• 한국연소학회:학술대회논문집
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• 2002.11a
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• pp.49-56
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• 2002

A two-dimensional direct numerical simulation is performed to investigate the formation characteristics of a single vortex interacting with $CH_4/N_2$-Air counterflow nonpremixed flame. The numerical method was based on a predictor-corrector scheme for a low Mach number flow. The detailed transport properties and a 16-step augmented reduced mechanism are adopted in this calculation. The budgets of the vorticity transport equation arc examined to reveal the mechanisms leading to the formation, evolution and dissipation of a single vortex interacting with counterflow nonpremixed flame. It is found that the stretching term, which depends on the azimuthal component of vorticity, and radial velocity, mainly generates vortieitv in non-reacting and reacting flows. The viscous and baroclinic torque term destroy the vorticity in non-reacting flow. In addition, the baroclinic torque term due to density and pressure gradient generates vorticity, while viscous and the volumetric expansion terms due to density gradient destroy vorticity in reacting flow.

• Journal of Environmental Science International
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• v.1 no.2
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• pp.105.2-117
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• 1992

The formation mechanism of the vortex streets in the lee of the mountain Is Investigated by a three-dimensional numerical model. The model is based upon the hydrostatic Boussinesq equations in which the vertical turbulent momentum flux is estimated by a turbulence parameterization scheme, but the horizontal viscosity is assumed to be constant. The results show that Karman vortex streets can form even without surface friction in a constant ambient flow with uniform stratification. The vortex formation is related to breaking of the mountain wave, which depends on the Froude number (Fr). In the case of a three-dimensional bell-shaped mountain, the wave breaking occurs when Fr is less than about 0.8, while a barman vortex forms when Fr is less than about 0.22. Vortex formation also depends on Reynolds number, which is estimated from the horizontal diffusivity. The vortex formation can be explained by the wave saturation theory given by Lindzen (1981) with some modification. Simulations in this study show that in the case of Karman vortex formation the momentum flux in the lower level is much larger than the saturated momentum flux, whereas it is almost equal to the saturated momentum at the upper levels as expected from the saturation theory. As a result, large flux divergence is produced in the lower layer, the mean flow is decelerated behind the mountain, and the horizontal wind shear forms between unmodified ambient wind. The momentum exchange between the mean flow and the mountain wave is produced by the turbulence within a breaking wave. From the result, well developed vortices like Karman vortex can be formed. . The results of the momentum budget calculated by the hydrostatic model are almost the same as nonhydrostatic results as long as horizontal scale of the mountain is 10 km. A well developed barman vortex similar to the hydrostatic one was simulated in the nonhydrostatic case. Therefore, we conclude that the hydrostatic assumption is adequate to investigate the origin of the Km8n vortex from the viewpoint of wave breaking.

• Environmental Sciences Bulletin of The Korean Environmental Sciences Society
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• v.1 no.2
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• pp.105-117
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• 1997

The formation mechanism of the vortex streets in the lee of the mountain is investigated by a three-dimensional numerical model. The model is based upon the hydrostatic Boussinesq equations in which the vertical turbulent momentum flux is estimated by a turbulence parameterization scheme, but the horizontal viscosity is assumed to be constant. The results show that Karman vortex streets can form even without surface friction in a constant ambient flow with uniform stratification. The vortex formation is related to breaking of the mountain wave, which depends on the Froude number (Fr). In the case of a three-dimensional bell-shaped mountain, the wave breaking occurs when Fr is less than about 0.8, while a Karman vortex forms when Fr is less than about 0.22. Vortex formation also depends on Reynolds number, which is estimated from the horizontal diffusivity. The vortex formation can be explained by the wave saturation theory given by Lindzen (1981) with some modification. Simulations in this study show that in the case of Karman vortex formation the momentum flux in the lower level is much larger than the saturated momentum flux whereas it is almost equal to the saturated momentum at the upper levels as expected from the saturation theory. As a result, large flux divergence is produced in the lower layer, the mean flow is decelerated behind the mountain, and the horizontal wind shear forms between unmodified ambient wind. The momentum exchange between the mean flow and the mountain wave is produced by the turbulence within a breaking wave. From the result, well developed vortices like Karman vortex can be formed. The results of the momentum budget calculated by the hydrostatic model are almost the same as nonhydrostatic results as long as horizontal scale of the mountain is 10 km. A well developed Karman vortex similar to the hydrostatic one was simulated in the nonhydrostatic case. Therefore, we conclude that the hydrostatic assumption is adequate to investigate the origin of the Karman vortex from the viewpoint of wave breaking.

• Journal of the Korean Society of Visualization
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• v.13 no.1
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• pp.21-25
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• 2015

Three-dimensional vortex structures in the corner region of translating normal plates are visualized experimentally with defocusing digital particle image velocimetry. Vortex formation processes for three plates with corner angle $60^{\circ}$, $90^{\circ}$, and $120^{\circ}$ are compared in order to study the effect of corner shape on vortex formation. In all cases, the self-induction of the starting vortex and its interaction with the potential flow induced by the moving plate cause the vortex to change its form dynamically after the plate starts to translate. While the vortex near a corner follows the plate in the low corner angle of $60^{\circ}$, the vortex separates early from the plate and its forward motion becomes slow in the high corner angle of $120^{\circ}$. It is also found that the starting vortex can transport inward at the corner, which depends on the corner angle.

• 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.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.245-248
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• 2002

Vortex lock-on or resonance in the flow behind a circular cylinder is visualized by a time-resolved PIV when a single frequency oscillation is superimposed on the mean incident velocity. Measurements are made of the $K{\'{a}}rm{\'{a}}n$ vortices in the wake-transition regime at the Reynolds number 360. Basically, natural shedding state is observed to compare with lock-on state. Wake motion by the change of the shedding frequency of lock-on state is investigated. When lock-on occurs, the vortex shedding frequency is found to be half the oscillation frequency as expected from previous experiments. The physical flow phenomena of natural shedding and lock-on states are analyzed with physical parameters of recirculation and vortex formation region. Consequently, it is found that the change of wake bubble plays an important role in the flow at the lock-on state. Vortex formation region is also actively changed like recirculation region as the lock-on occurs. Therefore, it is deduced that the recirculation region is closely related with the vortex formation region.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.3
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• pp.357-366
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• 2003

An experimental study is performed to investigate characteristics of near of wakes of circular cylinders with serrated fins using a hot-wire anemometer for various freestream velocities. The main focus of this paper is to investigate a reason why a vortex formation length is increased suddenly. Velocity of the fluid which flow through fins decreases as fin's height and freestream velocity increases and fin pitch decreases, and a thickness of boundary layer increases. The finned tube has a lower velocity gradient when the higher boundary layer grows. This velocity gradient on finned tube makes a weak shear force in the wake and moves to downstream in a state of lower momentum transfer between the freestream and the wake. The phenomenon makes a vortex formation length increased suddenly. The fluctuations of the velocity distributions on the finned tube and (equation omitted) = 1.0 contour line in the vortex formation region decreases when the fin height increases and the pitch decreases.

• Journal of Mechanical Science and Technology
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• v.19 no.2
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• pp.674-681
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• 2005

The unsteady evolution of trailing vortex sheets behind wings in close formation flight near the ground is simulated using a discrete vortex method. The ground effect is included by an image method. The method is validated by comparing computed results with other numerical results. For a lifting line with an elliptic loading, the ground has an effect of moving wingtip vortices laterally outward and suppressing the development of vortex evolution. The gap between wings in close formation flight has an effect of moving up wingtip vortices facing each other. For wings flying in parallel, the ground effect causes the wingtip vortices facing each other to move up, and it makes the opposite wing tip vortices to move laterally outward. When there is a relative height between the wings in ground effect, right-hand side wingtip vortices from a mothership move laterally inward.

• Journal of Institute of Convergence Technology
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• v.10 no.1
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• pp.1-5
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• 2020

The wake shapes behind wings in formation flight are very important to the aerodynamics and performances of aircrafts. In the present study, a discrete vortex methood is extended to handle the wake rollups behind multiple wings. It was found that the relative distance between the wings and the rotational direction of the wingtip vortices have significant effect on the movement of the wingtip vortices. When the wings are close to each other, the wingtip vortices moved faster than the wings of large relative distances. The vortex pair of opposite signs generated from each wingtip has an effect of moving the wingtip vortices upward. The relative height between the wings has an effect of moving the wingtips along the centerline of each vortex. The wakeshape behind multiple wings is a function of the relative distances and thus is dependent on the configuration of the formation flight. In the futhre, a study on the vortex movement pattern will be studied.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.11
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• pp.1645-1654
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• 2003

When vortex shedding is locked-on to a single frequency oscillatory flow, the variations of vortex dynamics are investigated using a time-resolved PIV system. Wake regions of recirculation and vortex formation, dynamic behavior of the shed vortices and the Reynolds stress fields are measured in the wake-transition regime at the Reynolds number 360. In the lock-on state, reduction of the wake region occurs and flow energy distributed downstream moves upstream being concentrated near the cylinder base. To observe the dynamic behavior of the shed vortices, the trajectory of the vortex center extended to the inside of the wake bubble is considered, which describes well the formation and evolution processes. The Reynolds stresses and their contributions to overall force balance on the wake bubble manifest the increase of the drag force by the lock-on.