Hyoung Gwon Choi

Laminar convective heat transfer of a Bingham plastic in a circular pipe—II. Numerical approach hydrodynamically developing flow and simultaneously developing flow

Abstract Hydrodynamically developing and simultaneously (that is, hydrodynamically and thermally) developing laminar flows of a Bingham plastic in a circular pipe have been investigated numerically. Solutions have been obtained by using a four-step fractional method combined with an equal order bilinear finite element method. For hydrodynamically developing flow, shorter entrance length is required to reach fully developed velocity field and thicker unyielded region appears closer to the inlet as the yield stress becomes larger. For simultaneously developing flow, the heat transfer characteristics show the same trends as those predicted from the thermally developing flow.

Finite element method simulation of turbulent wavy core–annular flows using a k–ω turbulence model method

Abstract A numerical simulation of wavy core flow was carried out previously. They calculated the interfacial wave shape for laminar flow. In our present simulation, the shear stress transport turbulence model is used to solve the turbulent kinetic energy and dissipation rate equations and a splitting method is used to solve Navier–Stokes equations for the wave shape, pressure gradient and the profiles of velocity and pressure in turbulent wavy core flows. The wavelength decreases with Reynolds number R and with the volume ratio η. The pressure gradient increases with Reynolds number R and with the volume ratio η. High pressures are generated at a stagnation point leading to wave steepening, while low pressures are generated at a reattachment point. The computed wave shapes and frictional losses are in satisfactory agreement with experiments and greatly improve on previous results.

A correlation for the lift-off of many particles in plane Poiseuille flows of Newtonian fluids

Choi & Joseph reported a two-dimensional numerical investigation of the lift-off of 300 circular particles in plane Poiseuille flows of Newtonian fluids. We perform similar simulations. Particles heavier than the fluid are initially placed in a closely packed ordered configuration at the bottom of a periodic channel. The fluid-particle mixture is driven by an external pressure gradient. The particles are suspended or fluidized by lift forces that balance the buoyant weight perpendicular to the flow. Pressure waves corresponding to the waves at the fluid-mixture interface are observed. During the initial transient, these waves grow, resulting in bed erosion. At sufficiently large shear Reynolds numbers the particles occupy the entire channel width during the transient. The particle bed eventually settles to an equilibrium height which increases as the shear Reynolds number is increased. Heavier particles are lifted to a smaller equilibrium height at the same Reynolds number. A correlation for the lift-off of many particles is obtained from the numerical data. The correlation is used to estimate the critical shear Reynolds number for lift-off of many particles

Numerical analysis of two-dimensional motion of a freely falling circular cylinder in an infinite fluid

The two-dimensional motion of a circular cylinder freely falling or rising in an infinite fluid is investigated numerically for the range of Reynolds number Re < 188 (Galileo number G < 163), where the wake behind the cylinder remains two-dimensional, using a combined formulation of the governing equations for the fluid and the dynamic equations for the cylinder. The effect of vortex shedding on the motion of the freely falling or rising cylinder is clearly shown. As the streamwise velocity of the cylinder increases due to gravity, the periodic vortex shedding induces a periodic motion of the cylinder, which is manifested by the generation of the angular velocity vector of the cylinder parallel to the cross-product of the gravitational acceleration vector and the transverse velocity vector of the cylinder. Correlations of the Strouhal-Reynolds-number and Strouhal-Galileo-number relationship are deduced from the results. The Strouhal number is found to be smaller than that for the corresponding fixed circular cylinder when the two Reynolds numbers based on the streamwise terminal velocity of the freely falling or rising circular cylinder and the free-stream velocity of the fixed one are the same. From numerical experiments, it is shown that the transverse motion of the cylinder plays a crucial role in reducing the Strouhal number. The effect of the transverse motion is similar to that of suction flow on the low-pressure side, where a vortex is generated and then separates, so that the pressure on this side recovers with the vortex separation retarded. The effects of the transverse motion on the lift, drag and moment coefficients are also discussed. Finally, the effect of the solid/fluid density ratio on Strouhal-Reynolds-number relationship is investigated and a plausible correlation is proposed.

Performance Analysis of the Parallel CUPID Code for Various Parallel Programming Models in Symmetric Multi-Processing System

A parallelization of the bi-conjugate gradient solver for the pressure equation of the CUPID (component unstructured program for interfacial dynamics) code, which was developed for analyzing the components of a pressurized water-cooled reactor, was studied in a symmetric multi-processing system. The parallel performance was investigated for three typical parallel programming models (MPI, OpenMP, Hybrid) by solving incompressible backward-facing step flow at various grid resolutions. It was confirmed that parallel performance was low when problem size was small or the memory requirement for each thread was considerably higher than the cache memory. Furthermore, it was shown that MPI was better than OpenMP regardless of the problem size, and Hybrid was the best when the number of threads was relatively small.

Study on the segregation algorithms of the incompressible Navier-Stokes equations using P1P1/P2P1 finite element formulation

Since velocity and pressure fields are obtained at each separate step in a segregated formulation, it produces a small and simple matrix at each step than in an integrated formulation. And the memory and cost requirements of computations are significantly reduced because the pressure equation for the mass conservation of the Navier-Stokes equations is constructed only once if the mesh is fixed. However, segregation formulation solves Poisson equation of elliptic type so that it always needs a pressure boundary condition along a boundary even when no physical information on pressure is given. In general, open boundary problems sometimes have no information on both the pressure and pressure gradient at the exit because velocities on downstream boundary are unknown. Therefore, the problem of proper imposition of outflow boundary condition is not clearly stated, which is an important issue in the segregation approach [1, 2].

Numerical Analysis of Conjugate Heat Transfer for Various Ice-Ball Shapes

In this study, numerical simulations were conducted for conjugate heat transfer around ice balls in an encapsulated ice thermal storage system. Four shapes of ice balls were modeled; the default one was a sphere, and the other three shapes were designed to enhance convective heat transfer through the ball surface. The flow around the ball was laminar, for which the Reynolds number was 300, and both forced and natural convections inside and outside the balls were considered. The simulations revealed that the magnitude of convective heat transfer for the different shapes decreased in the following order: bone, dimple, hole, and sphere. For the entire simulation, the maximum difference in the average temperatures of water inside the capsules was found to be 0.9°C. Therefore, it can be said that the effect of ice-ball shape on the performance of the ice thermal storage system is significant, considering that more than 0.3 million balls are used in this system. Corresponding Author, hgchoi@seoultech.ac.kr C 2016 The Korean Society of Mechanical Engineers 1. 서 론 심야의 잉여전력을 이용하여 여름철 피크전력 부하를 효율적으로 감소시키는 중앙집중식 축열 냉방시스템 중 빙축열시스템은 에너지 합리화 시 스템으로서 1990년대 초반부터 보급되어 왔으며 지금까지 시스템 효율의 증대와 운전 시뮬레이션 을 통하여 에너지 비용을 분석하고 최적운전기 법을 이용한 비용절감에 대한 연구 등을 수행해 왔다. 빙축열시스템에서 가장 핵심은 빙축열조 내 박서원 · 김명수 · 전병진 · 최형권 606 Case 1. Sphere Case 2. Bone Case 3. Hole Case 4. Dimple Fig. 1 Shapes of ice capsule for simulation 에서 얼음과 물의 전체 용적에 대한 최대 얼음량 으로 정의되는 빙충전율(Ice Packing Factor)의 향 상과 축열조의 축열량에 대한 방열량으로 해빙능 력과 축열조 단열성능의 정량적 값인 방냉 효율 의 향상이다. 이를 위한 연구 또한 꾸준히 진행 되어 왔다. 구(sphere)형 캡슐(capsule)인 아이스 볼(ball) 시 스템에서 Lee 등은 실험을 통해 축열조로 유입 되는 유량인 유입수와 축열조 내의 저장수와의 기준 온도차 그리고 아이스 볼의 크기를 달리 하 면서 축열조 내의 온도분포와 무차원 출구 온도 를 통해 열저장율을 비교하였다. Park 등은 냉 각유체인 브라인(brine)과 캡슐의 열전도에 대한 전열량을 비교하였는데, 그 결과 유입수가 저온 일수록, 유량이 작을수록 축열 효율이 증가하며 또한 캡슐의 크기가 작을수록 급격한 해빙이 진 행되어 방열효율이 감소함을 알았다. Heo 등은 축열조 내에서 작동유체인 브라인의 유동형태와 방향에 중점을 두어 축열 시에는 빙충전율을 향 상시킬 수 있는 Perforator의 Upward 방식이 좋으 며, 방열 시에는 성층도에서 앞서는 Distributor의 Downward방식이 이상적인 것을 확인하였다. 더 나아가 Jang 등은 축열조 형상과 연계하여 사각 형보다는 원통형 축열조가 더 안정적이고, 높이 는 높을수록 그리고 저온의 유입수가 Upward 방 식일 때 빙충전율이 높다고 제시하였다. 캡슐형 시스템은 기하학적 형상이 구형뿐만 아 니라 여러 가지 형상이 있다. Choi는 축열조 내 에 벽돌같이 쌓을 수 있는 아이스렌즈(ice Lens) 형상의 시스템에 대해서 장단점 및 특성을 소개 하였고, Kim은 캡슐의 형상이 스크류형인 밀폐 식 시스템에 대하여 한전에서 제시한 기준에 따 라 성능시험을 하여 축열밀도, 방냉효율 그리고 총괄에너지 이용효율 등을 연구하였다. Kim 등 은 효율을 위한 주요 인자로 캡슐의 용기 형상, 용기들의 배열구조, 브라인의 원활한 유동 확보 에 착안하여 스크류형 캡슐에 도넛형 링 캡슐을 삽입하는 모듈형을 개발하여 수치해석을 통해 단 일캡슐들 보다 열전달 효율이 높다는 것을 증명 하였다. 또한 Yoo 등은 큰 구형 캡슐 사이의 공극을 작은 캡슐로 채우는 방법으로 빙충전율 향상을 제시하였고 축열밀도의 향상으로 축열조 의 크기를 줄일 수 있었으며 방열 성능도 향상시 키는 결과를 확인하였다. 본 연구의 해석 대상인 캡슐은 전체적으로 기 본 형상이 구형으로 되어있기 때문에 주위 유동 도 중요하다. 3차원 형상인 구형은 간단한 형상 임에도 불구하고 후류에서 두 개의 서로 반대방 향으로 회전하는 와류(vortex)가 흐르는 실린더와 는 달리 오직 하나의 와류 고리(loop)가 주기적으 로 흐른다. 또한 낮은 레이놀즈 수에서는 레이놀 즈 수가 바뀜에 따라 다양한 유동구조를 가지는 데 Jeon 등은 레이놀즈 수가 증가하면서 변하 는 유동구조 중 항력과 양력의 섭동이 존재하는 비정상 면대칭, 비정상 비대칭 유동에서 선형비 례제어방법이라는 제어방식을 적용하였다. 그 결 과 섭동을 크게 감소시킬 수 있고, 이 섭동의 감 소는 양력의 방향과 밀접한 관계가 있는 와류 진동의 감소로 얻어짐을 확인하였다. Kwag은 수중에서 한 개 및 두 개의 구를 유동방향으로 직렬로 나열하여 구 주위의 층류유동을 3차원으 로 수치해석 하였다. 구 사이의 간격이 같더라도 레이놀즈 수에 따라 유동장은 다르며, 구 한 개 의 유동보다 간격이 작은 두 개의 구 유동에서 오히려 안정된 현상을 보여 주었으며, 구 사이 의 간격이 멀어질수록 앞의 구에서 생기는 와류 의 영향이 커서 불안정한 유동을 보임을 알 수 있었다. 다양한 아이스 볼 형상에 대한 복합열전달의 수치해석 607 Table 2 Materials of ice capsule & brine Thermal Conductivity Density Specific Heat Viscosity Thermal Diffusion Coefficient            BRINE 0.491 1042.0 3920 1.2×10 HDPE 0.33 96

Numerical Analysis for the Conjugate Heat Transfer of Skin Under Various Temperature Conditions of Contrast Therapy

In this paper, the contrast therapy of skin was numerically investigated by solving the conjugate heat transfer problem. A finite volume method based on the SIMPLE algorithm was adopted to solve the axisymmetric incompressible Navier-Stokes equations, coupled with an energy equation. These equations are strongly coupled with the Pennes bio-heat equation in order to consider the effect of blood perfusion rate. We investigated the thermal response of skin at some selected depths for various input temperature profiles of a stimulator for contrast therapy. From the numerical simulations, the regions with cold/hot threshold temperatures were found for five input temperature profiles. It was shown that the temperature varies mildly for different input profiles as the depth increases, owing to the Pennes effect. The input temperatures for effective hot/cold stimulation of dermis layer were found to be and , respectively. The present numerical results will be used for finding an optimal temperature profile of a stimulator for contrast therapy.

Comparison of Message Passing Interface and Hybrid Programming Models to Solve Pressure Equation in Distributed Memory System

* Dept. of Energy System, Graduate School of Energy and Environment, Seoul Nat’l Univ. of Science and Technology** Dept. of Mechanical/Automotive Engineering, Seoul Nat’l Univ. of Science and Technology. (Received August 21, 2014 ; Revised November 1, 2014 ; Accepted November 3, 2014)Key Words: Distributed Memory System(분산 메모리 시스템), Bi-Conjugate Gradient(이중공액구배법), Hybrid Parallel Model(하이브리드 병렬 모델), Message Passing Interface(MPI), OpenMP Directives(OpenMP 지시어)초록: 본 연구에서는 분산 메모리시스템에서의 압력 방정식의 병렬해법을 위하여 MPI(Message Passing Interface)와 하이브리드 병렬기법을 사용하였다. 두 모델은 영역분할 기법을 활용하며, 하이브리드 기법은 성능이 양호한 두 가지 영역분할에 대해 수행하였다. 두 병렬기법의 성능을 비교하기 위해서 다양한 문제 크기에 대해 최대 96개의 쓰레드를 사용하여 속도향상을 측정하였다. 병렬 성능은 캐쉬 메모리에 따른 문제의 크기 및 MPI 통신, OpenMP 지시어의 부하에 대해 영향을 받음을 확인하였다. 문제의 크기가 작은 경우에는 쓰레드가 증가할수록 MPI 통신 및 OpenMP 지시어 부하에 대한 비율이 상대적으로 크기 때문에 병렬 성능이 좋지 않으며, MPI 통신 부하보다는 OpenMP 지시어 부하가 상대적으로 크므로 MPI 병렬 기법의 병렬 성능이 더 우수하다. 문제의 크기가 큰 경우에는 캐쉬 메모리의 활용도가 높고 MPI 통신 및 OpenMP 지시어 부하에 대한 비율이 낮아 병렬 성능이 좋으며, OpenMP 지시어보다 MPI 통신에 의한 부하가 더 지배적이어서 하이브리드 병렬 성능이 MPI 병렬 성능보다 더 양호하다.Abstract: The message passing interface (MPI) and hybrid programming models for the parallel computation of a pressure equation were compared in a distributed memory system. Both models were based on domain decomposition, and two numbers of the sub-domain were selected by considering the efficiency of the hybrid model. The parallel performances for various problem sizes were measured using up to 96 threads. It was found that in addition to the cache-memory size, the overhead of the MPI communication/OpenMP directives affected the parallel performance. For small problems, the parallel performance was low because the percentage of the overhead of the MPI communication/OpenMP directives increased as the number of threads increased, and MPI was better than the hybrid model because it had a smaller communication overhead. For large problems, the parallel performance was high because, in addition to the cache effect, the percentage of the communication overhead was relatively low compared to that for small problems, and the hybrid model was better than MPI because the communication overhead of MPI was more dominant than that of the OpenMP directives in the hybrid model.

Numerical Analysis of Flow Rate Distribution of Diffusers with Various Shapes

:In this study, the distribution characteristics of diffusers with various shapes that are installed in an open-type thermal storage system are numerically investigated. Four diffusers are designed to distribute a working fluid evenly through the holes on bifurcated pipes. Three-dimensional steady simulations of incompressible laminar flow are conducted using commercial software (ANSYS-FLUENT). The simulation results show that both the bidirectional header-type diffuser and the H-type diffuser distribute the working fluid evenly whereas both the unidirectional and the bidirectional diffusers distribute the working fluid unevenly. The results also show that the H-type diffuser requires a higher head of pump than the bidirectional header-type diffuser. Therefore, the bidirectional header-type diffuser is recommended for use because it enables even distribution of the working fluid and requires a low head of pump. Corresponding Author, hgchoi@seoultech.ac.kr 2014 The Korean Society of Mechanical Engineers C