• 설비공학논문집
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• 제2권1호
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• pp.37-48
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• 1990

The drag and heat transfer reduction phenomena and degradation effects of drag reducing polymer solutions which are known as the viscoelastic fluids are investigated experimentally for the turbulent circular tube flows. Two stainless steel tubes are used for the experimental flow loops. Aqueous solutions of Polyacrylamide Separan AP-273 with concentrations from 300 to 1000 wppm are used as working fluids. Flow loops are set up to measure the friction factors and heat transfer coefficients of test tubes in the once-through system and the recirculating flow system. Test tubes are heated by power supply directly to apply constant heat flux boundary conditions on the wall. Capillary tube viscometer and falling ball viscometer are used to measure the viscous characteristics of fluids and the characteristic relaxation time of a fluid is determined by the Powell-Eyring model. The order of magnidude of the thermal entrance length of a drag reducing polymer solution is close to the order of magnitude of the laminar entrance length of Newtonian fluids. Dimensionless heat transfer coefficients of the viscoelastic non-Newtonian fluids may be represented as a function of flow behavior index n and newly defined viscoelastic Graetz number. As degradation continues viscosity and the characteristic relaxation time of the testing fluids decrease and heat transfer coefficients increase. The characteristic relaxation time is used to define the Weissenberg number and variations of friction factors and heat transfer coefficients due to degradation are presented in terms of the Weissenberg number.

• 전산구조공학
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• 제10권2호
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• pp.233-242
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• 1997

이 논문은 연속보의 동적 최적설계에 관한 연구이다. 본 연구에서는 연속보의 근사적인 최적제원을 결정하기 위하여 컴퓨터 프로그램의 실행결과를 이용한 기법을 이용하였다. 경간길이, 질량 및 휨강성이 변화하는 연속보의 자유진동해석과 이동 집중하중이 작용하는 경우 동적응답해석을 실행할 수 있는 컴퓨터 프로그램을 개발하였다. 모형실험 결과와 이론적인 해석결과가 잘 일치하여 해석결과의 타당성을 검증할 수 있었다. 설계효율을 나타내기 위하여 최대 동적응력, 경간사이의 응력차이, 이동 집중하중이 작용하는 점에서의 rms 처짐 및 전체지간의 총질량 등 4가지의 무차원 변수들의 선형결합으로 이루어지는 최적함수를 정의하였다. 3경간 연속보에 대한 해석결과 개략적인 최적제원을 갖는 연속보로 설계하는 경우 등간격의 등단면 연속보에 비해 설계효율을 개선할 수 있음을 알 수 있었다.

• 태양에너지
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• 제12권2호
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• pp.43-50
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• 1992

공기(空氣)가 통과(通過)하는 소결금속(燒結金屬)에서의 압력손실(壓力損失)에 대한 기초(基礎) 지식(知識)을 얻기 위하여 실험적(實驗的)으로 연구(硏究)를 수행(遂行)하여, 소결금속(燒結金屬)의 공극율(空隙率)과 소결금속(燒結金屬)을 통과(通過)하는 공기(空氣)의 유속(流速)이 압력손실(壓力損失)에 미치는 영향(影響)을 고찰(考察)하고, 또 본(本) 실험(實驗) 결과(結果)를 비소결금속(非燒結金屬)에서의 압력손실(壓力損失)에 대한 대표적(代表的)인 Ergun의 식(式)과 마찰계수(摩擦係數)의 관점(觀點)에서 비교(比較)하였다. 본(本) 연구결과(硏究結果), 소결금속(燒結金屬)에서의 압력손실(壓力損失)은 유속(流速)이 증가(增加)할 수록 증대(增大)하며, 또 동일 유속(流速)에서는 공극율(空隙率)이 감소(減少)(입자직경(粒子直徑)이 감소(減少))할 수록 증대(增大)한다. 특히 본(本) 실험(實驗) 결과(結果)를 소결금속(燒結金屬)의 두께와 직경(直徑) 및 입자직경(粒子直徑)의 무차원수(無次元數)들로 표현되는 무차원(無次元) 실험식(實驗式)으로 유도(誘導)하였으며, 이 실험식(實驗式)은 본(本) 연구결과(硏究結果)와 ${\pm}15%$ 범위에서 일치하고 있다.

• International Journal of Naval Architecture and Ocean Engineering
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• 제6권3호
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• pp.529-561
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• 2014

The two-phase turbulent flow past an interface-piercing circular cylinder is studied using a high-fidelity orthogonal curvilinear grid solver with a Lagrangian dynamic subgrid-scale model for large-eddy simulation and a coupled level set and volume of fluid method for air-water interface tracking. The simulations cover the sub-critical and critical and post critical regimes of the Reynolds and sub and super-critical Froude numbers in order to investigate the effect of both dimensionless parameters on the flow. Significant changes in flow features near the air-water interface were observed as the Reynolds number was increased from the sub-critical to the critical regime. The interface makes the separation point near the interface much delayed for all Reynolds numbers. The separation region at intermediate depths is remarkably reduced for the critical Reynolds number regime. The deep flow resembles the single-phase turbulent flow past a circular cylinder, but includes the effect of the free-surface and the limited span length for sub-critical Reynolds numbers. At different Froude numbers, the air-water interface exhibits significantly changed structures, including breaking bow waves with splashes and bubbles at high Froude numbers. Instantaneous and mean flow features such as interface structures, vortex shedding, Reynolds stresses, and vorticity transport are also analyzed. The results are compared with reference experimental data available in the literature. The deep flow is also compared with the single-phase turbulent flow past a circular cylinder in the similar ranges of Reynolds numbers. Discussion is provided concerning the limitations of the current simulations and available experimental data along with future research.

• 한국수자원학회논문집
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• 제44권6호
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• pp.477-486
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• 2011

본 연구는 인공수로 또는 인공호수와 같은 환경 친화적 친수공간이 건설됨에 따라 발생되는 수질오염 문제를 수질모형실험을 통해 해소할 수 있는 방법을 연구하였다. 호소수의 개념으로 도입되는 인공수로 및 호수는 제한된 수량 공급으로 인해 수질악화, 악취발생 및 녹조 현상이 일어날 수 있다. 하지만, 이러한 현상을 예상할 수 있는 방법은 수치해석으로만 의지해 왔고 물리적 해석에 의한 검증이 이뤄지지 않아 실제 적용에 대한 어려움이 있었다. 따라서 본 연구에서는 오염된 물과 오염되지 않은 물이 서로 희석되어 영양염류의 농도를 낮추는 현상에 착안하여 원형과 모형의 상사에 영향을 받지 않는 무차원의 물리적 수질모형실험을 실시하였다. 또한 수체 내에서 흐름을 인공적으로 발생시키는 흐름유발기기의 효과 검증과 적용방법을 연구하여 목표수질을 유지할 수 있는 방안을 제시하였다.

• 대한기계학회논문집
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• 제16권2호
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• pp.325-335
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• 1992

In this work, direct contact 4-stage fluidized bed heat exchanger is experimentally studied to develop a new type of heat exchanger which recovers the energy contained in the high temperature waste gas exhausted from the industrial furnaces. A sand is used as a heat transfer medium in this experiment. To determine the optimum operating condition, 11 different perforated plates which have a different free area ratio with different hole diameter are used in the experiment. From the room temperature experiment, the pressure drop which is caused by fluidized bed formation is observed. The high temperature experiment is carried out to seek the optimum operating condition of high heat efficiency at low heat exchanger operation cost. The results of experiment are as following. The pressure drop in the high temperature condition can be predicted from the results of the room temperature experiment. And Nusselt number becomes smaller due to the increased interference between sand particles as Reynolds number increases when the dilute phase fluidized beds are formed in nigh temperature condition. But heat transfer amount through the total sand surface area become larger due to the large resident amount of sand. Considering the heat transfer amount and the heat exchanger operation cost, perforated plates which have either a 30% or 35% of free area ratio with 15mm of hole diameter are best fitted for our goal of this work. The values of .phi. which is a dimensionless number representing the absorption heat amount per unit sand rate are in the range from 0.4 to 0.5, when Reynolds number of waste gas ranges from 25-30 with these perforated plates.

• 한국물환경학회지
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• 제32권2호
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• pp.135-141
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• 2016

This study investigated changes of hydrology, soil characteristics, riparian vegetation communities, and geomorphology in sandbars before and after sand-mining to determine the effect of sand-mining at upstream of Guemgang and Bochungcheon streams in Korea. Sand-mining events affected the mining area. They supplied organic matters and nutrients during flood. Sediment deposition caused soil texture change and expansion of vegetation area. However, riverbeds were stabilized after the disturbance. According to the analyses of aerial photographs, the vegetation area was significantly expanded in both dam-regulated streams and dam-unregulated streams after sand-mining. Willow shrubs advanced in disturbed area at an average of 10 years after sand-mining. It took willows trees 10.6 years to become dominant communities. Therefore, it took a total of 20.6 years for new riparian forest to form in sandbar after sand-mining. Our results confirmed that stream flow condition were dependent on vegetation recruitment in dam-regulated streams and dam-unregulated streams. For willow recruitment in unregulated streams, calculation of water level below dimensionless bed shear stress is important because low water level variation is a limiting factor of vegetation recruitment.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2011년도 학술발표회
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• pp.16-16
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• 2011

Statistics of extreme rainfall play a vital role in engineering practice from the perspective of mitigation and protection of infrastructure and human life from flooding. While flood frequency assessments, based on river flood flow data are preferred, the analysis of rainfall data is often more convenient due to the finer spatial nature of rainfall recording networks, often with longer records, and potentially more easily transferable from site to site. The rainfall frequency analysis as a design tool has developed over the years in New Zealand from Seelye's daily rainfall frequency maps in 1947 to Thompson's web based tool in 2010. This paper will present a history of the development of New Zealand rainfall frequency analysis methods, and the details of the latest method, so that comparisons may in future be made with the development of Korean methods. One of the main findings in the development of methods was new knowledge on the distribution of New Zealand rainfall extremes. The High Intensity Rainfall Design System (HIRDS.V3) method (Thompson, 2011) is based upon a regional rainfall frequency analysis with the following assumptions: $\bullet$ An "index flood" rainfall regional frequency method, using the median annual maximum rainfall as the indexing variable. $\bullet$ A regional dimensionless growth curve based on the Generalised Extreme Value (GEV), and using goodness of fit test for the GEV, Gumbel (EV1), and Generalised Logistic (GLO) distributions. $\bullet$ Mapping of median annual maximum rainfall and parameters of the regional growth curves, using thin-plate smoothing splines, a $2km\times2km$ grid, L moments statistics, 10 durations from 10 minutes to 72 hours, and a maximum Average Recurrence Interval of 100 years.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2018년도 학술발표회
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• pp.149-149
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• 2018

Prediction on initial motion of sediment is crucial to evaluate sediment transport and channel stability. The condition of incipient movement of sediment is characterized by bed shear stress, which is generated from force of moving water against the bed of the channel, and by critical shear stress, which depends on force resisting motion of sediment due to the submerged weight of the grains. When the bed shear stress exceeds the critical shear stress, sediment particles begin rolling and sliding at isolated and random locations. In Mountain River, debris flow frequently occurs due to heavy rainfall and can lead some natural stones from mountain slope into the bed river. This phenomenon could add additional forces to sediment transport system in the bed of river and also affect or change direction and magnitude of sediment movement. In this paper, evaluations on incipient motion of uniform coarse gravel under falling spheres impacts using small scale flume channel were conducted. The drag force of falling spheres due to water flow and length movement of falling spheres were investigated. The experiments were carried out in flume channel made by glass wall and steel floor with 12 m long, 0.6 m wide, and 0.6 m deep. The bed slopes were selected with the range from 0.7% to 1.5%. The thickness of granular layer was at least 3 times of diameter of granular particle to meet grain placement condition. The sphere diameters were chosen to be 4cm, 6 cm, 8 cm, 10 cm. The spheres were fallen in to the bed channel for critical condition and under critical condition of motion particle. Based on the experimental results, the Shields curve of particles Reynold number and dimensionless critical shear stress were plotted. The relationship between with drag force and the length movement of spheres were plotted. The pathways of the bed material Under the impact of spheres falling were analyzed.

• 대한조선학회논문집
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• 제31권4호
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• pp.119-129
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• 1994

본 연구의 목적은 좌초시 선저 보강판 부재를 대상으로 강체 웨지가 종방향으로 보강된 판으로 진입할 때의 흡수에너지-찢김길이 관계를 나타내는 실험식을 도출하는 것이다. 본 연구는 제1보에서 수행한 시리즈 실험 결과 및 찢김응답에 관한 주요 인자들의 중요도 조사 결과를 바탕으로 하고 있다. 준정적 하중상태에서 수행된 실험 결과를 바탕으로 무차원 해석을 수행하여 찢김이 진행되는 동안의 판부재의 흡수에너지를 찢김길이, 항복응력, 등가판두께, 웨지각도의 함수로 정식화하였다. 또한, 이식을 충격하중 작용시에도 적용할 수 있도록 동적 효과를 고려하였다. 특히, 본 연구에서는 동적 마찰 효과를 고려하기 위하여 동적 마찰 감소 계수의 개념을 도입하였다. 본 제안식의 유용성을 확인하기 위하여 기존의 제안식 및 충격 실험에 의해 얻어진 결과와 본 제안식의 결과를 비교하였다.