• Journal of Mechanical Science and Technology
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• 제15권12호
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• pp.1882-1891
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• 2001

The present paper investigated the effect of ultrasonic vibrations on the melting process of a phase-change material (PCM). Furthermore, the present study considered constant heat flux boundary conditions unlike many of the previous researches adopted constant wall temperature conditions. Therefore, in the present study, modified dimensionless parameters such as Ste* and Ra* were used. Also, general relationships between melting with ultrasonic vibrations and melting without ultrasonic vibrations were established during the melting of PCM. Experimental observations show that the effect of ultrasonic vibrations on heat transfer is very important throughout the melting process. The results of the present study reveal that ultrasonic vibrations accompany the effects like agitation, acoustic streaming, cavitation, and oscillating fluid motion. Such effects are a prime mechanism in the overall melting process when ultrasonic vibrations are applied. They enhance the melting process as much as 2.5 tildes, compared with the result of natural melting. Also, energy can be saved by applying ultrasonic vibrations to the natural melting. In addition, various time-wise dimensionless numbers provide conclusive evidence of the important role of ultrasonic vibrations on the melting phenomena.

• 설비공학논문집
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• 제2권3호
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• pp.188-198
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• 1990

Turbulent characteristics of turbulent pulsating flows were studied experimentally in a square duct. Velocity waveforms, velocity profiles, and turbulent intensity of turbulent pulsating flow were investigated by using a hot-wire anemometer with data acquisition and a processing system in a square duct with a ratio of 1 ($40mm{\times}40mm$) to 4,000mm long. Turbulent components were shown to be larger in decelerating than in accelerating regions and also larger for a large phase of velocity and U'rms distribution of turbulent flow. The effect of velocity amplitude ratio does not exist for specified time [${\theta}(z^{\prime})$], amplitude ratio (${\mid}U^{\prime}_{rms.os.1}{\mid}/{\mid}U_{m.os.1}{\mid}$), and phase difference (${\Delta}U^{\prime}_{rms.os.1}-{\Delta}U_{m.os.1}$) in either turbulent oscillating or cross-sectional mean velocity components. The effect of dimensionless angular frequency for specified time [${\theta}(z^{\prime})$] can be disregarded because the dimensionless angular frequency does not affect the specified time. The velocity distributions of turbulent pulsating flows for various time-averaged Reynolds numbers are in approximate agreement with the velocity distributions for equivalent Reynolds numbers and 1/7th power law of steady flow.

• 한국운동역학회지
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• 제27권2호
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• pp.109-116
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• 2017

Objective: This study aimed to investigate the relationship between leg stiffness and kinematic variables according to load while running. Method: Participants included eight healthy men (mean age, $22.75{\pm}1.16years$; mean height: $1.73{\pm}0.01m$; mean body weight, $71.37{\pm}5.50kg$) who ran with no load or a backpack loaded with 14.08% or 28.17% of their body weight. The analyzed variables included leg stiffness, ground contact time, center of gravity (COG) displacement and Y-axis velocity, lower-extremity joint angle (hip, knee, ankle), peak vertical force (PVF), and change in stance phase leg length. Results: Dimensionless leg stiffness increased significantly with increasing load during running, which was the result of increased PVF and contact time due to decreased leg lengths and COG displacement and velocity. Leg length and leg stiffness showed a negative correlation (r = -.902, $R^2=0.814$). COG velocity showed a similar correlation with COG displacement (r = .408, $R^2=.166$) and contact time (r = -.455, $R^2=.207$). Conclusion: Dimensionless leg stiffness increased during running with a load. In this investigation, leg stiffness due to load increased was most closely related to the PVF, knee joint angle, and change in stance phase leg length. However, leg stiffness was unaffected by change in contact time, COG velocity, and COG displacement.

• 대한기계학회논문집B
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• 제22권12호
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• pp.1726-1734
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• 1998

This paper focuses on the unsteady close-contact melting phenomenon occurring between a phase change material kept at its melting temperature and a flat surface on which constant heat flux is imposed. Based on the same simplifications and framework of analysis as the case of constant surface temperature, an approximate analytical solution which depends only on the liquid-to-solid density ratio is successfully derived. In order to keep consistency with the known solution procedure, both the dimensionless wall heat flux and the Stefan number are properly redefined. The obtained solution proves to agree quite well with the published numerical data and to be capable of resolving the fundamental features of unsteady close-contact melting, especially in the presence of the solid-liquid density difference. The density ratio directly affects the film growth rate and the initial value of solid descending velocity, thereby controlling the duration of unsteady process. The effects of other parameters can be evaluated readily from the steady solution which is implied in the normalized result. Since the dimensionless surface temperature for the present boundary condition increases from zero to unity along the evolution path of the liquid film thickness, the unsteady process lasts longer than that for the case of isothermal heating.

• 한국수산과학회지
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• 제38권5호
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• pp.316-322
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• 2005

In order to develop a floating breakwater, which can efficiently control long period waves, vertical plates are attached in pontoon. Wave control and dynamic behaviors of the newly developed vertical plates type are verified from numerical analysis and hydraulic experiment. As a result, for the wave control and energy dissipation, the newly developed vertical plates type is more efficient than the conventional pontoon type. For the floating body motion, the wave transmission, depending on incident wave period, is decreased at the natural frequency. Dimensionless drift distance has similar trend of the reflection rate of wave transformation near natural frequency except maximum and minimum values. Dimensionless maximum tension is 17 percent of the weight of floating breakwater in case of the conventional pontoon type and 18 percent or 14 percent in case of the newly developed vertical plates type. Thus, it is shown that the wave control is improved by the vertical plates type. In addition, by adjusting the interval of the front and back vertical plate, we would control proper wave control.

• 대한기계학회논문집B
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• 제25권12호
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• pp.1861-1868
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• 2001

Vortical structures in the near-wake region of a cubic obstacle are studied using numerical simulation. We consider flow between two parallel plates with a cube mounted on one of the plates. In the turbulent near-wake region of the flow, coherent structures such as hairpin vortices are found. Quasi-periodic behavior of the hairpin vortices is noticed; its dimensionless frequency at Re=1,000 is about 0.82 which is consistent with the result of Elavarasan of et al.'s experiment [Fluid Dyn. Research, 2000, 27] although their geometry is somewhat different from on. In the case of Re=3,500, the dimensionless frequency of the hairpin vortex is about 1.60. It is observed that the translating speed of the head of the hairpin vortex is lower than the streamwise mean velocity at that location. In the vicinity of the lower plate downstream of the cube, various length scales are identified thus less coherent. However, it is noticed that the vortical structures become gradually elongated downstream of the new reattachment.

• 대한기계학회논문집B
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• 제22권3호
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• pp.373-385
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• 1998

The local Nusselt numbers have been measured for a round turbulent jet impinging on the convex surface with and without rib. Liquid crystal/transient method was used to determine the Nusselt number distributions along the surface. The temperature on the surface was measured to within .+-.0.25 deg. C accuracy using liquid crystal and a digital color image processing system. The experiments were made for the jet Reynolds number (Re) 23,000, the dimensionless nozzle-to-surface distance (L/d) from 6 to 10, the dimensionless surface curvature (d/D) 0.056, and the various rib types (height(d$_{1}$) from 1 to 2 mm, pitch (p) from 6 to 32 mm). It was found that the average Nusselt numbers on the convex surface with rib are higher than those without rib, mainly due to an increase in the turbulent intensity caused by flow separation, recirculation and reattachment on the wall surface. In addition, we compared the results by the steady-state method using the gold-film Intrex with those by the transient method.

• 물과 미래
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• 제28권4호
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• pp.147-154
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• 1995

본 연구에서는 특별한 에너지 감세를 위한 수공구조물이 설치되지 않은 중 소규모 단락부 하류부에서 자유낙하하는 수맥으로 인하여 발생되는 비점착성 하상재료의 세굴특성을 수리모형실험을 통하여 분석하였다. 단락수맥 하류부에서의 세굴특성을 분석하기 위하여 먼저 직사각형 단락수맥의 흐름특성을 조사하였으며, 하상재료, 유량, 하류수심을 변화시키면서 세굴공의 직하류부에 생성되는 둔덕(mound)의 유무로 인한 세굴특성을 비교 분석하였다. 평형세굴심 뿐만 아니라 둔덕의 높이도 밀도 후르드수의 함수로 표시되며, 밀도 후르드수는 다른 무차원량 보다 평형세굴심과 비교적 깊은 관계가 있는 것을 확인하였다. 단락부 하류부 하상 보호공의 설계시 둔덕의 영향을 고려해야 한다는 결론을 얻었다.

• 한국자동차공학회논문집
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• 제1권3호
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• pp.34-45
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• 1993

This study aims to analyze the mixing characteristics of hydrogen considered as a new fuel for internal combustion engines. As the physical property of helium gas is similar to that of hydrogen, helium gas was used in this study. To analyze the steady and unsteady behavior of jet, helium gas was injected into the stationary atmosphere at the normal temperature and pressure. Concentration of helium gas in the center of jet flow is in inverse proportion with axial distance from the nozzle tip. This agrees with the free jet theory of Schlichting. The relative equation for dimensionless concentration to radial/axial distance the axial distance of potential core region, the cone angle a of the jet flow and the relative equation for arriving distance of the front of jet flow to the lapse of time are obtained. But free jet theory of Schlichting in the dimensionless concentration is not in agreement with the present experimental results of the distance of the radial direction. It needs more study. When the arrival frequency of jet flow is used as a parameter, the transition area changing from unsteady flow area into steady flow area becomes gradually wider downstream, but its ratio for the whole unsteady flow area gradually decreases.

• 대한기계학회논문집B
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• 제26권1호
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• pp.45-54
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• 2002

The breakup phenomena of a vertical laminar jet issuing from capillary tubes in a quiescent ambient air are investigated using a forced vibration analysis of the surface wave. Using a linear approach to the transient jet velocity, an approximate solution fur the longitudinal motion of a vertical liquid jet is theoretically derived, thus performing an instability analysis by a vibration method. The damping term of this equation is nonlinear as it depends on dimensionless parameters, a Weber number, and an Ohnesorge number. The instability condition is determined based on whether the coefficient of the damping term is less than zero or not. Uniform drop formation is dependent on the vibration frequency fur the forced vibration case.