11 no.2
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• pp.43-54
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• 2024

Recently, most of the rivers in Korea are experiencing various problems in dimension and river environment, such as expansion of stable area where disturbance does not occur during flood, increase of excessive trees in river channel, fixation of river channel, reduction of sand bar. When the soil supplied by the flood is deposited in the river, the plant is settled in the formed terrain, and when another disturbance of the scale that does not erode there occurs after the plant is settled, the river gradually grows and the vegetation zone is formed there. In particular, in terms of river management, river forestation and river aggradation are objects that must be managed because they are disadvantageous in terms of flood control by lowering the flow rate and raising the water level. Therefore, in this study, the area of vegetation occupied by the year of sandbar was analyzed in the process of river aggradation in Jang-Hang wetland. In addition, the correlation between the growth of Jang-Hang wetland was analyzed through the analysis of the flow rate and the flooding frequency that directly affect the growth of Jang-Hang wetland. Through this, the management plan of Jang-Hang wetland, which is registered in Ramsar Wetland but has been river forestation and is undergoing river aggradation, was proposed.

• 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.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.1882-1887
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• 2007

In the present experimental study, the effect of interfacial tensions on pressure drop of air-water two-phase flow in round mini-channels was investigated. A glass (highly wettable) tube and a Teflon (poorly wettable) tube, both in 350 mm length but 1.8 mm and 1.59 mm in inner diameters each, were used for the tests. All the experiments were performed only in the plug flow regime, confirmed by visualization. In the glass tube, the gas plugs were surrounded by the liquid film along the inner periphery. On the other hand, the inner wall remained dry at the gas portion in the Teflon tube. The pressure drop of the plug flow in the Teflon tube without the liquid film) appeared much larger than in the glass tube (with the liquid film) due to dissipation of energy by movement of the wetting lines. In this paper, various correlations on the two-phase pressure drop of plug flows were compared and a modified correlation was proposed, taking account of the surface wettability.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.49 no.5
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• pp.91-98
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• 2012

Since the major cellular data service providers in U.S, Japan as well as in Korea started the LTE (Long Term Evolution) service, there has been more strong need for the methods that can accurately measure the MIMO (Multi Input-Multi Output) OTA (Over The Air) performance of LTE handsets because the performance of the MIMO antenna determines the data throughput in the downlink. In this paper, the hand effect on the MIMO antenna performance is analyzed by numerically and experimentally. The hand effect on the LTE mobile handset is analyzed by measuring the link level performance in the MIMO OTA system.

• Journal of Positioning, Navigation, and Timing
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• v.4 no.3
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• pp.115-122
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• 2015

Global Positioning System (GPS) is used in various fields such as communications systems, transportation systems, e-commerce, power plant systems, and up to various military weapons systems recently. However, GPS receiver is vulnerable to jamming signals as the GPS signals come from the satellites located at approximately 20,000 km above the earth. For this reason, various anti-jamming techniques have been developed for military application systems especially and it is also required for commercial application systems nowadays. In this paper, we proposed a dual-channel Global Navigation Satellite System (GNSS) RF ASIC for digital pre-correlation anti-jam technique. It not only covers all GNSS frequency bands, but is integrated low-gain/attenuation mode in low-noise amplifier (LNA) without influencing in/out matching and 14-bit analogdigital converter (ADC) to have a high dynamic range. With the aid of digital processing, jamming to signal ratio is improved to 77 dB from 42 dB with proposed receiver. RF ASIC for anti-jam is fabricated on a 0.18-μm complementary metal-oxide semiconductor (CMOS) technology and consumes 1.16 W with 2.1 V (low-dropout; LDO) power supply. And the performance is evaluated by a kind of test hardware using the designed RF ASIC.

• Journal of Astronomy and Space Sciences
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• v.37 no.2
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• pp.117-129
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• 2020

In a Satellite communication system, a link budget analysis is the detailed investigation of signal gains and losses moving through a channel from a sender to receiver. It inspects the fading of passed on data signal waves due to the process of spreading or propagation, including transmitter and receiver antenna gains, feeder cables, and related losses. The extent of the proposed tool is to make an effective, efficient, and user-friendly approach to calculate link budget analysis. It is also related to the satellite communication correlation framework by building up a graphical interface link analysis tool utilizing STK^® software with the interface of C# programming. It provides better kinds of graphical display techniques, exporting and importing data files, printing link information, access data, azimuth-elevation-range (AER), and simulation is also possible at once. The components of the link budget analysis tool include transmitter gain, effective isotropic radiated power (EIRP), free space loss, propagation loss, frequency Doppler shift, flux density, link margin, elevation plot, etc. This tool can be useful for amateur users (e.g., CubeSat developers in the universities) or nanosat developers who may not know about the RF communication system of the satellite and the orbital mechanics (e.g., orbit propagators) principle used in the satellite link analysis.

• Journal of Fisheries and Marine Sciences Education
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• v.25 no.3
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• pp.625-631
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• 2013

Two-phase pressure drop experiments were performed during flow boiling to deionized water in a microchannel having a hydraulic diameter of $500{\mu}m$. Tests were made in the ranges of heat fluxes from 100 to $400kW/m^2$, vapor qualities from 0 to 0.2 and mass fluxes of 200, 400 and $600kg/m^2s$. The frictional pressure drop during flow boiling is predicted by using two models; the homogeneous model that assumes equal phase velocity and the separate flow model that allows a slip velocity between two phases. From the experimental results, it is found that the two phase multiplier decreases with an increase in mass flux. Measured data of pressure drop are compared to a few available correlations proposed for macroscale and mini/microscale. Among the separated flow models, the correlation model suggested by Lee and Garimella predicted the frictional pressure drop within MAE of 47.2%, which is better than other correlations.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.19 no.4
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• pp.461-469
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• 2008

This paper proposes a hybrid symbol offset estimation algorithm for MIMO(Multiple Input Multiple Output) OFDM system. As MIMO OFDM systems are multiple transmitter and receiver antenna systems, apart from SISO(Single Input Single Output) system, it is possible to use several combining techniques which are used in multiple receive antenna system. In this paper, we propose hybrid symbol offset estimation algorithms using combining techniques in multiple receive antenna systems, simulate and show the performances in MIMO system environments. The proposed equal gain combining correlation algorithm has better performance 1.8 times in searching the ideal symbol offset rather than the conventional early symbol offset algorithm in severe ISI channel.

• ETRI Journal
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• v.36 no.1
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• pp.62-68
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• 2014

This paper presents a novel user selection method based on the signal-to-interference-plus-noise ratio (SINR), which is approximated using limited feedback data at the base stations (BSs) of multiple user multiple-input multiple-output (MU-MIMO) systems. In the proposed system, the codebook vector index, the quantization error obtained from the correlation between the measured channel and the codebook vector, and the measured value of the largest singular value are fed back from each user to the BS. The proposed method not only generates precoding vectors that are orthogonal to the precoding vectors of the previously selected users and are highly correlated with the codebook vector of each user but also adopts the quantization error in approximating the SINR, which eventually provides a significantly more accurate SINR than the conventional SINR-based user selection techniques. Computer simulations show that the proposed method enhances the sum rate of the conventional SINR-based methods by at least 2.4 (2.62) bps/Hz when the number of transmit antennas and number of receive antennas per user terminal is 4 and 1(2), respectively, with 100 candidate users and an SNR of 30 dB.