• Title/Summary/Keyword: boundary condition

### Free surface effects on 2-D airfoils and 3-D wings moving over water

• Bal, Sakir
• Ocean Systems Engineering
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• v.6 no.3
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• pp.245-264
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• 2016
• The iterative boundary element method (IBEM) developed originally before for cavitating two-dimensional (2-D) and three-dimensional (3-D) hydrofoils moving under free surface is modified and applied to the case of 2-D (two-dimensional) airfoils and 3-D (three-dimensional) wings over water. The calculation of the steady-state flow characteristics of an inviscid, incompressible fluid past 2-D airfoils and 3-D wings above free water surface is of practical importance for air-assisted marine vehicles such as some racing boats including catamarans with hydrofoils and WIG (Wing-In-Ground) effect crafts. In the present paper, the effects of free surface both on 2-D airfoils and 3-D wings moving steadily over free water surface are investigated in detail. The iterative numerical method (IBEM) based on the Green's theorem allows separating the airfoil or wing problems and the free surface problem. Both the 2-D airfoil surface (or 3-D wing surface) and the free surface are modeled with constant strength dipole and constant strength source panels. While the kinematic boundary condition is applied on the airfoil surface or on the wing surface, the linearized kinematic-dynamic combined condition is applied on the free surface. The source strengths on the free surface are expressed in terms of perturbation potential by applying the linearized free surface conditions. No radiation condition is enforced for downstream boundary in 2-D airfoil and 3-D wing cases and transverse boundaries in only 3-D wing case. The method is first applied to 2-D NACA0004 airfoil with angle of attack of four degrees to validate the method. The effects of height of 2-D airfoil from free surface and Froude number on lift and drag coefficients are investigated. The method is also applied to NACA0015 airfoil for another validation with experiments in case of ground effect. The lift coefficient with different clearance values are compared with those of experiments. The numerical method is then applied to NACA0012 airfoil with the angle of attack of five degrees and the effects of Froude number and clearance on the lift and drag coefficients are discussed. The method is lastly applied to a rectangular 3-D wing and the effects of Froude number on wing performance have been investigated. The numerical results for wing moving under free surface have also been compared with those of the same wing moving above free surface. It has been found that the free surface can affect the wing performance significantly.

### Construction of Surface Boundary Conditions for the Regional Climate Model in Asia Used for the Prevention of Disasters Caused by Climate Changes (기상방재 대책수립을 위한 아시아지역 기상모형에 필요한 지표경계조건의 구축)

• Choi, Hyun-Il
• Journal of the Korean Society of Hazard Mitigation
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• v.7 no.5
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• pp.73-78
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• 2007
• It has been increasing that significant loss of life and property due to global wanning and extreme weather, and the climate and temperature changes in Korea Peninsula are now greater than the global averages. Climate information from regional climate models(RCM) at a finer resolution than that of global climate models(GCM) is required to predictclimate and weather variability, changes, and impacts. The new surface boundary conditions(SBCs) development is motivated by the limitations and inconsistencies of existing SBCs that have influence on model predictability. A critical prerequisite in constructing SBCs is that the raw data should be accurate with physical consistency across all relevant parameters and must be appropriately filled for missing data if any. The aim of this study is to construct appropriate SBCs for the RCM in Asia domain which will be used for the prevention of disasters due to climate changes. As all SBCs have constructed onto the 30km grid-mesh of the RCM suitable for Asia applications, they can be also used for other distributed models for climate and hydrologic studies.

### A Study on the Viscosity Characteristics of Dewatered Sewage Sludge according to Thermal Hydrolysis Reaction (열가용화 반응에 의하여 탈수된 하수슬러지의 점도 특성에 관한 연구)

• Song, Hyoung Woon;Han, Seong Kuk;Kim, Choong Gon;Shin, Hyun Gon
• Journal of the Korea Organic Resources Recycling Association
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• v.22 no.1
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• pp.27-34
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• 2014
• demand for a low-cost treatment technology is high because the sewage sludge has an 80% or higher water content and a high energy consumption cost. This study apply the thermal hydrolysis reaction that consumes a small amount of energy for sludge treatment. The purpose of this study is to quantify the viscosity of sewage sludge according to reaction temperature. we measured continuously the torque of dewatered sludge by the reaction temperature. As the reaction temperature increased, the dewatered sludge is thermal hydrolysis under a high temperature and pressure. Therefore, the bond water in the sludge cells comes out as free water, which changes the dewatered sludge from a solid phase to slurry of a liquid phase. The results of the viscosity measurements according to the reaction temperature showed that the viscosity was very high at $270,180kg/m{\cdot}sec$ at a temperature of 293K, but rapidly decreased with increases in the reaction temperature to $12kg/m{\cdot}sec$ at a temperature of 400K and to $4kg/m{\cdot}sec$ at a temperature of 460K or higher, similar to the changes in the viscosity of water. And we was obtained the viscosity function of boundary condition for the optimal design of thermal hydrolysis reactor by numerical modeling based on the this results.

### A Study on the Natural Frequency of Wind Turbine Tower Regarding to Modeling Method (풍력터빈타워의 모델링에 따른 고유진동수 특성에 관한 연구)

• Lee, Yun-Woo;Jang, Min-Seo;Kang, Soung-Yong;Kim, Pyoung-Hwa;Kang, Young-Jong
• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.3
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• pp.2272-2278
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• 2015
• Recently the importance of renewable energy is stood out regarding to the international concern about global environmental issues. Wind power is beginning to receive attention as one of renewable energy, and world-wide researches about wind power are being carried out. In the wind power system, tower structure plays an important roles for continuous and stable generation of electricity. Researchers use various analytical models to research and develop about tower structures. In this study, the effects of natural frequencies of various wind turbine tower models have been analyzed. It is possible to simplify the detailed parts of models by using modified tower unit weight since the results of 1st natural frequency show that the difference is only 0.14%. Since the difference in natural frequency is greater than 10%, according to the boundary condition, the simple fix end support is not appropriate to represent the real structure of the tower. It is expected that the result of this study may be utilized to establish the criterion about appropriate modelling method.

### Development of Impact Factor Response Spectrum based on Frequency Response of Both Ends-Fixed Beam for Application to Continuous Bridges (연속교 적용을 위한 양단고정지지 보의 진동수 기반 충격계수 응답스펙트럼 개발)

• Roh, Hwasung;Lee, Huseok;Park, Kyung-Hoon
• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.12
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• pp.301-306
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• 2016
• In bridge performance assessments, a new load carrying capacity evaluation model of simple bridges was proposed, which is based on the developed simple support impact factor spectrum. In this paper, a conservative assumption that the inner span with the both ends fixed boundary condition is ideal for applying the impact factor response spectrum for continuous bridges. The impact factor response spectrum has been proposed based on this assumption. The response spectrum by comparing the numerical analysis result and actual measurement data verified the applicability. The analysis was loading the moving load of DB-24 in a six-span continuous bridge, which was the same as the actual measurement data, the dynamic response was measured in the fourth span. The frequency of the bridge was obtained by FFT on the acceleration response and the span-frequency of sample bridge was calculated by the frequency. The impact factor of the sample bridge was determined by applying the span-frequency of the bridge to the proposed response spectrum; it was similar to the result of comparing the actual measured impact factor. Therefore, the method using the impact factor response spectrum based on the frequency response of both ends-fixed beam was found to be applicable to an actual continuous bridge.

### A 8b 1GS/s Fractional Folding-Interpolation ADC with a Novel Digital Encoding Technique (새로운 디지털 인코딩 기법을 적용한 8비트 1GS/s 프랙셔널 폴딩-인터폴레이션 ADC)

• Choi, Donggwi;Kim, Daeyun;Song, Minkyu
• Journal of the Institute of Electronics and Information Engineers
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• v.50 no.1
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• pp.137-147
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• 2013
• In this paper, an 1.2V 8b 1GS/s A/D Converter(ADC) based on a folding architecture with a resistive interpolation technique is described. In order to overcome the asymmetrical boundary-condition error of conventional folding ADCs, a novel scheme with an odd number of folding blocks and a fractional folding rate are proposed. Further, a new digital encoding technique with an arithmetic adder is described to implement the proposed fractional folding technique. The proposed ADC employs an iterating offset self-calibration technique and a digital error correction circuit to minimize device mismatch and external noise The chip has been fabricated with a 1.2V 0.13um 1-poly 6-metal CMOS technology. The effective chip area is $2.1mm^2$ (ADC core : $1.4mm^2$, calibration engine : $0.7mm^2$) and the power dissipation is about 350mW including calibration engine at 1.2V power supply. The measured result of SNDR is 46.22dB, when Fin = 10MHz at Fs = 1GHz. Both the INL and DNL are within 1LSB with the self-calibration circuit.

### Numerical Simulation of Dynamic Soil-pile Interaction for Dry Condition Observed in Centrifuge Test (원심모형실험에서 관측된 건조 지반-말뚝 동적 상호작용의 수치 모델링)

• Kown, Sun-Yong;Kim, Seok-Jung;Yoo, Min-Taek
• Journal of the Korean Geotechnical Society
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• v.32 no.4
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• pp.5-14
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• 2016
• Numerical simulation of dynamic soil-pile-structure interaction embedded in a dry sand was carried out. 3D model of the dynamic centrifuge model tests was formulated in a time domain to consider nonlinear behavior of soil using the finite difference method program, FLAC3D. As a modeling methodology, Mohr-Coulomb criteria was adopted as soil constitutive model. Soil nonlinearity was considered by adopting the hysteretic damping model, and an interface model which can simulate separation and slip between soil and pile was adopted. Simplified continuum modeling (Kim et al., 2012) was used as boundary condition to reduce analysis time. Calibration process for numerical modeling results and test results was performed through the parametric study. Verification process was then performed by comparing numerical modeling results with another test results. Based on the calibration and validation procedure, it is identified that proposed modeling method can properly simulate dynamic behavior of soil-pile system in dry condition.

### Assessment of Surface Boundary Conditions for Predicting Ground Temperature Distribution (지중온도 변화 예측을 위한 지표면 경계조건 검토)

• Jang, Changkyu;Choi, Changho;Lee, Chulho;Lee, Jangguen
• Journal of the Korean Geotechnical Society
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• v.29 no.8
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• pp.75-84
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• 2013
• Soil freezing is a phenomenon arising due to temperature difference between atmosphere and ground, and physical properties of soils vary upon the phase change of soil void from liquid to solid (ice). A heat-transfer mechanism for this case can be explained by the conduction in soil layers and the convection on ground surface. Accordingly, the evaluation of proper thermal properties of soils and the convective condition of ground surface is an important task for understanding freezing phenomenon. To describe convection on ground surface, simplified coefficient methods can be applied to deal with various conditions, such as atmospheric temperature, surface vegetation conditions, and soil constituents. In this study, two methods such as n-factor and convection coefficient for the convective ground surface boundary were applied within a commercial numerical program (TEMP/W) for modeling soil freezing phenomenon. Furthermore, the numerical results were compared to laboratory testing results. In the series of the comparison results, the convection coefficient is more appropriate than n-factor method to model the convective boundary condition.

### Impact of Boundary Conditions and Cumulus Parameterization Schemes on Regional Climate Simulation over South-Korea in the CORDEX-East Asia Domain Using the RegCM4 Model (CORDEX 동아시아 영역에서 경계조건 및 적운모수화방안이 RegCM4를 이용한 남한 지역 기후모의에 미치는 영향 분석)

• Oh, Seok-Geun;Suh, Myoung-Seok;Myoung, Ji-Su;Cha, Dong-Hyun
• Journal of the Korean earth science society
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• v.32 no.4
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• pp.373-387
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• 2011
• In this study, four types of sensitivity experiments (EG, EE, NG, NE; E: ERA-Interim, N: NCEP/DOE2, G: Grell scheme, E: Emanuel scheme) were performed to evaluate the simulation skills of RegCM4 released in July 2010 over the CORDEX (COordinated Regional Downscaling EXperiment) East Asia domain based on the combinations of boundary conditions (BC: ERA-Interim, NCEP/DOE2) and the cumulus parameterization schemes (CPS: Grell, Emanuel) for the 1989. The surface air temperature and precipitation data observed by the Korea Meteorological Adminstration were used to validate the simulation results over South Korea. The RegCM4 well simulates the seasonal and spatial variations of temperature but it fails to capture the seasonal and spatial variations of precipitation without consideration of the BC and CPS. Especially the simulated summer precipitation amount is significantly less in EG, NG, and NE experiments. But the seasonal variation of precipitation including summer precipitation is relatively well simulated in the EE experiment. The EE experiment shows a better skill in the seasonal march of East Asia summer monsoon, distribution of precipitation intensity and frequency than other experiments. In general, the skills of RegCM4 for temperature and precipitation are better during winter than summer, and in Emanuel than Grell schemes. The simulation results are more impacted by cumulus parameterization schemes than boundary conditions.

### Analysis Method for Speeding Risk Exposure using Mobility Trajectory Big Data (대용량 모빌리티 궤적 자료를 이용한 과속 위험노출도 분석 방법론)

• Lee, Soongbong;Chang, Hyunho;Kang, Taeseok
• Journal of the Society of Disaster Information
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• v.17 no.3
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• pp.655-666
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• 2021
• Purpose: This study is to develop a method for measuring dynamic speeding risks using vehicle trajectory big data and to demonstrate the feasibility of the devised speeding index. Method: The speed behaviors of vehicles were analysed in microscopic space and time using individual vehicle trajectories, and then the boundary condition of speeding (i.e., boundary speed) was determined from the standpoint of crash risk. A novel index for measuring the risk exposure of speeding was developed in microscopic space and time with the boundary speed. Result: A validation study was conducted with vehicle-GPS trajectory big data and ground-truth vehicle crash data. As a result of the analysis, it turned out that the index of speeding-risk exposure has a strong explanatory power (R2=0.7) for motorway traffic accidents. This directly indicates that speeding behaviors should be analysed at a microscopic spatiotemporal dimension. Conclusion: The spatial and temporal evolution of vehicle velocity is very variable. It is, hence, expected that the method presented in this study could be efficaciously employed to analyse the causal factors of traffic accidents and the crash risk exposure in microscopic space using mobility trajectory data.