• 대기
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• 제17권4호
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• pp.315-326
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• 2007

The effects of windbreaks on the reduction of suspended particles are investigated using a computational fluid dynamics (CFD) model with the ${\kappa}-{\varepsilon}$ turbulence closure scheme based on the renormalization group (RNG) theory. In the control experiment, the recirculation zones behind the storage piles are generated and, as a whole, relatively monotonous flow patterns appear. When the windbreaks with the 0% porosity are constructed, the recirculation zones are generated by the windbreaks and very complicated flow patterns appear due to the interference between the windbreaks and storage piles. The porosity of the windbreaks suppresses the generation of the recirculation zone and decreases the wind velocity in the windbreaks as well as that outside the windbreaks. As the emission of suspended particles from the storage piles are closely related with the friction velocity at the surfaces of the storage piles, variation of the friction velocity and total amount of the emission of the suspended particles with the height and porosity of the windbreaks are investigated. The results show that higher and more porous windbreaks emit less suspended particles and that the reduction effect of the porosity is still more effective than that of the height. In the case of the windbreak with 30 m height and 50% porosity, friction velocities above the storage piles are smaller than the critical friction velocity above which particles would be suspended. As a result, total amount of suspended particles are much fewer than those in other cases.

• 한국대기환경학회지
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• 제22권1호
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• pp.73-84
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• 2006

Hazardous Air Pollutants (HAPs) are characterized by being relatively heavier and denser than that of ambient air due to the various reasons such as higher molecular weight, low temperature and other complicated chemical transformations (Witlox, 1994). In an effort to investigate transport and diffusion from instantaneous emission of heavy gas, Lagrangian Particle Dispersion Model (LPDM) coupled with the RAMS output was employed. Both deposition process and buoyancy term were added on the atmospheric diffusion equations of LPDM, and the locations and concentrations of dense gas particle released from instantaneous single point source (emitting initially for 10 minutes only) were analyzed. The result overall shows that adding deposition process and buoyancy terms on the diffusion equation of LPDM has very small but detectable effect on the vertical and horizontal distribution of Lagrangian particles that especially transported for a fairly long traveling time. Also the slumping of dense gas can be found to be ignored horizontally compared to the advection by the horizontal wind suggesting that it was essential to couple the Lagrangian particle dispersion model coupled with the RAMS model in order to explain the dispersion of HAPs more accurately. However, during the initial time of instantaneous emission, buoyancy term play an important role on the vertical locations of dense particles for near surface atmosphere and around source area, indicating the importance of densities of HAPs in the beginning stage or short duration for the risk assessment of HAPs or management of heavy vapors during the explosive accidents.

• Wind and Structures
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• 제26권5호
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• pp.267-277
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• 2018

Numerical simulations are performed of a long flexible cylinder undergoing vortex-induced vibration at a Reynolds number of 500. The cylinder is pinned at both ends, having an aspect ratio of 100 (cylinder length to cylinder diameter) and a mass ratio of 4.2 (structural mass to displaced fluid mass). Temporal and spatial information on the cross-flow (CF) and in-line (IL) vibrations is extracted. High modal vibrations up to the $6^{th}$ in the CF direction and the $11^{th}$ in the IL direction are observed. Both the CF and IL vibrations feature a multi-mode mixed pattern. Mode competition is observed. The $2^{nd}$ mode with a low frequency dominates the IL vibration and its existence is attributed to a wave group propagating back and forth along the span. Distributions of fluid force coefficients are correlated to those of the CF and IL vibrations along the span. Histograms of the x'-y motion phase difference are evaluated from the total simulation time and a complete vibration cycle representing the standing or travelling wave pattern. Correlations between the phase difference and the vibrations are discussed. Vortex structures behind the cylinder show an interwoven near-wake pattern when the standing wave pattern dominates, but an oblique near-wake pattern when the travelling wave pattern prevails.

• Wind and Structures
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• 제26권5호
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• pp.293-304
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• 2018

A numerical study is conducted on the flow characteristics of a rectangular cylinder (chord-to-width ratio C/W = 2 - 10) mounted close to a rigid wall at gap-to-width ratios G/W = 0.25 - 6.25. The effects of G/W and C/W on the Strouhal number, vortex structure, and time-mean drag and lift forces are examined. The results reveal that both G/W and C/W have strong influences on vortex structure, which significantly affects the forces on the cylinder. An increase in G/W leads to four different flow regimes, namely no vortex street flow (G/W < 0.75), single-row vortex street flow ($0.75{\leq}G/W{\leq}1.25$), inverted two-row vortex street flow ($1.25<G/W{\leq}2.5$), and two-row vortex street flow (G/W > 2.5). Both Strouhal number and time-mean drag are more sensitive to C/W than to G/W. For a given G/W, Strouhal number grows with C/W while time-mean drag decays with C/W, the growth and decay being large between C/W = 2 and 4. The time-mean drag is largest in the single-row vortex street regime, contributed by a large pressure on the front surface, regardless of C/W. A higher C/W, in general, leads to a higher time-mean lift. The maximum time-mean lift occurs for C/W = 10 at G/W = 0.75, while the minimum time-mean lift appears for C/W = 2 at the same G/W. The impact of C/W on the time-mean lift is more substantial in single-row vortex regime. The effect of G/W on the time-mean lift is larger at a larger C/W.

• Wind and Structures
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• 제30권5호
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• pp.465-474
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• 2020

Vortex-induced vibrations of a yawed flexible cylinder near a plane boundary are numerically investigated at a Reynolds number Re_n= 500 based on normal component of freestream velocity. Free to oscillate in the in-line and cross-flow directions, the cylinder with an aspect ratio of 25 is pinned-pinned at both ends at a fixed wall-cylinder gap ratio G/D = 0.8, where D is the cylinder diameter. The cylinder yaw angle (α) is varied from 0° to 60° with an increment of 15°. The main focus is given on the influence of α on structural vibrations, flow patterns, hydrodynamic forces, and IP (Independence Principle) validity. The vortex shedding pattern, contingent on α, is parallel at α=0°, negatively-yawed at α ≤ 15° and positively-yawed at α ≥ 30°. In the negatively- and positively-yawed vortex shedding patterns, the inclination direction of the spanwise vortex rows is in the opposite and same directions of α, respectively. Both in-line and cross-flow vibration amplitudes are symmetric to the midspan, regardless of α. The RMS lift coefficient C_L,rms exhibits asymmetry along the span when α ≠ 0°, maximum C_L,rms occurring on the lower and upper halves of the cylinder for negatively- and positively-yawed vortex shedding patterns, respectively. The IP is well followed in predicting the vibration amplitudes and drag forces for α ≤ 45° while invalid in predicting lift forces for α ≥ 30°. The vortex-shedding frequency and the vibration frequency are well predicted for α = 0° - 60° examined.

• Wind and Structures
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• 제30권5호
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• pp.533-546
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• 2020

Large Eddy Simulation (LES) is used to study the effects of steady slot suction on the aerodynamic forces of and flow around a wall-mounted finite-length square cylinder. The aspect ratio H/d of the tested cylinder is 5, where H and d are the cylinder height and width, respectively. The Reynolds number based on free-stream oncoming flow velocity U_∞ and d is 2.78×10⁴. The suction slot locates near the leading edge of the free end, with a width of 0.025d and a length of 0.9d. The suction coefficient Q (= U_s/U_∞) is varied as Q = 0, 1 and 3, where U_s is the velocity at the entrance of the suction slot. It is found that the free-end steady slot suction can effectively suppress the aerodynamic forces of the model. The maximum reduction of aerodynamic forces occurs at Q = 1, with the time-mean drag, fluctuating drag, and fluctuating lift reduced by 3.75%, 19.08%, 40.91%, respectively. For Q = 3, all aerodynamic forces are still smaller than those for Q = 0 (uncontrolled case), but obviously higher than those for Q = 1. The involved control mechanism is successfully revealed, based on the comparison of the flow around cylinder free end and the near wake for the three tested Q values.

• 한국농림기상학회지
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• 제2권3호
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• pp.80-86
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• 2000

에디 공분산 방법은 생태계와 대기간의 질량과 에너지 교환을 측정하는데 널리 사용되고 있다. 이 방법은 다른 미기상학적 방법과는 달리 많은 가정을 필요로 하지 않는 직접 측정으로서, 스칼라의 농도 변화를 측정하기 위해 고속 반응의 개회로 또는 폐회로 기기를 필요로 한다. 후자를 사용할 경우, 흡입된 공기가 관을 통과하면서 스칼라의 농도 변동의 감쇠가 일어난다. 이러한 관 감쇠 효과는 측정하고자 하는 난류 플럭스를 과소 평가하게 한다. 난류 흐름의 감쇠 효과를 정량화하기 위해서 개회로 기기와 폐회로 기기로 측정된 수증기 농도를 각각 분석하였다. 통계적 분석에 의하면, 폐회로 기기에서 얻어진 스펙트럼이 0.5 Hz 이상의 영역에서 개회로 기기에서 얻어진 스펙트럼과 서로 다름을 보였다. 낮에는 관 감쇠에 의한 수증기 플럭스의 손실이 5% 이내였으나, 밤에는 풍속이 작고, 난류의 강도가 약하여 플럭스 손실이 상대적으로 크게 나타났다. 이론적으로 계산된 플럭스 손실은 관측 결과와 고주파수 영역에서 약간의 차이를 보였는데, 이것은 수증기가 관의 벽과 상호 작용하면서 플럭스 측정에 영향을 주었기 때문인 것으로 추정된다. 결론적으로, 개회로나 폐회로기기 모두 5% 오차 내에서 수증기 플럭스 관측에 사용할 수 있다. 그러나 대기가 안정할 때는 플럭스 산출시 고주파수에서의 영향을 신중히 고려해 주어야 한다.

• 한국항공우주학회지
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• 제43권5호
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• pp.405-412
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• 2015

본 논문은 회전안정성을 갖는 탄도수정탄의 대기속도 추정을 위한 필터 설계에 대하여 설명한다. 대상 시스템은 운용상의 제약(공간, 파워)으로 인하여, 대기속도 측정을 위한 센서를 사용할 수 없다. 따라서 한정된 센서를 이용한 대기속도 추정이 필요하다. 따라서 본 연구에서는 IMU(가속도계, 자이로)에서 측정하는 3축 가속도와 각속도 데이터만 이용하여, 대기속도 추정을 위한 필터를 설계하였다. 대상 시스템의 경우, 넓은 속도, 고도의 운용범위를 커버하기 위한 추정 필터가 필요하므로 본 연구에서는 확장 칼만필터를 설계하여 기존의 연구와의 차별성을 두었다. 확장 칼만필터 설계를 위한 자코비안 행렬은 NRF(No-roll frame)에서의 간략화된 선형모델을 이용하여 구성하였다. 최종적으로 센서 오차와 바람 모델을 포함한 시뮬레이션을 통해 그 성능을 검토하였다. 이때, 시뮬레이션은 설계한 대기속도와 각속도 모델 오차의 영향을 분석하기 위하여 네 가지 경우의 프로세스 공분산 행렬 값에 대한 영향을 분석하였다.

• 한국항공우주학회지
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• 제45권3호
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• pp.259-267
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• 2017

본 연구에서는 카이스트에서 개발된 비정렬격자 기반의 유동 해석자를 이용하여 KARI-11-180 익형, SDM과 천음속 비행체 주변 유동장에 대한 수치해석을 수행하였다. 유동장을 해석하기 위하여 RANS가 수치적으로 풀이되었으며, Roe가 제안한 FDS 방법을 사용하여 비점성 플럭스를 계산하였다. 난류 모델은 SA 모델, SST 모델, ${\gamma}-{\widetilde{Re}}_{{\theta}t}$모델이 사용되었다. KARI-11-180 익형 유동 해석 결과 천이현상을 고려하였을 때 항력 계수가 더 작게 예측되었으며, 계산된 공력 특성은 전반적으로 실험 결과와 잘 일치하였다. SDM의 경우 날개 앞전에서 유동 박리현상이 발생하였으며, 계산된 공력 특성이 EFD 결과와 유사한 경향을 보였다. 천음속 비행체의 경우 자유류 마하수가 0.9일 때 주 날개에서 발생하는 충격파를 성공적으로 포착하였으며, 실험 결과와 해석된 결과 사이의 유사성을 확인하였다.

• 대한기계학회논문집
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• 제8권5호
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• pp.442-449
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• 1984

본 연구에서는 두 분류가 교우되어 배합이 이루어지는 유동역에서 3차원 방향 에 대하여 평균속도분포, 난류강도분포, 난류전단응력분포, 상관계수의 분포 및 난류 운동에너지와 운동량의 변화 등을 측정 분석하였다.