• Wind and Structures
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• 제19권5호
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• pp.505-522
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• 2014

While effects of the atmospheric boundary layer flow on engineering infrastructure are more or less known, some local transient winds create difficulties for structures, traffic and human activities. Hence, further research is required to fully elucidate flow characteristics of some of those very unique local winds. In this study, important characteristics of observed vertical velocity profiles along the main wind direction for the gusty Bora wind blowing along the eastern Adriatic coast are presented. Commonly used empirical power-law and the logarithmic-law profiles are compared against unique 3-level high-frequency Bora measurements. The experimental data agree well with the power-law and logarithmic-law approximations. An interesting feature observed is a decrease in the power-law exponent and aerodynamic surface roughness length, and an increase in friction velocity with increasing Bora wind velocity. This indicates an urban-like velocity profile for smaller wind velocities and rural-like velocity profile for larger wind velocities, which is due to a stronger increase in absolute velocity at each of the heights observed as compared to the respective velocity gradient (difference in average velocity among two different heights). The trends observed are similar during both the day and night. The thermal stratification is near neutral due to a strong mechanical mixing. The differences in aerodynamic surface roughness length are negligible for different time averaging periods when using the median. For the friction velocity, the arithmetic mean proved to be independent of the time record length, while for the power-law exponent both the arithmetic mean and the median are not influenced by the time averaging period. Another issue is a large difference in aerodynamic surface roughness length when calculating using the arithmetic mean and the median. This indicates that the more robust median is a more suitable parameter to determine the aerodynamic surface roughness length than the arithmetic mean value. Variations in velocity profiles at the same site during different wind periods are interesting because, in the engineering community, it has been commonly accepted that the aerodynamic characteristics at a particular site remain the same during various wind regimes.

• 한국해안·해양공학회논문집
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• 제29권6호
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• pp.279-285
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• 2017

일반적으로 멱법칙과 대수함수 프로파일은 유속 연직 프로파일 모델로 많이 사용된다. 그러나 해역특성에 따라 모델별 매개변수의 값들이 상이하므로 실제 관측치를 통해 이들 값을 추정할 필요가 있다. 본 연구에서는 울돌목 해역에서 관측한 30분 평균 유속자료를 이용하여 창 낙조 시 멱법칙의 고도분포지수(n) 값과 대수함수 프로파일의 마찰속도($u^*$)와 조도길이($z_0$)를 추정하였다. 또한, 수심평균유속과 매개변수간의 상관관계를 분석하였으며, 관측치와의 오차분석을 수행하였다. 그 결과, power law의 고도분포지수(n)는 창조 시에는 평균 10.75, 낙조 시에는 평균 9.3의 고도분포지수 값을 가지는 것으로 나타났다. 한편, 대수함수 프로파일의 $u^*$는 창 낙조 시, 0.084 m/s와 0.105 m/s로 각각 추정되었으며, $z_0$는 0.004 m, 0.006 m로 각각 추정되었다.

• 한국유체기계학회 논문집
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• 제5권4호
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• pp.54-60
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• 2002

Experimental study was conducted to obtain the air velocity profiles in turbulent pipe flow. The acrylic smooth pipe (${\phi}=80mm$) was used for the test section of the flow loop. It was known that the velocity profiles of turbulent flow were different with Reynolds numbers and the viscous sublayer was usually quite thin. The following conclusions were drawn from the experimental investigations. Maximum velocity of the pipe center and flow-rate are useful for the duct design on the spot. The velocity profiles of high Reynolds number was flatter than those of low Reynolds number. It was known that the exponent, n, for power-law velocity profiles was $6{\sim}9$ depending on Reynolds number ranging from $10^4$ to $10^5$ in the turbulent flow, However, in this experiment study, it was $9{\sim}14$ depending on Reynolds number ranging from 17,000 to 123,727 in the turbulent flow, and $1.7{\sim}3.5$ depending on Reynolds number ranging from 2,442 to 4,564 in the transition region.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2003년도 춘계학술대회논문집
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• pp.461-464
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• 2003

A new control volume finite element method is proposed for three dimensional analysis of polymer flow. Tetrahedral finite element is employed and co-located interpolation procedure for pressure and velocity is implemented. Inclusion of pressure gradient term in the velocity shape functions prevents the checkerboard pressure field from being developed. Vectorial nature of pressure gradient is considered in the velocity shape function so that velocity profile in the limit of very small Reynolds number becomes physically meaningful. The proposed method was verified through three dimensional simulation of pipe flow problem for Newtonian and power-law fluid. Calculated pressure and velocity field showed an excellent agreement with analytic solutions for pressure and velocity. Driven-cavity problem, which is reported to yield checkerboard pressure filed when conventional finite element method is applied, could be solved without yielding checkerboard pressure field when the proposed control volume finite element method was applied. The proposed method could be successfully applied to the three dimensional mold filling problem.

• 한국기계가공학회지
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• 제11권2호
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• pp.118-124
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• 2012

The simulation of the air flow over models in atmospheric boundary layer wind tunnel is a research region based on advanced scientific technologies imposed by the necessity of studying the turbulent fluid dynamics in the proximity of the Earth's surface. In this study, the atmospheric boundary layer wind tunnel of UCD is used, the mean velocities are measured by augmentation devices such as roughness blocks and spires. The experimental results of mean velocity profile are well fitted with the value of power law.

• Wind and Structures
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• 제24권6호
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• pp.579-611
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• 2017

Bora is a strong, usually dry temporally and spatially transient wind that is common at the eastern Adriatic Coast and many other dynamically similar regions around the world. One of the Bora main characteristics is its gustiness, when wind velocities can reach up to five times the mean velocity. Bora often creates significant problems to traffic, structures and human life in general. In this study, Bora velocity and near-ground turbulence are studied using the results of three-level high-frequency Bora field measurements carried out on a meteorological tower near the city of Split, Croatia. These measurements are analyzed for a period from April 2010 until June 2011. This rather long period allows for making quite robust and reliable conclusions. The focus is on mean Bora velocity, turbulence intensity, Reynolds shear stress and turbulence length scale profiles, as well as on Bora velocity power spectra and thermal stratification. The results are compared with commonly used empirical laws and recommendations provided in the ESDU 85020 wind engineering standard to question its applicability to Bora. The obtained results report some interesting findings. In particular, the empirical power- and logarithmic laws proved to fit mean Bora velocity profiles well. With decreasing Bora velocity there is an increase in the power-law exponent and aerodynamic surface roughness length, and simultaneously a decrease in friction velocity. This indicates an urban-like velocity profile for smaller wind velocities and a rural-like velocity profile for larger wind velocities. Bora proved to be near-neutral thermally stratified. Turbulence intensity and lateral component of turbulence length scales agree well with ESDU 85020 for this particular terrain type. Longitudinal and vertical turbulence length scales, Reynolds shear stress and velocity power spectra differ considerably from ESDU 85020. This may have significant implications on calculations of Bora wind loads on structures.

• 상하수도학회지
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• 제20권2호
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• pp.295-302
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• 2006

Over the past decades, a flocculation and/or sedimentation process have been adopted to remove pollutants from CSOs. It has been learned that major factors affecting settlement of pollutants are the particle size distribution, their settling velocities and their specific gravity. It is, therefore, a good idea to analyze the particle size distribution and settleability of CSOs pollutants in order to develop details in designing a process. Discussed in this study are pollutant characteristics of CSOs such as particle size distribution and settleability of pollutants. The power law function is applied and is found to be an effective and reliable index for expressing the particle size distribution of pollutants in CSOs. Based on the regression analysis it is observed that the derived constants of curves representing settling velocity profile are proportional to the initial concentration of particles and to the ${\beta}$-values of power law distributions.

• 한국유체기계학회 논문집
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• 제17권3호
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• pp.33-40
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• 2014

In this study, a new model for calculating the liquid film thickness and condensation heat transfer coefficient in a vertical condenser tube is proposed by considering the effects of gravity, liquid viscosity, and vapor flow in the core region of the flow. In order to introduce the radial velocity profile in the liquid film, the liquid film flow was regarded to be in Couette flow dragged by the interfacial velocity at the liquid-vapor interface. For the calculation of the interfacial velocity, an empirical power-law velocity profile had been introduced. The resulting liquid film thickness and heat transfer coefficient obtained from the proposed model were compared with the experimental data from other experimental study and the results obtained from the other condensation models. In conclusion, the proposed model physically explained the liquid film thinning effect by the vapor shear flow and predicted the condensation heat transfer coefficient from experiments reasonably well.

• 한국대기환경학회지
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• 제12권4호
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• pp.459-472
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• 1996

An experimental investigation on the wind flow over smooth bell-shaped two-dimensional hills with hill slopes (the ratio of height to half width) of 0.3 and 0.5 is performed in an atmospheric boundary-layer wind tunnel. Two categories of the models are used in the present investigation; six two-dimensional single-hills, and four continuous double-hills. The measurements of the flow field and surface static-pressure distribution are carried out over the Reynolds number (based on the hill height) of 1.9 $\times 10^4, 3.3 \times 10^4, and 5.6 \times 10^4$. The velocity profiles and turbulence characteristics are measured by the pitot-tube and X-type hot-wire anemometer, respectively. The undisturbed boundary-layer profile on the bottom surface of the wind tunnel is reasonably consistent with the power-law profile with $\alpha = 7.0 (1/\alpha$ is the power-law exponent) and shows good spanwise uniformities. The profiles of turbulent intensity are found to be consistent along the centerline of the wind tunnel. The measured non-dimensional speed-up profiles at the hill crest show good agreements with the predictions of Jackson and Hunt's linear theory. The flow separation occurs in the hill slope of 0.5, and the oil-ink dot method is used to find the reattachment points in the leeside of the hill. The measured reattachment points are compared with the numerical predictions. Comparisons of the mean velocity profiles and surface pressure distributions between the numerical predictions and the experimental results show good agreements.

• 신재생에너지
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• 제5권4호
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• pp.11-18
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• 2009

Wind energy resources are recently considered as an important power generation alternative in the future. The fact that the investment of wind turbine installation continues to increase has motivated a need to develop more widely applicable methodologies for evaluating the actual benefits of adding wind turbines to conventional generating systems. This study is aiming to estimate the future wind resources with various estimation methods. The wind power is calculated at the hub height 75m of 800KW and 1,500KW wind turbines in Wolryong site, Jeju island, South Korea. Three equations - logarithmic, profile, and power law methods are applied for the accurate prediction of wind profile. In addition, yearly wind power can be calculated by using Weibull & Rayleigh distribution. It is found that predicted wind speed is highly affected by friction velocity, atmospheric stability, and averaged roughness length. It is concluded that Rayleigh distribution provides greater power generation than the Weibull distribution, especially for low wind-speed condition.