• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.66-71
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• 2008

Two cases with circumferential grooves were designed for a transonic compressor, and 3-D numerical simulations were conducted for stall mechanism at three representative speeds. A conclusion can be drawn from the comparison between compressors with or without casing treatment that: with the rising of rotation speed, stall margin increases dramatically under the help of casing treatments, and the case with middle grooves has reasonable compromise between stall margin increment and efficiency cutting. At lower speed, the increment reduces, and grooves at the back of blade tip have more influence on stall margin. Further investigation shows there is a transition in mechanism of compressor stall with the decline of rotational speed: at high rotation speed, the expansion of stall margin mainly results from the suppression of tip leakage vortex by casing treatments, yet it benefits more from the depression of boundary layer separation from suction surface of blade tip.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.2567-2572
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• 2007

The suppression of fluid force acting on a square prism near plane wall was studied by attaching fences on the corners of the prism. The height of the fence was 10% of the square width and the range of Reynolds number considered was Re=$2.0{\times}10^4$. The experimental parameters were the attaching position and numbers of fences, the space ratios G/B(G/B=0.1${\sim}$1.2) between prism and plane wall. The average drag coefficients were increased and the average lift coefficients were decreased and increased with the space ratios toward plane wall. The drag of the prism was reduced average 7.6% with the space ratios by attaching the normal fence at the rear and upper corner and the horizontal normal fence at the rear and lower corner on the prism. In this case, the separated flow at the front and upper corner was reattached on the upper side of the prism and the vortex streets between the prism and plane wall were appeared more slowly than that of prism without fences.

• 한국공간구조학회논문집
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• 제13권1호
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• pp.97-104
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• 2013

Recent tall buildings tend to have unconventional shapes as a prevailing, which is effective for suppressing across-wind responses. Suppression of across-wind responses is a major factor in tall building projects, and the so called aerodynamic modification method is comprehensively used. The purpose of the present study is to investigate the pressure fluctuations on tapered and setback tall buildings, including peak pressures, power spectra and coherences through the synchronous multi-pressure sensing system techniques. And flow measurements around the models were conducted to investigate the condition of vortex shedding. The results show that by tapering and setback, different distributions of mean pressure coefficients at leeward surface were found, which is caused by the geometric characteristics of the models. And the power spectra of wind pressures at sideward surface become wideband and the peak frequencies are different depending on heights, which makes the correlation near the Strouhal component low or even negative. The differences in shedding frequencies were also confirmed by the flow fields around the models.

• 한국해양공학회지
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• 제33권4호
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• pp.350-357
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• 2019

The development of marine technology is expected to increase the demand for marine plants because of increasing oil prices. Therefore, there is also expected to be an increase in the demand for cylindrical structures such as URF (umbilical, riser, flowline) structures and spars, which are used operating in various seas. However, a cylindrical structure experiences vortex induced motion (VIM) in a current. In particular, for risers and umbilicals, it is important to identify the characteristics of the VIM because interference between structures can occur. In addition, various studies have been conducted to reduce VIM because it is the cause of fatigue damage to structures. The helical strake, which was developed for VIM reduction, has an excellent VIM reduction performance, but is difficult to install on structures and has a negative effect on heave motion. Therefore, the purpose of this study was to supplement the shortcomings of the helical strake and develop a high-performance reduction device. In the reduction device developed in this study, a string is placed around the structure inside the flow, causing vibration. The vibration of this string causes a small turbulence in the flow field, reducing the VIM effect on the structure. Finally, in this study, the 2-DOF motion characteristics of models without a suppression device, models with a helical strake, and models with a string were investigated, and their reduction performances were compared through model tests.

• Wind and Structures
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• 제23권2호
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• pp.127-142
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• 2016

Aerodynamic effects, such as drag force and flow-induced vibration (FIV), on civil engineering structures can be minimized by optimally modifying the structure shape. This work investigates the turbulent wake of a square prism with its faces modified into a sinusoidal wave along the spanwise direction using three-dimensional large eddy simulation (LES) and particle image velocimetry (PIV) techniques at Reynolds number $Re_{Dm}$ = 16,500-22,000, based on the nominal width ($D_m$) of the prism and free-stream velocity ($U_{\infty}$). Two arrangements are considered: (i) the top and bottom faces of the prism are shaped into the sinusoidal waves (termed as WSP-A), and (ii) the front and rear faces are modified into the sinusoidal waves (WSP-B). The sinusoidal waves have a wavelength of $6D_m$ and an amplitude of $0.15D_m$. It has been found that the wavy faces lead to more three-dimensional free shear layers in the near wake than the flat faces (smooth square prism). As a result, the roll-up of shear layers is postponed. Furthermore, the near-wake vortical structures exhibit dominant periodic variations along the spanwise direction; the minimum (i.e., saddle) and maximum (i.e., node) cross-sections of the modified prisms have narrow and wide wakes, respectively. The wake recirculation bubble of the modified prism is wider and longer, compared with its smooth counterpart, thus resulting in a significant drag reduction and fluctuating lift suppression (up to 8.7% and 78.2%, respectively, for the case of WSP-A). Multiple dominant frequencies of vortex shedding, which are distinct from that of the smooth prism, are detected in the near wake of the wavy prisms. The present study may shed light on the understanding of the underlying physical mechanisms of FIV control, in terms of passive modification of the bluff-body shape.