Jae Hwan Jung

Natural Convection in a Square Enclosure with Differentially Heated Two Horizontal Cylinders

The present study numerically investigates two-dimensional natural convection in a cooled square enclosure with two inner circular cylinders, which have different isothermal conditions for different Rayleigh numbers in the range of 103 ≤ Ra ≤ 105. The cooled and heated isothermal conditions are imposed on the upper and lower cylinders, respectively. The centers of two equidiameter cylinders are placed at those of the lower and upper half of the enclosure, respectively, and the radius of inner cylinders varies. For all Rayleigh numbers, as the radius increases, the cold upper cylinder occupies a much wider area in the upper half of the cold enclosure, resulting in the formation of a wider dead zone of the heat transfer and fluid flow in the upper half of the enclosure. Regardless of the radius variation, the circulation of the flow shows two overall rotating symmetric eddies with the secondary vortices over the upper surface of the upper cylinder. The trajectories of the primary eddy and secondary vortices considerably depend on the radius, especially at a higher Rayleigh number. The dependence of the Nusselt number on the radius and the Rayleigh number is presented.

Natural Convection in a Square Enclosure with Two Horizontal Cylinders

This study investigates natural convection in a cooled square enclosure with two inner heated circular cylinders with the same diameter. The centers of two equidiameter cylinders are placed at those of the lower and upper half of the enclosure, respectively. The immersed boundary method (IBM) to model the inner circular cylinders based on the finite volume method is used to study a two-dimensional natural convection for different Rayleigh numbers varying in the range of 103 ≤ Ra ≤ 105. The effect of the radius of inner circular cylinders in an enclosure on heat transfer and fluid flow at different Rayleigh numbers has been examined. As the Rayleigh number increases, the horizontal symmetry is broken and the asymmetry occurred from the smaller radius. As the radius decreases, the dependence of the convection on the Rayleigh number is considerable. The dependence of the Nusselt number on the radius and the Rayleigh number is presented.

Effect of natural frequency modes on sloshing phenomenon in a rectangular tank

Abstract Liquid sloshing in two-dimensional (2-D) and three-dimensional (3-D) rectangular tanks is simulated by using a level set method based on the finite volume method. In order to examine the effect of natural frequency modes on liquid sloshing, we considered a wide range of frequency ratios (0.5 ≤ fr ≤ 3.2). The frequency ratio is defined by the ratio of the excitation frequency to the natural frequency of the fluid, and covers natural frequency modes from 1 to 5. When fr = 1 which corresponds to the first mode of the natural frequency, strong liquid sloshing reveals roof impact, and significant forces are generated by the liquid in the tank. The liquid flows are mainly unidirectional. Thus, the strong bulk motion of the fluid contributes to a higher elevation of the free surface. However, at fr = 2 the sloshing is considerably suppressed, resulting in a calm wave with relatively lower elevation of the free surface, since the waves undergo destructive interference. At fr = 2 the lower peak of the free surface elevation occurs. At higher modes of fr3, fr4, and fr5 the free surface reveals irregular deformation with nonlinear waves in every case. However, the deformation of the free surface becomes weaker at higher natural frequency modes. Finally, 3-D simulations confirm our 2-D results.

Large Eddy Simulation of Flow around Twisted Offshore Structure with Drag Reduction and Vortex Suppression

Abstract A twisted cylinder has been newly designed by rotating the elli ptic cross section along the spanwise direction in order to red uce thedrag and vorticies in wake region. The flow around the twisted cylinder at a subcritical Reynolds number (  ) of 3000 is investigatedto analyze the effect of twisted spiral pattern on the drag reduction and vortex suppression using large eddy simulation (LES) . The instantaneous wake structures of the twisted cylinder are compa red with those of a circular and a wavy cylinder at the same  . Theshear layer of the twisted cylinder covering the recirculation region is more elongated than that of the circular and the wavy cylinder. Successively, vortex shedding of the twisted cylinder is consid erably suppressed, compared with those of the circular and the wavy cylinder. Consequently, the mean drag coefficient and the fluct uating lift of the twisted cylinder are less than those of the circular andthe wavy cylinder.Keywords: Twisted cylinder(트위스트 실린더), Large eddy simulation(대형와모사법), Vort ex suppression(와류 감쇠), Drag reduction(저항 저감)

Study on the Angle-of-Attack Characteristics of the Rudder in Rotating Propeller Flow

This study aims at numerically investigating the angle of attack characteristics of the rudder behind a rotating propeller. The rotating propeller of 5 blades and the full spade rudder are placed in the numerical water tunnel with a uniform flow condition to consider propeller-rudder interaction. The turbulence closure model is employed to simulate the three-dimensional unsteady incompressible viscous turbulent flow around the propeller and the rudder. The present numerical method are well verified by comparing with the experimental results. In order to identify the dependence of the angle of attack of the rudder on the rudder angle, a wide range of rudder angles is considered. The present study carried out the quantitative and qualitative analysis of the angle of attack in terms of the pressure distribution, streamlines and the evaluation of the flow incidence, resulting in that the angle of attack increases as we move from the root and the tip to the center of the rudder, regardless of the rudder angle. The distribution of the angle-of-attack along the span is strongly affected by rotating propeller flow and rudder angle. Consequently, the distribution of the angle-of-attack of the oncoming flow against the rudder leading edge plays a role in determination of rudder performance.

Effect of Chamfering Top Corners on Liquid Sloshing in the Three-dimensional Rectangular Tank

This study aims at investigating the effect of the chamfer on the liquid sloshing in the three-dimensional (3D) rectangular tank. In order to simulate the 3D incompressible viscous two-phase flow in the 3D tank with partially filled liquid, the present study has adopted the volume of fluid (VOF) method based on the finitevolume method which has been well verified by comparing with the results of the relevant previous researches. The effects of the chamfering top corners of the tank on the liquid sloshing characteristics have been investigated. The angle of the chamfering top corners () has been changed in the range of (

Numerical Study on Liquid Sloshing in the Three-dimensional Rectangular Tank with Various Baffle Heights

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Large eddy simulation of flow over a twisted cylinder at a subcritical Reynolds number

Abstract This study aims at investigating the effect of the baffle height on the liquid sloshing in the three-dimensional (3D) rectangular tank. In order to simulate the 3D incompressible viscous two-phase flow in the 3D tank with partially filled liquid, the present study has adopted the volume of fluid (VOF) method based on the finite-volume method which has been well verified by comparing with the results of the relevant previous researches. The ratio of the baffle height ( h B ) to filling level ( h ) has been changed in the range of 0/1.2 hh B  to observe the effect on the impact loads on the side wall and free surface behavior. Generally, as baffle height increases, the impact pressure on the wall decreases and the deformation of free surface becomes weaker. However it seemed that a critical ratio of the baffle height existed to reveal the lowest impact pressure on the wall. Consequently, hh B /0.8 among hh B / s considered in the study showed the lowest impact pressure. ※Keywords: Sloshing phenomena(슬로싱 현상), Baffle(배플), Free surface(자유수면), Three-dimensional tank(3차원 탱크) 접수일: 2009년 9월 21일, 승인일: 2009년 12월 16일 †교신저자: lesmodel@pusan.ac.kr, 051-510-3685