• Proceedings of the KSME Conference
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• 2003.11a
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• pp.956-961
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• 2003

Direct numerical simulations of flow over a circular cylinder are performed at two different Reynolds numbers (Re=220 and 300) that correspond to three-dimensional instabilities of mode A and mode B, respectively, to investigate the characteristics of drag and lift at these Reynolds numbers. The drag and lift coefficients are measured locally along the spanwise direction and their characteristics are studied in detail. The variation of total drag in time is large at Re=220, and the total drag becomes minimum when vortex dislocation occurs in the wake. The drag and lift variations in space are also closely associated with the evolution of vortex dislocation at this Reynolds number. At Re=300, vortex dislocation is not found in the wake and temporal variations of drag and lift are much smaller than those at Re=220, but their spatial variations are quite large due to the near-wake secondary vortices existing in the mode B instability.

• Transactions of the Korean Society of Automotive Engineers
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• v.4 no.3
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• pp.31-39
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• 1996

The effect of rear-spoiler attached at rear end of automobile trunk on the aerodynamic characteristics of a MIRA reference car model was experimentally investigated. For different shapes and positions(G/H) of the rear-spoiler, the aerodynamic forces on the automobile were measured at various flow speed(ReL). The effect of rear-spoiler on the wake structure behind the automobile was also investigated using flow visualization and hot-wire anemometer. The rear-spoiler modifies the near wake structure and decreases aerodynamic drag and increases driving stability compared with those of the conventional automobile without rear-spoiler. From the experimental results, rear-spoiler of airfoil shape installed at the position of G/H=0.084 shows the best aerodynamic performance.

• International Journal of Naval Architecture and Ocean Engineering
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• v.9 no.2
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• pp.127-134
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• 2017

Vortex Induced Vibration (VIV) is a typical flow-structure interference phenomenon which causes an unsteady flow pattern due to vortex shedding at or near the structure's natural frequency leading to resonant vibrations. VIV may cause premature fatigue failure of marine risers and pipelines. A test model was carried out to investigate the role of a stationary fairing by varying the caudal horn angle to suppress riser VIV taking into account the effect of wake interference. The test results show significant reduction of VIV for risers disposed in tandem and side-by-side. In general, fairing with a caudal horn of $45^{\circ}$ and $60^{\circ}$ are efficient in quelling VIV in risers. The results also reveal fairing can reduce the drag load of risers arranged side-by-side. For the tandem configuration, a fairing can reduce the drag load of an upstream riser, but will enlarge the drag force of the downstream riser.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.6
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• pp.757-770
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• 1998

A Navier-Stokes code with a low Reynolds number k-.epsilon. turbulence model was tested to investigate its predictability for the unsteady transitional boundary layer flow due to rotor-stator interaction. A preliminary calculation with three different numbers of time steps 300, 600, and 1000 for a rotor wake passing period was carried out to see the effects of time steps on the unsteady flow and pressure fields due to rotor-stator interaction. Numerical solutions showed that unsteady pressure was much more sensitive to the number of time steps and over 600 time steps should be used to get a numerical solution independent of the number of time steps for a rotor wake passing period. The original low Reynolds number k-.epsilon. turbulence model showed very poor prediction of the unsteady transitional boundary layer flow due to rotor-stator interaction. This was due to the excessive production of turbulent kinetic energy near the leading edge. A modification suggested by Launder was incorporated and the modified model captured well the wake induced transitional strip. Present solutions also showed improved prediction over previous Euler/boundary layer solution in terms of the onset of unsteady transition and its extent.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.11
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• pp.1540-1548
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• 2000

Near-wake flow field downstream of a circular cylinder in the wake-transition regime where fine-scale secondary vortices have a spanwise wavelength of around one diameter has been studied by means of phase-averaging from cinematic PIV data. A cross-correlation algorithm in conjunction with the FFT(Fast Fourier Transform)analysis and an offset correlation technique is used for obtaining the velocity vectors. Which the help of very high sampling rate compared to the shedding frequency, it is possible to obtain phase-averaged flow fields although the shedding is not forced but natural. Phase -locked three-dimensional vortical structures are reconstructed form the phase-averaged data in one x-y(cross-sectional) and several z-x(spanwise-streamwise)planes. In this process of phase-averaging in a z-x plane, a technique to freeze the secondary vortices relative to the centerline is applied. The formation process of the secondary vortices is shown by considering spatial relations between the primary Karman and the secondary vortices and their temporal evolutions.

• Journal of computational fluids engineering
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• v.11 no.4 s.35
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• pp.1-8
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• 2006

Clocking effects of a stator on the performance and internal flow in an UTRC 1.5 stage axial turbine are investigated using a three-dimensional unsteady flow simulation. Six relative positions of two rows of stator are investigated by positioning the second stator being clocked in a step of 1/6 pitch. The relative efficiency benefit of about 1% is obtained depending on the clocking positions. However, internal flows have some different characteristics from that in the previous study at the best and worst efficiency positions, since the first stator wake is mixed out with the rotor wake before arriving at the leading edge of the second stator. Instead of the first stator wake, it is found that the wake interaction of the first stator and rotor has a important role on a relative efficiency variation at each clocking position. The time-averaged local efficiency along the span at the maximum efficiency is more uniform than that at the minimum efficiency. That is, the spanwise efficiency distribution at the minimum efficiency has larger values in mid-span but smaller values near the hub and casing in comparison to those at the maximum efficiency. Moreover, the difference between maximum and minimum instantaneous efficiencies during one period is found to be smaller at the maximum efficiency than at the minimum efficiency.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.561-567
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• 2000

This paper is concerned with the viscous interaction between rotor and stator. The viscous interaction is caused by wakes from upstream blades. The rotor cascade in the experiment was composed with five blades, and cylinders were placed to make the stator wakes and their locations were about 50 percent upstream of blade chord. The locations of cylinders were varied in the direction of cascade axis with 0, 12.5, 25, 50, and 75 percent of pitch length. The static pressure distributions on the blade surfaces and the velocity distributions in the cascade flow were measured. From the experimental result it was found that the value of velocity defect by a cylinder wake might vary depending on the wake position within the cascade but the value at the cascade exit approached to some constant value regardless of the difference of wake locus. The momentum defect at the downstream from the cascade and the pressure distribution on the blade surfaces showed that the wake flowing near the blade surfaces caused the decrease of lift and the increase of drag regardless of the disappearance of flow separation.

• 유체기계공업학회:학술대회논문집
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• 2005.12a
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• pp.473-480
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• 2005

Clocking effects of a stator on the performance and internal flow in an UTRC 1.5 stage axial turbine are investigated using a three-dimensional unsteady flow simulation. Six relative positions of two rows of stator are investigated by positioning the second stator being clocked in a step of 1/6 pitch. The relative efficiency benefit of about 1% is obtained depending on the clocking positions. However, internal flows have some different characteristics from that in the previous study at the best and worst efficiency positions, since be first stator wake is mixed out with the rotor wake before arriving at the leading edge of the second stator. Instead of the first stator wake, it is found that the wake interaction of the first stator and rotor has a important role on a relative efficiency variation at each clocking position. The time-averaged local efficiency along the span at the maximum efficiency is more uniform than that at the minimum efficiency. That is, the spanwise efficiency distribution at the minimum efficiency has larger values in mid-span but smaller values near the hub and casing in comparison to those at the maximum efficiency. Moreover, the difference between maximum and minimum instantaneous efficiencies during one period is found to be smaller at the maximum efficiency than at the minimum efficiency.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.3
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• pp.346-359
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• 1998

The aerodynamic forces and wake structure of the non-rotating downstream circular cylinder, of which the uniform freestream flow is interfered with another spinning upstream cylinder having the same diameter that is located upstream in a line have been investigated experimentally. When the spin rate of the downstream cylinder defined as the ratio of tangential surface velocity of the spinning cylinder to the freestream velocity increases gradually from zero to 1.4, the change of surface pressure distribution, aerodynamic forces of the non-rotating downstream cylinder were measured in case of several distance ratios of 1.5, 3.0, and 4.5 defined as the ratio of distance between the centers of two cylinders to the diameter. The wake flow patterns behind the cylinder were also investigated in each case. From the present experiments, it has been found that the spin rate significantly influences the aerodynamic forces and near-wake flow phenomena of the downstream cylinder in such a way that the drag increases as the spin rate and distance ratio increase and the wake width increases as the distance ratio increases.

• International Journal of Naval Architecture and Ocean Engineering
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• v.6 no.1
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• pp.151-161
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• 2014

In this study, an experimental investigation has been made of the applicability of outer-layer vertical blades to real ship model. After first devised by Hutchins and Choi (2003), the outer-layer vertical blades demonstrated its effectiveness in reducing total drag of flat plate (Park et al., 2011) with maximum drag reduction of 9.6%. With a view to assessing the effect in the flow around a ship, the arrays of outer-layer vertical blades have been installed onto the side bottom and flat bottom of a 300k KVLCC model. A series of towing tank test has been carried out to investigate resistance (CTM) reduction efficiency and improvement of stern wake distribution with varying geometric parameters of the blades array. The installation of vertical blades led to the CTM reduction of 2.15~2.76% near the service speed. The nominal wake fraction was affected marginally by the blades array and the axial velocity distribution tended to be more uniform by the blades array.