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Numerical study of particle dispersion from a power plant chimney
심정보,유동현,Shim, Jeongbo,You, Donghyun Korean Association for Particle and Aerosol Resear 2017 Particle and Aerosol Research Vol.13 No.4
An Eulerian-Lagrangin approach is used to compute particle dispersion from a power plant chimney. For air flow, three-dimensional incompressible filtered Navier-Stokes equations are solved with a subgrid-scale model by integrating the Newton's equation, while the dispersed phase is solved in a Lagrangian framework. The velocity ratios between crossflow and a jet of 0.455 and 0.727 are considered. Flow fields and particle distribution of both cases are evaluated and compared. When the velocity ratio is 0.455, it demonstrates a Kelvin-Helmholtz vortex structure above the chimney caused by the interaction between crossflow and a jet, whereas the other case shows flow structures at the top of the chimney collapsed by fast crossflow. Also, complex wake structures cause different particle distributions behind the chimney. The case with the velocity ratio of 0.727 demonstrates strong particle concentration at the vortical region, whereas the case with the velocity ratio of 0.455 shows more dispersive particle distribution. The simulation result shows similar tendency to the experimental result.
비정상 압력 구배에 의한 난류 경계층 유동 박리 현상의 동적 모드 분할 분석
박준신(Junshin Park),유동현(Donghyun You) 대한기계학회 2020 大韓機械學會論文集B Vol.44 No.2
시간에 따라 변하는 비정상 압력 구배에 의해 발생한 난류 경계층 유동 박리 현상에 대해 직접수치 모사와 동적 모드 분할 분석을 수행하였다. 압력 구배가 시간에 따라 진동하며, 각기 다른 전환 주파수 k=wLb /2U를 가지는 두 경우에 대해 계산을 수행하였다. 터빈 유동의 작동 범위내로 k=0.75인 경우, 대조군으로 이보다 빠른 영역의 k=3.75인 경우에 대하여 수치 해석을 진행하였다. k=3.75인 경우 박리 기포는 시간에 따라 높이 변화가 없는 데 반해 k=0.75인 경우 박리 기포의 높이, 재점착된 전단층의 거동도 시간에 따라 큰 변화를 보였다. 이 현상은 동적 모드 분할 기법을 적용한 결과 유의한 모드들로 분할되었으며, 이는 전단층의 거동을 잘 표현할 뿐만 아니라 주요 물리현상의 위치 및 주파수를 특정할 수 있음을 확인하였다. Direct numerical simulation and dynamic mode decomposition are conducted for a separated turbulent boundary layer under unsteady adverse pressure gradients. Two simulation cases are chosen based on the reduced frequency k=wLb /2U . First case is when k=0.75 which lies inside the operating condition of turbomachinery. The second case is when k=3.75 for comparison. When k=3.75, the height of the shear layer above the separation bubble does not change with time. However, when k=0.75, the height of the shear layer and the response of the reattached shear layer changes with time. Flow fields are decomposed into significant modes using dynamic mode decomposition. It is shown that significant modes not only describe the response of the shear layer effectively, but also specify locations and frequencies of significant features of the separated turbulent boundary layer.
가상경계기법을 통한 협착된 미세혈관에서 적혈구 분포의 수치 해석
윤대근(Daegeun Yoon),유동현(Donghyun You) 한국전산유체공학회 2019 한국전산유체공학회지 Vol.24 No.2
In order to investigate the distribution of red blood cells in a stenosed microvessel, three-dimensional blood flow simulations are conducted using immersed boundary methods. To treat deformable structures like blood cells, a continuous-forcing immersed boundary method is employed. Also, a ghost-cell immersed boundary method is employed to treat rigid and complex structures like a stenosed vessel. In the present study, two immersed boundary methods are integrated into a single computational framework by treating each structure separately. The present framework is validated through the simulation of blood flow in a straight microvessel compared with related previous studies. Present simulations of blood flow in a stenosed microvessel reveal that distribution of RBCs passing the stenosis is changed uniformly and the cell-free layer has significant spatial variation across the stenosis, which might induce the change of wall shear stress and flow resistance.
VOF기법 및 식별 알고리즘을 이용한 쇄파에서의 기포 크기 분포도 분석
문호준(Hojun Moon),유동현(Donghyun You) 한국전산유체공학회 2020 한국전산유체공학회지 Vol.25 No.3
A numerical simulation of a three-dimensional breaking wave is conducted using a VOF (volume-of-fluid) method to investigate the wave breaking dynamics and the bubble size distribution. The wave is initialized using the third-order Stokes wave solution. The initial wave slope and velocity fields generate turbulent wave breaking. Various interfacial phenomena are observed including a jet forming, a jet impacting to the free-surface, ejecting spray, entraining air pocket, and breakup. Bubbles with various sizes are formed from turbulent breakup of the air pocket during active breaking time. To obtain the bubble size distribution, an identification algorithm is proposed to accurately count independent bubbles. The proposed algorithm successfully identified independent bubble structures. A joining algorithm is also introduced to consider bubble structures spanning multiple blocks for parallel computations. The obtained bubble size distribution averaged during active breaking time is proportional to r<SUP>-10/3</SUP> for radii larger than the Hinze scale and shows good agreement with previous experiment and simulation results as well as the theoretical model.