• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회B
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• pp.2471-2476
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• 2008

Turbulent flow and heat transfer to water at supercritical pressure flowing in vertical pipes is investigated using direct numerical simulation (DNS). A conservative space-time discretization scheme for variable-density flows at low Mach numbers is adopted in the present study to treat steep variations of fluid properties at supercritical pressure just above the thermodynamic critical point. The fluid properties at these conditions are obtained using PROPATH and used in the form of tables in the simulations. The buoyancy influence induced by strong variation of density across the pseudo-critical temperature proved to play an important role in turbulent flow and heat transfer at supercritical state. Depending on the degree of buoyancy influence, turbulent heat transfer may be enhanced or significantly deteriorated, resulting in local hot spots along the heated surface.

• 전기학회논문지
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• 제62권1호
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• pp.43-48
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• 2013

In this paper, magnetic buoyancy force on nonmagnetic solid object submerged in magnetic liquid was simulated and measured. For the evaluation of the force, a multi-physics approach of hydrostatic equilibrium considering magnetic body force as well as gravity is presented. The magnetic body force should be regarded as an additional forcing term in the momentum equation of hydrodynamics. It is also shown that the virtual air-gap based Kelvin's force formula is a useful method for the calculation of force distribution in the magnetic liquid. The experimental result which was performed by a load-cell measurement system agreed quantitatively well with the numerical one.

• Ocean Systems Engineering
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• 제7권3호
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• pp.195-223
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• 2017

This paper presents 3D numerical simulations of a Free Standing Hybrid Riser under Vortex Induced Vibration, with prescribed motion on the top to replace the motion of the buoyancy can. The model is calculated using a fully implicit discretization scheme. The flow field around the riser is computed by solving the Navier-Stokes equations numerically. The fluid domain is discretized using the overset grid approach. Grid points in near-wall regions of riser are of high resolution, while far field flow is in relatively coarse grid. Fluid-structure interaction is accomplished by communication between fluid solver and riser motion solver. Simulation is based on previous experimental data. Two cases are studied with different current speeds, where the motion of the buoyancy can is approximated to a 'banana' shape. A fully three-dimensional CFD approach for VIV simulation for a top side moving Riser has been presented. This paper also presents a simulation of a riser connected to a platform under harmonic regular waves.

• 한국HCI학회:학술대회논문집
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• 한국HCI학회 2007년도 학술대회 3부
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• pp.326-333
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• 2007

To simulate solid dynamics,a we must com-pute the mass, the center of mass, and the products of inertia about the axes of the body of interest. These mass property computations must be continuously re-peated for certain simulations with rigid bodies or as the shape of the body changes. We introduce a GPU-friendly algorithm to approximate the mass properties for an arbitrarily shaped body. Our algorithm converts the necessary volume integrals into surface integrals on a projected plane. It then maps the plane into a frame-buffer in order to perform the surface integrals rapidly on the GPU. To deal with non-convex shapes, we use a depth-peeling algorithm. Our approach is image-based; hence, it is not restricted by the mathematical or geometric representation of the body, which means that it can efficiently compute the mass properties of any object that can be rendered on the graphics hardware. We compare the speed and accuracy of our algorithm with an analytic algorithm, and demonstrate it in a hydrostatic buoyancy simulation for real-time applications, such as interactive games.

• 한국터널지하공간학회 논문집
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• 제20권2호
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• pp.501-511
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• 2018

최근 '도로터널 방재시설 설치 및 관리지침'의 개정으로 인해 터널 내 제연용 제트팬의 용량 산정 시에는 열부력을 고려하도록 규정하고 있다. 그러나 열부력을 고려하는 세부방법론에 대한 규정이 없어, 이에 대한 세부적인 추가 연구가 필요한 실정이다. 본 연구에서는 터널 내 화재 시 열부력을 고려하기 위하여 3차원 수치해석 시뮬레이션을 수행하여 터널 내열부력을 계산하고, 화재차량의 진행방향에 따라 열부력과 차량항력의 관계, 즉, 화재차량의 위치에 따른 터널 내 제연용 제트팬 용량 산정방법에 대하여 검토를 수행하였다. 분석결과에 따르면, 하향경사 터널의 경우에는 열부력이 터널 내 저항력으로 작용하며, 터널 내 화재연기를 제연하기 위해 필요한 제연팬의 승압력은 단순히 터널 입구부의 열부력 값과 출구부의 차량항력값에 의해 단순히 결정되지 않으며, 터널 내 화재차량의 위치에 따라 종합적인 검토가 필요한 것으로 분석되었다.

• 대한임베디드공학회논문지
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• 제12권5호
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• pp.343-350
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• 2017

Underwater glider with a single buoyancy engine could generally obtain propulsive forces by moving the center of buoyancy and gravity. Futhermore, The hull and internal structure of underwater glider are designed according to the purpose of long-time operation, high speed and a wide variety of payloads (sensors, communications and etc.). In this paper, Ray-type underwater glider featuring flatfish is considered in view of hydrodynamics. The hull design is especially performed by the analysis of fluid resistance and dynamic performance. The resistance performance is analyzed using the Computational Fluid Dynamics (CFD). In addition, a simulation program is implemented in order to verify the validity of dynamics modeling and dynamic performances.

• 대한기계학회논문집B
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• 제28권11호
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• pp.1302-1314
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• 2004

Turbulent heat transfer to $CO_2$ at supercritical pressure flowing in vertical tubes is investigated using direct numerical simulation (DNS). A conservative space-time discretization scheme for variable-density flows at low Mach numbers is adopted in the present study to treat steep variations of fluid properties at supercritical pressure just above the thermodynamic critical point. The fluid properties at these conditions are obtained using PROPATH and used in the form of tables in the simulations. The buoyancy influence induced by strong variation of density across the pseudo-critical temperature proved to play a major role in turbulent heat transfer at supercritical state. Depending on the degree of buoyancy influence, turbulent heat transfer may be enhanced or significantly deteriorated, resulting in local hot spots along the heated surface. Based on the results of the present DNS combined with theoretical considerations, the physical mechanism of this local heat transfer deterioration is elucidated.

• International Journal of Naval Architecture and Ocean Engineering
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• 제11권2호
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• pp.865-874
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• 2019

A controller for an underwater glider is presented. Considered underwater glider is a torpedo-shaped autonomous underwater vehicle installing adjustable buoyancy bag and movable battery in it. The controller is composed of an LQR controller to maintain zigzag vertical movement for gliding and two PD controllers to control elevator/rudder angles. The LQR controller controls the pumping speed into the buoyancy bag and the moving speed to locate the battery. One of the PD controller controls the elevator angle to assist the LQR controller, and the other controls the rudder angle to adjust the direction of the underwater glider. A reduced order Luenberger observer is adopted to estimates the center of gravity of the glider and the buoyancy mass that are essential but cannot be measured. Mathematical simulation using Matlab proved the validity of the proposed controller to obtain better performance than conventional LQR only controller under the influence of sea current.

• 전기학회논문지
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• 제65권12호
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• pp.2037-2045
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• 2016

This paper presents a fuzzy-model-based design approach to the buoyancy and moment controls of a class of nonlinear underwater glider. Through the linearization and the sector nonlinearity methodologies, the underwater glider dynamics is represented by a Takagi-Sugeno (T-S) fuzzy model. Sufficient conditions are derived to guarantee the asymptotic stability of the closed-loop system in the format of linear matrix inequality (LMI). Simulation results demonstrate the effectiveness of the proposed buoyancy and moment controllers for the underwater glider.

• 한국수자원학회논문집
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• 제57권8호
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• pp.493-507
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• 2024

RANS 기반의 CFD 해석은 계산 효율성이 높아 실무 수리해석을 포함한 다양한 공학 분야에서 널리 적용되고 있으나 자유수면과 같이 이상유체흐름 해석에서 비물리적인 거동이 나타나는 문제가 오랫동안 제기되어 왔다. 일반적인 RANS 기반의 해석에서 적용되는 2 방정식 난류 모형은 단상유체를 대상으로 개발되어 유체 밀도의 급격한 변화가 발생하는 이상유체에서는 경계면에서 실제와 다른 높은 난류 에너지 생성을 모의한다. 최근 이를 해결하기 위해 개발된 방법 중의 하나인 부력 수정 난류 모형은 해안 분야에 적용되어 일부 적합성이 검증되었으나 개수로 흐름에 적용된 사례는 없다. 본 연구에서는 오픈 소스 프로그램인 OpenFoam의 VOF 기법을 기반으로 부력 수정 난류 모형의 적용성을 평가하였다. 등류 흐름 적용 결과에 의하면 부력 수정 k-𝜖 모형과 부력 수정 k-ω SST 모형이 자유수면 부근에서의 난류 에너지 저감 현상을 잘 모의함을 확인하였으며, 특히 부력 수정 k-ω SST 모형은 연직 유속 분포를 잘 모의함을 확인하였다. 또한 댐 붕괴 흐름에 적용하여 수면형의 변동이 크고 공동이 형성되는 경우에 대해 검토하였다. 모의 결과 부력 수정 난류 모형은 VOF 기법에 따라 상이한 결과를 나타내며 실험결과와 다른 비물리적인 거동을 나타내었다. 부력 수정 난류 모형이 수면이 안정적인 형태인 경우에는 적용성이 있으나 자유 수면의 급격한 변화가 발생하는 경우에 범용적으로 적용하기에는 여전히 한계가 있는 것으로 나타났다. 수면형이 급격하게 변화하거나 공동이 형성되는 흐름의 경우에는 난류 모형에 대한 적절한 보정이 필요한 것으로 판단된다.