• 대기
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• 제27권4호
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• pp.445-453
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• 2017

This study investigates turbulent flow around a square cylinder mounted on a flat surface at high Reynolds number using a large-eddy simulation (LES) model, particularly focusing on vortex streets behind the square cylinder. Total 9 simulation cases with different inflow wind directions, inflow wind speeds, and cylinder widths in the x- and y-directions are considered to examine the effects of inflow wind direction, speed, and cylinder widths on turbulent flow and vortex streets. In the control case, the inflow wind parallel to the x-direction has a maximum speed of $5m\;s^{-1}$ and the width and height of the cylinder are 50 m and 200 m, respectively. In all cases, down-drafts in front of the cylinder and updrafts, wakes, and vortex streets behind the cylinder appear. Low-speed flow below the cylinder height and high-speed flow above it are mixed behind the cylinder, resulting in strong negative vertical turbulent momentum flux at the boundary. Accordingly, the magnitude of the vertical turbulent momentum flux is the largest near the cylinder top. In the case of an inflow wind direction of $45^{\circ}$, the height of the boundary is lower than in other cases. As the inflow wind speed increases, the magnitude of the peak in the vertical profile of mean turbulent momentum flux increases due to the increase in speed difference between the low-speed and high-speed flows. As the cylinder width in the y-direction increases, the height of the boundary increases due to the enhanced updrafts near the top of the cylinder. In addition, the magnitude of the peak of the mean turbulent momentum flux increases because the low-speed flow region expands. Spectral analysis shows that the non-dimensional vortex generation frequency in the control case is 0.2 and that the cylinder width in the y-direction and the inflow wind direction affect the non-dimensional vortex generation frequency. The non-dimensional vortex generation frequency increases as the projected width of the cylinder normal to the inflow direction increases.

• Wind and Structures
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• 제22권2호
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• pp.211-234
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• 2016

Super long-span bridges provide people with great convenience, but they also bring traffic safety problems caused by strong wind owing to their high decks. In this paper, the large eddy simulation together with dynamic mesh technology in computational fluid dynamics (CFD) is used to explore the mechanism of a moving vehicle's transient aerodynamic force in crosswind, the regularity and mechanism of the vehicle's aerodynamic forces when it passes through a bridge tower's wake zone in crosswind. By comparing the calculated results and those from wind tunnel tests, the reliability of the methods used in the paper is verified on a moving vehicle's aerodynamic forces in a bridge tower's wake region. A vehicle's aerodynamic force coefficient decreases sharply when it enters into the wake region, and reaches its minimum on the leeward of the bridge tower where exists a backflow region. When a vehicle moves on the outermost lane on the windward direction and just passes through the backflow region, it will suffer from negative lateral aerodynamic force and yaw moment in the bridge tower's wake zone. And the vehicle's passing ruins the original vortex structure there, resulting in that the lateral wind on the right side of the bridge tower does not change its direction but directly impact on the vehicle's windward. So when the vehicle leaves from the backflow region, it will suffer stronger aerodynamic than that borne by the vehicle when it just enters into the region. Other cases of vehicle moving on different lane and different directions were also discussed thoroughly. The results show that the vehicle's pneumatic safety performance is evidently better than that of a vehicle on the outermost lane on the windward.

• Wind and Structures
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• 제38권1호
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• pp.59-74
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• 2024

Large Eddy Simulation (LES) is used to explore the influence of vibration frequency and amplitude on the aerodynamic performance of a rectangular cylinder with an aspect ratio of B/D=5 (B: breadth; D: depth of cylinder) at a Reynolds number of 22,000 near resonance frequency. In smooth flow conditions, the research employs a sequence of three-dimensional simulations under forced vibration with diverse frequency ratios f_e / f_o = 0.8-1.2 (f_e : oscillation frequency; f_o : Strouhal frequency when the rectangular cylinder is stationary ) and oscillation amplitudes A_h/D = 0.05 - 0.3. The individual influences of f_e / f_o and A_h/D on the characteristics of integrated and distributed aerodynamic forces are the focal points of discussion. For the integrated aerodynamic force, particular emphasis is placed on the analysis of the dependence of velocity-proportional component C₁ and displacement-proportional component C₂ of unsteady aerodynamic force on amplitude and frequency ratio. Near the resonance frequency, the dependencies of C₁ and C₂ on amplitude are stronger than that of frequency ratio. For the distributed aerodynamic force, the increase in frequency and amplitude promotes the position of the main vortex core and reattachment to the leading edge in the streamwise direction. In the spanwise direction, vibration enhances the spanwise correlation of aerodynamic force to weaken the three-dimensional effect of the flow field, and a lower frequency ratio and larger amplitude amplify this effect.

• 한국철도학회:학술대회논문집
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• 한국철도학회 2008년도 추계학술대회 논문집
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• pp.66-72
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• 2008

본 연구에서는 대심도 지하역사에서 화재가 발생할 시 급/배기 팬의 동작 유무에 따른 승강장에서의 열 및 연기의 거시적인 거동을 화재시뮬레이션을 통하여 분석하였다. 시뮬레이션 분석결과를 토대로 현재 설치된 급/배기 팬에 대한 제연/배연능력에 대하여 고찰하였다. 본 연구의 대상은 숭실대 입구 역사(7호선, 도시철도공사운영)이며, 숭실대 역사의 승강장은 길이 165m, 폭 23.5m, 깊이 47m 이다. 본 연구에서 전산수치해석을 위한 모델은 선로부 지하터널를 감안하여 전후 각 100m를 추가하였다. 따라서 모델링의 크기는 길이 365m, 폭23.5m, 깊이 47m 이다. 격자는 육면체 정렬격자계를 사용하였으며, 격자의 수는 대략 10,000,000 개가 사용되었다. 빠른 수치전산처리를 위하여, 병렬처리기법을 적용하였다. 본 전산수치해석에 사용된 CPU자원에 Intel 3.0GHZ Dual CPU 6개(core 12개)가 사용되었다.

• 대한토목학회논문집
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• 제28권4B호
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• pp.429-439
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• 2008

본 연구에서는 잠제의 제원(천단수심, 천단폭, 사면경사, 제장)에 따라 해빈상을 전파하는 풍파의 처오름 높이 변화특성을 논의하기 위하여, 기존의 수리모형 실험치와 계산치를 비교 검토를 통해 타당성과 유효성이 검증된 수치모델로서, 파 투과성구조물 해빈의 상호간섭을 직접 해석할 수 있는 3D-수치모델(LES-WASS-3D; 허와 이, 2007)을 이용하여 잠제 2기의 제원에 따른 수치시뮬레이션을 실시하였다. 그 결과 천단수심, 천단폭, 사면경사, 제장의 변화가 연안에서의 처오름 높이에 미치는 영향에 관하여 검토하였으며, 아울러, 잠제 주변의 파고분포 및 상층흐름특성과의 관계에 대해서도 논의하였다.

• Wind and Structures
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• 제8권6호
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• pp.427-442
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• 2005

Numerical and wind tunnel simulations of pollutant dispersion around rectangular obstacles with five aspect ratios have been conducted in order to identify the effects of flow patterns induced by buildings on plume dispersion in the near wake of buildings. An emission from a low source located upwind of obstacles was used in this simulation. The local flow patterns and concentrations around a cubical obstacle were initially investigated using three RANS turbulence models, (the standard $k-{\varepsilon}$, Shear Stress Transport (SST), Reynolds-Stress RSM turbulence model) and also using Large-eddy simulation (LES). The computed concentrations were compared with those measured in the wind tunnel. Among the three turbulence models, the SST model offered the best performance and thus was used in further investigations. The results show, for normal aspect ratios of width to height, that concentrations in the near wake are appreciably affected because of plume capture by the horseshoe vortex and convection by the vertical vortex pairs. These effects are less important for high aspect ratios. Vertical vortex pairs present a strong ability to exchange mass vertically and acts efficiently to reduce ground-level concentrations in the near wake.

• 한국추진공학회지
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• 제21권5호
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• pp.1-9
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• 2017

초임계 환경에서 와류형 분사기의 극저온 질소 분무 동적 특성 분석을 위하여 3차원 LES 난류 모델을 적용하였다. 초임계 상태에서 질소의 상태량들을 정확하게 예측하기 위해 SRK 실기체 상태방정식을 사용하였고, 점성계수와 열전도도는 Chung의 고압 상태 혼합물에 대한 방정식, 그리고 확산 계수는 Fuller의 이론에 Takahashi의 보정식을 적용하였다. 질소 분무 결과, 분사기 내, 외부에서 유동장과 음향장 사이의 상호작용으로 복잡한 유동구조가 형성된다. 복잡한 유동 현상을 분석하기 위해 FFT, POD 그리고 DMD 기법을 적용하여 해석을 수행하였다. FFT 해석을 수행하여 분사기 내, 외부에서 나타나는 특정 주파수를 파악하였으며, POD와 DMD를 통해 각 주파수가 어떠한 유동 구조를 갖는지에 대한 연구를 수행하였다. 또한, DMD를 통해 각 주파수의 감쇠 계수를 파악하여 이를 실험 결과와 비교하였다.

• 대한기계학회논문집B
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• 제34권10호
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• pp.925-931
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• 2010

본 논문은 공동 주위 난류유동특성을 LES 기법으로 수치해석을 수행하여 알아보았다. 본 연구에 적용된 레이놀즈수는 공동 깊이만큼의 높이 h 에서의 유속을 기준으로 $1.0{\times}10^5$ 이며 3 차원 공동에서의 유동특성을 알아보았다. 적절한 비압축성 Filtered Navier-Stokes 방정식을 적용하기 위해, 계산격자를 공동 표면 근처에는 조밀하게 멀어질수록 성기게 생성하였으며, 이는 계산시간을 단축시키며 빠른 수렴을 도와준다. 또한, Boussinesq 가설을 subgrid-scale 난류모델에 적용하였고, Subgrid-scale 난류점성을 얻기 위해 smagorinsky-Lilly SGS 모델을 적용하였으며, 그 때의 CFL 수는 1.0 이다. 또한, 본 논문은 서로 다른 4 가지 형상의 공동의 및 입구조건의 변화에 따른 유동 특성도 함께 연구되었다.

• 항공우주시스템공학회지
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• 제13권1호
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• pp.1-10
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• 2019

교반기에서의 유동 특성은 광범위한 산업 분야에서 매우 유용하다. 일반적으로 교반되는 용기에서의 유동 패턴, 전력 소비 및 혼합 시간은 임펠러의 설계뿐만 아니라 용기 형상 및 내부 구조에 달려 있다. 본 연구에서는 베플 형상과 임펠러의 상호 작용에 의해 생성되는 불안정하고 비정상상태의 복잡한 유동 특성 분석을 ANSYS FLUENT LES 난류 모델을 사용하여 수행하였다. Axial Flow 와 Radial Flow 두 가지 타입의 회전 임펠러와 3가지 베플의 형상 사이의 상호 작용과 영향을 전산유체역학(CFD)으로 예측 비교함으로써 교반 시 임펠러와 베플 주변에서의 유동 특성과 혼합 유동장에서 상대적으로 효율적인 경향을 보이는 설계 모델을 검증할 수 있었다.

• Wind and Structures
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• 제23권6호
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• pp.559-575
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• 2016

In order to investigate the influence of different blade positions on aerodynamic load and wind loads and load-effects of large scale wind turbine tower under the halt state, we take a certain 3 MW large scale horizontal axis three-blade wind turbine as the example for analysis. First of all, numerical simulation was conducted for wind turbine flow field and aerodynamic characteristics under different halt states (8 calculating conditions in total) based on LES (large eddy simulation) method. The influence of different halt states on the average and fluctuating wind pressure coefficients of turbine tower surface, total lift force and resistance coefficient, circular flow and wake flow characteristics was compared and analysed. Then on this basis, the time-domain analysis of wind loads and load-effects was performed for the wind turbine tower structure under different halt states by making use of the finite element method. The main conclusions of this paper are as follows: The halt positions of wind blade could have a big impact on tower circular flow and aerodynamic distribution, in which Condition 5 is the most unfavourable while Condition 1 is the most beneficial condition. The wind loads and load-effects of disturbed region of tower is obviously affected by different halt positions of wind blades, especially the large fluctuating displacement mean square deviation at both windward and leeward sides, among which the maximum response occurs in $350^{\circ}$ to the tower top under Condition 8; the maximum bending moment of tower bottom occurs in $330^{\circ}$ under Condition 2. The extreme displacement of blade top all exceeds 2.5 m under Condition 5, and the maximum value of windward displacement response for the tip of Blade 3 under Condition 8 could reach 3.35 m. All these results indicate that the influence of halt positions of different blades should be taken into consideration carefully when making wind-resistance design for large scale wind turbine tower.