• 수산해양교육연구
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• 제20권2호
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• pp.156-167
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• 2008

Meorological data have been collected to monitor the wetland area in Goheung bay since 2003 and four intensive observations were conducted to study effects of the atmospheric turbulence on the energy budget and the ecological changes. We improved an algorithm to estimate the sensible heat flux with routine data. The sensible heat flux estimated by gradient method was in good agreement with that measured by precision instruments such as surface layer scintillometer and ultrasonic anemometer. Diurnal variations of sensible heat flux showed analogous tendency to those of temperature gradient. When the vertical wind shear of horizontal wind components was weak, even though temperature gradient was strong, the gradient method underestimated the sensible heat flux. A compensation for the cloud will make this gradient method be a helpful tool to monitor the ecosystem without expensive instruments except for weak wind shear and temperature gradient.

• Nuclear Engineering and Technology
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• 제23권3호
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• pp.275-284
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• 1991

핵연료봉의 정방형 또는 삼각형 배열내의 2차 난류 유동의 해석은 연료봉내의 온도분포와 열전달 과정의 해석에 있어서 중요한 문제이다. 비등방성 난류모델과 등방성 난류모델을 사용하여 속 도장을 구하였고 열수력학적 성질이 일정하다는 가정하에 지배방정식을 유한 요소법을 사용하여 수치해석적인 방법으로 해를 구하였다. 또한 연료봉 표면 근처에서는 유체의 유동이 난류가 아니기 때문에 축 방향 속도는 벽의 법칙에 의해서 계산하였다. 이러한 방법에 의해서 구해진 해는 실험 결과와 비교되었고 비교적 잘 일치하였다.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2007년도 추계학술대회 논문집
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• pp.449-452
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• 2007

The present study numerically investigate the effects of the Syngas chemical kinetics on the basic flame properties and the structure of the Syngas diffusion flames. In order to realistically represent the turbulence-chemistry interact ion and the spatial inhomogeneity of scalar dissipation rate. the Eulerian Particle Flamelet Model(EPFM) with multiple flamelets has been applied to simulate the combustion processes and NOx formation in the syngas turbulent nonpremixed flames. Due to the ability for interactively describing the transient behaviors of local flame structures with CFD solver, the EPFM model can effectively account for the detailed mechanisms of NOx format ion including thermal NO path, prompt and nitrous NOx format ion, and reburning process by hydrocarbon radical without any ad-hoc procedure. validation cases include the Syngas turbulent nonpremixed jet and swirling flames. Based on numerical results, the detailed discussion has been made for the sensitivity of the Syngas chemical kinetics as well as the precise structure and NOx formation characteristics of the turbulent Syngas nonpremixed flames.

• Wind and Structures
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• 제24권1호
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• pp.59-78
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• 2017

Non-stationarity and non-Gaussian property are two of the most important characteristics of wind. These two features are studied in this study based on wind speed records measured at different heights from a 325 m high meteorological tower during the synoptic wind storms. By using the time-frequency analysis tools, it is found that after removing the low frequency trend of the longitudinal wind, the retained fluctuating wind speeds remain to be asymmetrically non-Gaussian distributed. Results show that such non-Gaussianity is due to the weak-stationarity of the detrended fluctuating wind speed. The low frequency components of the fluctuating wind speeds mainly contribute to the non-zero skewness, while distribution of the high frequency component is found to have high kurtosis values. By further studying the decomposed wind speed, the mechanisms of the non-Gaussian distribution are examined from the phase, turbulence energy point of view.

• 한국전산유체공학회지
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• 제20권4호
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• pp.1-6
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• 2015

Centrifugal pumps consume considerable amount of energy in various industrial applications. Therefore, improvement of the efficiency of these machines has become a major challenge. Cavitation is a phenomenon which decreases the pump efficiency and even causes structural demage. Hence, the goal of this paper is to investigate the cavitation problem in the single-stage and double-stage centrifugal pumps. The Volume of Fraction (VOF) method has been used for the numerical simulations together with Rayliegh-Plesset model for the gas-liquid two-phase flow inside the pump. In order to capture the turbulent phenomena, the standard k-${\varepsilon}$ turbulence model has been adopted, and the simulations have been done as unsteady cases. In addition, the motion of the rotating parts has been simulated using Multi Reference Frame(MRF) method. The results are presented and compared in terms of hydraulic head and NPSH for both the single-stage and double-stage pumps. The H-Q curves show the effects of cavitation on decreasing the pumps performances.

• 대한기계학회논문집B
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• 제27권3호
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• pp.302-310
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• 2003

Experiments were conducted to show the characteristics of the flow on unventilated parallel plane jets. Measurements of mean velocity components and turbulent intensities were carried out with a particle image velocimetry to investigate the flow field generated by the air issued from two identical plane parallel nozzles and mixed with the ambient air. The measurements range of these experiments were Reynolds number of 5300 based on the nozzle width and the cases of nozzle-to-nozzle distance were four times. six times and eight times the width of the nozzle. Results show that a recirculation zone with a sub-atmospheric static pressure was bounded by the inner shear layers of the individual jets and the nozzles plate. The positions. where maximum value of mean turbulent intensities and mean turbulent kinetic energy show, were at the same position with the merging point. The spread of jets in the merging region increases more rapidly than that of Jets in the converging and the combined region. As nozzle-to-nozzle distances were increased. it was shown that merging and combined lengths were shorter.

• Wind and Structures
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• 제25권6호
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• pp.569-588
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• 2017

The aim of this paper is to present a method to obtain the dynamic response of a wind turbine tower in time domain by means of the generation of time series and to estimate the associated fatigue damage by means of a Rainflow counting algorithm. The proposed method is based on assuming the vortex shedding is a bidimensional phenomena and on following a classical modal superposition method to obtain the structure dynamic response. Four different wind turbine tower geometric configurations have been analyzed in a range of usual wind velocities and covering extreme wind velocities. The obtained results have shown that, depending on the turbulence intensity and the mean wind velocity, there are tower geometric configurations more advantageous from the fatigue load standpoint. Consequently, the presented model can be utilized to define assembly strategies oriented to fatigue damage minimization.

• Wind and Structures
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• 제29권6호
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• pp.405-416
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• 2019

Aerodynamic characteristic of a small scale wind turbine under the influence of an incoming uniform wind field is studied using k-ω Shear Stress Transport turbulence model. Firstly, the lift and drag characteristics of the blade section consisting of S826 airfoil is studied using 2D simulations at a Reynolds number of 1×10⁵. After that, the full turbine including the rotational effects of the blade is simulated using Multiple Reference Frames (MRF) and Sliding Mesh Interface (SMI) numerical techniques. The differences between the two techniques are quantified. It is then followed by a detailed comparison of the turbine's power/thrust output and the associated wake development at three tip speeds ratios (λ = 3, 6, 10). The phenomenon of blockage effect and spatial features of the flow are explained and linked to the turbines power output. Validation of wake profiles patterns at multiple locations downstream is also performed at each λ. The present work aims to evaluate the potential of the numerical methods in reproducing wind tunnel experimental results such that the method can be applied to full-scale turbines operating under realistic conditions in which observation data is scarce or lacking.

• 한국가시화정보학회지
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• 제17권3호
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• pp.12-18
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• 2019

Direct numerical simulation of a fully developed turbulent plane Couette-Poiseuille flow with a two-dimensional (2-D) rod-roughened wall is performed to investigate the impacts of the surface roughness. It is shown that the logarithmic region in the mean velocity profile over the rough wall Couette-Poiseuille flow is significantly shortened by the surface roughness compared to that over a turbulent Couette-Poiseuille flow with smooth wall. The Reynolds shear stress over the rough wall Couette-Poiseuille flow is decreased compared to that for a smooth case in the outer layer. These results are attributed to weakened turbulence activity or roll-cell mode over the rough wall Couette-Poiseuille flow near the channel centerline due to suppressed development of u'-structure on the top wall, as documented through spanwise energy spectra of the streamwise velocity fluctuations. Inspection of congregation motion near the bottom wall and time evolution of u'-structure reveal weakened co-supporting cycle for the rough wall case.

• 대한기계학회논문집B
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• 제29권11호
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• pp.1199-1208
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• 2005

The flow fields including velocities, turbulence intensities, Reynolds shear stress and turbulent kinetic energy were investigated using particle image velocimetry(PIV) to study the flow characteristics around two square cylinders in a tandem arrangement. The experiments were carried out in the range of the spacing from 1.0 to 4.0 widths of cylinder, Reynolds number of 5.3$\times$10$^{3}$ and 1.6$\times$10$^{4}$ respectively. Discontinuous jumping at the drag coefficient variation was found for two cylinders simultaneously when the spacing between two cylinders is varied. This phenomenon is attributed to a sudden change of the flow pattern which depends on the reattachment of the shear layer separated from the upstream cylinder. Near such a critical spacing, the changes of the flow fields as well as the effect of Reynolds number were studied in detail.