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유주식(Joo-Sik Yoo) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.10
This study investigates transition to chaos in a two-dimensional natural convection of a low-Prandtl number fluid (Pr=0.02) in a wide-gap horizontal annulus with constant heat flux inner wall. When Pr=0.02, dual steady-state flows are not found. After the first Hopf bifurcation from a steady to a time-periodic flow, five successive period-doubling bifurcations are recorded before chaos. The power spectrum shows the period-2⁴ and 2? flows clearly, and a window of period 3×2³ flow is found in the chaotic regime. The approximate value of the Feigenbaum number for the last three period-doubling bifurcations is 4.76. The transition route to chaos of the present simulations is consistent with the period-doubling route of Feigenbaum.
넓은 수평 환형 공간에서의 진동하는 자연 대류로의 천이 : Pr=0.1
유주식(Joo-Sik Yoo),김용진(Yong- Jin Kim),엄용균(Yong-Kyoon Eom) 한국전산유체공학회 2000 한국전산유체공학회지 Vol.5 No.3
Natural convection in a wide-gap horizontal annulus is considered, and the transition of flows from steady to oscillatory convection is investigated for the fluid with Pr=0.1. The unsteady stream function-vorticity equation is solved with finite difference method. As Rayleigh number is increased, the steady crescent-shaped flow bifurcates to a time-periodic flow with like-rotating eddies. And afterwards, a transition to an oscillatory multicellular flow with a counter-rotating eddy on the top of the annulus occurs. A transition from steady to an oscillatory flow occurs, but dual solutions and hysteresis phenomena are not observed.
하나의 실린더가 회전하는 수평 환형 공간에서의 혼합 대류
유주식(Joo-Sik Yoo),하대홍(Dae-Hong Ha) 한국전산유체공학회 2001 한국전산유체공학회지 Vol.6 No.3
Mixed convection in a horizontal annulus is considered, and the effect of a forced flow on the natural convective flow is investigated. The inner cylinder is hotter than the outer cylinder, and the outer cylinder is rotating with constant angular velocity with its axis at the center of the annulus. The unsteady streamfunction-vorticity equation is solved with a finite difference method. For the fluid with Pr=0.7, there appear flows with two eddies, one eddy, or no eddy according the Rayleigh and Reynolds numbers. The rotation of the outer cylinder reduces the heat transfer fate at the wall of the annulus. The oscillatory multicellular flow of a low Prandtl number fluid with Pr=0.01 can be effectively suppressed by the forced flow.
주기적인 온도를 갖는 두 수평 평판 사이에서의 자연 대류에서의 이례적인 분기 현상
유주식(Joo-Sik Yoo),김용진(Yong-Jin Kim) 한국전산유체공학회 2004 한국전산유체공학회지 Vol.9 No.4
This study investigates the transition of flows in a natural convection problem with periodic wall temperatures of the form,. T_L = T₁ + 8ΔTsinkx and T_U= T₂ + 8ΔTsinkx. The fluid considered is air with Pr=-0.7. In the conduction-dominated regime with a small Rayleigh number, two large cells are fanned over one wave length, for all wave numbers. When k≤1.8, the flow becomes unstable with increase of the Rayleigh number, and multicellular convection occurs above a critical Rayleigh number. The flow patterns are classified by the number of eddies over one wave length, and several kinds of transition phenomena, such as 2→3→4, 4→3→2, and 2→4 eddy flow, occur with increase( or decrease) of the Rayleigh number. Dual solutions are found above a critical Rayleigh number, and an anomalous bifurcation is observed.
유주식(Joo-Sik Yoo) 한국전산유체공학회 2006 한국전산유체공학회지 Vol.11 No.2
This study investigates the oscillatory thermal convection of a fluid with Pr=0.02 in a wide-gap horizontal annulus with constant heat flux inner wall. When Pr=0.02, dual steady-state flows are not found. After the first Hopf bifurcation from a steady to a time-periodic flow, five successive period-doubling bifurcations are recorded before chaos. The power spectrum shows the period- 2⁴ and 2? flows clearly, and a window of period 3×2³ flow is found in the chaotic regime. The approximate value of the Feigenbaum number for the last three period-doubling bifurcations is 4.76. The transition route to chaos of the present simulations is consistent with the period-doubling route of Feigenbaum.
일정 열 유속 벽을 갖는 수평 환형 공간에서의 준주기성과 흔돈대류
유주식(Joo-Sik Yoo) 한국전산유체공학회 2011 한국전산유체공학회지 Vol.16 No.3
This study investigates the bifurcation sequence to chaos in a horizontal annulus with a constant heat flux wall. After the first Hopf bifurcation from a steady to a simple time-periodic flow with a fundamental frequency, quasi-periodic flows with two or three incommensurable frequencies appear. A reverse transition from a quasi-periodic flow to a simple periodic flow is observed with increase of Rayleigh number. And finally, chaotic convection is established after appearance of three incommensurable frequencies at a high Rayleigh number. Simple periodic flows exist between quasi periodic flows. The transition route to chaos of the present simulations follows the Ruelle-Takens route.
아래 평판이 미소한 불균일 온도를 갖는 두 수평 평판 사이에서의 자연 대류 : Pr=0.7
유주식(Joo-Sik Yoo) 한국전산유체공학회 2013 한국전산유체공학회지 Vol.18 No.2
Natural convection of air with Pr=0.7 between two horizontal plates with small magnitude non-uniform temperature distribution[?ΔT sin(kX/H), H : gap width, X : horizontal coordinate] in the lower plate is numerically(?=0.01) investigated. In the conduction-dominated regime with Ra ≤ 1700, two upright cells are formed over one wave length(2π/k). For small wave number, the flow becomes unstable with increase of Rayleigh number, and multicellular convection occurs above a critical Rayleigh number. The flow patterns are classified by the number of eddies over one wave length. When k=1, a transition of 2→4→6 eddy flow occurs with increase of Rayleigh number, and no hysteresis phenomenon is observed. Dual and triple solutions are found for k=1, and transitions of 10→8, 8→6, 6→4→2 eddy flow occur with decrease of Rayleigh number.
위 평판이 작은 불균일 온도를 갖는 두 수평 평판 사이의 자연 대류에서의 다중해
유주식(Joo-Sik Yoo) 한국전산유체공학회 2016 한국전산유체공학회지 Vol.21 No.3
Multiple solutions in natural convection of water with Pr=7 between two horizontal plates with small magnitude non-uniform temperature distribution in the upper plate is numerically investigated. The dimensionless temperature of upper plate is θ=ϵsinkx. Two upright cells are formed over one wave length in the conduction-dominated regime of small Rayleigh number. However, multicellular convection occurs above a critical Rayleigh number for small wave number. When k=1.5, dual solutions are found and a transition of 6→4 eddy flow occurs with decrease of Rayleigh number. When k=0.75, two, three, four and five multiple solutions are observed. Transitions of 14→12, 12→10, 10→8 and 6→8 eddy flow occur with decrease of Rayleigh number.
유주식(Joo-Sik Yoo),김용진(Yong-Jin Kim) 한국전산유체공학회 2000 한국전산유체공학회지 Vol.5 No.1
This study investigates the problem of phase change from liquid to solid in the inviscid stagnation flow. The instantaneous location of the solid-liquid interface is fixed for all times by a coordinate transformation. Finite difference method is used to obtain the solution of the unsteady problem, and the growth rate of solid and the transient heat transfer from the surfaces of solid are investigated. The transient solution is dependent on the three dimensionless parameters, but the final steady state is determined by only one parameter of temperature ratio/conductivity ratio. It is observed that the instantaneous heat flux at the surface of solid can be obtained with sufficient accuracy by measuring the thickness of the solid or vice versa.
일정 온도 벽면을 갖는 수평 환형공간 내의 유체의 자연 대류에서의 삼중해
유주식(Joo-Sik Yoo) 한국전산유체공학회 2017 한국전산유체공학회지 Vol.22 No.1
Natural convection of a fluid with the Prandtl number of 7(water) in a horizontal annulus with constant temperature walls is numerically investigated. The inner cylinder is hotter than the outer cylinder. The flows are classified by the number of eddies in a half annulus. It is found that dual or triple solutions exists above a critical Rayleigh number for an annulus with a aspect ratio D<SUB>i</SUB>/L=4. Transitions of 3→1 and 2→1 eddy flow occur with decrease of Rayleigh number. However, reverse transitions of 1→3 and 1→2 eddy flow do not occur with increase of Rayleigh number, and no hysteresis phenomenon is observed. In the regime of triple solutions, the 3 eddy flow has the largest mean Nusselt number value and the 1 eddy flow has the smallest value.