• 대한조선학회논문집
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• 제53권1호
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• pp.37-44
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• 2016

Recently researchers are conducting a lot of research related to EEDI in order to satisfy IMO resolution MEPC. Especially they are interested in design of energy saving device. This paper is to design the asymmetric pre-swirl stator for 160K LNG carrier in order to reduce energy. Two types of the asymmetric pre-swirl stator are taken into account; constant and variable pitch angle stators. “constant” and “variable” mean state that the pitch of stators change by radius. The dimensions of the stators are initially determined using potential-flow code. The propulsion performances of the stators are predicted using viscous-flow code. The model test is carried out in towing tank in PNU. Prediction of ship performance generally follow ITTC recommended. Ship wake prediction was done by two method, ITTC 1978 and ITTC 1999. Therefore propulsion performances were compared ITTC 1978 with ITTC 1999 methods. Comparison components are delivered power and thrust deduction coefficient of the model. Final pre-swirl stator is selected by comparing experiment and CFD.

• 한국초전도ㆍ저온공학회논문지
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• 제24권3호
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• pp.62-67
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• 2022

Liquid hydrogen (LH₂) has a higher density than gaseous hydrogen, so it has high transport efficiency and can be stored at relatively low pressure. In order to use efficient bulk hydrogen in the industry, research for the LH₂ supply system is needed. In the high-pressure hydrogen station based on LH₂ currently being developed in Korea, a heat exchanger is used to heat up supercritical hydrogen at 700 bar and 60 K, which is pressurized by a cryogenic high-pressure pump, to gas hydrogen at 700 bar and 300 K. Accordingly, the heat exchanger used in the hydrogen station should consider the design of high-pressure tubes, miniaturization, and freezing prevention. A helical heat exchanger generates secondary flow due to the curvature characteristics of a curved tube and can be miniaturized compared to a straight one on the same heat transfer length. This paper evaluates the heat transfer performance through parametric study on the distance between coils, guide effect, and anti-icing design of helical heat exchanger. The helical heat exchanger has better heat transfer performance than the straight tube exchanger due to the influence of the secondary flow. When the distance between the coils is uniform, the heat transfer is enhanced. The guide between coils increases the heat transfer performance by increasing the heat transfer length of the shell side fluid. The freezing is observed around the inlet of distribution tube wall, and to solve this problem, an anti-icing structure and a modified operating condition are suggested.

• Wind and Structures
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• 제35권4호
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• pp.255-268
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• 2022

Light-weight or low-damped structures may encounter the unsteady galloping instability that occurs at low reduced wind speeds, where the classical quasi-steady assumption is invalid. Although this unsteady phenomenon has been widely studied for rectangular cross sections with one side perpendicular to the incidence flow, the effect of the mean wind angle of attack has not been paid enough attention yet. With four sectional models of different side ratios and geometric shapes, the presented research focuses on the effect of the wind angle of attack on unsteady galloping instability. In static tests, comparatively strong vortex shedding force was noticed in the middle of the range of flow incidence where the lift coefficient shows a negative slope. In aeroelastic tests with a low Scruton number, the typical unsteady galloping, which is due to an interaction with vortex-induced vibration and results in unrestricted oscillation initiating at the Kármán vortex resonance wind speed, was observed for the wind angles of attack that characterize relatively strong vortex shedding force. In contrast, for the wind angles of attack with relatively weak shedding force, an "atypical" unsteady galloping was found to occur at a reduced wind speed clearly higher than the Kármán-vortex resonance one. These observations are valid for all four wind tunnel models. One of the wind tunnel models (with a bridge deck cross section) was also tested in a turbulent flow with an intensity about 9%, showing only the atypical unsteady galloping. However, the wind angle of attack with the comparatively strong vortex shedding force remains the most unfavorable one with respect to the instability threshold in low Scruton number conditions.

• Nuclear Medicine and Molecular Imaging
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• 제40권4호
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• pp.200-204
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• 2006

목적: 산소포화도법과 비교하여 동위원소법을 이용한 Qp/Qs측정의 정확도를 알아보고, 이와 연관된 요인이 무엇인지, 추적 검사시 두 검사법의 결과를 서로 비교해서 사용할 수 있는지에 대하여 알아보고자 하였다. 대상 및 방법: 두 검사법으로 Qp/Qs를 측정한 207명의 환자를 대상으로 하였다. 동위원소를 이용한 방법은 폐에 관심영역을 설정하여 시간-방사능 곡선을 얻어서 gamma variate model에 적용하여 면적으로부터 Qp/Qs를 구하였다. 환자의 연령, 좌우 단락의 종류와 동반된 질환 등에 따라서 방사성 동위원소 조영술과 심도자술로 얻어진 Qp/Qs의 상관계수와 일치도를 분석하였다. 결과: 동위원소 검사법과 심도자술로 얻어진 Qp/Qs의 평균치는 각각 $1.85{\pm}0.50,\;1.74{\pm}0.51$이며, 두 검사법으로 얻은 Qp/Qs의 상관계수는 0.86 (p<0.001)으로 강한 상관관계를 보였다. Bland-Altman 분석에서 두 검사법에 의한 Qp/Qs의 일치의 한계값은 $-0.42{\sim}0.63$, 범위는 1.05였다. 심실중격 결손증 환자에서는 상관계수가 0.90으로 강한 상관관계를 보였으며, 일치의 한계값은 $-0.29{\sim}0.45$ 였다. 심방중격 결손증, 심실중격 결손증, 동맥관 개존증 환자와 삼첨판 부전증과 승모판 부전증이 동반된 환자에서의 상관계수는 각각 0.78, 0.90, 0.84, 0.63, 0.44 였으며, 일치의 범위는 1.52, 0.74, 0.96, 1.57. 1.50이였다. 결론: 동위원소 검사법과 심도자술로 얻어진 Qp/Qs는 전체적으로 좋은 상관관계를 보이나 일치의 한계값이 크므로 추적 검사시 서로 호환하여 사용하기 어렵다. 방실판막 부전증이 동반된 경우에는 낮은 상관관계와 낮은 일치도를 보였다.

• 한국수자원학회논문집
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• 제51권12호
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• pp.1181-1194
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• 2018

본 연구에서는 2015년에서 2017년 사이에 유럽항공우주국 Sentinel-1 위성이 촬영한 Synthetic Aperture Radar (SAR) 영상을 활용하여 한강 유역 내 하천의 유량을 추정하는 모형을 개발하였다. 한강 유역 내 15개 중소규모 하천을 연구지역으로 선정하였으며 SAR 인공위성 영상 자료와 수위 및 유량관측소에서 산정한 유량 자료를 모형 구축을 위하여 사용하였다. 우선, 오류 보정을 위해 다양한 전처리 과정을 거친 12장의 SAR 영상을 히스토그램 매칭 기법을 적용하여 이미지의 밝기 분포를 동일하게 만들었다. 이후 임계치 분류방식을 사용하여 추출된 하천 수체의 면적과 지상 관측유량자료와의 관계식을 도출하여 유량추정모형을 구축하였다. 그 결과, 1개소를 제외한 14개 관측소에서 인공위성에서 추출한 하천 면적을 입력 자료로 하는 멱함수 형태의 유량추정모형을 구축할 수 있었다. 14개 관측소의 최소, 평균, 최대 결정 계수($R^2$)는 0.3, 0.8, 0.99로 나타났다.

• Wind and Structures
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• 제28권2호
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• pp.71-87
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• 2019

The yaw and interference effects of blades affect aerodynamic performance of large wind turbine system significantly, thus influencing wind-induced response and stability performance of the tower-blade system. In this study, the 5MW wind turbine which was developed by Nanjing University of Aeronautics and Astronautics (NUAA) was chosen as the research object. Large eddy simulation on flow field and aerodynamics of its wind turbine system with different yaw angles($0^{\circ}$, $5^{\circ}$, $10^{\circ}$, $20^{\circ}$, $30^{\circ}$ and $45^{\circ}$) under the most unfavorable blade position was carried out. Results were compared with codes and measurement results at home and abroad, which verified validity of large eddy simulation. On this basis, effects of yaw angle on average wind pressure, fluctuating wind pressure, lift coefficient, resistance coefficient,streaming and wake characteristics on different interference zone of tower of wind turbine were analyzed. Next, the blade-cabin-tower-foundation integrated coupling model of the large wind turbine was constructed based on finite element method. Dynamic characteristics, wind-induced response and stability performance of the wind turbine structural system under different yaw angle were analyzed systematically. Research results demonstrate that with the increase of yaw angle, the maximum negative pressure and extreme negative pressure of the significant interference zone of the tower present a V-shaped variation trend, whereas the layer resistance coefficient increases gradually. By contrast, the maximum negative pressure, extreme negative pressure and layer resistance coefficient of the non-interference zone remain basically same. Effects of streaming and wake weaken gradually. When the yaw angle increases to $45^{\circ}$, aerodynamic force of the tower is close with that when there's no blade yaw and interference. As the height of significant interference zone increases, layer resistance coefficient decreases firstly and then increases under different yaw angles. Maximum means and mean square error (MSE) of radial displacement under different yaw angles all occur at circumferential $0^{\circ}$ and $180^{\circ}$ of the tower. The maximum bending moment at tower bottom is at circumferential $20^{\circ}$. When the yaw angle is $0^{\circ}$, the maximum downwind displacement responses of different blades are higher than 2.7 m. With the increase of yaw angle, MSEs of radial displacement at tower top, downwind displacement of blades, internal force at blade roots all decrease gradually, while the critical wind speed decreases firstly and then increases and finally decreases. The comprehensive analysis shows that the worst aerodynamic performance and wind-induced response of the wind turbine system are achieved when the yaw angle is $0^{\circ}$, whereas the worst stability performance and ultimate bearing capacity are achieved when the yaw angle is $45^{\circ}$.

• 한국항공우주학회지
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• 제33권12호
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• pp.76-82
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• 2005

본 연구에서는 축류형 마이크로터빈의 단 수를 단 단에서부터 최대 6단까지 변경하면서 각 단에서의 공력특성을 측정하였다. 실험에 사용된 마이크로터빈은 터빈입구에서 유량계수가 2.0, 부하계수가 3.25이며 유로의 평균직경이 25.8mm인 소형 축류형 다단터빈이 적용되었다. 정익과 동익의 솔리디티는 0.67~0.75 범위의 값이 적용되었으며 입구에 일정한 질유량과 전압력으로 조정한 후에 터빈의 부하를 변경하면서 탈설계 영역에서의 공력특성을 측정하였다. 본 실험에서는 단 당 최대 2kW/kg/sec의 비출력이 얻어졌으나 단수의 증가에 따라 비출력의 증가폭은 다소 완화되었으며, 토오크의 경우는 단수가 증가되면서 낮은 회전수 영역에서는 토오크의 증가폭이 일정하나 높은 회전수영역에서는 토오크의 증가폭이 둔화되었다. 블레이드의 높이에 비하여 팁간격의 영향이 크므로 터빈의 효율은 낮으나 단 수의 증가에 따라 증가가 가능하다.

• 상하수도학회지
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• 제23권5호
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• pp.599-608
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• 2009

This manuscript covers the results of field investigation and lab-scale experiments to design a double-layered biofilter system to control urban storm runoff. The biofilter system consisted of a coarse soil layer (CSL) for filtration and fine soil layer (FSL) for adsorption and biological degradation. The variations of flow rate and water quality of runoff from a local expressway were monitored for seven storm events. Laboratory column experiments were performed using seven kinds of soil and mulch to maximize pollutants removal. The site mean concentration (SMC) of storm runoff from the drainage area (runoff coefficient: 0.92) was measured to be 203 mg/L for SS, 307 mg/L for $TCOD_{Cr}$, 12.3 mg/L for TN, 7.3 mg/L for ${NH_4}^+-N$, and 0.79 mg/L for TP, respectively. This study employed a new design concept, to cover the maximum rainfall intensity with one month recurrence interval. Effective storms for last ten years (1998-2007) in seoul suggested the design rainfull intensity to be 8.8 mm/hr Single layer soil column showed the maximum removal rate of pollutants load when the uniformity coefficient of CSL was 1.58 and the silt/clay contents of FSL was virtually 7%. The removal efficiency during operation of double layer soil column was 98% for SS and turbidity, 75% for TCODCr, 56% for ${NH_4}^+-N$, 87% for TP, and 73-91% for heavy metals. The hydraulic conductivity of the soil column, 0.023 cm/sec, suggested that the surface area of the biofilter system should be about 1% of the drainage area to treat the rainfall intensity of one month recurrence interval.

• 한국수자원학회논문집
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• 제45권12호
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• pp.1227-1241
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• 2012

본 연구에서는 목자판과 수면이 포함되어 있는 시차를 가진 두 영상에 대해 직사각형 검사영역을 설정하고 그 상관계수를 분석하여 수면을 판단하는 새로운 수면인식 기법을 제안하였다. 상관계수의 수직적인 값들로부터 임계치, 첨두값, 기울기, 분산비 등 4가지 방법을 이용하여 수면의 위치를 판정하였다. 흔들림 등으로 인해 두 영상의 위치가 불일치하여 생기는 문제를 제거하기 위해 추가로 영상의 흔들림을 보정하는 알고리즘과 통계적 필터링 기법을 적용하였다. 저수시에 촬영한 28개 지점의 영상에 개발한 수면 인식 방법을 적용하였다. 이 방법으로 계산한 수면은 목측 수면과의 평균상대오차가 3.4~5.7 cm 정도로 나타났다. 수면의 요동이 있을 경우, 이 방법은 기존 방법을 보완하여 영상수위계의 수위 측정성능을 높이는데 활용될 수 있을 것이다.

• 설비공학논문집
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• 제8권1호
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• pp.1-12
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• 1996

In lots of application to heat exchanger systems, closed two-phase thermosyphons are tilted from a horizontal. If the tilt angle, especially, is less than 30$^{\circ}$, the operational performances of thermosyphon are highly dependent on tilt angle. The present study was conducted to better understand such operational behaviors as mech-anni는 of phase change, and flow patterns inside a tilted thermosyphon. For experiment, an ethanol thermosyphon with a 35% of fill charge rate was designed and manufactured, using a copper tube with a diameter 19mm and a length 1500mm. Through a series of test, the tilt angle was kept constant at each of 4 different values in the range 10~25deg. and the heat supply to the evaporator was stepwisely increased up to 30㎾/$m^2$. When a steady state was established to the thermosyphon for each step of thermal loads, the wall temperature distribution and vapor temperature at the condenser were measured. The wall temperature distributions demonstrated a formation of dry patch in the top end zone of the evaporator, with a values of temperature 20~4$0^{\circ}C$ higher than the wetted surface for a moderate heat flux q≒20㎾/$m^2$. Inspite of the presence of hot dry patch, however, the mean values of boiling heat transfer coefficient at the evaporator wall were still in a good agreement with those predicted by Rohsenow's formula, which was based on nucleate boiling. For the condenser, the wall temperatures were practically uniform, and the measured values of condensation heat transfer coefficient were 1.7 times higher than the predicted values obtained from Nusselt's film condensation theory on tilted plate. Using those two expressions, a correlation was formulated as a function of heat flux and tilt angle, to determine the total thermal resistance of a tilted thermosyphon. The correlation formula showed a good agreement with the experimental data within 20%.