• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.2
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• pp.1533-1540
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• 2015

In order to demonstrate the flow properties of the river bed and the design of hydraulic structures, the estimation of friction velocity is essentially required. However, existing friction velocity equations such as Log method and Power law have trouble to estimate the friction velocity because a boundary condition and various hydraulic properties are changed constantly in near the wall. In the present study, therefore, a new friction velocity equation that can minimize the parameters and reduce an error was suggested. To verify accuracy and reliability for the proposed equation, Clauser method, $\sqrt{gRI}$ method, reynolds stress method by Dr. Song were compared with the proposed method by estimated entropy parameter M for each channel. Consequently, the results show that uniform flow condition as well as non-uniform flow condition with highly accuracy nearly matched in case of accelerating non-uniform condition of $R^2=0.9621$, Decelerating Non Uniform condition of $R^2=0.9274$, Uniform condition of $R^2=0.8865$.

• Ocean Systems Engineering
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• v.6 no.1
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• pp.23-60
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• 2016

The major objective of this study was to develop further understanding of 3D nearshore hydrodynamics under a variety of wave and tidal forcing conditions. The main tool used was a comprehensive 3D numerical model - combining the flow module of Delft3D with the WAVE solver of XBeach - of nearshore hydro- and morphodynamics that can simulate flow, sediment transport, and morphological evolution. Surf-swash zone hydrodynamics were modeled using the 3D Navier-Stokes equations, combined with various turbulence models (${\kappa}-{\varepsilon}$, ${\kappa}-L$, ATM and H-LES). Sediment transport and resulting foreshore profile changes were approximated using different sediment transport relations that consider both bed- and suspended-load transport of non-cohesive sediments. The numerical set-up was tested against field data, with good agreement found. Different numerical experiments under a range of bed characteristics and incident wave and tidal conditions were run to test the model's capability to reproduce 3D flow, wave propagation, sediment transport and morphodynamics in the nearshore at the field scale. The results were interpreted according to existing understanding of surf and swash zone processes. Our numerical experiments confirm that the angle between the crest line of the approaching wave and the shoreline defines the direction and strength of the longshore current, while the longshore current velocity varies across the nearshore zone. The model simulates the undertow, hydraulic cell and rip-current patterns generated by radiation stresses and longshore variability in wave heights. Numerical results show that a non-uniform seabed is crucial for generation of rip currents in the nearshore (when bed slope is uniform, rips are not generated). Increasing the wave height increases the peaks of eddy viscosity and TKE (turbulent kinetic energy), while increasing the tidal amplitude reduces these peaks. Wave and tide interaction has most striking effects on the foreshore profile with the formation of the intertidal bar. High values of eddy viscosity, TKE and wave set-up are spread offshore for coarser grain sizes. Beach profile steepness modifies the nearshore circulation pattern, significantly enhancing the vertical component of the flow. The local recirculation within the longshore current in the inshore region causes a transient offshore shift and strengthening of the longshore current. Overall, the analysis shows that, with reasonable hypotheses, it is possible to simulate the nearshore hydrodynamics subjected to oceanic forcing, consistent with existing understanding of this area. Part II of this work presents 3D nearshore morphodynamics induced by the tides and waves.

• Journal of Korea Water Resources Association
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• v.56 no.12
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• pp.871-881
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• 2023

This study investigates the geomorphic changes and Bed Relief Index of the river downstream of the Yeongju Dam by Nays2DH, a two-dimensional numerical model, in order to grasp the dynamics of the downstream river while applying various flow patterns such as pulse discharge. It shows that the geomorphic and the bed elevations changes are the largest under the condition of the normalized pulse discharge. The total change in the riverbed is 29.88 m for uniform flow, 27.46 m for normalized hydrograph, 29.63 m for pulse flow and 31.87 m for pulse flow with normalized hydrograph which result in the largest variation in scour and deposition. The Bed Relief Index (BRI) increases with time under conditions of uniform flow, pulse flow and pulse flow with normalized hydrograph. However, BRI increased rapidly until 30 hrs after the peak flow (14 hrs), but decreased from 56 hrs under the condition of normalized hydrograph. Therefore, the condition of normalized hydrograph gives greater dynamics than the condition of a single flood or constant flow, and the dynamics increase downstream than upstream, resulting in an effect on improving the environment of the river downstream of the dam.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.65 no.1
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• pp.158-164
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• 2016

That's the reason why LED lighting has to employ AC power inlet. However, LED is a kind of diode, semiconductor, it's driven by DC power. With whis reason all of LED lighting should have AC/DC converter in its systems. This converter causes energy loss, it's the target for lesson the energy loss. To reduce this energy loss, DC power distribution structure can be used. LED lighting system using LVDC is a kind of DC power distribution structure, but LVDC has severe voltage drop which makes non-uniform brightness in lighting system. In this paper, we suggest a novel structure for the uniform brightness in LVDC LED lighting system using IoT based network system. The constructed test-bed system of suggested structure shows this structure can con control the brightness with uniformity.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.32 no.6
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• pp.446-457
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• 2020

Crescentic sand bar in the coastal zone of eastern Korea is a common morphological feature and the rhythmic patterns exist constantly except for high wave energy events. However, four consecutive typhoons that directly and indirectly affected the East Sea of Korea from September to October in 2019 impacted the formation of longshore uniform sand bar and overall shoreline retreats (approx. 2 m) although repetitive erosion and accretion patterns exist near the shoreline. Widely used XBeach to predict storm erosions in the beach is utilized to investigate the morphological response to a series of storms and each storm impact (NE-E wave incidence). Several calibration processes for improved XBeach modeling are conducted by recently reported calibration methods and the optimal calibration set obtained is applied to the numerical simulation. Using observed wave, tide, and pre & post-storm bathymetries data with optimal calibration set for XBeach input, XBeach successfully reproduces erosion and accretion patterns near MSL (BSS = 0.77 (Erosion profile), 0.87 (Accretion profile)) and observed the formation of the longshore uniform sandbar. As a result of analysis of simulated total sediment transport vectors and bed level changes at each storm peak Hs, the incident wave direction contributes considerable impact to the behavior of crescentic sandbar. Moreover, not only the wave height but also storm duration affects the magnitude of the sediment transport. However, model results suggest that additional calibration processes are needed to predict the exact crest position of bar and bed level changes across the inner surfzone.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.18 no.3
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• pp.179-188
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• 2006

The purpose of this study is to quantitatively estimate the settling and erosional properties for cohesive sediments in Shihwa lake. Settling tests are conducted by multi-depth method using a specially designed 1.8 m tall settling column, and erosion tests are conducted with annular flume under the uniform bed condition. As result of settling tests, it is confirmed that the settling velocity of the cohesive sediments has the range of $0.002 for suspended sediments concentration of 0.1$0.19{\sim}0.55N/m^{2}$ for bed shear stress of $1.14{\sim}1.32g/cm^{3}$, and the erosion rate coefficient decreases with logarithmic function in a range of $18.4{\sim}3.9mg/cm^{2}{\cdot}hr$ with increase of bed shear stress.

• Journal of the Korean Society of Hazard Mitigation
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• v.6 no.1 s.20
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• pp.69-81
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• 2006

The purpose of this study is to identify the downstream influences due to the dam discharge by using 2-dimensional model, SMS(Surface water Modeling System). RMA-2 and SED-2D in SMS were applied to Yongdam multipurpose dam watershed located in Gum river basin. Through the simulation, erosion and deposit quantitative analysis of sinuous channels and scour pattern analysis of bridges have been done. A differences erosion depths between deposit are simulated as $-102.4 mm{\sim}54.2 mm$ at No.176(1.4 km) and $-104.1 mm{\sim}28.9 mm$ at No.146(7.4 km), sinuous channel. The river bed at Kamdong bridge in straight channal is simulated as uniform erosion. However, the river bed at Dumdul bridge in sinuous channal has been shown as different erosion depths at each sides. Consequently, the parts that could not be simulated on the existing 1-dimensional model, can be improved results by using a 2-dimensional model, about weakness points for hydraulic modeling such as extreme bend, tributary confluence.

• Journal of Mechanical Science and Technology
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• v.20 no.10
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• pp.1765-1772
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• 2006

High-pressure die casting such as thixocasting and rheocasting is an effective process in the manufacturing automotive parts. Following the recent trend in the automotive manufacturing technologies, the product design subject to the die casting becomes more and more complex. Simultaneously the injection speed is also designed to be very high to establish a short cycletime. Thus, the requirement of the die design becomes more demanding than ever before. In some cases the product's shape can have multiple slender manifolds. In such cases, design of the inlet and outlet parts of the die is very important in the whole manufacturing process. The main issues required for the qualified products are to attain gentle and uniform flow of the molten liquid within the passages of the die. To satisfy such issues, the inlet cylinder ('bed cylinder' in this paper) must be as large as possible and simultaneously the outlet opening at the end of each passage must be as small as possible. However these in turn obviously bring additional manufacturing costs caused by re-melting of the bed cylinder and increased power due to the small outlet-openings. The purpose of this paper is to develop effective simulation methods of calculation for fluid flows in multiple columns, which mimic the actual complex design, and to get some useful information which can give some contributions to the die-casting industry. We have used a commercial code CFX in the numerical simulation. The primary parameter involved is the size of the bed cylinder. We will show how the very small opening of the outlet can be treated with the aid of the porous model provided in the code. To check the validity of the numerical results we have also conducted a simple experiment by using water.

• Journal of Powder Materials
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• v.28 no.2
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• pp.110-119
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• 2021

In this study, the high-temperature oxidation properties of austenitic 316L stainless steel manufactured by laser powder bed fusion (LPBF) is investigated and compared with conventional 316L manufactured by hot rolling (HR). The initial microstructure of LPBF-SS316L exhibits a molten pool ~100 ㎛ in size and grains grown along the building direction. Isotropic grains (~35 ㎛) are detected in the HR-SS316L. In high-temperature oxidation tests performed at 700℃ and 900℃, LPBF-SS316L demonstrates slightly superior high-temperature oxidation resistance compared to HR-SS316L. After the initial oxidation at 700℃, shown as an increase in weight, almost no further oxidation is observed for both materials. At 900℃, the oxidation weight displays a parabolic trend and both materials exhibit similar behavior. However, at 1100℃, LPBF-SS316L oxidizes in a parabolic manner, but HR-SS316L shows a breakaway oxidation behavior. The oxide layers of LPBF-SS316L and HR-SS316L are mainly composed of Cr₂O₃, Fe-based oxides, and spinel phases. In LPBF-SS316L, a uniform Cr depletion region is observed, whereas a Cr depletion region appears at the grain boundary in HR-SS316L. It is evident from the results that the microstructure and the high-temperature oxidation characteristics and behavior are related.

• Resources Recycling
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• v.18 no.1
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• pp.22-29
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• 2009

To design and construct a moving bed stoker incinerator for incineration treatment of the domestic oil fly ash, operating condition and moving bed area of incinerator were determined by performing incinerate experiment of the oil fly ash in the muffle furnace which simulates moving bed stoker incinerator in all conditions. Incineration process of the oil fly ash could be divided into 3 stages, every stage needs the appropriate operating condition for effective incineration. The optimum content of water in the heavy oil fly ash was found to be 20 wt% to prevent the ash from flying and reduce the volume. Science combustion rate of oil fly ash depends on the oxygen content, the incinerator must have a equipment to control the oxygen content in the combustion air. The optimum temperature was $750{\sim}800^{\circ}C$ in order to prevent adhesion to the stocker and evaporation of metal compounds of low melting point. Uniform combustion reaction and acceleration of combustion rate required agitation during the combustion of oil fly ash. The incineration rate was $12.53kg/m^2hr$ and the working area of moving bed incinerator was found to be $60m^2$ to incinerate 18 tons of oil fly ash per day.