• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.3033-3038
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• 2007

In this research, Rayleigh instability of gas-liquid flow in annular pipe is studied in film boiling using viscous potential flow. Viscous potential flow is a kind of approximation of gas-liquid interface considering velocity field as potential including viscosity. A dispersion relation is obtained including the effect of heat and mass transfer and viscosity. New expression for dispersion relation in film boiling and critical wave number is obtained. Viscosity and heat and mass transfer have a stabilizing effect on instability and its effect appears in maximum growth rate and critical wave number. And the existence of marginal stability region is shown.

• 한국산학기술학회논문지
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• 제21권10호
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• pp.415-420
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• 2020

군중 유동은 대도시의 철도 환승역, 터미널, 복합 다중 건물, 경기장 등에서 흔히 볼 수 있으며, 이러한 시설물에서의 이용객들의 원활한 흐름 뿐만아니라 안전 확보측면에서도 중요한 요소이다. 본 연구에서는 새로운 군중 유동 해석법을 개발하여 철도 환승 연결로 모델에 대하여 적용하였다. 해석법에서는 출구의 포텐셜 값을 가장 작은 값으로 입력하고, 주변 격자들의 포텐셜 값은 점진적으로 증가시켜서 전체적인 포텐셜 지도를 구성한다. 포텐셜 값이 큰 격자에서 작은 격자로 이동하는 방향 벡터를 구하여 이를 따르는 유적선을 구한다. 이 유적선이 여객 유동의 기본 경로가 된다. 해석 대상의 모든 모델에서, 보행자들은 처음 예측된 최단 거리 경로로 이동하지 않고, 시시각각의 상황에 따라 변경된 대체 경로를 이용하여 이동하였다. 양 방향의 보행자가 서로 마주치는 병목 구역에서도 진입 시차를 두어 분산시키면 보행이 훨씬 더 원활하게 되었다. 이상의 해석 결과로부터, 철도역의 하드웨어적 개량 공사를 하지 않고, 여객 유동 분석과 같은 소프트웨어적 해석으로도 혼잡 완화 방안을 찾을 수 있음을 보여준다.

• 오토저널
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• 제8권1호
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• pp.52-61
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• 1986

An iterative viscous-potential flow procedure has been developed and used to predict aerodynamic characteristics of automobiles in ground proximity. The method is capable of predicting the effects of separated flows. The viscous-potential flow iteration procedure provides the connection between potential flow, boundary layer and wake modules. The separated wake is modeled in the potential flow analysis by thin sheets across which exists a jump in velocity potential. The ground effect is properly accounted for by placing a body image in the potential flow calculation. The agreement between theory and experiment is good and, thus, demonstrates that the method can be used in the preliminary design stage.

• 대한기계학회논문집A
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• 제28권9호
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• pp.1320-1327
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• 2004

An efficient boundary-based technique is developed for addressing shape design sensitivity analysis in supercavitating flow problem. An analytical sensitivity formula in the form of a boundary integral is derived based on the continuum formulation for a general functional defined in potential flow problems. The formula, which is expressed in terms of the boundary solutions and shape variation vectors, can be conveniently used for gradient computation in a variety of shape design in potential flow problems. While the sensitivity can be calculated independent of the analysis means, such as the finite element method (FEM) or the boundary element method (BEM), the FEM is used for the analysis in this study because of its popularity and easy-to-use features. The advantage of using a boundary-based method is that the shape variation vectors are needed only on the boundary, not over the whole domain. The boundary shape variation vectors are conveniently computed by using finite perturbations of the shape geometry instead of complex analytical differentiation of the geometry functions. The supercavitating flow problem is chosen to illustrate the efficiency of the proposed methodology. Implementation issues for the sensitivity analysis and optimization procedure are also addressed in this flow problem.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 춘계학술대회
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• pp.1047-1052
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• 2004

An efficient boundary-based technique is developed for addressing shape design sensitivity analysis in supercavitating flow problem. An analytical sensitivity formula in the form of a boundary integral is derived based on the continuum formulation for a general functional defined in potential flow problems. The formula, which is expressed in terms of the boundary solutions and shape variation vectors, can be conveniently used for gradient computation in a variety of shape design in potential flow problems. While the sensitivity can be calculated independent of the analysis means, such as the finite element method (FEM) or the boundary element method (BEM), the FEM is used for the analysis in this study because of its popularity and easy-touse features. The advantage of using a boundary-based method is that the shape variation vectors are needed only on the boundary, not over the whole domain. The boundary shape variation vectors are conveniently computed by using finite perturbations of the shape geometry instead of complex analytical differentiation of the geometry functions. The supercavitating flow problem is chosen to illustrate the efficiency of the proposed methodology. Implementation issues for and optimization procedure are addressed in this flow problem.

• 한국해양공학회지
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• 제35권3호
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• pp.229-237
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• 2021

A numerical hydrodynamic performance analysis of the pitch-type multibody wave energy converter (WEC) is carried out based on both linear potential flow theory and computational fluid dynamics (CFD) in the unidirectional wave condition. In the present study, Salter's duck (rotor) is chosen for the analysis. The basic concept of the WEC rotor, which nods when the pressure-induced motions are in phase, is that it converts the kinetic and potential energies of the wave into rotational mechanical energy with the proper power-take-off system. This energy is converted to useful electric energy. The analysis is carried out using three WEC rotors. A multibody analysis using linear potential flow theory is performed using WAMIT (three-dimensional diffraction/radiation potential analysis program), and a CFD analysis is performed by placing three WEC rotors in a numerical wave tank. In particular, the spacing between the three rotors is set to 0.8, 1, and 1.2 times the rotor width, and the hydrodynamic interaction between adjacent rotors is checked. Finally, it is confirmed that the dynamic performance of the rotors slightly changes, but the difference due to the spacing is not noticeable. In addition, the CFD analysis shows a lateral flow phenomenon that cannot be confirmed by linear potential theory, and it is confirmed that the CFD analysis is necessary for the motion analysis of the rotor.

• 대한기계학회논문집B
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• 제25권11호
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• pp.1518-1526
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• 2001

In this paper, the mechanism of unsteady potential interaction and wake interaction in one stage axial turbine is numerically investigated at design point in two-dimensional viewpoint. The numerical technique used is the upwind scheme of Van-Leer's Flux Vector Splitting (FVS) and Cubic spline interpolation is applied on zonal interface between stator and rotor. The inviscid analysis is used to embody the influence of potential interaction only and viscous analysis is used to embody the influences of both potential interaction and wake interaction at the same time. The potential-flow disturbance from the stator into a rotor passage and the periodic blockage effect of rotor produce the unsteady pressure on the blade surface in inviscid analysis. After the wake is cut by rotor, two counterrotating votical patterns flanking the wake centerline in the passage are generated. So, these phenomena magnify the unsteady pressure in viscous analysis than that in inviscid analysis. The resulting unsteady forces on the rotor, generated by the combined interaction of the two effects by potential and wake interaction, are discussed.

• Journal of Ship and Ocean Technology
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• 제4권3호
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• pp.33-45
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• 2000

A computational system for wave and viscous flow analysis (WAVIS) has been developed. The system includes a pre-processor, flow solvers and a post-processor. The pre-processor is composed of full form presentation, surface mesh and field grid generation. The flow solvers are for potential and viscous flow calculation. The post-processor has graphic utility for result analysis. All the programs are integrated in a GUI-launcher package. To validate the developed CFD programs of WAVIS, the calculated results for modern commercial hull forms are compared with measurements. It is found that the results from WAVIS are in good agreement with the experimental data, illustrating the accuracy of the numerical methods employed for WAVIS.

• 한국해양공학회지
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• 제14권3호
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• pp.41-45
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• 2000

In solving the unsteady potential flow problem of the ship in waves with the panel method, in general one can consider the basic flow as the free stream or double body solution. For the double body solution, the body boundary condition has the 2nd derivatives of the velocity potential. Low order panel methods are known to suffer from the significant error in the 2nd derivatives computed at the body surface. This paper analyzes the numerical error in the 2nd derivatives for a 2-D cylinder and a 3-D sphere problem, and an extrapolation method to obtain the correct derivatives on the body surface is suggested.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2006년도 제26회 춘계학술대회논문집
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• pp.57-60
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• 2006

액상의 환경으로 고속의 기체가 분사될 때 기체-액체 표면에서 일어나는 불안정성에 대해 점성전위 유동의 이론을 이용하여 분석하였다. 기체의 속도가 낮을 경우 액상으로 기포로 형성되지만 속도가 증가하면서 기체는 제트의 형태로 변하게 되는데, 천음속 구간에서 제트로 변하게 되는 것으로 알려져 있다. 본 연구에서는 주로 액체 제트를 해석하는데 사용된 점성전위유동이론을 기체 제트의 불안정성 해석에 응용하였다. 천음속 구간에서 기체 제트의 성장률이 변하는 것을 확인하였다. 초음속 구간으로 가면서 성장률이 감소하는 것을 확인하였다. 그리고 이를 레이놀즈수와 같은 무차원수에 대해 기체 제트의 성장률의 변화에 대해 알아보았다.