• 한국가시화정보학회:학술대회논문집
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• 2007.11a
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• pp.134-137
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• 2007

The temporal evolution of wake behind a circular cylinder oscillating rotationally with a relatively high forcing frequency has been investigated experimentally using a dynamic PIV technique. Experiments were carried out with varying the frequency ratio $F_R\;(=f_f/f_n)$ in the range from 0.0 (stationary) to 1.6 at oscillation amplitude of ${\theta}_A=30^{\circ}$ and Reynolds number of $Re=4.14{\times}10^3$. Depending on the forcing condition ($F_R$), the flow was divided into three regimes; non-lock-on ($F_R=0.4$), transition ($F_R=0.8$, 1.6) and lock-on regimes ($F_R=1.0$) with markedly different flow structure in the near-wake region behind the cylinder. When the frequency ratio was less than 1.0 ($F_R{\le}1.0$), the rotational oscillatory motion of the cylinder decreased the length of the vortex formation region and enhanced the mutual interaction between large-scale vortices across the wake centerline. The entrainment of ambient fluid seemed to play an important role in controlling the near-wake flow and shear-layer instability. However, the flow characteristics changed markedly beyond the lock-on flow regime ($F_R=1.0$) due to high-frequency forcing. At $F_R=1.6$, the mutual interactions between the vortices shed from both sides of the cylinder were not so strong. Thereby, the flow entrainment and momentum transfer into the wake center region were reduced. In addition, the size of the large-scale vortices decreased since the lateral extent of the wake was suppressed.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.41 no.10
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• pp.761-769
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• 2013

Effects of helicopter wakes on helicopter aerodynamics are serious, but the wake configuration is very complicated and hard to predict. The purpose of this study is the detailed observation of wake using numerical methods. Vortex lattice method and freewake method are used to track the vortices in the wake. In this paper, the wake configuration is observed during hovering flight. In the case of hovering flight at the moderate thrust level, besides tip vortex, counter-rotating vortex can be observed at the inboard part of blade. When the vortices move downward, tip vortex and counter-rotating vortex get close and influence to each other. Therefore, vortices are highly distorted due to their own instability.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.5
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• pp.713-721
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• 2001

Characteristics of the conical vortices on the roof corner of a rectangular prism have been investigated by using a PIV(Particle Image Velocimetry) technique. The Reynolds number based on the free stream velocity and the height of the model was 5.3$\times$10$^3$. The mean, instantaneous velocity vector fields, vorticity fields, and turbulent kinetic energy distribution were measured for two different angles of attack, 30$^{\circ}$and 45$^{\circ}$. The PIV measurements clearly observed not only the conical main vortex and the secondary vortex but also the tertiary vortex which is firstly reported in this paper. Asymmetric formation of the corner vortex for the case of 30$^{\circ}$angle of attack produces relatively the high magnitude of vorticity and turbulent kinetic energy around the bigger vortex which generates the peak suction pressure on the roof. Fairly symmetric features of the roof vortex are observed in the case of 45$^{\circ}$angle of attack, however, the dynamic characteristics are proved to be asymmetric due to the rectangular shape of the roof.

• Bulletin of the Society of Naval Architects of Korea
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• v.27 no.4
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• pp.27-31
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• 1990

To calculate the lift of a thin lifting surface like the ship-rudder, it is popular to replace the lifting surface by a series of vortices. Two methods, which are vortex lattice method and mode function method, are frequently used to distribute the vortices on the lifting surface. In this paper, the intermediate way of two mentioned calculation method is carried out to exploit the merits of them. The basic concept of this method is to divide the lifting surface with several strips in span-wise and replace vortices to the chord-wise at each strips. A horn type semi-balanced rudder is chosen for the real method, and the validity of the proposed calculation is pursued by the open water test of the same rudder. Finall, this method is applied to the calculation of the interference between the two homogenous rudders siting parallel to the free stream.

• Journal of the Society of Naval Architects of Korea
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• v.51 no.1
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• pp.1-7
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• 2014

The present study numerically investigated the effect of the advance ratio on the wake characteristics of the marine propeller in the propeller open water test. Therefore, a wide range of the advance ratio(0.2${\kappa}-{\omega}$SST Model are considered. The three-dimensional vortical structures of tip vortices are visualized by the swirl strength, resulting in fast decay of the tip vortices with increasing the advance ratio. Furthermore, to better understanding of the wake evolution, the contraction ratio of the slip stream for different advance ratios is extracted from the velocity fields. Consequently, the slip stream contraction ratio decreases with increasing the advance ratio and successively the difference of the slip stream contraction ratio between J=0.2 and J=0.8 is about 0.1R.

• The KSFM Journal of Fluid Machinery
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• v.12 no.2
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• pp.8-16
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• 2009

The objective of this study is to investigate the three-dimensional turbulence flow characteristics of a rotor passage of an one-stage axial flow gas turbine and to investigate the effects of a boundary layer fence installed on the hub endwall of the rotor passage. Secondary flows occurring within the rotor passage (e.g. horseshoe vortex, passage vortex, and cross flow) cause secondary loss and reduce turbine efficiency. To control these secondary flows, a boundary layer fence measuring half the height of the thickness of the inlet boundary layer was installed on the hub endwall of the rotor passage. This study was performed numerically. The results show that the wake and secondary flows generated by the stator reduced the rotor load to constrain the development of cross flow and secondary flow reinforced by the rotor passage. In addition, the secondary vortices occurring within the rotor passage were reduced by the rotation of the rotor. Although, the boundary layer fence induced additional vortices, giving rise to an additional loss of turbine, its presence was shown to reduce the total pressure loss when compared to effects of the case without fence regardless of the relative position of blades by enervating secondary vortices occurred within the rotor passage.

• Journal of the Korean Society of Marine Environment & Safety
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• v.25 no.6
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• pp.795-801
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• 2019

The aim of this study was to investigate the possibility of the skin-friction reduction by vortex control. A vortical system such as a horseshoe vortex, a hairpin vortex, and a wake region was induced around a hemisphere attached on a Perspex flat plate in the circulating water channel. Hairpin vortices were developed from the wake region and horseshoe vortices were formed by an adverse pressure gradient in front of the hemisphere. The horseshoe vortices located on the flank of the hemisphere induced a high momentum flow in the wake region by the direction of their vorticity. This process increased the frequency of the hairpin vortices as well as the frictional drag on the surface of the wake region. To reduce the skin-friction drag, suction control in front of the hemisphere was applied through a hole. Flow visualization was performed to optimize the free-stream velocity, size of the hemisphere, and size of the suction hole. Once the wall suction control mitigated the strength of the horseshoe vortex, the energy supplied to the wake region was reduced, causing the frequency of the hairpin vortex generation to decrease by 36.4 %. In addition, the change in the skin-friction drag, which was measured with a dynamometer connected to a plate in the wake region, also decreased by 2.3 %.

• Journal of the Korean earth science society
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• v.33 no.6
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• pp.544-553
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• 2012

A simple experimental method was developed to help students understand the effect of the Coriolis force on typhoon genesis and movement. It consists of rotating tanks with and without a sloping bottom, and a small stirrer to produce cyclonic typhoon-like vortices by locally stirring the water. Vortices were able to last for more than 3 minutes without dissipation in the rotating tank. However, vortices were hardly maintained without rotation, and would rather disappear as soon as the stirrer stopped mixing. Since the dynamical properties of the rotating water are similar to those of the atmosphere influenced by the Coriolis force, the experiments show that the Coriolis force is indispensable to the typhoon genesis. When the tank had both the sloping bottom and rotation, vortices would move in a particular direction. Considering the topographical beta effect, this result indicates that typhoons are drifted not only by the steering wind but also by the meridional gradient of the Coriolis force. The methodology developed in this study, would be useful for both students and teachers to better the relationship between the Coriolis force and the typhoon genesis.

• Proceedings of the KSME Conference
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• 2005.11a
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• pp.33.1-33.1
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• 2005

• Proceedings of the Optical Society of Korea Conference
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• 2004.07a
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• pp.130-131
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• 2004