• Journal of Mechanical Science and Technology
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• v.18 no.2
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• pp.313-324
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• 2004

A rich phenomenon in the dynamics of azimuthal vortices in a circular cylinder caused by the inertial oscillation is investigated numerically at high Reynolds numbers and moderate Rossby numbers. In the actual spin-up flow where both the Ekman circulation and the bottom friction effects are included, the first appearance of a seed vortex is generated by the Ekman boundary-layer on the bottom wall and the subsequent roll-up near the corner bounded by the side wall. The existence of the small vortex then rapidly propagates toward the inviscid region and induces a complicated pattern in the distribution of azimuthal vorticity, i.e. inertial oscillation. The inertial oscillation however does not deteriorate the classical Ekman-pumping model in the time scale larger than that of the oscillatory motion. Motions of single vortex and a pair of vortices are further investigated under a slip boundary-condition on the solid walls. For the case of single vortex, repeated change of the vorticity sign is observed together with typical propagation of inertial waves. For the case of a pair of vortices with a two-step profile in the initial azimuthal velocity, the vortices' movement toward the outer region is resisted by the crescent-shape vortices surrounding the pair. After touching the border between the core and outer regions, the pair vortices weaken very fast.

• 한국전산유체공학회:학술대회논문집
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• 2007.10a
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• pp.127-133
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• 2007

This paper represents the numerical study on Taylor flow according to the radius ratio and the angular velocity for flow between tow cylinder. The numerical model is consisted of two cylinder which inner cylinder is rotating and outer cylinder is fix, and the axial direction is used the cyclic condition because of the length for axial direction is assumed infinite. The diameter of inner cylinder is assumed 86.8 mm, the numerical parameters are angular velocity and radius ratio. The numerical method is compared with the experimental results by Wereley, and the results are very good agreement. The critical Taylor number is calculated by theoretical and numerical analysis, and the results is showed the difference about ${\pm}10\;%$. As $Re/Re_c$ is increased, Taylor vortex is changed to wavy vortex, and then the wave number for azimuthal direction is increased. Azimuthal wave according to the radius ratio is showed high amplitude and low frequence in case of small radius ratio, and is showed low amplitude and high frequence in case of large radius ratio.

• Journal of the Korean Society of Visualization
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• v.14 no.2
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• pp.31-39
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• 2016

We investigate an internal flow pattern of an evaporating droplet where the contact line non-uniformly recedes. By using tomographic Particle Image Velocimetry, we observe a three-dimensional azimuthal vortex pair that is maintained until the droplet is completely dried. The non-uniformly receding contact line motion breaks the flow symmetry. Finally, a simplified scaling model presents that the mechanical stress along the contact line is proportional to the vorticity magnitude, which is validated by the experimental results.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.8
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• pp.1081-1088
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• 2003

Numerical investigation on the structures of various Taylor vortices induced in the flow between two concentric cylinders, with the inner one rotating and with a pressure-driven axial flow imposed, is carried out, and compared with the experiments of Wereley and Lueptow [Phys. fluid, 11(12), 1999] who studied the Taylor vortices using PIV in detail. Especially, the properties of helical vortices and random wavy vortices are discussed, and their three-dimensional structures are visualized using the numerical data. Our simulation also predicts that random wavy vortices have quasi-periodic movement which can be explained by traveling waves formed in the azimuthal direction. The numerical results are well consistent with the experimental findings of Wereley and Lueptow.

• 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.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.11
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• pp.1610-1624
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• 1998

An axisymmetric jet is forced with two helical fundamental waves of identical frequency spinning in opposite directions and an additional axisymmetric sub harmonic wave. The subharmonic component rapidly grows downstream from subharmonic resonance with the fundamental, significantly depending on the initial phase difference. The variations of the subharmonic amplitude with the initial phase difference show cusp-like shapes. The amplification of the sub harmonic results in 'vortex pairing of helical modes'. Furthermore, azimuthal variation of the amplification induces an asymmetric jet cross-section. When the initial subharmonics is imposed with an initial phase difference close to a critical value, the jet-cross section evolves into a three-lobed shape. One lobe is generated by the enhanced vortex pairing and the other two lobes are generated by the delayed vortex pairing. Thus, it is confirmed that the initial phase difference between the fundamental and the subharmonic plays an important role in controlling the jet cross-section.

• Wind and Structures
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• v.23 no.2
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• pp.109-125
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• 2016

Flow interference is investigated between two tripped cylinders of identical diameter D at stagger angle ${\alpha}=0^{\circ}{\sim}180^{\circ}$ and gap spacing ratio $P^*$ (= P/D) = 0.1 ~ 5, where ${\alpha}$ is the angle between the freestream velocity and the line connecting the cylinder centers, and P is the gap width between the cylinders. Two tripwires, each of diameter 0.1D, were attached on each cylinder at azimuthal angle ${\beta}={\pm}30^{\circ}$, respectively. Time-mean drag coefficient ($C_D$) and fluctuating drag ($C_{Df}$) and lift ($C_{Lf}$) coefficients on the two tripped cylinders were measured and compared with those on plain cylinders. We also conducted surface pressure measurements to assimilate the fluid dynamics around the cylinders. $C_D$, $C_{Df}$ and $C_{Lf}$ all for the plain cylinders are strong function of ${\alpha}$ and $P^*$ due to strong mutual interference between the cylinders, connected to six interactions (Alam and Meyer 2011), namely boundary layer and cylinder, shear-layer/wake and cylinder, shear layer and shear layer, vortex and cylinder, vortex and shear layer, and vortex and vortex interactions. $C_D$, $C_{Df}$ and $C_{Lf}$ are very large for vortex and cylinder, vortex and shear layer, and vortex and vortex interactions, i.e., the interactions where vortex is involved. On the other hand, the interference as well as the strong interactions involving vortices is suppressed for the tripped cylinders, resulting in insignificant variations in $C_D$, $C_{Df}$ and $C_{Lf}$ with ${\alpha}$ and $P^*$. In most of the (${\alpha}$, $P^*$ ) region, the suppressions in $C_D$, $C_{Df}$ and $C_{Lf}$ are about 58%, 65% and 85%, respectively, with maximum suppressions 60%, 80% and 90%.

• Atmosphere
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• v.24 no.2
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• pp.207-223
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• 2014

The bogussing method was further developed by incorporating the asymmetric component into the symmetric bogus tropical cyclone of the Structure Adjustable Balanced Vortex (SABV). The asymmetric component is separated from the disturbance field associated with the tropical cyclone by establishing local polar coordinates whose center is the location of the tropical cyclone. The relative importance of wave components in azimuthal direction was evaluated, and only two or three wave components with large amplitude are added to the symmetric components. Using the Weather Research and Forecast model (WRF), initialized with the asymmetric bogus vortex, the track and central pressure of tropical cyclones were predicted. Nine tropical cyclones, which passed over Korean peninsula during 2010~2012 were selected to assess the effect of asymmetric components. Compared to the symmetric bogus tropical cyclone, the track forecast error was reduced by about 18.9% and 17.4% for 48 hours and 72 hours forecast, while the central pressure error was not improved significantly. The results suggest that the inclusion of asymmetric component is necessary to improve the track forecast of tropical cyclones.

• Proceedings of the Korean Magnestics Society Conference
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• 2013.12a
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• pp.108-109
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• 2013

• 유체기계공업학회:학술대회논문집
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• 2006.08a
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• pp.401-402
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• 2006

The turbulent flow around a sphere was investigated using two experimental techniques: smoke-wire flow visualization in wind tunnel at Re=5300, 11000 and PIV measurements in a circulating water channel. The smoke-wire visualization shows flow separation points near an azimuthal angle of $90^{\circ}$, recirculating flow, transition from laminar to turbulent shear layer, evolving vortex roll-up and fully turbulent eddies in the sphere wake. The mean velocity field measured using a PIV technique in x-y center plane demonstrates the detailed near-wake structure such as nearly symmetric recirculation region, two toroidal vortices, laminar separation, transition and turbulent eddies. The PIV measurements of turbulent wake in y-z planes show that a recirculating vortex pair dominates the near-wake region.