• 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.

• 한국연소학회:학술대회논문집
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• 2002.06a
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• pp.68-75
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• 2002

The dynamic behaviors of the single vortex and flame-vortex interaction in a $CH_4/Air$ diffusion flame with addition of $CO_2$ were investigated numerically. The numerical method was based on a predictor-corrector for low Mach number flow. A two-step global reaction mechanism was adopted as a combustion model. Through comparison of results by effect of $CO_2$ added either on the fuel or oxidizer side, it was found that the growth of single vortex and entrainment of surrounding fluid by $CO_2$addition on the fuel side are larger than those by $CO_2$ addition on oxidizer side. Also, when $CO_2$ is added on fuel side, flame-vortex interaction becomes more significant than on air side.

• Journal of the Korean Society of Safety
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• v.25 no.1
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• pp.9-16
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• 2010

Flame structure of diffusion flame interacting with a single vortex was investigated with direct numerical simulation (DNS). A well-known counterflow diffusion flame was used as an initial flat flame and single vortices were made by issuing a high-velocity jet abruptly in fuel- and air-side. The variations in the maximum concentration of major species (CO and $CO_2$) and NOx (NO and $NO_2$) with the stoichiometric scalar dissipation rate were investigated. Unsteady effects in the species concentration variation of the flame interacting with a vortex were identified by comparing with that of steady flame. $NO_2$ formation characteristics of the flame interacting with a vortex were well understood by investigating the $HO_2$ formation. To enhance the prediction performance in the fire simulation, current turbulent combustion modelings are needed to be modified by adopting the unsteady effects in the species concentrations of diffusion flame interacting with a vortex.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.4
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• pp.486-493
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• 1997

Experimental investigations on the comparison of developments between transient jets and evolving jet diffusion flames have been made in initial injection period. To achieve this experiment, an ignition technique using a residual flame as the ignition source is devised. High speed Schlieren visualizations, and measurements including jet tip penetration velocities and jet widths of the primary vortex are employed to examine the developing processes for several flow conditions. It is seen that the developing behaviors in the presence of flame are greatly different from those in transient jet, and thus the flow characteristics in the transient part are also modified. The discernible differences are shown to consist of the delay of the rollup of the primary vortex, the faster spreading after the rollup due to exothermic expansion, and the survival of only a primary vortex. The growth of primary vortex in the transient jet is properly explained through an impulsively started laminar vortex prior to the interaction. It is also found that the jet tip penetration velocity varies with elapsed time and an increase in Res gives rise to a higher tip penetration velocity.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.17 no.12
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• pp.1223-1229
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• 2007

Flow traveling over a cavity opening forms a vortex due to unstable shear layer and induces an aerodynamic pressure excitation from the diffusion of the vortex convecting out of the trailing edge of the opening. The interaction between the excitation force and the cavity response sustains resonance in the resonator(cavity) and locked-in vortex shedding at the leading edge of the opening. The aerodynamic excitation force can be described from the diffusion of the vortex over the trailing edge and the level of its diffusivity is related to the strength of vorticity seeded at the leading edge. In this study, the control scheme of the internal pressure oscillation was proposed from regulating the vorticity at the leading edge by use of an oscillating spoiler. It was found that the relative motion between the spoiler and the air mass at the cavity opening influenced vorticity strength and the control was achieved by direct feedback of the cavity pressure fluctuation to the actuator.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.9
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• pp.1209-1219
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• 2003

The dynamic behaviors of the single vortex interacting with $CH_4-Air$ jet diffusion flame are investigated numerically. The numerical method is based on a predict-corrector scheme for a low Mach number flow. A two-step global reaction mechanism is adopted as a combustion model. Studies are conducted in fixed initial velocities for the three cases according as where $CO_2$ is added; (1) without dilution, (2) dilution in fuel stream and (3) dilution in oxidizer stream. A single vortex is generated by an axisymmetric jet, which is made by an impulse of a cold fuel when a flame is developed entirely in a computational domain. The simulation shows that $CO_2$ dilution in fuel stream results in somewhat larger vortex radius, and greater amount of entrainment of surrounding fluid than in other cases. Thus, the dilution of $CO_2$ in fuel stream enhances the mixing in single vortex and increases the stretching of the flame surface. The budgets of the vorticity transport equation are examined to reveal the mechanism of vortex formation when $CO_2$ is added. It is found that, in the case of $CO_2$ dilution in fuel stream, the vortex destruction due to volumetric expansion and the vortex production due to baroclinic torque are more dominant than in other cases.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.32 no.6
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• pp.9-15
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• 2004

In this study the development of the two dimensional flowfield around an impulsively translated circular cylinder is numerically simulated using the diffusion vortex method. A detailed streamline pattern of fore wake and main wake of Re=1200, 9500 flowfields are investigated. The results of streamline pattern, the size of main wake and the axial velocities along the rear symmetry axis of the circular cylinder show good agreement with the reported experimental results. The long term wake delvelopment for Re=1200 flowfield was calculated up to ${\tau}=50$. The vortex shedding frequency shows the similar value as that of reported.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.11a
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• pp.196-199
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• 2005

Flow traveling over a cavity opening forms a vortex due to unstable shear layer and induces an aerodynamic pressure excitation from the diffusion of the vortex convecting out of the trailing edge of the opening. The interaction between the excitation force and the cavity response sustains resonance in the resonator(cavity) and locked-in vortex shedding at the leading edge of the opening. The aerodynamic excitation force can be described from the diffusion of the vortex over the trailing edge and the level of its diffusivity is related to the strength of vorticity seeded at the loading edge. In this study, the control scheme of the internal pressure oscillation was proposed from regulating the vorticity at the leading edge by use of an oscillating spoiler. It was found that the relative motion between the spoiler and the air mass at the cavity opening influenced vorticity strength and the control was achieved by direct feedback of the cavity pressure fluctuation to the actuator.

• Proceedings of the KSME Conference
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• 2001.06e
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• pp.518-523
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• 2001

Vortex methods were originally conceived as a tool to model the evolution of unsteady, incompressible, high Reynolds number flows of engineering interest. Recently various methods have been proposed for simulating the diffusion in vortex methods for two-dimensional incompressible flows. We test the diffusion schemes of vortex methods. In this paper we directly compare the particle strength exchange scheme with the vorticity redistribution scheme in tenus of their accuracy and computational efficiency. Comparisons between both viscous models described are presented for short-time runs of impulsively started flows past a circular cylinder for Reynolds number of 60. The particle strength exchange scheme has been shown more accurate and efficient than the vorticity redistribution scheme.

• 한국연소학회:학술대회논문집
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• 2000.12a
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• pp.113-122
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• 2000

Dynamic structures of unsteady $CH_4$/Air jet diffusion flames with flame-vortex interaction were numerically investigated. A time-dependent, axisymmetric computational model was adopted for this calculation. Two step global reaction mechanism which considers 6 species, was used to calculate the reaction rates. The predicted results including gravitational effect show that the large outer vortices and the small inner vortex street can be well simulated without any additional disturbances in the downstream of nozzle tip. It was found that the temperature and species concentrations had various values for the same mixture fraction in flame-vortex interaction region. This unsteady jet flame configuration accompanying flame-vortex interaction is expected to give good implications for the structure of turbulent flames.