Choon-Bum Choi

Flow past a square cylinder with an angle of incidence

A parametric study has been carried out to elucidate the characteristics of flow past a square cylinder inclined with respect to the main flow in the laminar flow regime. Reynolds number and angle of incidence are the key parameters which determine the flow characteristics. Location of separation point is greatly affected by angle of incidence, thus determining the flow field around the square cylinder. The critical Reynolds number for periodic vortex shedding at each angle of incidence considered is obtained by using Stuart–Landau equation. Attempt is made to classify the related flow patterns from a topological point of view, resulting in three distinct patterns in total. A comprehensive analysis of the effects of Reynolds number and angle of incidence on flow-induced forces on the square cylinder is presented. Collecting all the results obtained, contour diagrams of force and moment coefficients, Strouhal number, rms of lift-coefficient fluctuation, as well as a flow-pattern diagram are proposed for th...

Heat Transfer Enhancement in Channel Flow Using an Inclined Square Cylinder

Heat transfer enhancement in channel flow by using an inclined vortex generator has been numerically investigated. A square cylinder is located on the centerline of laminar channel flow, which is subject to a constant heat flux on the lower channel wall. As the cylinder is inclined with some angle of attack with respect to the main flow direction, flow characteristics change downstream of the cylinder, and significantly affect heat transfer on the channel wall. A parametric study has been conducted to identify the cause, and to possibly find the optimal inclination angle. It turns out that the increased periodic fluctuation of the vertical velocity component in the vicinity of the channel walls is responsible for the heat transfer enhancement. The large fluctuation is believed to be induced by the large-scale vortices shed from the inclined square cylinder, as well as by the secondary vortices formed near the channel walls.

Secondary instability in the near-wake past two tandem square cylinders

Floquet stability analysis has been carried out to detect the onset of the secondary instability in the laminar flow past two identical square cylinders in tandem. Presence of a neighbouring cylinder alters flow topology, leading to change of stability characteristics. A parametric study has been performed with the gap between the two square cylinders as the key parameter. Three distinct patterns of base flow are found depending on the gap, and distinctive modes of the secondary instability are identified for each pattern of the base flow. The modes exhibit either odd reflection-translation symmetry or even RT symmetry. The critical Reynolds number and the corresponding dominant spanwise wavelength are presented for a wide range of the gap. Temporal and spatial characteristics of the dominant Floquet modes are described in detail. A hysteresis is also noticed in a certain range of the gap. Neutral stability curves are presented for some selected values of the gap. The averaged in-plane vorticity of the do...

Three-dimensional instability in flow past a rectangular cylinder ranging from a normal flat plate to a square cylinder

Floquet stability analysis has been carried out to detect the onset of the three-dimensional (3D) instability in flow past a rectangular cylinder ranging from a normal flat plate to a square cylinder with various aspect ratios (AR). The conventional modes A, B, and QP vanish as AR decreases towards a thin normal flat plate, whereas new modes A2 and QP2 emerge. For “thin” rectangular cylinders, mode QP2 becomes unstable first with increasing Reynolds number (Re). In a certain range of AR, mode A becomes unstable and then stable again as Re increases, resulting in a closed curve on the neutral stability diagram. For validation, 3D simulations were also performed.

Three-dimensional instability in the flow past two side-by-side square cylinders

The onset of the three-dimensional (3D) instability in the flow past two side-by-side square cylinders has been studied via Floquet stability analysis. The gap between the two cylinders is the key parameter that alters flow topology, leading to change of stability characteristics. Depending on the gap, four distinct flow patterns have been identified for base flow. For each pattern of base flow, distinctive Floquet modes of 3D instability have been found. The spatial and temporal characteristics of the Floquet modes are similar to those of mode A or mode B of the single square cylinder case, namely, odd or even Reflection-Translation symmetries. Six distinct Floquet modes have been identified in total, and the spatial and temporal characteristics of each mode are described in detail. The criticality of Floquet modes, including their critical Reynolds number and the associated dominant spanwise wave number, is presented for each gap considered in the current study. For a small gap, the flow past the two cy...

Numerical Study of Density-stratified Flow Past Two 3D Hills - Aligned in Tandem -

In this paper a parametric study using an immersed boundary method has been carried out to investigate the effects of stable density stratification on the wakes past two identical three-dimensional hills aligned in tandem. The Reynolds number based on the uniform inlet velocity and twice the hill height was fixed at Re

Effect of Domain Size on Flow Characteristics in Simulating Periodic Obstacle Flow

Effect of computational domain size in simulating of periodic obstacle flow has been investigated for the flow past tube banks. Reynolds number, defined by freestream velocity () and cylinder diameter (d), was fixed as 200, and center-to-center distance (P) as 1.5d. In-line square array and staggered square array were considered. Drag coefficient, lift coefficient and Strouhal number were calculated depending on domain size. Circular cylinders were implemented on a Cartesian grid system by using an immersed boundary method. Boundary condition is periodic in both streamwise and lateral directions. Previous studies in literature often use a square domain with a side length of P, which contains only one cylinder. However, this study reveals that the domain size is improper. Especially, RMS values of flow-induced forces are most sensitive to the domain size.

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(=