• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.6
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• pp.773-780
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• 2003

The three-dimensional spatial structures of impeller flow created by a six bladed Rushton turbine have identified based on the volumetric velocity information from multi-plane stereoscopic PIV measurements. A total of 10 planes with 2 mm space and a 50 mm by 64 mm size of the field of view were targeted. To reduce the depth of focus, we adopted an angle offset configuration which satisfied the Scheimpflug condition. The distortion compensation procedure was utilized during the in situ calibration. Phase-locked instantaneous data were ensemble averaged and interpolated in order to obtain mean 3-D. volumetric velocity fields on a 60 degree sector of a cylindrical ring volume enclosing the turbine blade. Using the equi-vorticity surface rendering, the spatial structure of the trailing vortices was clearly demonstrated. Detail flow characteristics of the radial jet reported in previous studies of mixer flows were easily identified.

• Proceedings of the KSME Conference
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• 2001.06e
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• pp.690-695
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• 2001

The three-dimensional spatial structures of impeller flow created by a six bladed Rushton turbine have identified based on the volumetric velocity information from multi-plane stereoscopic PIV measurements. A total of 10 planes with 2 mm space with a 50 mm by 64 mm size of the field of view were targeted. To reduce the depth of focus, we adopted an angle offset configuration which satisfied the Scheimpflug condition. The distortion compensation procedure was utilized during the in situ calibration. Phase-locked instantaneous data were ensemble averaged and interpolated in order to obtain mean 3-D, volumetric velocity fields on a 60 degree sector of a cylindrical ring volume enclosing the turbine blade. Using the equi-vorticity surface rendering, the spatial structure of the trailing vortices was clearly demonstrated. Detail flow characteristics of the radial jet reported in previous studies of mixer flows were easily identified.

• International Journal of Naval Architecture and Ocean Engineering
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• v.6 no.3
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• pp.529-561
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• 2014

The two-phase turbulent flow past an interface-piercing circular cylinder is studied using a high-fidelity orthogonal curvilinear grid solver with a Lagrangian dynamic subgrid-scale model for large-eddy simulation and a coupled level set and volume of fluid method for air-water interface tracking. The simulations cover the sub-critical and critical and post critical regimes of the Reynolds and sub and super-critical Froude numbers in order to investigate the effect of both dimensionless parameters on the flow. Significant changes in flow features near the air-water interface were observed as the Reynolds number was increased from the sub-critical to the critical regime. The interface makes the separation point near the interface much delayed for all Reynolds numbers. The separation region at intermediate depths is remarkably reduced for the critical Reynolds number regime. The deep flow resembles the single-phase turbulent flow past a circular cylinder, but includes the effect of the free-surface and the limited span length for sub-critical Reynolds numbers. At different Froude numbers, the air-water interface exhibits significantly changed structures, including breaking bow waves with splashes and bubbles at high Froude numbers. Instantaneous and mean flow features such as interface structures, vortex shedding, Reynolds stresses, and vorticity transport are also analyzed. The results are compared with reference experimental data available in the literature. The deep flow is also compared with the single-phase turbulent flow past a circular cylinder in the similar ranges of Reynolds numbers. Discussion is provided concerning the limitations of the current simulations and available experimental data along with future research.

• Journal of Korean Society for Atmospheric Environment
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• v.9 no.4
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• pp.295-302
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• 1993

The nature of the cavity region and dispersion around trianglular ridge was investigated using model. The artifical neutral boundary layer was simulated in water channel. Two dimensional trianglar ridges, having height of 1.2 cm and various width were placed normal to the flow. Mean velocity with many dimensionless parameters were measured and compared with wind tunnel results by other studies. Using vorticity generator and roughness, the neutral boundary layer was well represented by the water channel. concentration patterns resulting from dye source placed 0.2 cm height above were examined. Narrower the trianglar ridge width resulted in increased amplification factor and the larges amplification factor was observed near downward top of the ridge.

• Journal of Advanced Marine Engineering and Technology
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• v.26 no.4
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• pp.439-449
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• 2002

Experimental investigations of the flow structure and heat transfer enhancement in a channel with a built-in circular cylinder and a wing-let type vortex generator are presented. Without any vortex generators, relatively low heat transfer takes place in the downstream of the circular cylinder where is a recirculation region with low velocity fluid is formed. However with a wing-let type longitudinal vortex generator in the wake region behind the cylinder, heat transfer in the region can be enhanced. In order to control the strength of longitudinal vortices, the angle of attack of the vortex generators is varied from $20^{circ} to 45^{\circ}$, but spacings between the vortex generations are fixed to be 5 mm. The 3-dimensional mean velocity field downstream of the vortex generator is measured by a five-hole pressure probe, and the hue-capturing method using thermochromatic liquid crystals has been used to provide the local distribution of the heat transfer coefficient. The vorticity field and streamwise velocity contour are obtained from the velocity field. Streamwise distributions of averaged Stanton number on the measurement planes show very similar trends for all the experimental cases($\beta=20^{circ}, 30^{circ} and 45^{\circ}$). Circulation strength and heat transfer coefficient have the maximum values when the angle of attack($\beta$) is $30^{\circ}$.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.5
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• pp.611-618
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• 2001

The flow characteristics of a turbulent buoyant jet were experimentally investigated using a single-frame PIV system. The Reynolds number based on the nozzle exit velocity and nozzle diameter was about Re=5$\times$10$^3$. The instantaneous velocity fields in the streamwise plane passing the jet axis were measured in the near field X/D <11 with and without the temperature gradient. By ensemble averaging the instantaneous velocity fields, the spatial distributions of mean velocity, vorticity, and higher-order statistics up to third order were obtained. The temperature difference of 10$\^{C}$ does not affect a significant influence to the flow structure in the near field, but the total entrainment rate is increased slightly. The entrainment rate shows a linear variation with the streamwise distance in the region after X/D=5.0.

• Journal of the korean society of oceanography
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• v.34 no.1
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• pp.1-21
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• 1999

A two-layer quasi-geostrophic numerical model is used to investigate the temporal variability of the East Korea Warm Current (EKWC), especially the separation from the Korean coast and the generation of warm eddies. An attention is given on the active role of the nonlinear boundary layer process. For this, an idealized flat bottom model of the East Sea is forced with the annual mean wind curl and with the inflow-outflow specified at the Korea (Tsushima) and Tsugaru Straits. Two types of separation mechanisms are identified. The first one is influenced by the westward movement of the recirculating leg of the EKWC (externally driven separation),the second one is solely driven by the boundary layer dynamics (internally driven separation). However, these two processes are not independent, and usually coexist. It is hypothesized that 'internally driven separation' arises as the result of relative vorticity production at the wall, its subsequent advection via the EKWC, and its accumulation up to a critical level characterized by the separation of the boundary flow from the coast. It is found that the sharp southeastern corner of the Korean peninsula provides a favorable condition for the accumulation of relative vorticity. The separation of the EKWC usually accompanies the generation of a warm eddy with a diameter of about 120 km. The warm eddy has a typical layer-averaged velocity of 0.3 m/s and its lifespan is up to a year. In general, the characteristics of the simulated warm eddy are compatible with observations. A conclusion is therefore drawn that the variability of the EKWC is at least partially self-excited, not being influenced by any sources of perturbation in the forcing field, and that the likely source of the variability is the barotropic instability although the extent of contribution from the baroclinic instability remains unknown. The effects of the seasonal wind curl and inflow-outflow strength are also investigated.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.10
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• pp.1979-1989
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• 1992

An experimental study has been performed to investigate the horseshoe vortex system formed around cylindrical obstacles mounted vertically on the surface over which a boundary layer is formed. To measure the mean velocity of the flow field, a five-hole Pitot tube has been used. In addition, surface static pressure measurements and surface flow visualization were also performed. From the five-hole probe measurements, vorticity distribution was deduced numerically and the streamwise velocity distribution was also examined. To consider the effect of the leading-edge shape on the formation of the horseshoe vortex, a qualitative comparison was made between the three-dimensional flows around a circular cylinder and a wedge-type cylinder. The five-hole probe measurements showed a single primary vortex which exists immediately upstream of the obstacles, and endwall flow visualization showed the existence of a corner vortex. As the vortex passes around the obstacle, the vortex strength is reduced and the vortex core moves radially outward. Due to this horseshoe vortex, the fluid momentum is found to decrease along the streamwise direction. Since the horseshoe vortex formed around a wedge-type cylinder has weaker strength and is confined to a narrower region than that around a circular, the possibility that the secondary flow loss due to the horseshoe vortex can be reduced through a change of the leading- edge shape is proposed.

• Wind and Structures
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• v.31 no.3
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• pp.229-240
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• 2020

In the present work, the main features of primary vortices and the vorticity concentrations downstream of vortex bursting in crossflow plane of a delta wing with a sweep angle of Λ=70° were investigated under the variation of the sideslip angles, β. For the pre-review of flow structures, dye visualization was conducted. In connection with a qualitative observation, a quantitative flow analysis was performed by employing Particle Image Velocimetry (PIV). The sideslip angles, β were varied with four different angles, such as 0°, 4°, 12°, and 20° while angles of attack, α were altered between 25° and 35°. This study mainly focused on the instantaneous flow features sequentially located at different crossflow planes such as x/C=0.6, 0.8 and 1.0. As a summary, time-averaged and instantaneous non-uniformity of turbulent flow structures are altered considerably resulting in non-homogeneous delta wing surface loading as a function of the sideslip angle. The vortex bursting location on the windward side of the delta wing advances towards the leading-edge point of the delta wing. The trajectory of the primary vortex on the leeward side slides towards sideways along the span of the delta wing. Besides, the uniformity of the lift coefficient, C_L over the delta wing plane was severely affected due to unbalanced distribution of buffet loading over the same plane caused by the variation of the sideslip angle, β. Consequently, dissimilarities of the leading-edge vortices result in deterioration of the mean value of the lift coefficient, C_L.

• Bulletin of the Society of Naval Architects of Korea
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• v.27 no.3
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• pp.38-46
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• 1990

A numerical solution method of the boundary integral equation for axisymmetric potential flows is presented. Those are represented by ring source and ring vorticity distribution. Strengths of ring source and ring vorticity are approximated by linear functions of a parameter $\zeta$ on a segment. The geometry of the body is represented by a cubic B-spline. Limiting integral expressions as the field point tends to the surface having ring source and ring vorticity distribution are derived upto the order of ${\zeta}ln{\zeta}$. In numerical calculations, the principal value integrals over the adjacent segments cancel each other exactly. Thus the singular part proportional to $\(\frac{1}{\zeta}\)$ can be subtracted off in the calculation of the induced velocity by singularities. And the terms proportional to $ln{\zeta}$ and ${\zeta}ln{\zeta}$ can be integrated analytically. Thus those are subtracted off in the numerical calculations and the numerical value obtained from the analytic integrations for $ln{\zeta}$ and ${\zeta}ln{\zeta}$ are added to the induced velocity. The four point Gaussian Quadrature formula was used to evaluate the higher order terms than ${\zeta}ln{\zeta}$ in the integration over the adjacent segments to the field points and the integral over the segments off the field points. The root mean square errors, $E_2$, are examined as a function of the number of nodes to determine convergence rates. The convergence rate of this method approaches 2.