• 대한기계학회논문집B
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• 제23권5호
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• pp.610-620
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• 1999

Recently, numerous modifications of low Reynolds number $k-{\epsilon}$ model have boon carried out with the aid of DNS data. However, the previous models made in this way are too intricate to be used practically. To overcome this shortcoming, a new low Reynolds number $k-{\epsilon}$ model has boon developed by considering the distribution of turbulent properties near the wall. This study proposes the revised a turbulence model for prediction of turbulent flow with adverse pressure gradient and separation. Nondimensional distance $y^+$ in damping functions is changed to $y^*$ and some terms modeled for one dimensional flow in $\epsilon$ equations are expanded into two or three dimensional form. Predicted results by the revised model show an acceptable agreement with DNS data and experimental results. However, for a turbulent flow with severe adverse pressure gradient, an additive term reflecting an adverse pressure gradient effect will have to be considered.

• 대한수학회지
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• 제60권2호
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• pp.375-393
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• 2023

Rabinowitz action functional is the Lagrange multiplier functional of the negative area functional to a constraint given by the mean value of a Hamiltonian. In this note we show that on a symplectization there is a one-to-one correspondence between gradient flow lines of Rabinowitz action functional and gradient flow lines of the restriction of the negative area functional to the constraint. In the appendix we explain the motivation behind this result. Namely that the restricted functional satisfies Chas-Sullivan additivity for concatenation of loops which the Rabinowitz action functional does in general not do.

• 대한기계학회논문집
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• 제16권2호
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• pp.369-381
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• 1992

Characteristics of flow and heat transfer have been studied numerically in a square duct with a periodic pressure gradient. The flow in a duct was assumed to be fully developed and constant heat flux was imposed at the surfaces of a square duct. The distributions of axial velocity and time-space averaged temperature are investigated with angular velocity and amplitude ratio at a given Reynolds number 1000. When the periodic pressure gradient was imposed axially in a duct, the reverse flow may be occurred near the duct wall. The magnitude of this reverse flow increases as the amplitude ratio increases or as the angular frequency decreases. In the ranges of the amplitude ratio and the angular velocity in present investigation, the ratio of the periodic time space averaged temperature to the nonperiodic space averaged temperature has been found to be greater than one. This means that the cooling effect at the duct walls deteriorates with a periodic situation compared with nonperiodic one.

• 설비공학논문집
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• 제13권4호
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• pp.304-311
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• 2001

An experimental study on the countercurrent two-phase flow in narrow rectangular channels has been performed. The void fraction and the pressure gradient were investigated using air and water in 760 mm long, 100 mm wide. vertical test sections with 2, 3 and 5 mm channel gaps. Tests were systematically performed with downward liquid superficial velocities and upward gas velocities covering 0 to 0.08 and 0 to 2.5 m/s ranges. respectively. the experimental results were compared with the previous correlations, which were mainly for round tubes, and the qualitative trends were found to be in good agreements. However the quantitative discrepancies were hardly neglected. as the superficial gas velocities increased, the void fraction increased and the pressure gradient decreased, where the effects of the liquid superficial velocities were infinitesimal. as the gap width of the rectangular channel increased the void fraction and the 2-phase frictional pressure gradient approached those values for the round tubes. Equi-periphery diameter, rather than the hydraulic diameter, seemed to be more effective in the analysis of two-phase flow behavior.

• 대한전자공학회논문지SP
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• 제38권4호
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• pp.422-433
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• 2001

본 논문에서는 영상의 그래디언트 (gradient)를 구하여 그래디언트 값의 분포를 마치 3차원 지형과 같은 개념으로 간주하고 여기에 물이 흐르는 개념을 적용한 에지 (edge) 검출 방법을 제안하였다 영상에서 그래디언트 값이 큰 부분은 배경과 객체간의 에지라 볼 수 있으며, 이 에지에 물이 고이게 하기 위해서는 반전된 그래디언트 영상을 사용한다. 반전된 그래디언트 영상에서 물의 흐름을 기반으로 한 enhancing 작업과 국부적응 임계값 적용을 실시하여 잡음을 줄인 에지 영상을 찾는 방법을 제안한다. 합성영상과 실제영상에 대한실험을 통해 제안한 방법의 효율성을 검증하였다.

• Journal of Mechanical Science and Technology
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• 제19권4호
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• pp.1027-1035
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• 2005

Thermophoresis in dense gases is studied by using a multi-scale approach and Born- Yvon­Green (BYG) equation. The problem of a particle movement in an ambient dense gas under temperature gradient is divided into inter and outer ones. The pressure gradient in the inner region is obtained from the solutions of BYG equation. The velocity profile is derived from the conservation equations and calculated using the pressure gradient, which provides the particle velocity in the outer problem. It is shown that the temperature gradient applied to the quiescent ambient gas induces some pressure gradient and thus flow tangential to the particle surface in the interfacial region. The mechanism that induces the flow may be the dominant source of the thermophretic particle movement in dense gases. It is also shown that the particle velocity has a nonlinear relationship with the applied temperature gradient and decreases with increasing temperature.

• 대한기계학회논문집B
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• 제29권1호
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• pp.131-138
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• 2005

Thermo-osmosis of liquids in a microscale channel is investigated by theoretical and simulation study. From the basic set of conservation equations, the temperature and velocity distributions are derived in the function of the given temperatures and pressure gradient. The pressure gradient for a given temperature gradient is then obtained by the molecular simulation. It is shown that the temperature gradient tangential to the surface induces the pressure gradient and thus the flow in the interfacial region between the liquid and channel surface. The thermo-osmotic flow is proportional to the applied temperature gradient, and the factor of proportionality depends on temperature and intermolecular potential. The origin and characteristics of the phenomenon are discussed in molecular details.

• 대한기계학회논문집
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• 제18권3호
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• pp.690-700
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• 1994

The effects of thermal stratification on the flow of a stratified fluid past a circular cylinder were examined in a wind tunnel. In order to produce strong thermal stratifications, a compact heat exchanger type variable electric heater is employed. Linear temperature gradient of up to $250^{\circ}C/m$ can be well sustained. The velocity and temperature profiles in the cylinder wake with a strong thermal gradient of $200^{\circ}C/m$ were measured and the smoke wire flow visualization method was used to investigate the wake characteristics. It is found that the temperature field effects as an active contaminant, so that the mean velocity and temperature profiles can not sustain their symmetricity about the wake centerline when such a strong thermal gradient is superimposed. It is evident that the turbulent mixing in the upper half section is stronger than that of the lower half of the wake in a stably stratified flow.

• 대한의용생체공학회:학술대회논문집
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• 대한의용생체공학회 1992년도 춘계학술대회
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• pp.92-97
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• 1992

The pulsatile nature of blood flow makes artefacts in 2D Fourier transform image. Spatial presaturation is known to be effective in eliminating flow artefacts when the spin echo acquisition is employed. However. this method requires additional RF pulse and spoiling gradient for presaturation. In this paper a new flow saturation technique which does not require additional saturation-RF and gradient is proposed. The proposed technique is equivalent to the existing saturation technique but the elimination of the flow component is achieved by a pair of tailored $90^{\circ}-180^{\circ}$ RF pulses in tile spin echo sequence. By use of two tailored RF pulses with opposite phase polarity, a linear phase gradient is generated for those moving materials and consequently all the spins of moving materials become dephased thereby no signal is observable. Computer simulations and experimental results obtained using both a phantom and a human volunteer with a 2.0 T whole body system are also presented.

• International Journal of Fluid Machinery and Systems
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• 제8권4호
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• pp.304-310
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• 2015

Numerical simulation was applied to investigate the Taylor vortex flow inside the concentric cylinders with a constant radial temperature gradient. The reliability of numerical simulation method was verified by the experimental results of PIV. The radial velocity and temperature distribution in plain and 12-slit model at different axial locations were compared, and the heat flux distributions along the inner cylinder wall at different work conditions were obtained. In the plain model, the average surface heat flux of inner cylinder increased with the inner cylinder rotation speed. In slit model, the slit wall significantly changed the distribution of flow field and temperature in the annulus gap, and the radial flow was strengthen obviously, which promoted the heat transfer process at the same working condition.