• Wind and Structures
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• v.29 no.1
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• pp.43-53
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• 2019

Vortex-induced vibration of three circular cylinders (each of diameter D) in an equilateral triangular arrangement is investigated using the immersed boundary method. The cylinders, with one placed upstream and the other two side-by-side downstream, are free to vibrate in the cross-flow direction. The cylinder center-to-center spacing L is adopted as L/D = 2.0. Other parameters include the Reynolds number Re = 100, mass ratio $m^*=2.0$, reduced velocity $U_r=2{\sim}15$ and damping ratio ${\zeta}=0$. Cylinder vibration responses are dependent on $U_r$ and classified into five regimes, i.e. Regime I ($U_r{\leq}3.2$), Regime II ($3.2<U_r{\leq}5.0$), Regime III ($5.0<U_r{\leq}6.4$), Regime IV ($6.4<U_r{\leq}9.2$) and Regime V ($U_r>9.2$). Different facets of vibration amplitude, hydrodynamic forces, wake patterns and displacement spectra are extracted and presented in detail for each regime.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.12
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• pp.1950-1957
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• 2004

Analysis for structures composed of materials containing regularly spaced in-homogeneities is usually executed by using averaged material properties. In order to evaluate the effective properties, a unit cell is defined and loaded somehow, and its response is investigated. The imposed loading, however, should accord to the status of unit cells immersed in the macroscopic structure to secure the accuracy of the properties. In this study, mathematical description for the periodicity of the displacement field is derived and its direct implementation into FE models of unit cell is attempted. Conventional finite element code needs no modification, and only the boundary of unit cell should be constrained in a way that the periodicity is preserved. The proposed method is applicable to skew arrayed in-homogeneity problems. Homogenized in-plane elastic properties are evaluated for a few representative cases and the accuracy is examined.

• Wind and Structures
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• v.5 no.2_3_4
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• pp.379-392
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• 2002

Numerical flow computations around an aeroelastic 3D square cylinder immersed in the turbulent boundary layer are shown. Present computational code can be characterized by three numerical aspects which are 1) the method of artificial compressibility is adopted for the incompressible flow computations, 2) the domain decomposition technique is used to get better grid point distributions, and 3) to achieve the conservation law both in time and space when the flow is computed a with moving and transformed grid, the time derivatives of metrics are evaluated using the time-and-space volume. To provide time-dependant inflow boundary conditions satisfying prescribed time-averaged velocity profiles, a convenient way for generating inflow turbulence is proposed. The square cylinder is modeled as a 4-lumped-mass system and it vibrates with two-degree of freedom of heaving motion. Those blocks which surround the cylinder are deformed according to the cylinder's motion. Vigorous oscillations occur as the vortex shedding frequency approaches cylinder's natural frequencies.

• International Journal of Naval Architecture and Ocean Engineering
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• v.9 no.6
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• pp.598-612
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• 2017

The recently developed Harmonic Polynomial Cell (HPC) method has been proved to be a promising choice for solving potential-flow Boundary Value Problem (BVP). In this paper, a flux method is proposed to consistently deal with the Neumann boundary condition of the original HPC method and enhance the accuracy. Moreover, fixed mesh algorithm with free surface immersed is developed to improve the computational efficiency. Finally, a two dimensional (2D) multi-block strategy coupling boundary-fitted mesh and fixed mesh is proposed. It limits the computational costs and preserves the accuracy. A fully nonlinear 2D numerical wave tank is developed using the improved HPC method as a verification.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.11a
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• pp.760-763
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• 2010

The expansion of 1D numerical code to 2D or 3D is needed in order to improve the analysis accuracy of the interior ballistics. The cut cell method has been imposed for the code expansion to multi dimensions. The MUSCL-Hancock scheme as a high resolution method has been selected. A feasibility of the cut cell method has been verified by analyzing the free piston problem.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2845-2850
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• 2007

Flow-induced forces on two identical nearby circular cylinders immersed in the cross flow at Re =100 were numerically studied. We consider all possible arrangements of the two circular cylinders in terms of the distance between the two cylinders and the inclination angle with respect to the direction of the main flow. It turns out that significant changes in the characteristics of flow-induced forces are noticed depending on how the two circular cylinders are positioned, resulting in quantitative changes of force coefficients on both cylinders. Collecting all the numerical results obtained, we propose a contour diagram for drag coefficient and lift coefficient for each of the two cylinders. The perfect geometrical symmetry implied in the flow configuration allows one to use those diagrams to estimate flow-induced forces on two identical circular cylinders arbitrarily positioned in physical space with respect to the main flow direction.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2890-2895
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• 2007

Heat transfer on two identical nearby circular cylinders immersed in the uniform cross flow at Re = 100 and Pr = 7.0 was numerically studied. We consider all possible arrangements of the two circular cylinders in terms of the distance between the two cylinders and the inclination angle with respect to the direction of the main flow. It turns out that significant changes in the characteristics of heat transfer are noticed depending on how the two circular cylinders are positioned, resulting in quantitative changes of heat transfer coefficients on both cylinders. Collecting all the numerical results obtained, we propose a contour diagram for averaged Nusselt number for each of the two cylinders. The perfect geometrical symmetry implied in the flow configuration allows one to use those diagrams to estimate heat transfer rates on two identical circular cylinders arbitrarily positioned in physical space with respect to the main flow direction.

• Journal of computational fluids engineering
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• v.13 no.2
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• pp.42-47
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• 2008

Heat transfer on two identical nearby circular cylinders immersed in the uniform cross flow at Re=120 and Pr=0.7 was numerically studied. We consider all possible types of arrangements of the two circular cylinders in terms of the distance between the two cylinders and the inclination angle with respect to the direction of the main flow. It turns out that significant changes in the characteristics of heat transfer are noticed depending on how the two circular cylinders are positioned, resulting in quantitative changes of heat transfer coefficients on both cylinders. Collecting all the numerical results obtained, we propose a contour diagram for averaged Nusselt number for each of the two cylinders. The perfect geometrical symmetry implied in the flow configuration allows one to use those diagrams to estimate heat transfer rates on two identical circular cylinders arbitrarily positioned in physical space with respect to the main flow direction.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.60 no.10
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• pp.1838-1845
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• 2011

This paper deals with coupled electromagnetic-thermal field analysis for thermal fluid analysis of oil immersed power transformer. Electric power losses are calculated from electromagnetic field analysis and are used as input source of thermal field analysis based on computational fluid dynamics(CFD). Particularly, In order to accurately predict the temperature rise in oil immersed power transformer, the thermal problem should be coupled with the electromagnetic problem. Moreover, to reduce analysis region, the heat transfer coefficient is applied to boundary surface of the power transformer model. The coupling method results are compared with the experimental values for verifying the validity of the analysis. The predicted temperature rises show good agreements with the experimental values.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.1
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• pp.30-38
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• 2008

As a follow-up of our previous studies on flow-induced forces on two identical nearby circular cylinders immersed in the cross flow at Re=100 and flow patterns past them,$^{(1,2)}$ we present Reynolds-number effects on the forces and patterns by further computing flows with Re=40, 50, 160. We consider all possible arrangements of the two circular cylinders in terms of the distance between the two cylinders and the angle inclined with respect to the main flow direction. Collecting all the numerical results obtained, we propose contour diagrams for mean force coefficients and their rms of fluctuation as well as for flow patterns and Strouhal number for each Re. These diagrams shed light on a comprehensive picture on how the wake interaction between the two cylinders alters depending on Re.