• Journal of Ocean Engineering and Technology
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• v.28 no.4
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• pp.261-267
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• 2014

Because a change in the natural frequencies of a structure indicates structural health problems, monitoring the natural frequencies crucial. Long-term measurement for the Uldolmok tidal current power plant structure has shown that its natural frequencies fluctuate with a constant cycle twice a day. In this study, lab-scale tests to investigate the causes of these natural frequency fluctuations were carried out in a circulating water channel. Three independent variables in the tests that could affect the fluctuation of the natural frequencies were the water level, current velocity, and boundary condition between the specimen and the bottom of the circulating water channel. The experimental results were verified with numerical ones using ABAQUS. It was found that the fluctuation of the natural frequencies was governed by a decrease in stiffness due to the boundary condition much more than the effect of added mass. In addition, it was found that the natural frequency would decrease with an increase in the tidal current velocity because of its nonlinearity when the boundary condition was severely deteriorated due to damage.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.45 no.9
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• pp.8-13
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• 2008

Noise in homogeneous extrinsic semiconductor samples is calculated due to distributed diffusion noise sources. As the length of the device shrinks at a fixed bias voltage, the ac-wise short-circuit noise current shows excess noise as well as thermal noise spectra. This excess noise behaves like a full shot noise when the channel length becomes very small compared with the extrinsic Debye length. For the first time, the analytic formula of the excess noise in extrinsic semiconductors from velocity-fluctuation noise sources is given for finite frequencies. This formula shows the interplay between transit time, dielectric relaxation time, and velocity relaxation time in determining the terminal noise current as well as the carrier density fluctuation. As frequency increases, the power spectral density of the excess noise rolls off. This formula sheds light on noise in nanoscale MOSFETs where quasi-ballistic transport plays an important role in carrier transport and noise.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.6
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• pp.823-829
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• 1986

The Large-scale structure of the circular jet in the transition region, which influences the subsequent flow in the turbulent region, was studied experimentally. Measuring equipments are composed of the two channel hot-wire anemometer, the computer controlled two-directional traverse mechanism, the data acquisition system, and FFT-analyzer. The circular jet has 50mm diameter. The mean velocity distribution, the velocity fluctuation, the auto 'cross correlations and the power spectra were acquired at moderate Reynolds number of 10$^{4}$. And the VITA method was used to measure the convection velocity of Large-scale eddy. The phase of u'is in advance of that of v'in all regions. .over bar. $R_{u}$(.tau.=0) is approximately zero in the potential core region, but a small regular deviation is observed. At a position in the mixing layer region the convection velocity is different along the part of the eddy, and in this experiment the convection velocity of the inner region is larger than the outer region. The averge convection velocity of the eddy along y/D=0 was approximately constant in the transition region.D=0 was approximately constant in the transition region.

• Journal of the Society of Naval Architects of Korea
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• v.53 no.5
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• pp.371-379
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• 2016

In the ship design process, ship motion and propulsion performance in sea waves became very important issues. Especially, prediction of ship propulsion performance during real operation is an important challenge to ship owners for economic operation in terms of fuel consumption and route-time evaluation. Therefore, it should be considered in the early design stages of the ship. It is thought that the averaged value and fluctuation of effective inflow velocity to the propeller have a great effect on the propulsion performance in waves. However, even for the nominal velocity distribution, very few results have been presented due to some technical difficulties in experiments. In this study, flow measurements near the propeller plane using a stereo PIV system were performed. Phase-averaged flow fields on the propeller plane of a KVLCC2 model ship in waves were measured in the towing tank by using the stereo PIV system and a phase synchronizer with heave motion. The experiment was carried out at fully loaded condition with making surge, heave and pitch motions free at a forward speed corresponding to Fr=0.142 (Re=2.55×106) in various head waves and calm water condition. The phase averaged nominal velocity fields obtained from the measurements are discussed with respect to effects of wave orbital velocity and ship motion. The low velocity region is affected by pressure gradient and ship motion.

• Journal of Korea Foundry Society
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• v.24 no.1
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• pp.26-33
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• 2004

The pressure of the mold flux acting on the meniscus shell was investigated through the coupling analysis of heat transfer in the mold and fluid flow in the flux caused by the mold oscillation. Finite element method was employed to solve the conservation equation associated with appropriate boundary conditions. As reported by previous workers, the axial pressure is positive on the negative strip time and negative on the positive strip time. A maximum pressure is predicted toward the top of the meniscus shell which has the thin shell arid a maximum value is in proportion to the relative mold oscillation velocity. The relative mold oscillation velocity was changed by the effect of meniscus level fluctuation. Therefore the pressure of the mold flux acting on the meniscus shell was different each cycle of the mold oscillation due to the irregularity of relative mold oscillation velocity.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.10
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• pp.1210-1218
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• 2004

Experimental study was performed to investigate the flow behavior in boundary layer on the blade suction surface of a multi-stage axial flow compressor, which was focused on the third stage of the 4-stage Low Speed Research Compressor. Flow measurements in the boundary layer were obtained using a boundary layer hot wire probe, which was traversed normal to the blade suction surface at small increments by the probe traverse specially designed. Detailed boundary layer flow measurements covering most of the stator suction surface were taken and are described using time mean and ensemble averaged velocity profiles. Amplitude of the velocity fluctuation and turbulence intensity in the boundary layer flow are also discussed. At midspan, narrow but strong wake zone due to passing wake disturbances is generated in the boundary layer near the blade leading edge for the rotor blade passing period. Corner separation is observed at the tip region near the trailing edge, which causes to increase steeply the boundary layer thickness.

• Journal of the Korean Society of Safety
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• v.14 no.4
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• pp.158-167
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• 1999

In parallel flow condition, to estimate the stability of the extended embankment constructed on a permeable foundation ground, a laboratory model test was performed due to extended materials and water level increasing velocity of a flood period. A laboratory model test was peformed for different permeability coefficients ($K_1=2.0{\times}10^{-5}cm/sec,\;K_2=1.5{\times}10^{-4}cm/sec,\;K_3=2.3{\times}10^{-3}cm/sec$) using seepage. The fluctuation of water level occurring to an extended embankment was analyzed by laboratory model tests as vary the increasing velocity of water level with 0.6cm/min, 1.2cm/min, 2.4cm/min respectively. In analysis results, the increase of water level into embankment occurs rapidly because seepage water moving along with a permeable soil flow into embankment. The larger the permeability coefficient of an extended part is the longer initial seepage distance, and the exit point of downstream slope is gradually increased and then shows unstable seepage behavior as occurring partial collapse. As the increasing velocity of water level increase, the initial seepage line is formed low, and the discharge increases. Therefore, the embankment extended by a lower permeable soil than existing embankment shows stable seepage behavior because an existing embankment plays a role as filter for an extended part.

• Journal of Korean Tunnelling and Underground Space Association
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• v.6 no.1
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• pp.41-50
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• 2004

The numerical investigation for the effects of blasting-induced vibration on adjacent dam and pore water pressure fluctuation was conducted through solid-water coupled analysis under dynamic loading. The stability of dam was examined by peak particle velocity of core. Pore water pressure distributions were calculated by steady state flow analysis using coupled analysis on ground water and blasting-induced vibration. The influence of pore water pressure and the effective stress distribution in the ground were also investigated. Furthermore, effective stress alteration was examined by applying Finn & Byrne Model to monitor the generation and dissipation of pore water pressure.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.05a
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• pp.141-144
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• 2008

Atomization characteristics and spatial distribution of the spray emanating from an injector of liquid-propellant thruster are investigated by using dual-mode phase Doppler anemometry (DPDA). Spray characteristic parameters such as the mean velocity, Sauter mean diameter (SMD), and velocity fluctuation are measured at various locations along the spray axis as well as on the radial direction. Those data are quantified in radial profile and also used to scrutinize the correlation between diameter and turbulence intensity of spray droplets. For the better visual grasp, dynamic behavior of spray droplets along the spray stream is presented through the velocity vectors projected on the plane of geometric axis of nozzle orifice and radial coordinate.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.5
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• pp.1253-1263
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• 1994

The stability of the embankment is depended upon the location of seepage line. As the seepage flow occurs in the embankment, the slope of the embankment loses its stability. Of particular interest is the stability following a rapid change of embankment level. The variation of seepage line in the embankment model according to flow velocity was investigated. In addition to this non-steady state flow in embankment by a fluctuation of water level is discussed. The experimental model was construction with slopes of 1 : 2.5 and flow velocity is turned from 60 cm/sec~90 cm/sec. Analysis of the experimental results, the seepage line is influenced by flow velocity and coefficient of permeability.