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넓은 수평 환형 공간에서의 중간 Prandtl수 유체의 자연 대류의 천이
유주식,Yoo, Joo-Sik 대한기계학회 2000 大韓機械學會論文集B Vol.24 No.2
Natural convection in a wide-gap horizontal annulus is considered, and the transition of flows and the bifurcation phenomenon are investigated for the fluids with Pr=0.2 and 0.3. At Pr=0.2, a bicellular flow pattern is observed at high Rayleigh number, and the solution is unique. At Pr=0.3, both the steady unicellular and bicellular flows exist above a certain critical Rayleigh number. For the fluids of Pr=0.2, the bicellular flow can be obtained by the impulsive heating of the inner cylinder, but it is not obtained from the zero initial condition for Pr=0.3. Hysteresis phenomena have not been observed. A transition from a bicellular flow to a unicellular flow occurs for Pr=0.3.
Von-Karman 회전 유동 하에서의 액체 실리콘의 응고와 열전달
유주식,Yoo, Joo-Sik 대한기계학회 1999 大韓機械學會論文集B Vol.23 No.2
Heat transfer and solidification of liquid silicon in von-$K{\acute{a}}rm{\acute{a}}n$ swirling flow is investigated. The moving boundary is fixed for all times by a coordinate transformation, and finite difference method Is used to obtain the instantaneous location of the solid-liquid Interface and the heat transfer from the surfaces of solid and liquid. For small Stefan number or low wall temperature, the transient heat transfer from the surface of solid(QS(t)) is much larger than that from the liquid side of solid-liquid interface(QL(t)) and QL(t) reaches its quasi-steady-state value much faster than QS(t).
좁은 수평 환형공간에서의 낮은 Prandtl 수 유체의 자연 대류
유주식,Yoo, Joo-Sik 대한기계학회 1998 大韓機械學會論文集B Vol.22 No.12
Natural convection of low Prandtl number fluids with $Pr{\leq}0.2$ in a narrow horizontal annulus is numerically investigated. For $Pr{\leq}0.2$, hydrodynamic instability induces oscillatory multicellular flows consisting of multiple like-rotating cells. For a fluid with $Pr{\approx}0$, the region in which instability of conduction regime first forms is near the vertical section of annulus, and the multiple cells are distributed uniformly in the lower and upper regions of annulus. As Pr increases, however, the cells are shifted upwards. The like-rotating cells drift downward, as time goes on, and the speed of travel increases with increase of Pr. For a fluid with Pr=0.1, a flow with period-4 solution is observed between chaotic states.
수평 원주형 환형내에서의 정상상태의 공기의 자연대류에 대한 이중해
유주식,Yu, Ju-Sik 대한기계학회 1996 大韓機械學會論文集B Vol.20 No.9
Dual solutions for steady natural convection of air between two horizontal concentric cylinders are numerically investigated in the range of $D_i$/L(=diameter of inner cylinder/gap width)$\leq$10. It is found that, when the Rayleigh number based on the gap width exceeds a certain critical value, a new flow pattern forming two counter-rotating eddies in the half of the annulus can be realized, which is different from the crescent-shaped flow commonly observed. In the new flow pattern, the fluid near the top of the hot inner cylinder moves downward. This solution is found for D$_{i}$/L.geq.0.3, but not for$D_i$/L$\leq$0.2. As $D_i$/L increase, the critical Rayleigh number is decreased, and tends to a finite limit.t.
유주식,엄용균,Yoo, Joo-Sik,Eom, Yong-Kyoon 대한기계학회 2000 大韓機械學會論文集B Vol.24 No.6
The problem of phase change from liquid to solid in the inviscid plane-stagnation flow is theoretically investigated. The solution at the initial stage of freezing is obtained by expanding it in powers of time, and the final equilibrium state is determined from the steady-state governing equations. The transient solution is dependent on the three dimensionless parameters, but the equilibrium state is determined by one parameter of (temperature ratio/conductivity ratio). The effect of the fluid flow on the growth rate of the solid in the pure conduction problem can be clearly seen from the solution of the initial stage and the final equilibrium state. The characteristics of the transient heat transfer at the surface of the solid and the liquid side of the solid-liquid interface for all the dimensionless parameters are elucidated.
일정 열 유속으로 냉각되는 안쪽 실린더를 갖는 수평 환형 공간에서의 공기의 자연 대류
유주식,엄용균,김용진 대한설비공학회 2000 설비공학 논문집 Vol.12 No.8
Natural convection of air in a horizontal annulus with the inner cylinder cooled by the application of a constant heat flux and the isothermally heated outer cylinder is considered. The bifurcation phenomenon of flow patterns and the heat transfer characteristics are numerically investigated. The zero initial condition induces a unicellular flow in a half annulus. A bicellular flow consisting of two counter-rotating eddies in a half annulus can be obtained above a certain critical Rayleigh number. A transition from the bicellular to the unicellular flow occurs with a decrease in Rayleigh number. Hysteresis phenomena have not been observed. In the regime of dual flows, the overall Nusselt number of the bicellular flow is greater than that of the unicellular flow.
넓은 수평 환형 공간에서의 혼동 열 대류 : Pr=0.1
유주식,엄용균 대한설비공학회 2001 설비공학 논문집 Vol.13 No.2
Transition to chaotic convection is investigated for natural convection of a fluid with Pr=0.1 in a wide-gap horizontal annuls. The unsteady two-dimensional stream-function-vorticity equation is solved with finite difference method. As the Rayleigh number is increased, the steady 'downward flow' bifurcates to a time-periodic flow with a fundamental frequency, and afterwards a period-doubling bifurcation occurs. As the Rayleigh number is increased further, the chaotic flow regime is reached after a sequence of successive Hopf bifurcation to quasi-periodic and chaotic flow regimes. The route to chaos shows the Ruelle-Takens-Newhouse scenario. The flow of chaotic regime displays complex coalescence and separation of eddies in the side and lower region of the annulus.