• Journal of Mechanical Science and Technology
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• 제18권10호
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• pp.1869-1879
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• 2004

The present study has numerically investigated two-dimensional flow over three circular cylinders in an equidistant side-by-side arrangement at a low Reynolds number. For the study, numerical simulations are performed, using the immersed boundary method, in the range of g* < 5 at Re= 100, where g* is the spacing between two adjacent cylinder surfaces divided by the cylinder diameter. Results show that the flow characteristics significantly depend on the gap spacing and a total of five kinds of wake patterns are observed over the range: modulation-synchronized (g* (equation omitted) 2), inphase-synchronized (g* (equation omitted) 1.5) , flip-flopping (0.3 < g* (equation omitted) 1.2) , deflected (g* (equation omitted) 0.3), and single bluff-body patterns (g* < 0.3). Moreover, the parallel and symmetric modes are also observed depending on g* in the regime of the flip-flopping pattern. The corresponding flow fields and statistics are presented to verify the observations.

• 물과 미래
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• 제21권3호
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• pp.291-291
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• 1988

2차원 흐름이 원주주위를 지날 때 발생하는 흐름의 변화가 기본방정식인 연속방정식과 운동량방정식에 의하여 수치적으로 해석된다. 수치해석 과정은 기본방정식에 유함수, 와도 및 흐름의 특성을 나타내는 무차원 매개변수를 도입하여 무차원 유함수-와소수송식을 유도한 후, successive over relaxation scheme과 alternating direct implicit scheme으로 수행된다. 수치실험은 레이놀즈수 125-275를 기존의 수치해석에서는 주로 수치실험 결과와 비교한다. 원주표면의 압력을 구하는 방법에 있어서 기존의 수치해석에서는 주로 방사 운동량방정식만을 사용하였으나, 본 논문에서는 기존의 방법외에 방사 운동량방정식 및 접선 운동량방정식에 의해 압력을 계한하고, 두 값을 비교하여 레이놀즈수에 따른 압력을 구하는 방법을 제시한다. 또한 와도의 분포를 도시하여 원주에 의한 후류의 영향을 받지 않는 외부경계의 한계를 새로이 설정한다.

• 물과 미래
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• 제31권4호
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• pp.291-297
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• 1998

2차원 흐름이 원주주위를 지날 때 발생하는 흐름의 변화가 기본방정식인 연속방정식과 운동량방정식에 의하여 수치적으로 해석된다. 수채해석 과정은 기본방정식에 유함수, 와도 및 흐름의 특성을 나타내는 무차원 매개변수를 도입하여 무차원 유함수-와소수송식을 유도한후, successive over relaxation scheme과 alternating direct implicit scheme으로 수행된다. 수치실험은 레이놀즈수 125-275를 갖는 흐름에 대하여 수행되었으며, 시간에 따른 유선, 와도, 원주표면의 압력을 구하는 방법에 있어서 기존의 수치해석에서는 주로 방사 운동량방정식만을 사용하였으나, 본 논문에서는 기존의 방법외에 방사 운동량방정식 및 접선 운동량방정식에 의해 압력을 계한하고, 두값을 비교하여 레이놀즈수에 따른 압력을 구하는 방법을 제시한다. 또한 와도의 분포를 도시하여 원주에 의한 후류의 영향을 받지 않는 외부경계의한계를 새로이 설정한다.

• 대한기계학회논문집B
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• 제30권5호
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• pp.438-445
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• 2006

We performed the simulation of the unsteady three dimensional flow over a square cylinder in a wind tunnel in moderate Reynolds number range, $100{\sim}2500$ by using LBM. SGS model was applied for the turbulent flow. Frist of all we compared LBM(Lattice Boltzmann Method) solution of Poiseuille flow applied Farout and bounce back boundary conditions with the analytical and FOAM solutions to verify the applicability of the boundary conditions. For LBM simulation the calculation domain was formed by structured grids and prescribed uniform velocity and density inlet and Farout boundary conditions were imposed on the in-out boundaries. Bounceback and wind tunnel boundary conditions were applied to the cylinder walls and the boundaries of calculation domain respectively. The maximum Strouhal number of the vortex shedding is 0.2025 at Re = 750. and the number maintains the constant value of 0.18 when Re>1000. We also predicted that the critical reynolds number of the turbulent flow is in the range of $250{\sim}500$.

• 한국전산유체공학회지
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• 제16권3호
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• pp.22-28
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• 2011

Laminar Flow over an equilateral triangular cylinder is studied for several inflow angles. Under an uniform flow of $Re_d$=50,75,100,125,150, the triangular cylinder is rotated by ${\theta}$=$0^{\circ}$,$15^{\circ}$,$30^{\circ}$,$45^{\circ}$,$60^{\circ}$,$75^{\circ}$,$90^{\circ}$,$105^{\circ}$. The governing equations are solved by the PISO algorithm based on the finite volume method of the unstructured grid system. The effects of the inflow angle on the vortex-shedding flows are investigated. The Strouhal number shows a minimum at ${\theta}$=$60^{\circ}$. It is closely related to the variation of pressure and flow structure induced by the movement of separation points.

• 한국전산유체공학회지
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• 제17권2호
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• pp.71-77
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• 2012

Numerical simulations are carried out for flow over a circular cylinder controlled by the momentum forcing which is generated by a pair of plasma actuators symmetrically mounted on the cylinder surface. A popular and empirical plasma model is used for the spatial distribution of momentum forcing. In this study, we consider two different types of actuation, i.e., steady and unsteady (or pulsed) actuation. In the unsteady actuation, the actuation is turned on and off periodically, its frequency being a control parameter. The objective of this study is to investigate the effects of actuator location and actuation frequency on the flow structures and the forces on the cylinder. Results show that the cylinder wake can be effectively controlled by proper actuator location. For example, when the actuators are located at $120^{\circ}$ from the stagnation point, vortex shedding is completely suppressed with the boundary layer almost fully attached to the surface, resulting in drag reduction and lift elimination.

• 에너지공학
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• 제24권3호
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• pp.109-114
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• 2015

배열회수 보일러의 전열관군은 외부에 고온의 배기가스가 흐르면서 유동 유발 진동을 야기 시키며 배열회수 보일러의 전열관군에서 파손을 야기할 수 있어서 열교환기의 구조적 안정성을 위해 열교환기의 전열관군에서 유동 유발 진동 특성을 규명할 필요가 있다. 일반적인 열교환기 전열관군에서 유동 유발 진동에 관한 실험적 연구는 기존에 많이 진행되어 오고 있으며 유동 유발 진동에 대한 무차원 PSD(Power Spectral Density) 함수를 무차원 주파수인 Strouhal 수, fU/U의 함수로 도출된 실험적 결과들이 도출되어 있다. 본 연구는 열교환기 전열관군에서 유동 유발 진동에 관한 기존의 연구들의 결과를 전산유체해석을 통해 검증하고 배열회수 보일러의 전열관군의 유동 유발 진동 특성에 적용하기 위한 기반을 마련하는 것을 목적으로 한다. 이러한 것을 위해 단일 원관에서 비정상 상태 유동해석을 수행하여 주기적인 와동 발생 특성과 원관에서의 양력 변화 특성을 살펴보았다. 또한 원관에서 양력 변동 특성으로부터 유동 유발 진동에 따른 PSD 특성 결과를 도출하여 기존의 연구들과 비교를 통해 원관 주위의 PSD 특성을 정립하였다.

• Journal of Advanced Marine Engineering and Technology
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• 제22권4호
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• pp.436-444
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• 1998

Applicability of the RNG k-$\varepsilon$ model to the analysis of unsteady axisymmetric turbulent flow of a reciprocating engine including port/valve assembly is studied numerically. The governing equations based on non-orthogonal including port/valve assembly is studied numerically. The governing equations based on a non-orthogonal coordinate formulation with Cartesian velocity components are used and discretised by the finite volume method with non-staggered variable arrangements. The predicted results using the RNG k-$\varepsilon$ model of the unsteady axisymmetric turbulent flow within a cylinder of reciprocating model engine including port/valve assembly are compared to these from the modified k-$\varepsilon$ model and experimental data. Using the RNG k-$\varepsilon$ model seems the have some potential for the simulations of the unsteady turbulent flow within a port/valve-cylinder assembly over the modified k-$\varepsilon$model.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.956-961
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• 2003

Direct numerical simulations of flow over a circular cylinder are performed at two different Reynolds numbers (Re=220 and 300) that correspond to three-dimensional instabilities of mode A and mode B, respectively, to investigate the characteristics of drag and lift at these Reynolds numbers. The drag and lift coefficients are measured locally along the spanwise direction and their characteristics are studied in detail. The variation of total drag in time is large at Re=220, and the total drag becomes minimum when vortex dislocation occurs in the wake. The drag and lift variations in space are also closely associated with the evolution of vortex dislocation at this Reynolds number. At Re=300, vortex dislocation is not found in the wake and temporal variations of drag and lift are much smaller than those at Re=220, but their spatial variations are quite large due to the near-wake secondary vortices existing in the mode B instability.

• 한국전산유체공학회지
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• 제7권1호
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• pp.20-27
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• 2002

Flow over a circular cylinder is studied numerically using a turbulence model. Based on the κ-ε-f/sub μ/ model of Park and Sung[6], a new damping function is used. The efficiency of the strain dependent damping function is addressed for vortex-shedding flows past a circular cylinder. The mean velocity and Reynolds stresses are compared with available experimental data at Re/sub D/= 3900. Also, the computational results for the Strouhal number are evaluated at several Reynolds number. The predictions by κ-ε-f/sub μ/ model are in good agreement with the experiments.