Kyongjun Lee

Heat Transfer Characterization of Two Isothermal Circular Cylinders in Proximity

Heat transfer on two nearby isothermal circular cylinders of equal diameter immersed in a uniform crossflow at Re = 120 and Pr=0.7 was numerically studied. We consider all possible arrangements of the two cylinders in terms of the distance between the two cylinders and the inclination angle with respect to the direction of the main flow. It turns out that significant changes in the characteristics of heat transfer are noticed depending on how they are positioned, resulting in quantitative changes in heat transfer coefficients of both cylinders. Collecting all of the numerical results obtained, we propose a contour diagram for averaged Nusselt number for each of the two cylinders. The geometrical symmetry implied in the flow configuration allows one to use those diagrams to estimate heat transfer rates on two isothermal circular cylinders of equal diameter arbitrarily positioned in physical space with respect to the main flow direction.

Forces Induced by Flows Past Two Nearby Circular Cylinders

Flow-induced forces on two identical nearby circular cylinders immersed in the cross flow at Re

Development of a Cartesian-based Code for Effective Simulation of Flow Around a Marine Structure - Integration of AMR, VOF, IBM, VIV, LES

Simulation of flow past a complex marine structure requires a fine resolution in the vicinity of the structure, whereas a coarse resolution is enough far away from it. Therefore, a lot of grid cells may be wasted, when a simple Cartesian grid system is used for an Immersed Boundary Method (IBM). To alleviate this problems while maintaining the Cartesian frame work, we adopted an Adaptive Mesh Refinement (AMR) scheme where the grid system dynamically and locally refines as needed. In this study, We implemented a moving IBM and an AMR technique in our basic 3D incompressible Navier-Stokes solver. A Volume Of Fluid (VOF) method was used to effectively treat the free surface, and a recently developed Lagrangian Dynamic Subgrid-scale Model (LDSM) was incorporated in the code for accurate turbulence modeling. To capture vortex induced vibration accurately, the equation for the structure movement and the governing equations for fluid flow were solved at the same time implicitly. Also, We have developed an interface by using AutoLISP, which can properly distribute marker particles for IBM, compute the geometrical information of the object, and transfer it to the solver for the main simulation. To verify our numerical methodology, our results were compared with other authors’ numerical and experimental results for the benchmark problems, revealing excellent agreement. Using the verified code, we investigated the following cases. (1) simulating flow around a floating sphere. (2) simulating flow past a marine structure.Simulation of flow past a complex marine structure requires a fine resolution in the vicinity of the structure, whereas a coarse resolution is enough far away from it. Therefore, a lot of grid cells may be wasted, when a simple Cartesian grid system is used for an Immersed Boundary Method (IBM). To alleviate this problems while maintaining the Cartesian frame work, we adopted an Adaptive Mesh Refinement (AMR) scheme where the grid system dynamically and locally refines as needed. In this study, We implemented a moving IBM and an AMR technique in our basic 3D incompressible Navier-Stokes solver. A Volume Of Fluid (VOF) method was used to effectively treat the free surface, and a recently developed Lagrangian Dynamic Subgrid-scale Model (LDSM) was incorporated in the code for accurate turbulence modeling. To capture vortex induced vibration accurately, the equation for the structure movement and the governing equations for fluid flow were solved at the same time implicitly. Also, We have developed an interface by using AutoLISP, which can properly distribute marker particles for IBM, compute the geometrical information of the object, and transfer it to the solver for the main simulation. To verify our numerical methodology, our results were compared with other authors’ numerical and experimental results for the benchmark problems, revealing excellent agreement. Using the verified code, we investigated the following cases. (1) simulating flow around a floating sphere. (2) simulating flow past a marine structure.

Effects of Schmidt Number on Turbulent Mass Transfer Around a Rotating Circular Cylinder

Characteristics of turbulent mass transfer around a rotating circular cylinder have been investigated by Direct Numerical Simulation. The concentration field was computed for three different cases of Schmidt number, Sc = 1, 10 and 100 at Re* = 336. Our results confirm that the thickness of the Nernst diffusion layer decreases as Sc increases. Wall-limiting behavior within the Nernst diffusion layer was examined and compared with those of channel flow. Concentration fluctuation was found to be time-scaled with (r+ )2 while the time scale ratio equals the Schmidt number throughout the Nernst diffusion layer. Scalar modeling closure constants were determined, and turned out to vary considerably within the diffusion layer.Copyright

Reynolds-number Effect on the Flow Past Two Nearby Circular Cylinders

As a follow-up of our previous studies on flow-induced forces on two identical nearby circular cylinders immersed in the cross flow at Re=100 and flow patterns past them, we present Reynolds-number effects on the forces and patterns by further computing flows with Re=40, 50, 160. We consider all possible arrangements of the two circular cylinders in terms of the distance between the two cylinders and the angle inclined with respect to the main flow direction. Collecting all the numerical results obtained, we propose contour diagrams for mean force coefficients and their rms of fluctuation as well as for flow patterns and Strouhal number for each Re. These diagrams shed light on a comprehensive picture on how the wake interaction between the two cylinders alters depending on Re.

Effective Heat Transfer Using Large Scale Vortices

Abstract A numerical study has been carried out to investigate heat transfer enhancement in channel flow using large-scale vortices. A square cylinder, inclined with respect to the main flow direction, is located at the center of the channel flow, generating a separation region and Karman vortices. Two cases are considered; one with a fixed blockage ratio and the other one with a fixed cylinder size. In both cases, the flow characteristics downstream of the cylinder significantly change depending on the inclination angle. As a result, heat transfer from channel wall is significantly enhanced due to increased vertical-velocity fluctuations induced by the large-scale vortices shed from the cylinder. Quantitative results as well as qualitative physical explanation are presented to justify the effectiveness of the inclined square cylinder as a vortex generator to enhance heat transfer from channel wall.기호설명 A : 정방형 실린더 변의 길이 C d : 항력 계수(=    )  : 평균 항력 계수 C f : 마찰 계수(=