• 한국전산유체공학회:학술대회논문집
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• 2007.10a
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• pp.21-26
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• 2007

This study focuses on the near-field vortical structure and dynamics of coaxial jets. The characteristics of laminar flow and mixing in coaxial jets are investigated using a unsteady flow simulation. In order to analyze the geometic effects on the vortical structure, several cases of different configurations are selected for various values of the velocity ratio of inner jet to outer jet. From the result, it is confirmed that mixing is promoted by the development of vortical structure and the interaction between inner jet and outer jet. This feature is strongly related to the vortex frequency in the shear-layers. The vortex frequency depends on the velocity ratio and the lip thickness of inner nozzle, but the outer pipe length has no effect on the frequency variation.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.10
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• pp.1223-1228
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• 1999

As part of study on separated flow using the vortex particle method, stability of the method and particle redistribution were scrutinized. Stability was investigated by choosing different combination of numerical parameters. The Gaussian vortex was considered to make the problem simple by eliminating the complexity due to presence of walls. It was shown that the numerical method was stable when $v{\Delta}t/h^2{\leq}0.5$. In all the stable cases the circulation and the linear momentum were conserved. Without the particle redistribution, the angular momentum was severely attenuated.

• Journal of computational fluids engineering
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• v.14 no.3
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• pp.49-55
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• 2009

This study focuses on the near-field vortical structure and dynamics of coaxial jets. The characteristics of laminar flow and mixing in coaxial jets are investigated using a unsteady flow simulation. In order to analyze the geometric effects on the vortical structure, several cases of different configurations are selected for various values of the velocity ratio of inner jet to outer jet. From the result, it is confirmed that the flow mixing is promoted by the development of vortical structure and the interaction between inner jet and outer jet. This feature is strongly related to the vortex frequency in the shear-layers. The vortex frequency depends on the velocity ratio and the lip thickness of inner nozzle, but the outer pipe length has no effect on the frequency variation.

• Wind and Structures
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• v.16 no.5
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• pp.457-476
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• 2013

This paper presents theoretical background for a semi-empirical, mathematical model of critical vortex excitation of slender structures of compact cross-sections. The model can be applied to slender tower-like structures (chimneys, towers), and to slender elements of structures (masts, pylons, cables). Many empirical formulas describing across-wind load at vortex excitation depending on several flow parameters, Reynolds number range, structure geometry and lock-in phenomenon can be found in literature. The aim of this paper is to demonstrate mathematical background of the vortex excitation model for a theoretical case of the structure section. Extrapolation of the mathematical model for the application to real structures is also presented. Considerations are devoted to various cases of wind flow (steady and unsteady), ranges of Reynolds number and lateral vibrations of structures or their absence. Numerical implementation of the model with application to real structures is also proposed.

• Journal of the Korea Institute of Military Science and Technology
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• v.16 no.4
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• pp.481-485
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• 2013

Rotor wake causes unsteady aerodynamics of rotor blade. So, accurate prediction of wake is very important and vortex method is good solution for this problem. Aerodynamic force of the rotor blade is calculated by potential panel method and the rotor wake is simulated by vortex particle method. The vortex particle method is easier to treat wake-body interaction and has better performance to expect the effect of ground and fuselage interaction. Rotor in hovering and forward flight condition is simulated through these methods. Thrust and surface pressure of rotor are compared with experiment data.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.7
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• pp.771-779
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• 2004

Unsteady behavior of the early wake in the viscous flow field past an impulsively started semicircular cylinder is studied numerically. In this paper, we propose the hybrid diffusion scheme to simulate dynamic characteristics of wake such as a fishtail-like flapping and an alternate vortex-shedding more accurately. This diffusion scheme based on particle strength exchange is mixed with the stochastic nature of random walk method. Also, the viscous splitting algorithm which calculates convective and diffusion terms successively is applied in order to handle random walk method effectively. Consequently, the early behavior of wake due to the breakdown of symmetrical vortici balance is more practically simulated with the vortex particle method.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.3039-3044
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• 2007

A few fluid structure interaction analyses have been developed for turbomachinery blades in comparison with aircraft wings. Also, the existing aeroelastic analyses for turbomachinery blades have been mostly limited to cases with a steady freestream. In reality, however, the inflowing freestream is often pulsating. Therefore, this paper presents stability and forced response analyses of an isolated three-dimensional blade under pulsating freestream conditions. A new three-dimensional unsteady vortex lattice model under a pulsating freestream has been developed in discrete time domain to examine unsteady aerodynamic forces acting on a vibrating blade. The blade's structural behaviors have been analyzed by using a three-dimensional plate model. In the aeroelastic analysis, the flutter onset of a blade under pulsating freestream is predicted by the Floquet analysis. The new time domain method can predict aeroelastic stability as well as time history.

• Journal of Ocean Engineering and Technology
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• v.12 no.1
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• pp.113-119
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• 1998

An experimental study was carried out in a three-dimensional cubic cavity driven by 2-dimensional plane Poiseuille flow for three kinds of Reynolds number, $10^4$, 3 $\times$ $10^4$ and 5 $\times$ $10^4$ based on the cavity width and cavity inlet mean flow velcoity. Instant simultaneous velocity vectors at whole field were measured by 2-D PIV system. Laser based illumination and two-frame grey-level cross correlation algorithm are adopted. Severe unsteady flow fluctuation within the cavity are remarkable at above Re = 3 $\times$ $10^4$ Reynolds numbers and sheared mixing layer phenomena are also found at the region where inlet driving Poiseuille flow is collided with the clock-wise rotating main primary vortex at upper center area. Instant velocity profiles reveal that deformed forced vortex formation is observed throughout the separate two areas.

• Journal of Ocean Engineering and Technology
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• v.9 no.2
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• pp.71-74
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• 1995

A numerical model capable of simulating a 2-D airfoil flying over in the vicinity of the waves is discussed. Instead of treating the problem as a heaving oscillation one above the rigid flat wall, sources are distributed on the prescribed wave profile. The wave deformation due to the airfoil is assumed to be negligible and treated as a rigid undulated wall. The source and vortex are distributed on the surface of the foil. It is found that the variation of $C_L$ with wave steepness in severe and that the lift variation due to waves decreases as the wing height above the water surface increases.

• Journal of Advanced Marine Engineering and Technology
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• v.22 no.3
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• pp.371-380
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• 1998

The flow patterns and dynamic properties of ship's propulsion mechanism of two-stage Weis-Fogh type are studied by the discrete vortex method. In order to study the effects of the interaction of the two wings two cases of the phase differences of the wing's motion are considered the same phase and the reverse phase. The flow patterns by simulations correspond to the photographs obtained by flow visualization and flowfield of the propulsion mechanism which is unsteady and complex is clearly visualized by numerical simulations. The time histories of the thrust an the drag coefficients on the wings are also calculated and the effects of the interaction of the two wings are numerically clarified.