3 research outputs found
λ€μμ λ λ° κ³ μ²΄ ννΈλ¬Όμ μλ ₯νμ κ±°λ ν΄μμ μν GPU κΈ°λ°μ SPH-DEM μ°κ³ν΄μ μ½λ κ°λ°
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곡νλΆ, 2020. 8. κΉμμ.In the late phase severe accident of LWR, the massive corium releases out of the reactor pressure vessel (RPV) and falls to the coolant if the In-Vessel Retention (IVR) strategy fails. The melt jet can be fragmented into debris particles based on the assumption that the ex-vessel pool is sufficiently deep. It is known that there are various three-phase flow issues associated with the fragmented debris particles under the influence of phase change of cavity coolant. In such cases, the vapor phase forms a sharp and dynamic interface with the liquid phase while the transient relocation behavior of debris particles is the main concern. Thus, coupling Lagrangian-based multi-phase CFD techniques and Discrete Element Method (DEM) can be an effective approach in terms of numerical modeling of such behaviors. In this respect, an integrated numerical code for incompressible 3-phase flow has been developed in this study by two-way phase coupling of multi-phase Smoothed Particle Hydrodynamics (SPH) and DEM model.
Smoothed Particle Hydrodynamics (SPH) is one of the best-known meshless CFD methods in which the fluid system is represented as the finite number of Lagrangian particles. The SPH code developed in this study proposes a new density estimation model and improves the surface tension model for accurate simulation of incompressible two-phase flow behavior. The demonstration of its applicability has been performed through several V&V simulations including multi-phase dam-break and sloshing simulations.
Discrete Element Method (DEM) is a direct simulation method for a rigid body that can analyze the translation, rotation, and collision behavior of solid particles in detail. The soft-sphere collision model with Hertz-Mindlin contact force equations is adopted for developed DEM code in this study. To precisely estimate the wall boundary interactions of bed-formed debris particles, a versatile wall boundary model is newly proposed in this study that also covers the sliding and rolling behavior of solid particles. The inter-particle collision behavior and sliding & rolling motion of particles are well proven in several V&V cases.
The numerical code system for incompressible 3-phase flow is newly developed by two-way phase coupling of the above two models (SPH-DEM). The unresolved coupling approach between two methods was adopted for the analysis of the overall behavior of particulate solid debris. The fundamental validation of the phase coupled model was performed for both single-particle behavior and particulate granular flow such as dam-breaking motion of particle-fluid.
The SPH-DEM coupled code in this study has been parallelized based on Graphical Process Unit (GPU) in order to overcome the inherent efficiency problem of the Lagrangian-based numerical method. Parallel mapping and reduction are applied for solving discretized summation equations of each SPH particle, solving contact force equations for each DEM particle, and also for solving coupling equations between SPH and DEM particles. The efficiency of code parallelization was evaluated through the scalability analysis based on the benchmark calculation.
Finally, the simulation of the vapor-driven leveling behavior of spherical solids was performed as a case study to demonstrate the applicability of the developed code. The time-variant surface shape of solid particles was compared with the benchmark experiments both qualitatively and quantitatively. The effect of gas flow rate on the tendency of leveling behavior also has been analyzed.
The developed numerical system in this study is expected to be a good alternative for the simulation of such phenomena that were difficult to handle with traditional numerical methods since the numerical schemes used in the code have a high potential for simulation of complicated physics with highly deformable geometry. In addition, this validated code system can contribute to hydrodynamic modeling studies for severe accident technology by performing numerical experiments on conditions that hard to be conducted on a laboratory scale.κ°μκ²½μλ‘ μ€λμ¬κ³ νκΈ° κ³Όμ μμ ν΅μ°λ£ μ©μ΅λ¬Ό λ
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μμ μ 체 μ¬μ΄μ μ°κ³λ mm μ΄νμ μ€μΌμΌμ κ°μ§λ ν΅μ°λ£ ννΈλ¬Όμ νμμ νΉμ±μ κ³ λ €νμ¬ λ μ μ¬μ΄μ κ²ΉμΉ¨μ νμ©νμ¬ μ΄λλ κ΅νμ λͺ¨λΈλ§νλ λΉν΄μ(unresolved) λ°©μμΌλ‘ μ΄λ£¨μ΄μ‘λ€. λν, SPH μ 체 λͺ¨λΈ, DEM κ°μ²΄ λͺ¨λΈ, SPH-DEM μ°κ³ λͺ¨λΈ κ°κ°μ λν κ²μ¦μ λ€μν μ€μΌμΌμμ λ€μν μ€ν μ°κ΅¬λ€κ³Όμ λΉκ΅λ₯Ό ν΅ν΄ μννμλ€.
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μμ 체λμν(SPH) κΈ°λ²κ³Ό μ΄μ°μμλ²(DEM) λͺ¨λΈμ λΌκ·Έλμ§μ ν΄μ κΈ°λ²μ νΉμ±μ μ»΄ν¨ν° μ±λ₯μ λΉμ½μ μΈ λ°μ μλ λΆκ΅¬νκ³ μ€μΌλ¬λ¦¬μ ν΄μ κΈ°λ²μ λΉν΄ κ³μ° ν¨μ¨ λ° μκ°μ λν μλμ μΈ μ μ½μ΄ μ‘΄μ¬νλ€. νΉν μ‘체-기체μ μ΄μμ λ ν΄μμ λ€λ£° κ²½μ° κΈ°μ²΄ μμ λ°λκ° μκΈ° λλ¬Έμ λΌκ·Έλμ§μ μ 체ν΄μ κΈ°λ²μμλ λ μμ μκ° κ°κ²©μ΄ μꡬλλ€. μ΄μ λ³Έ μ°κ΅¬μμλ κ·Έλν½ μ²λ¦¬ μ₯μΉ (Graphics Processing Unit, GPU)λ₯Ό νμ©νμ¬ SPH ν΄μ, DEM ν΄μ, SPH-DEM μ°κ³ ν΄μμ΄ λͺ¨λ κ° λΌκ·Έλμ§μ μ§μ μ λν΄ λμμ μνλ μ μλλ‘ GPU κΈ°λ°μ μ°κ³μ½λ λ³λ ¬ν λ° κ°μνλ₯Ό μννμλ€.
λ§μ§λ§μΌλ‘ κ°λ°ν λΌκ·Έλμ§μ κΈ°λ°μ 3μμ λ ν΄μ 체κ³μ μ μ©μ± μ
μ¦μ μν΄ μμλ‘ μ€λμ¬κ³ νκΈ° κ³Όμ μμ λ°μν μ μλ ν΅μ°λ£ ννΈλ¬Ό μΈ΅(debris bed)μ ννν(self-leveling) κ±°λμ λν κ²μ¦ ν΄μμ μννμλ€. μκ°μ λ°λ₯Έ ννΈλ¬Ό μΈ΅ νλ©΄μ νμ λ³νλ₯Ό ν κΈ°κ΄μμ μνλ 기체주μ
μ€νκ³Ό λΉκ΅νλ ννλ‘ κ²μ¦μ΄ μ΄λ£¨μ΄μ‘λ€. λΆμ κ²°κ³Ό, λ³Έ μ°κ΅¬μμ κ°λ°ν SPH-DEM μ°κ³ν΄μ μ½λκ° κ³ μ²΄ μ
μ μμ ν¬ν¨ν μλ ₯νμ 3μ κ±°λμ μ λμ μΌλ‘, μ μ±μ μΌλ‘ μ ν΄μνλ κ²μ νμΈνμλ€.
λ³Έ μ°κ΅¬μμ κ°λ°ν λΌκ·Έλμ§μ κΈ°λ°μ SPH-DEM 3μμ λ ν΄μ 체κ³λ μμλ‘ μ€λμ¬κ³ μ ν΄μμ κ΄μ μμ κΈ°μ‘΄μ μμΉν΄μ κΈ°λ²λ€μ΄ λ€λ£¨κΈ° μ΄λ €μ λ νμλ€μ λν λμ λλ μνΈ λ³΄μμ μν μ ν μ μλ€. λν, λ³Έ μ°κ΅¬μμ κ°λ°ν μ½λλ μ 1μ리 κΈ°λ°μ 물리 λ²μΉμ κΈ°λ°μΌλ‘ μ λ λ° κ°μ²΄μ κ±°λμ ν΄μνκΈ° λλ¬Έμ μ€νμΌλ‘ ꡬννκΈ° μ΄λ €μ΄ 쑰건μ΄λ μ€μΌμΌμ λν μμΉμ μ¬νμ΄ κ°λ₯νκ³ , μ΄λ₯Ό λ°νμΌλ‘ κΈ°μ‘΄μ μ€μΌμΌλ§ λ²μΉμ κ²μ¦νκ±°λ μ€ν κ²°κ³Όκ° μλ μμμμ μμΉ λ°μ΄ν°λ₯Ό μμ±νμ¬ κΈ°μ‘΄μ μκ΄μμ κ°μ νλλ° νμ©ν μ μλ€. μ΄λ¬ν μ μμ λ³Έ μ°κ΅¬λ μμλ‘ μ€λμ¬κ³ μ ν΄μμ΄λ μμ μ± νκ°μ κ΄λ ¨νμ¬ λΆνμ€μ±μ μ κ°νλλ° κΈ°μ¬νλ€.Chapter 1 Introduction 1
1.1 Background and Motivation 1
1.2 Previous Studies 3
1.2.1 Numerical Studies on Particulate Debris Bed 3
1.2.2 SPH-DEM Phase Coupling 4
1.3 Objectives and Scope 5
Chapter 2 Fluid Phase: Smoothed Particle Hydrodynamics 8
2.1 Smoothed Particle Hydrodynamics (SPH) 9
2.1.1 SPH Particle Approximation 9
2.1.2 SPH Particle Approximation of Derivatives 10
2.1.3 Kernel Function 11
2.1.4 Accuracy of SPH Approximation 12
2.1.5 Governing Equations for Incompressible Flow 14
2.2 Multi-phase SPH Governing Equations 16
2.2.1 Treatment of Multi-Phase Flow 16
2.2.2 Normalized Density Model 18
2.2.3 Continuum Surface Force (CSF) Model 19
2.3 Implementation of SPH Model 21
2.3.1 Algorithm of SPH Code 21
2.3.2 Nearest Neighboring Particles Searching (NNPS) 22
2.3.3 Time Integration 23
2.4 V&V Simulations 24
2.4.1 Rayleigh-Taylor Instability 25
2.4.2 Bubble Terminal Velocity 25
2.4.3 Dam-Break Simulation 25
2.4.4 Centralized Sloshing Simulation 26
Chapter 3 Solid Phase: Discrete Element Method 45
3.1 Discrete Element Method (DEM) 46
3.2 DEM Contact Force 47
3.2.1 Soft-sphere Contact Model 47
3.2.2 Contact Force Model 48
3.2.3 Hertz-Mindlin Contact Force Model 49
3.3 Wall Boundary Conditions 52
3.3.1 Versatile Wall Boundary Model 52
3.3.2 Particle Collision with the Wall 54
3.3.3 Sliding and Rolling on the Wall Boundary 56
3.4 DEM Implementation Algorithm 57
3.4.1 Contact Detection 58
3.4.2 Estimation of Relative Velocity 59
3.4.3 Calculation of Contact Force 60
3.4.4 Wall Boundary Conditions and Time Integration 60
3.5 V&V and Simulations 61
3.5.1 Conservation of Momentum and Angular Momentum 62
3.5.2 Conservation of Energy in Elastic Collision 63
3.5.3 V&V Simulations for Wall Boundary Model 63
3.5.4 Granular Collapse of Spherical Particles 64
Chapter 4 Two-way Phase Coupling of SPH and DEM 76
4.1 Unresolved Coupling of SPH and DEM 76
4.2 Governing Equations 78
4.2.1 SPH Particles: Locally Averaged N-S Equations 78
4.2.2 DEM Particles: Coupling Forces Acting on Solid Particles 80
4.2.3 SPH Particles: Reaction Force from Momentum Exchange 82
4.3 Algorithm of SPH-DEM Coupled Model 83
4.4 V&V Simulations for SPH-DEM Coupled Model 84
4.4.1 Single DEM Particle Behavior 85
4.4.2 Pressure Drop through Packed Bed 87
4.4.3 Granular Flow in Liquid: 3D Dam-Break 89
Chapter 5 GPU Parallelization of Coupled SPH-DEM Code 103
5.1 Parallelization of Governing Equations 104
5.1.1 GPU-based Parallelization 104
5.1.2 Parallelization of SPH-DEM Governing Equations 104
5.2 Parallelization of NNPS and Contact Detection 105
5.3 Results of GPU Parallelization 107
5.3.1 Speedup in Computation Time 107
5.3.2 Parallelization Factors 107
Chapter 6 Code Application to Vapor-Driven Leveling Behavior of Spherical Debris 113
6.1 Self-Leveling Behavior of Debris Bed 114
6.1.1 Self-Leveling Issue in LWR 114
6.1.2 Self-Leveling Behavior in Terms of Debris Coolability 114
6.2 Benchmark Experiment 116
6.3 SPH-DEM Simulation Setup 118
6.3.1 Properties and Simulation Conditions 118
6.3.2 Sequence of SPH-DEM Leveling Simulation 120
6.3.3 Determination of Inclined Angle 121
6.4 Validation Results and Discussions 121
6.4.1 SPH-DEM Simulation Results 121
6.4.2 Validation Result 122
6.4.3 Effect of Gas Flow Rate 122
Chapter 7 Summary 129
7.1 Summary 129
7.2 Recommendations 131
References 134
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