• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.7
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• pp.927-936
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• 1998

In the fully developed pipe flow, when jet is injected in cross to the flow there are complex transition flows caused by interaction of the cross flow and jet. These interactions are studied by means of the flow visualization methods and frequency analysis using a hot-wire anemometer. The velocity range of cross flow of the pipe is 0.3 m/s ~ 1.2 m/s and the corresponding Reynolds number, R$\sub$p/, based on the pipe diameter is 2.25 * 10$\^$3/ ~ 9.02 * 10$\^$3/. The velocity ratio (R), jet velocity/cross flow velocity, is chosen from 2 to 10. A circular cylinder is placed in the pipe instead of jet to observe the vortex shedding from the solid body. To compare the jet and circular cylinder flow, the vortical structure is analyzed in both cases and the structure of vortices and the origin of its formation are investigated, especially. The vortex shedding of the dominant coherent structure is compared between the jet flow and the circular cylinder flow. In the case of the jet flow, the Strouhal numbers are different depending on the existence of the upright vortex as well as the velocity ratio (R).

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.2
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• pp.649-658
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• 1996

Small-amplitude harmonic oscillations of multi-cylinders are considered both experimentally and theoretically. For the theoretical model, the flow regime is separated into inner and outer regions. In the inner region, the flow is governed by the generalized Stokes boundary layer equation. In the outer region, the full Navier-Stokes equation for the steady streaming flow is solved numerically by using ADI scheme and FVM coupled with the boundary integral method. Flow visualization experiments are conducted by using the Laser Sheet Image Technique. The case of two circular cylinders and square cylinders with variable distances are chosen as a typical example. Although experimental results are based on the flow in the finite domain, both experimental and numerical results agree well qualitatively. As the separation of cylinders is increased, a numerical result shows the asymptotic convergence to a single cylinder case.

• Journal of Ocean Engineering and Technology
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• v.20 no.2 s.69
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• pp.29-35
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• 2006

The flow patterns around multi-circular cylinders are studies, in order to obtain a global view on the structure of wave control and circulation of sea flow in coastal region. The flow force depends upon the vortex shedding exerted on the structure, especially how the vortex shedding affects the erosion when the structure sets on the sand bottom. Therefore, it is necessary that the flow pattern be hocked. In order to simulate the flow around multi-circular cylinders, the CFX and FLUENT of the computational fluid dynamics (CFD) program were used and compared with the experimental results of the flow visualization installation. The phenomena of flow around the multi-cylinders will be applied to fundamental data for predicting the flow force acting against the structure, erosion and sedimentation around cylinders in arrangement.

• Journal of Advanced Marine Engineering and Technology
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• v.31 no.2
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• pp.159-165
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• 2007

The Characteristics of the flowfield around two circular cylinders in tandem arrangement was investigated by PIV. Strouhal numbers. velocity vectors and velocity profiles were observed at centre-to-centre space ratios of P/D= 1.5. 2.0 and 2.5, and Reynolds number of $Re=3.0{\times}10^3{\sim}5.0{\times}10^3$. As the results the Strouhal numbers measured in the rear region of the cylinder of wake side were decreased with the space ratios. The flow between two cylinders was almost stagnated and the size of the stagnated region was larger in the close side than in the far side of the front cylinder. The direction of vortex between two cylinders was opposed each other with the small difference(${\alpha}\;{\pm}1.0^{\circ}$) of the attack angle ${\alpha}$.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.2
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• pp.131-136
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• 2008

The characteristics of the flowfields around two circular cylinders in various arrangements were investigated by PIV. In tandem arrangement, the Strouhal numbers measured in the rear region of the cylinder of wake side were decreased with the space ratios, the flow between two cylinders was almost stagnated and the closer to upstream cylinder, the larger the width of the stagnated region was. The direction of vortex between two cylinders was opposed each other with the small difference(${\alpha}$=${\pm}1.0^{\circ}$) of the attack angle . In side by side arrangement, the flow velocity between two circular cylinders were increased with the space ratios.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.4
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• pp.1458-1471
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• 1996

The present study is concerned with the heat transfer enhancement associated with a symmetrical or asymmetrical horseshoe vortex in front of and around the circular cylinder centered between the side walls of a wind tunnel. The static pressure measurements and the flow visualization in front of and around cylinders have been performed to determine the existence of horseshoe vortex. The hue-capturing method using the thermochromatic liquid crystals with great spatial resolution was used to obtain the local information of the endwall heat transfer coefficients. In case of one cylinder, the convective heat transfer coefficients of the region where the horseshoe vortex exists are larger than those of any other region. In case of two cylinders with tandem arrangement, the heat transfer rate of gap spacing (d/D= 1.5) is higher than that of gap spacings (d/D=2.0 or 2.5).

• Journal of Mechanical Science and Technology
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• v.19 no.5
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• pp.1169-1181
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• 2005

Experimental and numerical studies on the unsteady wake field behind a square cylinder near a wall were conducted to find out how the vortex shedding mechanism is correlated with gap flow. The computations were performed by solving unsteady 2-D Incompressible Reynolds Averaged Navier-Stokes equations with a newly developed ${\epsilon}-SST$ turbulence model for more accurate prediction of large separated flows. Through spectral analysis and the smoke wire flow visualization, it was discovered that velocity profiles in a gap region have strong influences on the formation of vortex shedding behind a square cylinder near a wall. From these results, Strouhal number distributions could be found, where the transition region of the Strouhal number was at $G/D=0.5{\sim}0.7$ above the critical gap height. The primary and minor shedding frequencies measured in this region were affected by the interaction between the upper and the lower separated shear layer, and minor shedding frequency was due to the separation bubble on the wall. It was also observed that the position (y/G) and the magnitude of maximum average velocity $(u/u_{\infty})$ in the gap region affect the regular vortex shedding as the gap height increases.

• Journal of Advanced Marine Engineering and Technology
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• v.25 no.6
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• pp.1353-1361
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• 2001

Vortex lock-on or resonance behind a circular cylinder is visualized using a time-resolved PW when a single frequency oscillation is superimposed on the mean incident velocity. For vector processing, a cross-correlation algorithm in conjunction with a recursive correlation and interrogation window shifting techniques is used. Measurements are made of the Karmas and streamwise vertices in the wake-transition regime at Reynolds lumber 360. When lock-on occurs, the vortex shedding frequency is found to be half the oscillation frequency as expected from previous experiments. At the lock-on state, the Karman vortices are observed to be more disordered by the increased strength and spanwise wavelength of the streamwiee vortices, which lead? to a strong three-dimensional motion.

• Journal of the Korean Society of Visualization
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• v.22 no.2
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• pp.63-73
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• 2024

The injection of polymer can significantly reduce drag, particularly in the turbulent flow region where the mutual interaction between the polymer and turbulent vortices occurs. In this study, Taylor-Couette flow of PEO-in-water solutions with a rotating inner cylinder was analyzed. Despite the shear-thinning behaviour of PEO-in-water solutions being well-documented, for a given range of shear rates their viscosity remains nearly constant. By varying the polymer concentration, we analyzed the torque evolution of different solutions followed by the viscoelasticity effects of the polymer on the interphase transition points. The torque was analyzed using a dimensionless torque scaling method, which allows for the assessment of the fluid's momentum transport capabilities. It was observed that for low concentrations of PEO, the flow behaviour exhibited only minor differences in comparison to that of water, the Newtonian fluid. However, once the PEO concentration exceeded the polymer overlap concentration, the flow behaviour was significantly altered.

• Wind and Structures
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• v.28 no.6
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• pp.355-367
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• 2019

Control of horseshoe vortex in the circular cylinder-plate juncture using vortex generator (VG) was studied at $Re_D$(where D is the diameter of the cylinder) = $2.05{\times}10^5$. Impact of a number of parameters e.g., the shape of the VG's, number of VG pairs (n), spacing between the VG and the cylinder leading edge (L), lateral gap between the trailing edges of a VG pair (g), streamwise gap between two VG pairs ($S_{VG}$) and the spacing between the two VG's in parallel arrangement ($Z_{VG}$) etc. were investigated on the horseshoe vortex control. The study is conducted using surface oil flow visualization and surface pressure measurements in low speed wind tunnel. It is observed that all the parameters studied have significant control effect, either by reduction in separation region or by lowering the adverse pressure along the symmetric axis upstream of the juncture.