• Steel and Composite Structures
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• v.42 no.6
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• pp.805-826
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• 2022

The free and live load-forced vibration behaviour of porous functionally graded (PFG) higher order nanobeams in the thermal and magnetic fields is investigated comprehensively through this work in the framework of nonlocal strain gradient theory (NLSGT). The porosity effects on the dynamic behaviour of FG nanobeams is investigated using four different porosity distribution models. These models are exploited; uniform, symmetrical, condensed upward, and condensed downward distributions. The material characteristics gradation in the thickness direction is estimated using the power-law. The magnetic field effect is incorporated using Maxwell's equations. The third order shear deformation beam theory is adopted to incorporate the shear deformation effect. The Hamilton principle is adopted to derive the coupled thermomagnetic dynamic equations of motion of the whole system and the associated boundary conditions. Navier method is used to derive the analytical solution of the governing equations. The developed methodology is verified and compared with the available results in the literature and good agreement is observed. Parametric studies are conducted to show effects of porosity parameter; porosity distribution, temperature rise, magnetic field intensity, material gradation index, non-classical parameters, and the applied moving load velocity on the vibration behavior of nanobeams. It has been showed that all the analyzed conditions have significant effects on the dynamic behavior of the nanobeams. Additionally, it has been observed that the negative effects of moving load, porosity and thermal load on the nanobeam dynamics can be reduced by the effect of the force induced from the directed magnetic field or can be kept within certain desired design limits by controlling the intensity of the magnetic field.

• International Journal of Naval Architecture and Ocean Engineering
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• v.13 no.1
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• pp.641-649
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• 2021

Fuel oil consumption (FOC) must be minimized to determine the economic route of a ship; hence, the ship power must be predicted prior to route planning. For this purpose, a numerical method using test results of a model has been widely used. However, predicting ship power using this method is challenging owing to the uncertainty of the model test. An onboard test should be conducted to solve this problem; however, it requires considerable resources and time. Therefore, in this study, a deep feed-forward neural network (DFN) is used to predict ship power using deep learning methods that involve data pattern recognition. To use data in the DFN, the input data and a label (output of prediction) should be configured. In this study, the input data are configured using ocean environmental data (wave height, wave period, wave direction, wind speed, wind direction, and sea surface temperature) and the ship's operational data (draft, speed, and heading). The ship power is selected as the label. In addition, various treatments have been used to improve the prediction accuracy. First, ocean environmental data related to wind and waves are preprocessed using values relative to the ship's velocity. Second, the structure of the DFN is changed based on the characteristics of the input data. Third, the prediction accuracy is analyzed using a combination comprising five hyperparameters (number of hidden layers, number of hidden nodes, learning rate, dropout, and gradient optimizer). Finally, k-means clustering is performed to analyze the effect of the sea state and ship operational status by categorizing it into several models. The performances of various prediction models are compared and analyzed using the DFN in this study.

• Journal of the Korean Society of Hazard Mitigation
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• v.9 no.4
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• pp.81-89
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• 2009

The seepage should be analyzed to design or reinforce the levees. The steady seepage analysis is an usual application in USA and European countries where the large scaled dams and levees are existed. However, Korea and Japan, where the reaching time is short, the excessive forces are applied on the levees at the short reaching time if the seepages are analyzed in steady condition. Accordingly, the unsteady analysis based on the variation of time is necessitated. In the unsteady analysis, the flood wave type is necessary. No criteria and standards, however, are derived for the unsteady seepage in Korea. In the study, the flood wave type is derived for the unsteady seepage. The major reliable flood surface data are collected in 5 stations including Jindong of the Nakdong river basin. The data are sorted in duration, and they are non-dimensionalized. The statistical method is also applied to derive the waves. To verify the study, the seepage is analyzed by the derived wave and applied to the prototype. The results are also compared with the Japanese Method. The errors between the hydraulic gradient and critical velocity method are $0{\sim}0.7%$, $0{\sim}0.7%$ at the Jindong, $1.6{\sim}4.0%$, $1.7{\sim}4.1%$ at the Hyunpoong, $0.6{\sim}3.6%$, $0.6{\sim}3.7%$ at the Waegwan, $2.0{\sim}8.1%$, $2.0{\sim}8.1%$ at the Nakdong, and $1.2{\sim}9.8%$, $1.3{\sim}9.9%$ at the Jeongam, respectively. The relationship($R^2$) between each method is relatively high as $0.983{\sim}0.999$. This means the results are more logical than the Japanese method, and the study is applicable to the design of hydraulic structures.

• Journal of the Korean Society of Radiology
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• v.81 no.5
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• pp.1151-1163
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• 2020

Purpose This study aimed to evaluate changes of subprosthetic pannus on cardiac CT and determine its relationship to echocardiographic findings in patients with mechanical aortic valve replacement (AVR). Materials and Methods Between April 2011 and November 2017, 17 AVR patients (56.8 ± 8.9 years, 12% male) who showed pannus formation on CT and had undergone both follow-up CT and echocardiography were included. The mean interval from AVR to the date of pannus detection was 10.5 ± 7.1 years. In the initial and follow-up CT and echocardiography, the pannus extent and echocardiographic parameters were compared using paired t-tests. The relationship between the opening angle of the prosthetic valve and the pannus extent was evaluated using Pearson correlation analysis. Results The pannus extent was significantly increased on CT (p < 0.05). The peak velocity (3.9 ± 0.8 m/s vs. 4.2 ± 0.8 m/s, p = 0.03) and mean pressure gradient (36.4 ± 15.5 mm Hg vs. 42.1 ± 15.8 mm Hg, p = 0.03) were significantly increased. The mean opening angles of the mechanical aortic leaflets were slightly decreased, but there was no statistical significance (73.1 ± 8.3° vs. 69.4 ± 12.1°, p = 0.12). The opening angle of the prosthetic leaflets was inversely correlated with the pannus extent (r = -0.57, p < 0.001). Conclusion The pannus extent increases over time, increasing transvalvular peak velocity and the pressure gradient. CT can be used to evaluate the pannus extent associated with hemodynamic changes that need to be managed by surgical intervention.

• Journal of Korea Water Resources Association
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• v.47 no.6
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• pp.537-546
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• 2014

Surface image velocimetry was introduced as an efficient and sage alternative to conventional river flow measurement methods during floods. The conventional surface image velocimetry uses a pair of images to estimate velocity fields using cross-correlation analysis. This method is appropriate to analyzing images taken with a short time interval. It, however, has some drawbacks; it takes a while to analyze images for the verage velocity of long time intervals and is prone to include errors or uncertainties due to flow characteristics and/or image taking conditions. Methods using spatio-temporal images, called STIV, were developed to overcome the drawbacks of conventional surface image velocimetry. The grayscale-gradient tensor method, one of various STIVs, has shown to be effectively reducing the analysis time and is fairly insusceptible to any measurement noise. It, unfortunately, can only be applied to the main flow direction. This means that it can not measure any two-dimensional flow field, e.g. flow in the vicinity of river structures and flow around river bends. The present study aimed to develop a new method of analyzing spatio-temporal images in two-dimension using cross-correlation analysis. Unlike the conventional STIV, the developed method can be used to measure two-dimensional flow substantially. The method also has very high spatial resolution and reduces the analysis time. A verification test using artificial images with lid-driven cavity flow showed that the maximum error of the method is less than 10 % and the average error is less than 5 %. This means that the developed scheme seems to be fairly accurate, even for two-dimensional flow.

• Journal of Korean Society of Environmental Engineers
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• v.30 no.2
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• pp.207-212
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• 2008

This study focused on shorten the period of the struvite crystal birth and development by adding seed materials. For this purpose, three different seed materials were selected: sand, anthracite and struvite. The experiments has been conducted to evaluate the effect of the particle size of the selected seed material on the struvite crystallization, and to study the mixing effect which can be expressed by the value of $G{\cdot}t_d$(the multiple of mean velocity gradient(G) and mixing time($t_d$)). It was observed in this study that the removal efficiency of ammonia nitrogen increased by 9%, 11%, and 20% for sand, anthracite, and struvite added as the seed material, respectivley. This indicated that the struvite crystallization efficiency had a close correlation with the specific surface area of the seed particle. It was found that when struvite was selected as the seed material, the struvite crystallization proceeded at lower $G{\cdot}t_d$ value as compared with other seed materials. This observation implied that the secondary crystal birth would be dominated in this reaction. It was concluded in this study that the particle size was not significant factor on the struvite crystallization, while the $G{\cdot}t_d$ value was a considerably important factor in terms of the theory of the struvite crystal birth.

• Investigative Magnetic Resonance Imaging
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• v.6 no.1
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• pp.41-46
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• 2002

The Fourier transform arteriography (FTA) exploits the periodic variation of arterial flow velociety of arterial flow velocity in stnchronized with cardiac cycles. This technique is intrinsically unique compared to other modern techniques. This technique separates the arteries from the veins using the pulsatile arterial flow without using the presaturation RF pulses. Therefore, it has less RF deposition and is free from the dark band artifacts that can arise from retrograde flow and curved arteries. Furthermore, it is free from the artifacts induced by eddy currents. However, there are some drawbacks such as a single projection view and the saturation of arteries at the end of an imaging slab. These drawbacks are circumvented by applying recently developed techniques. The fast gradient switching capability of modern MRI systems enabled us to incorporate dual projection views into the conventional FTA sequence without increasing the repetition time. In addition, signals from the distal arteries were enhanced by use of a ramped RF pulse and therefore the distal arteries were less saturated. By use of the FTA sequence with dual projection views and the ramped RF pulse, we acquired the sagittal and coronal projection views of femoral arteriograms simultaneously with more enhanced signals of distal arteries than the conventional FTA.

• Current Research on Agriculture and Life Sciences
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• v.10
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• pp.41-47
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• 1992

In general, the behavior of moisture transmission is estimated by vapor permeability or vapor transmission resistance, but its values obtained by experiments do not have great adaptability for practical situations because of changes in the experimental conditions. This fact is why only theoretical discussions have advanced. Thus, the fundamental study of the moisture transmission phenomenon has been treated lightly. Here, as the first step toward the basic research of moisture transmission, the amount of moisture transmission and the moisture distribution in specimens were investigated. The experiment was conducted in a steady state, and the moisture distribution was measured by slicing and weighing the specimens. From the examination of the vapor transmission resistance, the phenomenon of moisture transmission was dealt with devide the moisture transfer on the wood surface and moisture diffusion in wood. The following results were obtained. 1) The phenomenon of moisture transmission should be approached by its division into moisture transfer on the wood surface and moisture diffusion in the wood because the positive values of vapor transmission resistance exist in the extrapolation of thickness 0mm. 2) The distribution of moisture in wood can be illustrated by two straight lines intersecting at the point of nine percent moisture content : namely, diffusion coefficients have two constant values at moisture contents below and above nine percent. The shape of the distribution curve of moisture content is similar irrespective of the wood thickness. On the other hand, when the moisture contents on both sides was more than nine percent, the distribution of the moisture content could be illustrated by one straight lines. 3) The amount of moisture movement is determined by the moisture gradient in wood. 4) Coefficients of the moisture transfer depend on the thickness of the specimens.

• Water for future
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• v.14 no.1
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• pp.39-49
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• 1981

The purpose of this study is to examine the meander charactericstics for the rivers in Korea..In this study, the new characteristics factors of meander are proposed, and the relationships among the factors proposed in this study and the existing factors are derived. An attempt is made to find considerable relation among meander characteristics, but width and meander belt did not show any defined trend and considerable scatter of points was observed. Relationships among the meander length, belt and flowrate, etc., which are factors of meander characteristics, are analyzed the 67 rivers above 30km in length. Channel shape factor which is the ratio of the length from the starting point to the end to the channel lenght, tortuosity which is the ratio of the curved channel length against the channel length are suggested for a new characteristics factor of meander. They are well correlated with channel length, Horton's shape facotr and meander gradient, consequently have to be important measures of river meander. The result of the detailed comparison and the analysis of degree of sinuosity, velocity and water surface slop are brought light on the fact show that the curved reach is morestable than the straight one. The ratio of the meander length to the meander belt and its accumulative frequency showed excellent correlationship when plotted on the semi-log paper. The results of regression analysis of meander belt and meander length show linear for the South Han river branches and power curve for the Geum river and the Nakdong river branches.

• Journal of the Korean Society of Marine Environment & Safety
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• v.25 no.6
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• pp.795-801
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• 2019

The aim of this study was to investigate the possibility of the skin-friction reduction by vortex control. A vortical system such as a horseshoe vortex, a hairpin vortex, and a wake region was induced around a hemisphere attached on a Perspex flat plate in the circulating water channel. Hairpin vortices were developed from the wake region and horseshoe vortices were formed by an adverse pressure gradient in front of the hemisphere. The horseshoe vortices located on the flank of the hemisphere induced a high momentum flow in the wake region by the direction of their vorticity. This process increased the frequency of the hairpin vortices as well as the frictional drag on the surface of the wake region. To reduce the skin-friction drag, suction control in front of the hemisphere was applied through a hole. Flow visualization was performed to optimize the free-stream velocity, size of the hemisphere, and size of the suction hole. Once the wall suction control mitigated the strength of the horseshoe vortex, the energy supplied to the wake region was reduced, causing the frequency of the hairpin vortex generation to decrease by 36.4 %. In addition, the change in the skin-friction drag, which was measured with a dynamometer connected to a plate in the wake region, also decreased by 2.3 %.