• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.28 no.8
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• pp.646-652
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• 2017

The effect of plasma on mono-static radar cross section under atmospheric pressure is demonstrated when the dielectric barrier discharge actuator has plasma layer. The volume of plasma layer is increased by using planar array of electrodes. Because the incident wave has electric field which is perpendicular to the electrode array, the undesired effect on radar cross section caused by structure of plasma actuator is minimized. In experiments, mono-static radar cross section is measured at the frequencies from 2 GHz to 25 GHz. The generated plasma reduces the radar cross section at frequencies above 18 GHz, and the amount of reduction reaches to 8 dB in maximum. The reduction can be controlled by changing the peak-to-peak voltage from high voltage generator. The result shows the possibility of plasma as a flexible radar cross section controller.

• Journal of Navigation and Port Research
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• v.29 no.5 s.101
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• pp.383-388
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• 2005

To install a radar reflector on small ships, such as a small fishing vessel, a fishery buoy, and a barge ship for fishery, it is very important to develop the optimal system which may determine a proper installation location. For this, the response characteristics how waves have an effect on the small ships should be accurately analyzed. In this paper, we analyze the dynamic behaviors of small ships, which may be caused by irregular waves. To do this, we investigate how a barge ship responses to wavelength, water depth, and directions of incoming waves. The analyzed results shall be utilized to evaluate an effect on a radar cross section when we install an radar reflector on a barge ship for fishery and a small ship.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.32 no.1
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• pp.11-16
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• 2020

The rotational stability of an interlocking caisson breakwater was studied. Using the analytical solution for the linear wave incident to the infinite breakwater, the phase difference effect of wave pressures in the direction of the breakwater baseline is considered, and Goda's wave pressure formula in the design code is adopted to consider the nonlinearity of the design wave. The rotational safety factor of the breakwater was defined as the ratio of the rotational frictional resistance moment due to caisson's own weight and the acting rotational moment due to the horizontal and vertical wave forces. An analytical solution for the rotational center point location and the minimum safety factor is presented. Stability assessment formula were proposed to be applicable to all design wave conditions used in current port and harbor structure design such as regular waves, irregular waves and multi-directional irregular waves.

• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.4
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• pp.352-357
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• 2008

The detection of micro cracks in materials at the early stage of fracture is important in many structural safety assurance problems. The nonlinear ultrasonic technique (NUT) has been considered as a positive method for this, since it is more sensitive to micro crack than conventional linear ultrasonic methods. The basic principle is that the waveform is distorted by nonlinear stress-displacement relationship on the crack interface when the ultrasonic wave transmits through, and resultantly higher order harmonics are generated. This phenomenon is called the contact acoustic nonlinearity (CAN). The purpose of this paper is to prove the applicability of CAN experimentally by detection of micro fatigue crack artificailly initiated in Aluminum specimen. For this, we prepared fatigue specimens of Al6061 material with V-notch to initiate the crack, and the amplitude of second order harmonic was measured by scanning along the crack direction. From the results, we could see that the harmonic amplitude had good correlation with COD and it can be used to detect the crack depth in more accurately than the common 6 dB drop echo method.

• Journal of radiological science and technology
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• v.45 no.6
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• pp.483-489
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• 2022

This study proposes a new skull lateral examination, and provides an improved examination environment for patients and radiologists. The study was divided into three groups. One group was divided into the SID (source to image receptor distance) 110 ㎝ and 180 ㎝ in the skull lateral posture, the other group The other group was divided into an position in contact with the detector and an position without contact with the detector, and the other group was divided into male and female groups, considering that the difference in shoulder width between adult males and females would affect the dose and image quality. For dose evaluation, the ESD (entrance surface dose) was measured at the EAM (external auditory meatus), and the conditions were applied equally at 70 ㎸p, 200 ㎃, and 10 ㎃s. For image quality evaluation, SNR (signal to noise ratio) and CNR (contrast to noise ratio) were measured in frontal sinus, EAM, and sella turcica. As a result of ESD comparison, when sid 110 ㎝ to sid 180 ㎝ was changed among the three groups, ESD values decreased the most to 729.18±4.62 μ㏉ and 224.18±0.74 μ㏉ at 180 ㎝ (p<0.01). The values of SNR and CNR were statistically significant (p<0.01), but there was no qualitative difference. This shows that when the SID is 180 ㎝, it is possible to reduce the dose without lowering the image quality. So, It is suggested that the SID 180 ㎝ is used without contacting the detector when examining the skull lateral.

• The Journal of the Acoustical Society of Korea
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• v.27 no.3
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• pp.129-139
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• 2008

The purpose of the present work is to derive the reflection and transmission coefficients of $S_0\;and\;A_0$ mode Lamb waves in relation to the geometry of a rectangular notch when the waves propagate across the notch in an elastic plate. Firstly, the excitable modes of the Lamb wave were analyzed with respect to the plate thickness. The scattering phenomena were divided into three independent processes according to the boundary shape of the notch and the direction of the wave propagation. Linear equations for each process were derived with corresponding free or continuous boundary conditions to analyze the scattered waves. By the rule of linear superposition, the waves scattered at each process were summed for each mode. Then the steady-state reflection and transmission coefficients of the scattered waves were determined so that the difference of energy flux between the incident and the scattered waves would remain within 4%.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.16 no.1
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• pp.27-38
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• 2004

In this study, the reliability design method developed by Shimosako and Takahashi in 1999 for calculation of the expected sliding distance of the caisson of a vertical breakwater is extended to take into account the variability in wave direction such as directional spreading of waves, obliquity of the deep-water design principal wave direction from the shore-normal direction, and its variation about the design value. To calculate the transformation of random directional waves, the model developed by Kweon et al. in 1997 is used instead of Goda's model, which was developed in 1975 for unidirectional random waves normally incident to a straight coast with parallel depth contours and has been used by Shimosako and Takahashi. The effects of directional spreading and the variation of deep-water principal wave directions were minor compared with those of the obliquity of the deep-water design principal wave direction from the shore-normal direction, which tends to reduce the expected sliding distance as it increases. Especially when we used the field data in a part of east coast of Korea, considering the variability in wave directions reduced the expected sliding distance to about one third of that not considering the directional variability. Reducing the significant wave height calculated at the design site by 6％ to correct the effect of wave refraction neglected in using Goda's model was found to be proper when the deep-water design principal wave direction is about 20 degrees. When it is smaller than 20 degrees, a value smaller than 6％ should be used, or vice versa. When we designed the caisson with the expected sliding distance to be 30㎝, in the area of water depth of 25 m or smaller, we could reduce the caisson width by about 30％ at the maximum compared with the deterministic design, even if we did not consider the variability in wave directions. When we used the field data in a part of east coast of Korea, considering the variability in wave directions reduced the necessary caisson width by about 10% at the maximum compared with that not considering the directional variability, and is needed a caisson width smaller than that of the deterministic design in the whole range of water depth considered (10∼30 m).

• Korean Journal of Optics and Photonics
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• v.20 no.5
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• pp.266-271
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• 2009

We designed a thermo-optic switch based on a directional coupler with not only a high extinction ratio but also significantly low power consumption. The switch operates by using the thermo-optic effect of the polymer which the refractive index changes by heating the electrode. If the electrode is not powered (OFF), the input light will be coupled completely to the other waveguide. When the electrode is powered at a certain level (ON), input light launched into the input waveguide will remain in that waveguide due to the lower index adjusted in the other waveguide. The switch based on the directional coupler was designed using the generalized extinction ratio curve and the lateral shift of the input waveguide. The coupling length is 1,610 ${\mu}m$ and the extinction ratios are -28 and -30 dB for ON and OFF states, respectively. The electrode structures were optimized by thermal analysis. The transported heat into the waveguide is increased, as the electrode width (w) is increased and the center distance between the electrode and the waveguide (d) is decreased. Also, because the heat generated in the electrode affects the other waveguide, the temperature difference between two waveguides is varied as the given w and d. There are specific conditions which have the maximum of the temperature difference. That of the temperature difference is increased as the width and the temperature of the electrode are increased. Especially, when the switch is designed using the condition with the maximum of the temperature difference for switching, the temperature of the electrode can be decreased. We expect this condition will be the novel method for the reduction of the power consumption in a thermo-optic switch.

• Journal of radiological science and technology
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• v.29 no.2
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• pp.57-62
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• 2006

Purpose : This study was to understand about the measurement of shoulder motion fraction and motion ratio. We proposed the radiological criterior of glenohumeral and scapulothoracic movement ratio. Materials and Methods : We measured the motion fraction of the glenohumeral and scapulothoracic movement using CR(computed radiological system) of arm elevation at neutral, 90 degree, full elevation. Central ray was $15^{\circ},\;19^{\circ},\;22^{\circ}$ to the cephald for the parallel scapular spine, and the tilting of torso was external oblique $40^{\circ},\;36^{\circ},\;22^{\circ}$ for perpendicular to glenohumeral surface. Healthful donor of 100 was divided 5 groups by age(20, 30, 40, 50, 60). The angle of glenohumeral motion and scapulothoracic motion could be taken from gross arm angle and radiological arm angle. We acquired 3 images at neutral, $90^{\circ}$ and full elevation position and measured radiographic angle of glenoheumeral, scapulothoracic movement respectively. Results : While the arm elevation was $90^{\circ}$, the shoulder motion fraction was 1.22(M), 1.70(W) in right arm and 1.31, 1.54 in left. In full elevation, Right arm fraction was 1.63, 1.84, and left was 1.57, 1.32. In right dominant arm(78%), $90^{\circ} and Full motion fraction was 1.58, 1.43, in left(22%) 1.82, 1.94. In generation 20, $90^{\circ} and Full motion fraction was 1.56, 1.52, 30' was 1.82, 1.43, 40' was 1.23, 1.16, 50' was 1.80, 1.28, 60' was 1.24, 1.75. There was not significantly by gender, dominant arm and age. Conclusion : The criterior of motion fraction was useful reference for clinical dignosis the shoulder instability.

• Journal of the Korean Society of Marine Environment & Safety
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• v.22 no.6
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• pp.615-624
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• 2016

In this study, two rectangular barges in close proximity were simulated to analyze the characteristics of motion responses due to hydrodynamic interactions. Using a numerical solution from DNV-GL SESAM, coupled stiffness matrix terms for these same FEM models were added to the multiple body modes in the surge direction. Potential theory was used to calculate the first order radiation and diffraction effects on the simulated barge models. In the results, the sheltering effect of the barges was not shown at 1.3 rad/s with hull separation of 20 m in transverse waves. The separation effect between the barges was more clear with longitudinal waves and a shallow water depth. However, sway forces were influenced by hull separation with transverse waves. The peaks for sway and heave motion and sway force occurred at higher frequencies as hull separation narrowed with longitudinal and transverse waves. Given a depth of 10 m, the sway motion on the lee side of a coupled barge made a significant difference in the range of 0.2-0.8 rad/s with transverse and oblique waves. Also, the peaks for sway force were situated at lower frequencies, even when incident waves changed.