• 태양에너지
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• 제5권2호
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• pp.11-19
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• 1985

In this paper, the interface stability not to occur mixing and entrainment between the adjacent layers has been studied in the case of the selective withdrawal of a stratum and the injection in stratified fluid formed by the density difference in a small solar pond. There are stability parameter, Richardson number, Rayleigh number and Froude number as the parameters governing stability in order to measure the interface stability on the stratified fluid. The model which could measure the interface stability on the stratified fluid was the small solar pond composed by 1 meters wide, 2 meters high, and 5 meters long. In order to measure the interface stability on the stratified fluid at the inlet port, the middle section and the outlet port, Richardson number, Rayleigh number, and Froude number involved in the parameters governing the stability were calculated by means of the data resulted from the test of the study on hydrodynamic stability between the convective and nonconvective layers in that solar pond. Richardson number written by the ratio of inertia force to buoyancy force can be used in order to measure the stability on the stratified fluid related to the buoyancy force generated from the injection of fluid. Rayleigh number written by the product of Grashof number by Prandtl number can be used in order to measure the stability of the fluid related to the heat flux and diffusivity of viscosity. Froude number written by the ratio of gravity force to inertia force can be used in order to measure the stability of the nonhomogeneous fluid related to the density difference. As the result of calculating the parameters governing stability, the interface stability on the stratified fluid couldn't be identified below the 70cm height from the bottom of the solar pond, but it could be identified above the 70cm height from it at the inlet port, the middle section and the outlet port. When compared with such the three parameters as Richardson number, Rayleigh number, Froude number, the calculated result was in accord with them at inlet port, the middle section and the outlet port. Henceforth, it is learned that even though any of the three parameters is used for the purpose of measuring the interface stability on the stratified fluid, the result will be the same with them. It is concluded that all the use of Richardson number, Rayleigh number, and Froude number, is desirable and infallible to measure the interface stability on the stratified fluid in the case of considering the exist of the fluid flow and the heat flux like the model of the solar pond.

• Nuclear Engineering and Technology
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• 제26권3호
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• pp.401-410
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• 1994

실험실 규모의 인공균열 추적자 이동실험을 통해 지하매질의 단일균열을 위한 핵종이동 모델중 수리 분산확산모델 및 국부통로확산모델의 타당성이 연구되었다. 사용된 추적자는 비수착성으로 알려진 우라닌과 소디움 리그노설포네이트 이었으며 적용유량은 0.4에서 1.5cc /min의 범위였다. 최적화법으로 모델들의 관련 매개변수들을 구하였으며 모델과 실험결과를 비교하였다. 실험에서 우라닌은 암석내로의 확산이 중요한 인자임을 보여주었고, 두 물질 모두 균열 표면에서의 수착은 중요하게 작용하지 않았다. 두 모델의 암석 확산을 나타내는 매개변수값들은 서로 잘 일치하였다. 모사결과에 따르면 유량변화시 두 모델은 정량적으로 같은 결과를 주었으나, 균열길이는 다른 크기로 두 모델에 영향을 주었다. 즉, 균열길이가 커짐에 따라 국부통로화산모델의 파과곡선 퍼짐성 (dispersivity)이 수리분산화산모델의 파과곡선 퍼짐성보다 증가하였다. 실험과 모델들의 일치는 위 두모델들이 단일 균열 시스템을 해석하는데 매우 유용함을 입증하였다.

• 한국항해항만학회지
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• 제38권1호
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• pp.19-27
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• 2014

선박이 천수 및 수로를 운항하는 경우, 바닥과 측벽의 영향으로 인해 선체침하 및 비대칭적인 힘이 선체 주위에 발생하여 바닥이나 다른 선박 혹은 수로의 벽에 충돌하는 현상이 발생한다. 특히, 수로가 많은 유럽이나 북미를 운항하는 해운회사와 항해사들은 선박의 충돌을 방지하기 위해서 중요한 문제로 다루고 있다. 본 연구에서는 선박의 안전한 항해를 위해 수치해석을 이용하여 선박과 수로의 벽 사이에 발생하는 유체역학적 힘인 횡력(Sway Force)과 선회 모멘트(Yaw Moment)를 정성적으로 추정하고자 하였다. 천수와 측벽효과(Bank Effect)를 해석할 수 있는 프로그램을 개발하여 크기가 다른 원유 운반선 2척에 대하여 선속, 수심 그리고 선박과 측벽 사이 거리의 변화에 따른 다양한 해석을 수행하였다. 계산된 결과는 시험 결과 및 기 발표된 수치해석 결과와 비교하였다.

• Tribology and Lubricants
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• 제31권5호
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• pp.213-220
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• 2015

This study evaluates the effect of grooves on the stem part of a plunger on the lubrication characteristics of a fuel injection pump (FIP) by using hydrodynamic lubrication analysis. The current study uses the two-dimensional Reynolds equation to evaluate the changes in lubrication characteristics with variations in clearance, viscosity, and grooves for a laminar, incompressible, and unsteady state flow. This study investigates the lubrication characteristics by comparing the dimensionless minimum film thickness or the film parameter, which is the ratio of the minimum film thickness to surface roughness. The analysis method for the groove section differs depending on the depth of the groove. For instance, in the case of a shallow groove, the film thickness equation considers the depth of the groove, while in the case of a deep grove, it considers the flow continuity. The lubrication characteristics of the FIP are more sensitive to changes in the groove width than to changes in other design variables. Moreover, the application of a groove is more effective under low viscosity conditions. The smaller the distance from the edge of the stem part to the first groove in the case of shallow grooves, the better are the lubrication characteristics of the FIP. In contrast, in the case of deep grooves, the lubrication characteristics of the FIP improve as the distance increases. The application of shallow grooves is more effective for improving the lubrication characteristics than the application of deep grooves.

• Korea-Australia Rheology Journal
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• 제21권4호
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• pp.213-223
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• 2009

Taylor-Couette flow (i.e., flow between concentric, rotating cylinders) has long served as a paradigm for studies of hydrodynamic stability. For Newtonian fluids, the rich cascade of transitions from laminar, Couette flow to turbulent flow occurs through a set of well-characterized flow states (Taylor Vortex Flow, wavy Taylor vortices, modulated wavy vortices, etc.) that depend on the Reynolds numbers of both the inner and outer cylinders ($Re_i$ and $Re_o$). While extensive work has been done on (a) the effects of weak viscoelasticity on the first few transitions for $Re_o=0$ and (b) the effects of strong viscoelasticity in the limit of vanishing inertia ($Re_i$ and $Re_o$ both vanishing), the viscoelastic Taylor-Couette problem presents an enormous parameter space, much of which remains completely unexplored. Here we describe our recent experimental efforts to examine the effects of drag reducing polymers on the complete range of flow states observed in the Taylor-Couette problem. Of particular importance in the present work is 1) the rheological characterization of the test solutions via both shear and extensional (CaBER) rheometry, 2) the wide range of parameters examined, including $Re_i$, $Re_o$ and Elasticity number E1, and 3) the use of a consistent, conservative protocol for accessing flow states. We hope that by examining the stability changes for each flow state, we may gain insights into the importance of particular coherent structures in drag reduction, identify simple ways of screening new drag reducing additives, and improve our understanding of the mechanism of drag reduction.

• 천문학회보
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• 제43권1호
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• pp.43.1-43.1
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• 2018

Star formation in galaxies predominantly takes place in giant molecular clouds (GMCs). While it is widely believed that UV radiation feedback from young massive stars can destroy natal GMCs by exciting HII regions and driving their expansion, our understanding on how this actually occurs remains incomplete. To quantitatively assess the effect of UV radiation feedback on cloud disruption, we conduct a series of theoretical studies on the dynamics of HII regions and its role in controlling the star formation efficiency (SFE) and lifetime of GMCs in a wide range of star-forming environments. We first develop a semi-analytic model for the expansion of spherical dusty HII regions driven by the combination of gas and radiation pressures, finding that GMCs in normal disk galaxies are destroyed by gas-pressure driven expansion with SFE < 10%, while more dense and massive clouds with higher SFE are disrupted primarily by radiation pressure. Next, we turn to radiation hydrodynamic simulations of GMC dispersal to allow for self-consistent star formation as well as inhomogeneous density and velocity structures arising from supersonic turbulence. For this, we develop an efficient parallel algorithm for ray tracing method, which enables us to probe a range of cloud masses and sizes. Our parameter study shows that the net SFE, lifetime (measured in units of free-fall time), and the importance of radiation pressure (relative to photoionization) increase primarily with the initial surface density of the cloud. Unlike in the idealized spherical model, we find that the dominant mass loss mechanism is photoevaporation rather than dynamical ejection and that a significant fraction of radiation escapes through low optical-depth channels. We will discuss the astronomical.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2006년 창립20주년기념 정기학술대회 및 국제워크샵
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• pp.311-314
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• 2006

This paper concerns about total dynamic analysis of integrated mining system. This system consists of vertical steel pipe, intermediate buffer station, flexible pipe and self-propelled miner. The self-propelled miner and buffer are assumed as rigid-body of 6-dof. Discrete models of vertical steel pipe and flexible pipe are adopted, which are obtained by means of lumped-parameter method. The motion of mining vessel is not considered. Instead, the motion of mining vessel is taken into account in form of various boundary conditions (e.g. forced excitation in slow motion and/or fast oscillation and so on). A terramechanics model of extremely soft cohesive soil is applied to the self-propelled miner. The hydrodynamic forces and moments are included in the dynamic models of vehicle and lifting pipe system. Hinged and fixed constraints are used to define the connections between sub-systems (vertical steel pipe, buffer, flexible pipe, miner). Equations of motion of the coupled model are derived with respect to the each local coordinates system. Four Euler parameters are used to express the orientations of the sub-systems. To solve the equations of motion of the total dynamic model, an incremental-iterative formulation is employed. Newmark-b method is used for time-domain integration. The total dynamic responses of integrated mining system are investigated.

• 한국수자원학회논문집
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• 제37권11호
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• pp.915-927
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• 2004

남한강 수계를 대상으로 대표 어종의 하천 어류 서식처 환경에 대한 서식처 적합도 기준을 작성하고, 어류 서식을 위한 최적유량의 산정방법을 확립하기 위하여, 생태학적 접근 방법 중 어류서식처 환경평가방법의 한가지인 점유량점증론적방법론(IFIM)의 물리적서식처모의시스템(PHABSIM)을 적용하여 어류서식처 보호 및 유지를 위한 최적유량을 산정하였다. 서식처 평가에 요구되는 수리인자의 모의를 위해 2차원 동수역학적 모델을 이용하였으며, 현장조사를 실시하여 대표어종으로 선정된 피라미의 단일변량곡선법을 적용한 서식처 적합도 기준을 작성하였다. 이러한 결과를 토대로 유황변화에 어류 서식환경이 가장 민감하게 반응하는 여울 구간과 여울이 형성되지 않음으로서 어류의 서식처 평가의 고려대상에서 제외되었던 본류 구간을 동시에 모의하여 상$\cdot$하류간의 수문학적 물질균형이 이루어짐을 확인할 수 있었다.

• Tribology and Lubricants
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• 제31권4호
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• pp.148-156
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• 2015

In the study, a design process for ensuring optimal clearance in a fuel injection pump(FIP) is suggested. Structure analysis and hydrodynamic lubrication analysis are performed to determine the optimal clearance. The FIP is simulated using Hypermesh, Abaqus 6.12 to evaluate the reduction of clearance when the maximum supply pressure is applied. The reduction in clearance is caused by the difference in the deformations between the barrel and plunger. When the deformation of the plunger is larger than that of the barrel, a reduction in clearance at the head part occurs. On the other hand, the maximum clearance reduction equals the maximum deformation in the stem part, because the deformation of barrel does not occur in this region. The clearance of FIP should be designed to be larger than maximum reduction of clearance in order to avoid contact between the plunger and barrel. In addition, the two-dimensional Reynolds equation is used to evaluate lubrication characteristics with variations of viscosity, clearance and nozzle for a laminar, incompressible, unsteady state flow. The equation is discretized using the finite difference method. The lubrication characteristics of FIP are investigated by comparing film parameter, which is the ratio of the minimum film thickness and surface roughness. The optimal clearance of FIP is to be designed by considering the maximum reduction in clearance, lubrication characteristics, machining limits and tolerance of clearance.

• 한국해안해양공학회지
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• 제13권1호
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• pp.61-72
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• 2001

천해영역에서 적용 가능한 동수역학적 수치모형을 이용하여 규칙 입사파군이 수직으로 입사할 때 잠제에 의하여 발생되는 반사와 투과의 거동 특성을 해석하였다. 여러 입사파군 조건에서 계산된 잠제의 반사율과 투과율이 파군의 발달정도를 해석하기 위하여 제시된 변수들의 함수로 표시되었다. 입사파군과 잠제의 반사율 및 투과율의 관계를 정성적으로 해석한 결과, 고파군이 발달될수록 잠제에 의하여 발생되는 입사파랑의 반사율과 투과율의 크기가 감소하는 것으로 나타났다. 특히 파군으로 입사하는 경우의 반사율과 투과율이 단일 파랑으로 입사하는 경우의 반사율과 투과율에 비하여 상대적으로 작게 나타났다. 그러나 고파군과 고파군의 간격에 따른 반사율과 투과율의 차이는 크지 않은 것으로 나타났다. 비록 제한적이지만 본 연구에서 해석된 결과에 의하면 파랑 연이음의 평균을 이용할 때 입사파군과 잠제의 반사율 및 투과율의 관계가 가장 잘 해석될 수 있다.