• 한국분무공학회지
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• 제12권4호
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• pp.198-203
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• 2007

In the present work, surface wetting effect on spread-splash regime and transition criterion of the water and ethanol droplets impacting an unheated dry wall has been experimentally investigated. The droplet was directed on a polished STS plate and a glass slide, and the impinging behavior was visualized and recorded using a CCD camera. Droplet diameter and velocity approaching the wall were measured as well. The critical Sommerfeld number representing the spread-splash boundary for the ethanol droplet impinging on the substrates turned out to be smaller compared to that for the water droplet impinging on the substrates with the surface roughness condition remained unchanged. The shift of the transition boundary is considered to be due to the effect of the surface wettability represented by static contact angle and surface tension of droplet.

• 태양에너지
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• 제20권1호
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• pp.81-89
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• 2000

In the present study, the characteristics of conductive and convective heat transfer occurred in a circular absorber of PTC (parabolic trough concentrator) for medium temperature solar energy utility were numerically investigated. A circular tube was considered as an absorber and the shape of PTC modeled in this study was based on the system that was installed in Korea Institute of Energy Research. Not only convection inside the tube but also conduction through the wall of the tube were analyzed, simultaneously. Circumferentially non-uniform heat flux that was simulated from the non-uniform solar disc model proposed by Jose was applied as thermal boundary condition on the tube surface. And, hydrodynamically fully developed laminar velocity profile was used as the inlet boundary condition and it was assumed that the working fluid was water. And, local heat fluxes at the interface of the tube and the working fluid were calculated for different wall thickness and thermal conductivity of the tube at various Reynolds number. Based on the results, the effects of thermal conduction of the tube on the local heat transfer were investigated.

• Structural Engineering and Mechanics
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• 제81권4호
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• pp.443-459
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• 2022

The paper studies the dispersion and attenuation of propagating waves in the "plate+compressible viscous fluid layer" system in the case where the fluid layer flow is restricted with a rigid wall, and in the case where the fluid layer has a free face. The motion of the plate is described by the exact equations of elastodynamics and the flow of the fluid by the linearized Navier-Stokes equations for compressible barotropic Newtonian viscous fluids. Analytical expressions are obtained for the amplitudes of the sought values, and the dispersion equation is derived using the corresponding boundary and compatibility conditions. To find the complex roots of the dispersion equation, an algorithm based on equating the modulus of the dispersion determinant to zero is developed. Numerical results on the dispersion and attenuation curves for various pairs of plate and fluid materials under different fluid layer face conditions are presented and discussed. Corresponding conclusions on the influence of the problem parameters on the dispersion and attenuation curves are made and, in particular, it is established that the change of the free face boundary condition with the impermeability condition can influence the dispersion and attenuation curves not only in the quantitative, but also in the qualitative sense.

• 대한기계학회논문집B
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• 제29권2호
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• pp.169-179
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• 2005

Nucleate boiling experiments on heating surface of constant wall temperature were performed using R113 for almost saturated pool boiling conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain a constant wall temperature condition of heating surface and to measure the heat flow rate with high temporal and spatial resolutions. Bubble images during the bubble growth were taken as 5000 frames per second using a high-speed CCD camera synchronized with the heat flow rate measurements. The bubble growth behavior was analyzed using the new dimensionless parameters for each growth regions to permit comparisons with previous experimental results at the same scale. We found that the new dimensionless parameters can describe the whole growth region as initial and later (thermal) respectively. The comparisons showed good agreement in the initial and thermal growth regions. In the initial growth region including surface tension controlled, transition and inertia controlled regions as divided by Robinson and Judd, the bubble growth rate showed that the bubble radius was proportional to $t^{2/3}$ regardless of working fluids and heating conditions. And in the thermal growth region as also called asymptotic region, the bubble showed a growth rate that was proportional to $t^{1/5}$, also. Those growth rates were slower than the growth rates proposed in previous analytical analyses. The required heat flow rate for the volume change of the observed bubble was estimated to be larger than the heat flow rate measured at the wall. Heat, which is different from the instantaneous heat supplied through the heating wall, can be estimated as being transferred through the interface between bubble and liquid even with saturated pool condition. This phenomenon under a saturated pool condition needs to be analyzed and the data from this study can supply the good experimental data with the precise boundary condition (constant wall temperature).

• 한국가시화정보학회지
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• 제20권2호
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• pp.45-54
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• 2022

We analyzed the low wall-shear stress area in the intracranial aneurysm that occurred at an anterior communicating artery with a special emphasis on vortical structures close to the wall. We reconstructed the aneurysm model from patient CTA data. We assumed blood as an incompressible Newtonian fluid and treated the blood vessel as a solid wall. The pulsatile boundary condition was applied at the inlet of the anterior cerebral artery. From the instantaneous flow field, we computed the histogram of the wall-shear stress over the aneurysm wall and found the low wall-shear stress event (< 0.4 Pa). This extreme event was due to the low wall-shear stress area that occurred at the daughter sac. We found that the merging of two vortices induced the low wall-shear stress area; one arises from the morphological characteristics of the daughter sac, and the other is formed by a jet flow into the aneurysm sac. The latter approaches the daughter sac, which ultimately leads to the strong ejection event near the daughter sac.

• Journal of Mechanical Science and Technology
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• 제20권5호
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• pp.692-709
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• 2006

Nucleate pool boiling experiments with constant wall temperature were performed using pure R1l3 for subcooled, saturated, and superheated pool conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant wall temperature and to measure the instantaneous heat flow rate accurately with high temporal and spatial resolutions. Images of bubble growth were taken at 5,000 frames per second using a high-speed CCD camera synchronized with the heat flow rate measurements. The bubble geometry was obtained from the captured bubble images. The effect of the pool conditions on the bubble growth behavior was analyzed using dimensionless parameters for the initial and thermal growth regions. The effect of the pool conditions on the heat flow rate behavior was also examined. This study will provide good experimental data with precise constant wall temperature boundary condition for such works.

• 대한기계학회논문집
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• 제19권4호
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• pp.1071-1082
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• 1995

Numerical analyses have been performed to obtain the absorption heat and mass transfer coefficients and the absorption mass flux from a falling film of the LiBr aqueous solution which is cooled by cooling air. Heat flux at the wall is specified in terms of the heat transfer coefficient of cooling air and the cooling air temperature. Effects of operating conditions, such as the heat transfer coefficient, the cooling air temperature, the system pressure and the solution inlet concentration have been investigated in view of the local absorption mass flux and the total mass transfer rate. Effects of film thickness and film Reynolds number on the heat and mass transfer coefficients have been also estimated. Analyses for the constant wall temperature condition have been also carried out to examine the reliability of present numerical method by comparing with previous investigations.

• 유변학
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• 제5권1호
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• pp.34-48
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• 1993

최근 CAD/CAM의 발전과 더불어 사출성형공정은 여러분야에 폭넓게 응용되고 있 다. 사출성형공정은 크게 충전과정(filling stage), 냉각과정(cooling stage), 보압과정(packing stage)로 나누어 지는데 이중 충전과정은냉각과정과 보압과정에서 나타날 물리적인 현상과 최종 성형품의 기계적 성질에 중요한 영향을 끼치게 된다. 충전과정의 수치 해석 방법은 대 표적으로 control volume method, branching flow method, transient moving boun-dary method로 구분된다. 본 연구에서는 격자의 형태를 양호하게 형성시키고 유동선단의 형태를 개선하기위한 기법인 Spline 곡선을 이용한 선단격자 재구성(frontal remeshing using spline curve)과 수치해석에 소요되는 시간을 줄이기 위하여 벽면경계조건 처리를 위한 선단 유동 장생성(frontal flow field construction for wall boun-dary condition treatment)기법을 개발 하고 transient moving voundary method에 적용시켜 원형 평판과 인장 및 굽힘시편 그리고 두께가 변하는 사각 형상을 가진 캐비터에서의 충전과정을 수치해석하였다. 그결과 압력 분 포, 온도분포, 속도장, 유동선단의 진전형태 등이 기존에 제출된 해석결과와 비교하여 볼 때 만족스러운 수치해석결과를 보였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.3116-3121
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• 2007

In this paper, the inverse shape design is introduced using the permeable wall boundary condition. Inverse shape design defines the blade shape for the prescribed Mach numbers or pressure distribution on its surface. It calculates the normal mass flux from the difference between the calculated and prescribed pressure at the surface. A new geometry can be achieved after applying the quasi one-dimensional continuity equation from the leading edge to the trailing edge. For validation of this method, two test cases are studied. The first test case of inverse shape design illustrates the cosine bump with a strong shock. After seven geometry modifications, the shock-free bump geometry can be obtained. The second example concerns the redesign of a transonic turbine cascade. The initial isentropic Mach distribution has a peak on the upper surface. The target isentropic Mach number distribution was imposed smoothly. The peak of Mach distribution has disappeared at the final geometry. This proposed inverse design method has proven to be an efficient and robust tool in turbomachinery design fields.

• 전자공학회논문지D
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• 제34D권10호
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• pp.60-67
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• 1997

This paper presents a 3-D finite difference time domain (FDTD) method used for indoor propagation simulations where the electromagnetic wav eis uniformly excited on th eone of the wall in a building and affected by an indoor obstacles. In cases of simulation and measurement, the frequency of 851 MHz is used. The conductivities of walls, floor, ceiling and indoor obstacles are measured and used for simulations. These simulations are carried out using different boundary condition such as mur's absorbing boundary condition (ABC) and perfectly matched layer (PML) technique. The PML technique is found to be well-suited to this analysis because of it's smaller computational domain than mur's ABC. The measured signal strengths are compared to simulated values with good agreement.