• 대기
• /
• 제18권3호
• /
• pp.207-224
• /
• 2008

Severe downslope windstorms observed at Gangneung, Korea in the springtime during the last 30 years are studied to understand their generation mechanisms. 92 severe wind cases are selected for which the maximum instantaneous wind speeds exceed two standard deviation of total mean plus ($18.7ms^{-1}$). They are categorized into the three mechanisms (hydraulic jump, partial reflection, and critical-level reflection) proposed in previous studies based on the flow condition, which is calculated using the wind and temperature profile observed at one upstream rawinsonde station, Osan. Among the three, partial reflection is found to be the most frequent mechanism for the last 30 years (1976 - 2005). To understand the role of inversion in generating severe downslope windstorms, horizontal velocity perturbation was calculated analytically for the atmosphere with an inversion layer. It turned out that the intensity of downslope wind was increased by inversion layer of specific heights, which are well matched with the observations. For better understanding the generation mechanisms, two-dimensional numerical simulations are conducted for the 92 severe wind cases using the ARPS model. In most simulations, surface wind speed exceeds the value of the severe-wind criterion, and each simulated case can be explained by its own generation mechanism. However, in most simulations, the simulated surface wind speed is larger than the observed, due to ignoring the flow-splitting effect in the two-dimensional framework.

• Wind and Structures
• /
• 제9권2호
• /
• pp.95-108
• /
• 2006

Wind turbines are highly complex structures for numerical flow simulation. They normally comprise of a turbine mounted on a tower thus the movement of the turbine blades and the blade/tower interaction must be captured. In addition the ground effect should also be included. There are many more important features of wind turbines and it is difficult to include all of them. A simplified set of features is chosen here for both the turbine and the tower to show how the method can begin to identify the main points connected with wind turbine wake generation and tip vortex tower interaction. An approach to modelling the rotating blades of a turbine is proposed here. The model uses point forces based on blade element theory to model the blades and takes into account their time dependent motion. This means that local instantaneous velocities can be used as a basis for the blade element theory. The model is incorporated into a large eddy simulation code and, although many important features are left out of the model, the velocity/power performance relation is generally of the correct order of magnitude. Suggested improvements to the method are discussed.

• 생물환경조절학회지
• /
• 제15권2호
• /
• pp.138-148
• /
• 2006

대립계 포도 비가림하우스에 부착된 일체형 방풍벽과 노지에 설치된 분리형 방풍벽의 구조 안전성을 설계풍속 $30.9m{\cdot}s^{-1}$와 $50m{\cdot}s^{-1}$ 조건에서 각각 분석하였다. 비닐하우스 부착형 방풍벽의 경우, 방풍벽을 설치했을 때 방풍벽의 경사각이 측면부의 유동분포에 미미한 변화를 주어 측면부가 받는 풍압면적이 다소 감소하는 것으로 나타났으나 큰 차이를 보이지는 않았다. 그러나 구조강도 측면에서는 방풍벽 설치를 위한 부가적인 파이프 투입 효과로 약 11%정도 구조 안전성이 향상되는 것으로 분석되었다. 주기둥 간격이 3m인 분리형 방풍벽의 경우,대형 태풍수준인 $50m{\cdot}s^{-1}$에서 구조적으로 불안정한 것으로 나타났으며 분리형 방풍벽의 이론적 고찰 결과와 해석 결과와는 큰 차이를 보여 3차원으로 구성된 구조물의 2차원 모델 시 그 정확성에 한계가 있음을 알 수 있었다. 추후 분리형 방풍벽의 효용성을 증대하기 위하여 최적 파이프 규격 설정에 관한 세부적인 연구가 필요할 것으로 판단되며 비닐하우스 부착형 방풍벽의 경우, 태풍으로 인한 비닐하우스 피해 발생 시 정밀한 피해실태 조사를 통하여 분석 결과의 정확성을 향상시킬 수 있는 구조 안전성 분석 기법 개발에 관한 연구가 필요한 것으로 판단되었다.

• Wind and Structures
• /
• 제23권1호
• /
• pp.1-18
• /
• 2016

This work experimentally and numerically analyzes the flow configurations and the dynamic wind loads on panels of rectangular L/h 5:1 cross section mounted on a structural frame of rectangular bars of L/h 0.5:1, corresponding to a radar structure. The fluid dynamic interaction between panels and frame wakes imposes dynamic loads on the panels, with particular frequencies and Strouhal numbers, different from those of isolated elements. The numerical scheme is validated by comparison with mean forces and velocity spectra of a panel wake obtained by wind tunnel tests. The flow configuration is analyzed through images of the numerical simulations. For a large number of panels, as in the radar array, their wakes couple in either phase or counter-phase configurations, changing the resultant forces on each panel. Instantaneous normal and tangential force coefficients are reported; their spectra show two distinct peaks, caused by the interaction of the wakes. Finally, a scaled model of a rectangular structure comprised of panels and frame elements is tested in the boundary layer wind tunnel in order to determine the influence of the velocity variation with height and the three-dimensionality of the bulk flow around the structure. Results show that the unsteady aerodynamic loads, being strongly influenced by the vortex shedding of the supporting elements and by the global 3-D geometry of the array, differ considerably on a panel in this array from loads acting on an isolated panel, not only in magnitude, but also in frequency.

• 생물환경조절학회지
• /
• 제4권2호
• /
• pp.167-174
• /
• 1995

This study was carried out to find a way of improving the windproof capability of greenhouse foundations. Generally, greenhouses are often collapsed due to the strong winds, because they are very light weight structures. In such a critical situations, the foundations are very often subjected to uplift and vibration at the same time. This paper describes both the wind disaster of greenhouses by the typhoon FAEY and the uplift resistance of greenhouse foundations. Followings are the results obtained from this study ; Judging from the view point of year round cultural aspects, it is recommended that some measures be taken for the preventions of greenhouse film ruptures because greenhouse structural damages are found to be directly associated with the local rupture of cover film. In the case of surveyed area, movable pipe-houses or pipe-houses of 1-2W type were found to be completely destroyed when the maximum instantaneous wind velocity was over 30m/sec or so. In the case of movable pipe-houses, the uplift resistance of greenhouse was expected to increase with the increase of pipe diameter and/or the embedment pipe length. But at present situations there is a limitation in raising the uplift resistance of movable pipe-house, because pipe diameters as well as pipe lengths customarily selected by farmers are quite a much limited.

• Wind and Structures
• /
• 제22권6호
• /
• pp.663-684
• /
• 2016

Conical vortices generated at the corner regions of large-span flat roofs have been investigated by using the Particle Image Velocimetry (PIV) technique. Mean and instantaneous vector fields for velocity, vorticity, and streamlines were measured at three visual planes and for two different flow angles of $15^{\circ}$. The results indicated that conical vortices occur when the wind is not perpendicular to the front edge. The location of the leading edge corresponding to the negative peak vorticity and maximum turbulent kinetic energy was found at the center of the conical vortex. The wind pressure reaches the maximum near the leading edge roof corner, and a triangle of severe suctions zone appears downstream. The mean pressure in uniform flow is greater than that under turbulent flow condition, while a significant increase in the fluctuating wind pressure occurs in turbulent streams. From its emergence to stability, the shape of the vortex cross-section is nearly elliptical, with increasing area. The angle that forms between the vortex axis and the leading edge is much smaller in turbulent streams. The detailed flow structures and characteristics obtained through FLUENT simulation are in agreement with the experimental results. The three dimensional (3-D) structure of the conical vortices is clearly observed from the comprehensive arrangement of several visual planes, and the inner link was established between the vortex evolution process, vortex core position and pressure distribution.

• 대한기계학회논문집B
• /
• 제23권2호
• /
• pp.218-225
• /
• 1999

Flow visualization study has been conducted to simulate the turbulent dispersion behavior of a crossflow jet physically under the conditions of various thermal stratification in a wind tunnel. A smoke jet with the constant ratio of the jet to freestream velocity is injected normally to the cross flow of the thermally stratified wind tunnel(TSWT) for flow visualization. The typical natures of the smoke dispersion under different thermal stratifications such as neutral, weakly stable, strongly stable, weakly unstable, strongly unstable and inversion layer are successfully reproduced in the TSWT. The Instantaneous velocity and temperature fluctuations are measured by using a cold and hot-wire combination probe. The time averaged dispersion behaviors, the centerline trajectories, the spreading angles and the virtual origins of the cross jet are deduced from the edge detected images with respect to the stability parameter. All the general characteristics of the turbulent dispersion behavior reveal that the definitely different dispersion mechanisms are inherent in both stable and unstable conditions. It is conjectured that the turbulent statistics obtained in the various stability conditions quantitatively demonstrate the vertical scalar flux plays a key role in the turbulent dispersion behavior.

• 한국가시화정보학회:학술대회논문집
• /
• 한국가시화정보학회 2005년도 추계학술대회 논문집
• /
• pp.46-49
• /
• 2005

Vents at outer walls of a large factory building are very important for natural ventilation. But, if a full-open vent is used, rain comes through the vents. We tried to utilize the natural ventilation effectively using a louver. A 1/120 scale-down building model was placed inside an atmospheric boundary layer simulated in a wind tunnel test section. The effect of louver angle on the ventilation flow inside the factory building was investigated experimentally. Instantaneous velocity fields inside the building model were measured using a 2-frame PIV system with varying the louver angles ($\theta=20^{\circ},\;40^{\circ},\;60^{\circ}$). For the case of $\theta=60^{\circ}$, as the incoming flow into the factory building increases, the inside velocity distribution becomes uniformly.

• 생물환경조절학회지
• /
• 제14권3호
• /
• pp.166-173
• /
• 2005

길이 40m,폭 5.5m의 단일피복 구조의 8연동 무가온하우스 상단부에 설계적설심 19.1 cm의 눈이 쌓인다는 조건과 시설 측면으로 설계풍속 $36.6\;m{\cdot}s^{-1}$의 바람이 분다는 조건 그리고 참고자료로 활용하기 위해 적용한 최대적설심 37.8cm의 눈이 쌓인다는 조건과 순간최대풍속 $60.0\;m{\cdot}s^{-1}$의 강풍이 분다는 조건에서 유동 및 구조강도 해석을 수행하였다. 적설하중 조건에서는 설계적설심 19.1 cm와 최대적설심 37.8cm에서 파이프에 걸리는 최대응력이 각각 $53.8\;N{\cdot}mm^{-2}$과 $107\;N{\cdot}mm^{-2}$으로 재료의 허용응력 보다 작은 것으로 나타나 안전한 것으로 분석되었으나, 설계풍속 $36.6\;m{\cdot}s^{-1}$와 순간최대풍속 $60.0\;m{\cdot}s^{-1}$의 풍하중 조건에서는 파이프에 걸리는 최대응력이 각각 $250\;N{\cdot}mm^{-2}$과 $672\;N{\cdot}mm^{-2}$으로 재료의 허용응력을 모두 초과하여 플라스틱하우스가 불안전한 것으로 분석되었다.

• 대한기계학회논문집B
• /
• 제25권5호
• /
• pp.713-721
• /
• 2001

Characteristics of the conical vortices on the roof corner of a rectangular prism have been investigated by using a PIV(Particle Image Velocimetry) technique. The Reynolds number based on the free stream velocity and the height of the model was 5.3$\times$10$^3$. The mean, instantaneous velocity vector fields, vorticity fields, and turbulent kinetic energy distribution were measured for two different angles of attack, 30$^{\circ}$and 45$^{\circ}$. The PIV measurements clearly observed not only the conical main vortex and the secondary vortex but also the tertiary vortex which is firstly reported in this paper. Asymmetric formation of the corner vortex for the case of 30$^{\circ}$angle of attack produces relatively the high magnitude of vorticity and turbulent kinetic energy around the bigger vortex which generates the peak suction pressure on the roof. Fairly symmetric features of the roof vortex are observed in the case of 45$^{\circ}$angle of attack, however, the dynamic characteristics are proved to be asymmetric due to the rectangular shape of the roof.