• 한국해양환경ㆍ에너지학회지
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• 제9권2호
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• pp.85-91
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• 2006

본 연구는 자연지형에 존재하는 다양한 형태의 크고 작은 거칠기 요소들로 인하여 기본 유동장이 변화하는 양상을 밝히기 위한 연구로서 회류수조에서 PIV 기법을 사용하여 수행되었다. 먼저 제1보에서는 동일한 2차원 사각단면을 가지는 많은 양의 거칠기 요소를 평판위에 규칙적으로 배열한 후 거칠기요소의 높이와 거칠기 사이의 간격이 다른 세 경우에 대하여 유속, 유선 및 와도분포를 계측하여 거칠기 유동의 특성을 살펴보았다. 본 2보에서는 난류유동특 성을 비롯하여 벽법칙을 이용한 해석과 경계층 적분변수 도출을 통하여 거칠기 간격이 유동장 변화에 미치는 영향을 정량적으로 비교하고자 하였다. 실험결과 거칠기 간격이 거칠기 높이의 7배와 14배인 경우 3.5배에 비하여 거칠기로 인한 유동장의 변화가 큼을 재차 확인할 수 있었다.

• 한국수자원학회논문집
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• 제44권5호
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• pp.417-427
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• 2011

침수식생 개수로 흐름은구조적으로상부영역과식생영역으로구분되는데, 상부영역은 일반 개수로의 흐름특성을 보이며 식생영역은 줄기에 의해 난류가 억제되어 정수식생 조건의 흐름과 유사하게 단순한 흐름특성을 보인다. 본 논문은 침수식생 개수로 흐름의 난류 및 유사이동 특성에 관한 실험연구이다. 이를 위해 폭 0.5m, 길이 12m인 개수로 실험장치를 이용하여수리실험을 실시하였다. 다양한 유량에 대하여 수심비 2～3인 흐름을 구현하였고, 나무원형 실린더를 이용하여 식생을 재현하였다. 평형상태에서 부유사 농도 분포를 측정하기 위해서중앙입경이 75 ${\mu}M$인 모래를 한계조건에 도달하기까지 지속적으로 투입하였다. 유속 성분의 계측을 위하여 레이저 도플러 유속계를 사용하였고, 튜브를 이용한 사이포닝 방식으로 시료를 직접 채취하여 부유사 농도를 측정하였다. 측정된 자료를 이용하여 실험조건에 따른 평균흐름과 난류량의 특성을 살펴보았고, Rouse 수에 따른 부유사 농도 분포를 제시하였다.

• 한국전자통신학회논문지
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• 제16권1호
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• pp.157-164
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• 2021

순천만 갯벌에서 3차원 초음파풍속계로 바람과 기온을 측정하고, 적외선 센서를 이용하여 이산화탄소 농도를 측정하였다. 일반적으로 기온이 증가하면 대기 중의 이산화탄소 농도가 증가하고, 기온이 감소하면 이산화탄소도 감소한다. 그러나 일몰 직후에는 광합성이 줄어들기 때문에, 기온이 감소함에도 이산화탄소 농도는 증가하였다. 또한 갯벌이 해수로 덮이게 되는 고조기에는 기온 증가에도 불구하고 대기 난류가 강하게 나타나 이산화탄소 농도가 감소하였다. 이산화탄소의 농도 변화에 미치는 갯벌 생태계의 광합성과 호흡 그리고 대기 난류의 영향에 대한 정량적 평가가 필요하다.

• 한국해양공학회지
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• 제36권5호
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• pp.326-339
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• 2022

Recently, not only Korea but also the world has been suffering from problems related to coastal erosion. The hard defense method has been primarily used as a countermeasure against erosion. However, this method is expensive and has environmental implications. Hence, interest in other alternative methods, such as the eco-friendly vegetation method, is increasing. In this study, we aim to analyze the hydraulic characteristic of submerged rigid vegetation according to the cross-sectional change through a hydraulic experiment and numerical simulation. From the hydraulic experiment, the reflection coefficient, transmission coefficient, and energy dissipation coefficient were analyzed according to the density, width, and multi-row arrangement of the vegetation zone. From numerical simulations, the flow field, vorticity distribution, turbulence distribution, and wave distribution around the vegetation zone were analyzed according to the crest depth, width, density, and multi-row arrangement distance of the vegetation zone. The hydraulic experiment results suggest that the transmission coefficient decreased as the density and width of the vegetation zone increased, and the multi-row arrangement condition did not affect the hydraulic characteristics significantly. Moreover, the numerical simulations showed that as the crest depth decreased, the width and density of vegetation increased along with vorticity and turbulence intensity, resulting in increased wave height attenuation performance. Additionally, there was no significant difference in vorticity, turbulence intensity, and wave height attenuation performance based on the multi-row arrangement distance. Overall, in the case of submerged rigid vegetation, the wave energy attenuation performance increased as the density and width of the vegetation zone increased and crest depth decreased. However, the multi-row arrangement condition did not affect the wave energy attenuation performance significantly.

• Journal of Korea Trade
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• 제26권7호
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• pp.59-76
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• 2022

Purpose - As ahigher-order ability, improvisational capability is often employed to help a firm cope with unexpected or unanticipated issues. In light of the severe global business challenges and turbulent environmental changes, this study aimed to explore whether home-based network ties could shape the influence of improvisational capability on international entrepreneurship, alongside investigating the boundary conditions of competitive turbulence in their moderating effects. Design/methodology - The sample for the international entrepreneurship sector was obtained from the Industry and Information Technology Department of Jiangsu. In September 2021, the questionnaires were sent to the targeted ventures and required the top managers to complete the survey by email or telephone. The final research sample comprised 113 international new ventures. To test the hypotheses, moderated hierarchical regression analysis was conducted. Findings - Our empirical results suggested that (1) unlike some previous literature, a positive effect of improvisational capability on the performance of international new ventures was not found; (2) home country-based networks (both political ties and business ties) are contingent factors that may partially stimulate the value creation of improvisational capability; and (3) in a highly competitive environment, the moderating role of business ties at home may become much stronger, however, the contingent role of political ties at home was not found. Originality/value - This study mainly concentrates on the two important types of home country-based networks, political and business ties at home, that may help international new ventures access strategic resources necessary for supporting the performance implications of improvisational capability. Thus, it extends the existing improvisational theory to encompass international entrepreneurship.

• Wind and Structures
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• 제37권4호
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• pp.303-314
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• 2023

In this study, two steady RANS turbulence models (SST k-ω and Realizable k-ε) and four unsteady turbulence models (URANS SST k-ω and Realizable k-ε, SST-SAS, and SST-IDDES) are evaluated with respect to their capacity to predict crosswind characteristics on high-speed trains (HSTs). All of the numerical simulations are compared with the wind tunnel values and LES results to ensure the accuracy of each turbulence model. Specifically, the surface pressure distributions, time-averaged aerodynamic coefficients, flow fields, and computational cost are studied to determine the suitability of different models. Results suggest that the predictions of the pressure distributions and aerodynamic forces obtained from the steady and transient RANS models are almost the same. In particular, both SAS and IDDES exhibits similar predictions with wind tunnel test and LES, therefore, the SAS model is considered an attractive alternative for IDDES or LES in the crosswind study of trains. In addition, if the computational cost needs to be significantly reduced, the RANS SST k-ω model is shown to provide relatively reasonable results for the surface pressures and aerodynamic forces. As a result, the RANS SST k-ω model might be the most appropriate option for the expensive aerodynamic optimizations of trains using machine learning (ML) techniques because it balances solution accuracy and resource consumption.

• 대한토목학회논문집
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• 제36권4호
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• pp.645-650
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• 2016

본 연구에서는 기존의 수리모형실험에서 나타나는 파랑-흐름 상호작용에 의한 파고감쇠현상을 분석하기 위하여 Navier-Stokes Solver를 이용한 수치시뮬레이션을 수행하였다. 파랑과 흐름이 만날 경우, 유속의 난류성분이 증가하여 난류강도가 커지는 것을 확인하였다. 이것으로부터 파랑이 전파될수록 난류의 영향으로 파랑에너지가 감소하고, 파고가 줄어드는 현상을 이해할 수 있었다. 그리고 흐름의 유속이 증가할수록 난류강도가 증가함으로 파고감쇠효과가 크게 나타났다. 또한 파랑이 동일한 거리를 전파할 때에 파고가 작을수록, 주기가 짧을수록 파고감쇠현상이 심화되는 것을 확인할 수 있었다.

• Wind and Structures
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• 제30권1호
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• pp.99-117
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• 2020

This paper focuses on the processes of wind flow in atmospheric boundary layer, to produce realistic full scale pressures for design of low-rise buildings. CFD with LES turbulence closure is implemented on a scale 1:1 prototype building. A proximity study was executed computationally in CFD with LES that suggests new recommendations on the computational domain size, in front of a building model, apart from common RANS-based guidelines (e.g., COST and AIJ). Our findings suggest a location of the test building, different from existing guidelines, and the inflow boundary proximity influences pressure correlation and reproduction of peak loads. The CFD LES results are compared to corresponding pressures from open jet, full scale, wind tunnel, and the ASCE 7-10 standard for roof Component & Cladding design. The CFD LES shows its adequacy to produce peak pressures/loads on buildings, in agreement with field pressures, due to its capabilities of reproducing the spectral contents of the inflow at 1:1 scale.

• 한국대기환경학회지
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• 제18권5호
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• pp.401-410
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• 2002

Concerns for energy conservation, environmental pollution, and the fact that organic wastes account for a major portion of our waste materials, have created the interest of biogas, which usually contains about 60 to 70 percent methane, 30 to 40 percent carbon dioxide, and other gases, including ammonia, hydrogen sulfide, mercaptans and other noxious gases. Cyclone combustors are used for homing a wide range of fuels such as low calorific value gas, waste water, sludge. coal, etc. The 3-dimensional swirling flow, combustion and emission in a tangential inlet cyclone incinerator under different inlet conditions are simulated using a standard k-s turbulence model and ESCRS (Extended Simple Chemically-Reacting System) model. The commercial code Phoenics Ver.3.4 was used for the present work. The main parameters considered in this work are inlet velocity and air to fuel ratio. The results showed that the change of operating conditions had an influence on the shape and size of recirculation zones, mixture fraction and axial velocity which are important factors for combustion efficiency and emission behavior. The application of this kind of computer program seams to be promising as a potential tool for the optimum design of a cyclone combustor with low emission.

• Wind and Structures
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• 제23권4호
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• pp.301-311
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• 2016

The numerous benefits of Savonius turbine such as simple in structure, has appropriate self-start ability, relatively low operating velocity, water acceptance from any direction and low environmental impact have generated interests among researchers. However, it suffers from a lower efficiency compared to other types of water turbine. To improve its performance, parameters such flow pattern, pressure and velocity in different conditions must be analyzed. For this purpose, a detailed description on the flow field of various types of Savonius rotors is required. This article presents a numerical study on a nonlinear two-dimensional flow over a classic Savonius type rotor and a Benesh type rotor. In this experiment, sliding mesh was used for solving the motion of the bucket. The unsteady Reynolds averaged Navier-Stokes equations were solved for velocity and pressure coupling by using the SIMPLE (Semi-Implicit Method for Pressure linked Equations) algorithm. Other than that, the turbulence model using $k-{\varepsilon}$ standard obtained good results. This simulation demonstrated the method of the flow field characteristics, the behavior of velocity vectors and pressure distribution contours in and around the areas of the bucket.