• Atmosphere
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• v.25 no.4
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• pp.685-692
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• 2015

In this study, the effects of trees on flow and scalar dispersion in an urban street canyon were investigated using a computational fluid dynamics (CFD) model. For this, we implemented the drag terms of trees to the CFD model, and compared the CFD-simulated results to the wind-tunnel results. For comparison, we considered the same building configuration as the wind-tunnel experiment. The trees were located at the center of street canyon with the aspect ratio (defined as the ratio of the street width to the building height) of 1. First, the flow characteristics were analyzed in the tree-free and high-density tree cases and the results showed that the CFD model reproduced well the flow pattern of the wind-tunnel experiment and reflected the drag effect of trees in the street canyon. Then, the dispersion characteristics of scalar pollutants were investigated for the tree-free, low-density tree and medium-density tree cases. In the tree-free case, the nondimensionalized concentration distribution simulated by the CFD model was quite similar to that in the wind-tunnel experiment in magnitude and pattern. The correlation coefficients between the measured and simulated concentrations are more than 0.9 in all the cases. As the tree density increased, nondimensionalized concentration increased (decreased) near the wall of the upwind (downwind) building, which resulted from the decrease in wind speed case by the drag effect of trees. However, the CFD model underestimated (overestimated) the concentration near the wall of upwind (downwind) building.

• Wind and Structures
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• v.28 no.5
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• pp.299-313
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• 2019

In order to examine the effects of different wind deflectors on the wind load distribution characteristics of extra-large cooling towers, a comparative study of the distribution characteristics of wind pressures on the surface of three large cooling towers with typical wind deflectors and one tower without wind deflector was conducted using wind tunnel tests. These characteristics include aerodynamic parameters such as mean wind pressures, fluctuating wind pressures, peak factors, correlation coefficients, extreme wind pressures, drag coefficients and vorticity distribution. Then distribution regularities of different wind deflectors on global and local wind pressure of extra-large cooling towers was extracted, and finally the fitting formula of extreme wind pressure of the cooling towers with different wind deflectors was provided. The results showed that the large eddy simulation (LES) method used in this article could be used to accurately simulate wind loads of such extra-large cooling towers. The three typical wind deflectors could effectively reduce the average wind pressure of the negative pressure extreme regions in the central part of the tower, and were also effective in reducing the root of the variance of the fluctuating wind pressure in the upper-middle part of the windward side of the tower, with the curved air deflector showing particularly. All the different wind deflectors effectively reduced the wind pressure extremes of the middle and lower regions of the windward side of the tower and of the negative pressure extremes region, with the best effect occurring in the curved wind deflector. After the wind deflectors were installed the drag coefficient values of each layer of the middle and lower parts of the tower were significantly higher than that without wind deflector, but the effect on the drag coefficients of layers above the throat was weak. The peak factors for the windward side, the side and leeward side of the extra-large cooling towers with different wind deflectors were set as 3.29, 3.41 and 3.50, respectively.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.58 no.4
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• pp.289-298
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• 2022

Sea anchor for fishery is commonly used in jigging fishery and purse seine. The study of sea anchor was studied for improvement of opening efficiency and drag by changing the type of shape and the diameter of vent. However, standard specification of sea anchor is not set and has not been studied for underwater stability. Therefore, this study aimed to improve underwater stability of sea anchor by changing a vent diameter and weight of sinker. The experiment was conducted in flume water tank. The experiment model of sea anchor was made from actual model of sea anchor which is used in fishery by similarity law. The model of sea anchor was designed to different types of vent diameter and weight of sinker in different current speed. The value of movement of side to side (X-axis), drag of sea anchor (Y-axis) and movement of up and down (Z-axis) was measured for 30 seconds. Each value of X, Y, Z-axis was analyzed through t-test and ANOVA analysis to verify that each value had a significant difference according to the difference compositions. There was correlation between the movement of X-axis and Z-axis. The drag of sea anchor was stronger as the current speed increased. However, the larger the vent diameter, the weaker the drag. From the result of the standard deviation, the movement of X-axis was inversely proportional to the vent diameter. However, movement of Z-axis was larger as the weight of sinker was the heaviest or lightest from the result of the standard deviation. These results suggest that the sea anchor should be combined with proper size of the vent diameter and the weight of sinker to improve the stability.

• Journal of Astronomy and Space Sciences
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• v.24 no.2
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• pp.125-134
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• 2007

In this paper, we analyze the orbital variation of the Korea Multi-Purpose SATellite-1(KOMPSAT-1) in a strong space environment due to satellite drag by solar and geomagnetic activities. The satellite drag usually occurs slowly, but becomes serious satellite drag when the space environment suddenly changes via strong solar activity like a big flare eruption or coronal mass ejections(CMEs). Especially, KOMPSAT-1 as a low earth orbit satellite has a distinct increase of the drag acceleration by the variations of atmospheric friction. We consider factors of solar activity to have serious effects on the satellite drag from two points of view. One is an effect of high energy radiation when the flare occurs in the Sun. This radiation heats and expands the upper atmosphere of the Earth as the number of neutral particles is suddenly increased. The other is an effect of Joule and precipitating particle heating caused by current of plasma and precipitation of particles during geomagnetic storms by CMEs. It also affects the density of neutral particles by heating the upper atmo-sphere. We investigate the satellite drag acceleration associated with the two factors for five events selected based on solar and geomagnetic data from 2001 to 2002. The major results can be summarized as follows. First, the drag acceleration started to increase with solar EUV radiation with the best cross-correlation (r = 0.92) for 1 day delayed F10.7. Second, the drag acceleration and Dst index have similar patterns when the geomagnetic storm is dominant and the drag acceleration abruptly increases during the strong geomagnetic storm. Third, the background variation of the drag accelerations is governed by the solar radiation, while their short term (less than a day) variations is governed by geomagnetic storms.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.27 no.4
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• pp.238-246
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• 1991

In order to make clear the mechanism forming the fishing ground of anchovy drag net in connection with water temperature, catch and oceanographic data for the years of 1970-1988 in the Southern Sea of Korea were analyzed. The annual catch of anchovy drag net was about 21,000M/T in 1970 but it was increasing largely so far 100,000M/T in recent years. The fishing season of anchovy drag net is the whole year beside the prohibition season established in aims to protect the spawning group of anchovy, and then in the best season of the period from July to December, the fishing ground is made up in the coastal area joining Yosu, Namhaedo and Bangeojin. There were some evidences that the fishing condition was controlled by the oceanographic condition, especially water temperature, that is, when there was large difference in water temperature between the south area of the Eastern Sea and the west area of the southern Sea and the thermocline is formed strongly in the larger less than 20m, the fishing condition was good. On the other hand, there was a very effective correlation between the catch(X) of anchovy spawning group in Spring by other fishing gears, mainly drift net and that(Y) of the little size of anchovy by drag net in Autumn, expressed by the relative equation, Y=62,246+1.3X, r=0.63.

• Journal of the Korean Society of Marine Environment & Safety
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• v.25 no.3
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• pp.344-353
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• 2019

The wakes behind a square cylinder were simulated using two-equation turbulence models, $k-{\varepsilon}$ and RNG $k-{\varepsilon}$ models. For comparisons between the model predictions and analytical solutions, we employed three skill assessments:, the correlation coefficient for the similarity of the wake shape, the error of maximum velocity difference (EMVD) of the accuracy of wake velocity, and the ratio of drag coefficient (RDC) for the flow patterns as in the authors' previous study. On the basis of the calculated results, we discussed the feasibility of each model for wake simulation and suggested a suitable value for an eddy viscosity related constant in each turbulence model. The $k-{\varepsilon}$ model underestimated the drag coefficient by over 40 %, and its performance was worse than that in the previous study with one-equation and mixing length models, resulting from the empirical constants in the ${\varepsilon}-equation$. In the RNG $k-{\varepsilon}$ model experiments, when an eddy viscosity related constant was six times higher than the suggested value, the model results were yielded good predictions compared with the analytical solutions. Then, the values of EMVD and RDC were 3.8 % and 3.2 %, respectively. The results of the turbulence model simulations indicated that the RNG $k-{\varepsilon}$ model results successfully represented wakes behind the square cylinder, and the mean error for all skill assessments was less than 4 %.

• Journal of the Korea Institute of Military Science and Technology
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• v.21 no.1
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• pp.10-16
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• 2018

The Republic of Korea Marine Corps is planning to develop a new amphibious assault vehicle which is able to operate with higher water speed than current KAAV. In order to achieve a higher water speed for hydrodynamically bulff-body vehicles, it is essential to develop drag reduction strategies. In this paper, resistance characteristics including trim angles of amphibious assault vehicles with several appendage designs are investigated using a commercial CFD code, STAR-CCM+. The computed results are compared with experimental data conducted at the towing tank with 1:4.5 scaled model and show good correlation. Comparing with the results of bare hull, 3.4 % of hydrodynamic drag and 52 % of trim angle are reduced by the application of double angled bow flap and a hydrofoil attached at the transom.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.11a
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• pp.386-392
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• 2010

The multiphase simulation of a liquid jet in a lab-scale ramjet combustor with a plain orifice type injector was studied with a commercial CFD tool, a FLUENT program. The objectives of the current study are to analysis the breakup characteristics of a hexane liquid jet in a cross flow and to derive the correlation between flow conditions and drag force coefficients in a test section. From the result of a numerical simulation, we concluded that a DPM and Realizable $k-{\varepsilon}$ model with an enhanced wall treatment were available to simulate the multiphase flow simulation. And the calculated distribution of a hexane vapor concentration was well-matched with experimental results.

• Nuclear Engineering and Technology
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• v.55 no.12
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• pp.4395-4407
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• 2023

Bubble columns are widely encountered in several industries, especially in the field of nuclear safety. The Eulerian-Eulerian and the Eulerian-Lagrangian methods are commonly used to investigate bubble columns. Eulerian approaches require additional tasks such as strict volume conservation at the interface and a predefined well-structured grid. In contrast, the Lagrangian approach can be easily implemented. Hence, we introduce a fully Lagrangian approach for the simulation of bubble columns using the discrete bubble model (DBM) and moving particle semi-implicit (MPS) methods. Additionally, we propose a rigorous method to estimate the volume fraction accurately, and verified it through experimental data and analytical results. The MPS method was compared with the experimental data of Dambreak. The DBM was verified by analyzing the terminal velocity of a single bubble for each bubble size. It agreed with the analytical results for each of the four drag correlations. Additionally, the improved method for calculating the volume fraction showed agreement with the Ergun equation for the pressure drop in a packed bed. The implemented MPS-DBM was used to simulate the bubble column, and the results were compared with the experimental results. We demonstrated that the MPS-DBM was in quantitative agreement with the experimental data.

• Proceedings of the Korean Nuclear Society Conference
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• 1997.05a
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• pp.409-414
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• 1997

Reflood model in RELAP5/MOD3.1 are modified to improve the unrealistic prediction results of the model. In the new method, the modified Zuber pool boiling critical heat flux (CHF) correlation is adopted. The reflood drop size is characterized by the use of We=1.5 and the minimum drop size of 0.0007 m for $p^{*}\;{\leq}\;0.025$. To describe the wall to vapor heat transfer at low pressure and low flow condition, the Webb-Chen correlation is utilized . The suggested method has been verified through the simulations of the Lehigh University rod bundle reflood tests. Through sensitivity study it is shown that the effect of drag coefficients is dominant in the reflood model. It is proved that the present modifications result in much more improved quench behavior and accurate wan and vapor temperature predictions.