• The Journal of the Acoustical Society of Korea
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• v.21 no.4
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• pp.421-429
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• 2002

Sea surface backscattering signal measurements were conducted in the shallow waters off the east coast of Korea to study the acoustic wave scattering from the sea surface. The grazing angles of wave range from 20° to 40° with a frequency of 60 kHz. The wind speed and surface roughness of the experiment area were 3 m/os and below 1 m, respectively. The measured acoustic backscattering strengths greatly exceed the composite roughness predictions at low grazing angles. To account for this discrepancy, the scattering strengths due to a near-surface bubble layer were considered. The prediction with bubble contribution was found to be in good agreement with the experimental measurement.

• Tribology and Lubricants
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• v.35 no.4
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• pp.244-266
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• 2019

The purpose of this study is to estimate the wear volumes of engine journal bearings operating at variable angular velocity of a shaft in the beginning of firing start-up cycle. To do this, first we find the potential region of wear scar on engine journal bearings where the applied bearing load and crank shaft velocity are variable. The potential wear regions are discovered by finding minimum oil film thickness at every crank angle existing below most oil film thickness scaring wear (MOFTSW) obtained based on the concept of the centerline average surface roughness. Then we calculate the wear volume from the wear depth and two wear angles decided by the magnitude of each film thickness lower than MOFTSW at every crank angle. The results show that the expected wear region is located at a few bearing angles after and/or behind the upper center of a big-end bearing and the lower center of a main bearing. And the real wear region is similar to the estimated wear region. Further we find that the wear scar on an engine journal bearing may occur at re-starting time after switch-off of a start motor especially under the condition of high oil temperature.

• Korean Journal of Applied Biomechanics
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• v.15 no.2
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• pp.129-138
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• 2005

The purpose of this study was to investigate the techniques used by long jumpers who recorded over 8meters in 2002 Busan Asian Game and 2003 Daegu Universiade. The kinematic characteristics from the last three stride to takeoff at the takeoff board were analyzed such as velocities, heights and angles. The real-life three-dimensional coordinates of 20 body landmarks during each trial were collected using a Direct Linear Transformation procedure. The conclusion were as follows; 1. The height variation who recorded over 8 meters of center of gravity of the jumpers at the last stride was under 8cm. In order to record over 8meters the national long jumpers should have under 10cm height variation. 2. In the approach phase the horizontal velocity of the jumpers should reach to 10.5m/s in last three strides and 9.79m/s in touch down at take off board. 3. The horizontal velocity at take off board must have over 8.51m/s and the vertical velocity must have 3.75m/s simultaneously in order to record 8meters. 4. The forward body lean angle should have over 20degrees with pushing the take off board in forwarding movement. The appropriate body variation range ratio between take off and touch down should be 1.2 vs 1 and the trunk angle at touch down on the board should be close to the erect posture for higher body flight.

• Journal of Biosystems Engineering
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• v.35 no.4
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• pp.231-238
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• 2010

The purpose of this paper was to investigate experimentally the steering torque characteristics of a tractor operated in various ground conditions. The experiments were conducted with the tractor reconstructed for steering torque test of the tractor at two different off-road conditions (ground-I and ground-II) and a on-road condition (ground-III), three different levels of tire inflation pressures (69 kPa, 138 kPa and 207 kPa), and four different levels of axle loads (4120 N, 4730 N, 5340 N and 5950 N). The results of this study are summarized as follows: 1) The steering torque was increased with the increase in steering angle for all experimental levels of ground conditions, axle loads and inflation pressures of tire. 2) As the axle load increased, the steering torque of the tractor increased for all ground conditions, and the increasing rate of the steering torque with the increase of axle load was greater at on-road than at off-road. 3) As the tire inflation pressure decreased, the steering torque increased. Also the increasing tendency of the steering torque with decreasing the tire inflation pressure showed that the harder the ground was, the larger the effect was. But for the soft ground condition, ground-I, no specific trend with inflation pressures was found. 4) Steering angle-steering torque relationship with ground conditions showed that the increasing rate of the steering torque was greater at on-road than off-road for small steering angle under 10 degree, and was greater at off-road than on-road for large steering angles over 10 degree.

• The Journal of the Korea Contents Association
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• v.15 no.8
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• pp.556-563
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• 2015

The purpose of this study was to analyzed body movement of the right and left directions relative to stance width. Seven college students(male, height $174.9{\pm}4.8cm$, weight $70.9{\pm}6.6kg$) participated in this study. For the purpose of this study, nine high-speed cameras and two force plates was utilized. Stance width was limited into small stance width(mean 42.9cm) and large stance width(mean 65cm). The results showed that (1) the small stance width took shorter time than the large stance during landing of the right feet to toe-off; (2) the small stance width was took longer movement of the right feet after split step comparing to the large stance; (3)there was no significant difference in the speed of center of gravity between toe-off and maximum speed; (4) the small stance width was greater angles to hip joint and knee joint during toe-off of the right feet comparing to the large stance; (5) the was no significant difference in values for force plates between the small stance and the large stance width during toe-off of the right feet.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.20 no.3
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• pp.141-146
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• 2020

In brushless DC motors (BLDC), the on/off angle of the switch is determined by the optimal alignment of the stator and rotor, while switched reluctance motors (SRM) are complex parameters with many on/off angles of the switch. It appears as a function and therefore the switching angle is variable for optimal operation. Therefore, in order to operate the switched reluctance motor (SRM) optimally, the rotor position can be detected using a high resolution position sensor and a complicated additional circuit. In this paper, rotor position detection and related position detection circuits are applied and detected by using various sensors such as encoder, hall sensor and opto interrupter among several methods to drive switched reluctance motor (SRM). Also a study on securing the initial position of the rotor was conducted.

• Journal of Internet Computing and Services
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• v.21 no.6
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• pp.41-50
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• 2020

The consequences of wearing high heels can be different according to the heel height, gait speed, shoe design, heel base area, and shoe size. This study aimed to focus on the knee extension and flexion range of motion (ROM) during gait, which were challenged by wearing five different shoe heel types and two different self-selected gait speeds (comfortable and fast) as experimental conditions. Measurement standards of knee extension and flexion ROM were individually calibrated at the time of heel strike, mid-stance, toe-off, and stance phase based on the 2-minute video recordings of each gait condition. Seven healthy young women (20.7 ± 0.8 years) participated and they were asked to walk on a treadmill wearing the five given shoes at a self-selected comfortable speed (average of 2.4 ± 0.3 km/h) and a fast speed (average of 5.1 ± 0.2 km/h) in a random order. All of the shoes were in size 23.5 cm. Three of the given shoes were 9.0 cm in height, the other two were flat shoes and sneakers. A motion capture software (Kinovea 0.8.27) was used to measure the kinematic data; changes in the knee angles during each gait. During fast speed gait, the knee extension angles at heel strike and mid-stance were significantly decreased in all of the 3 high heels (p<0.05). The results revealed that fast gait speed causes knee flexion angle to significantly increase at toe-off in all five types of shoes. However, there was a significant difference in both the knee flexion and extension angles when the gait in stiletto heels and flat shoes were compared in fast gait condition (p<0.05). This showed that walking fast in high heels leads to abnormal knee ROM and thus can cause damages to the knee joints. The findings in this preliminary study can be a basis for future studies on the kinematic changes in the lower extremity during gait and for the analysis of causes and preventive methods for musculoskeletal injuries related to wearing high heels.

• Journal of the Society of Naval Architects of Korea
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• v.37 no.3
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• pp.57-68
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• 2000

The Power Augmented Ram(PAR) effect, which blows the down stream of the propellers into the underside of the wings and hence increases the pressure between the lower surface of the wings and the sea surface, is known significantly to enhance the performance of the WIG concept by reducing the take-off and landing speeds. The aerodynamic characteristics of a 20 passenger PARWIG are investigated by wind tunnel tests with the 1/20 scale model. The efflux of the forward mounted propellers are simulated by jet flows with a blower and duct system. The lift, drag, and pitch moment of the model with various ground clearances, angles of attack and flap angles are measured for the various jet velocities, jet nozzle angles, horizontal and vertical positions of the nozzle, and the nozzle diameters. The aerodynamic characteristics of the PARWIG due to these parametric changes are compared and pertinent discussions are included. It is shown that the proper use of the PAR can increase the lift coefficient of as much as up to 4.

• The Journal of the Acoustical Society of Korea
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• v.23 no.8
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• pp.562-569
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• 2004

An experiment for observation of the distribution of the seagrass (zostera marina) beds and characteristics of high-frequency backscattering by the photosynthesis was conducted off the coast. Acoustic data were taken as a function of the grazing angles and the relative azimuth angles on the seagrass beds of which bottom type was sandy-mud. The transmitted source signal was a 120 kHz CW waveform. Mapping of the seagrass beds distribution was drawn up using the seagrass backscattering strength with azimuth and grazing angles. The result of the comparison backscattering strength distribution of the seagrass beds was shown to be the similar to the photograph of real seagrass beds. The seagrass backscattering strength was also compared between day and night to verify the effects of the acoustical scattering by the bubbles of Photosynthetic oxygen formed on the seagrass. In these results. it is clear that observation of the seagrass beds between day and night showed the different characteristics because the bubbles of Photosynthetic oxygen affect the acoustical scattering.

• Journal of Power Electronics
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• v.16 no.2
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• pp.464-472
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• 2016

This study applies the selective harmonic elimination (SHE) technique to design and operate a regulated AC/DC/AC power supply suitable for maritime military applications and underground trains. The input is a single 50/60 Hz AC voltage, and the output is a 400 Hz regulated voltage. The switching angles for a multi-level inverter and transformer turns ratio are determined to operate with special connected transformers with equal power ratings and produce an almost sinusoidal current. As a result of its capability of directly controlling harmonics, the SHE technique is applicable to apparatus with congenital immunity to specific harmonics, such as series-connected transformers, which are specially designed to equally share the total load power. In the present work, a single-phase 50/60 Hz input source is rectified via a semi-controlled bridge rectifier to control DC voltage levels and thereby regulate the output load voltage at a constant level. The DC-rectified voltage then supplies six single-phase quazi-square H-bridge inverters, each of which supplies the primary of a single-phase transformer. The secondaries of the six transformers are connected in series. Through off-line calculation, the switching angles of the six inverters and the turns ratios of the six transformers are designed to ensure equal power distribution for the transformers. The SHE technique is also employed to eliminate the higher-order harmonics of the output voltage. A digital implementation is carried out to determine the switching angles. Theoretical results are demonstrated, and a scaled-down experimental 600 VA prototype is built to verify the validity of the proposed system.