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박상로(Sangro Park),이창훈(Changhoon Lee) 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11
Coherent structures in Rayleigh-Benard convection are investigated. Especially, thermal plumes, vortical structures and their relation are studied using direct numerical simulation. The flow regime is soft turbulence with Rayleigh number106, Prandtl number 0.7 and the aspect ratio 4. Thermal boundary layer makes an important influence on heat flux on the whole domain, therefore thermal boundary layer and wave propagation caused by impinging thermals from the opposite side are studied. An important result from the study is that vertical and horizontal movement of thermal plumes make vortical structures which enhance thermal plume activities in return. Topology analysis of vortical structures are also conducted with invariants of velocity gradient tensor. And conditional averaged joint probability density function is used to see the effect of thermal plume activity into vortical structures.
직접 수치 모사를 이용한 자연 대류에서의 입자 거동 해석
박상로(Sangro Park),이창훈(Changhoon Lee) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
Particles falling under gravity in a flow of weakly turbulent Rayleigh-Bénard convection are studied with a direct numerical simulation and pseudo-spectral method via dealiased-Fourier and Chebyshev-tau expansion. The particle motion is tracked by using four point-Hermite and fifth order-Lagrangian interpolation scheme. The flow condition is Rayleigh number 2 × 106, Prandtl number 0.678 with a large aspect ratio of Γ = 6. Particles are forced by drag and gravity for the range of Stokes number 0.1, 0.3, 0.5, 1, and 5. Froude number, which determines the ratio of surrounding flow inertia and gravity, is set to 0.19, 0.29, and 0.39. Collisions of particles or force on fluid by particles are not considered. In this paper, we mainly concerned the effect of gravity on particle motion and importance of drag force caused by thermal structures in natural convection. The results are suggested by analysis of time- and space- averaged particle settling velocity compared with terminal settling velocity of particles in a still fluid. And the strong correlation of temperature, drag force and particle settling velocity indicates strong influence of thermal structures on particle motion.
Poisson 방정식을 Explicit 하게 풀기 위한 새로운 Projection 수치 기법
박상로(Sangro Park),이창훈(Changhoon Lee) 대한기계학회 2011 대한기계학회 춘추학술대회 Vol.2011 No.10
A new scheme of projection method for incompressible fluid flow is presented. When divergence of Navier-Stokes equation is taken, Poisson’s equation between pressure term and nonlinear tern appears to satisfy the continuity condition. But solving the Poisson’s equation with implicit numerical method requires a lot of computational cost. By substituting Poisson’s equation into the Navier-Stokes equation, we can remove the pressure gradient term from the equation; instead divergence free projection operator appears. Because of the ‘Convolution Theorem’, multiplication between divergence free projection operator and nonlinear term in wave space can be replaced with calculation in physical space by convolution. Also, the divergence free projection operator in physical space has a fast decaying distribution, thus the operator can be truncated in physical space. With the truncated divergence free projection operator, the Poisson’s equation can be solved explicitly. In this study, the truncated operator is tested to check the ability of reducing divergence. And this scheme was applied into direct numerical simulation of isotropic turbulence to investigate how seriously the truncation affects the statistical properties of the flow.
Park, Chan-Young,Park, Sangro 慶北大學校 1982 論文集 Vol.33 No.-
P. Bankston(1)이 Cardinal을 이용한 새로운 개념인 Spectrum을 정의하고 이것을 이용한 反-공간을 처음 소개함으로서 종래의 위상공간으로 부터 새로운 위상적 성질을 소개하였다. 그 이후 I.L. Reilly와 K.M. Vamanamurthy (8) (9) 등에 의하여 Anti-Compact, Anti-Lindelo¨f 공간 등에 대한 여러성질들이 연구되어 졌다. 본 논문에서는 S-closed 공간, quasi H-closed공간 등에 Anti-개념을 적용시켜 여러성질들을 연구하였다.