• 산업기술연구
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• 제22권B호
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• pp.27-34
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• 2002

3-D numerical studies are performed to investigate the effect of the air dam height and approaching air velocities on the pressure distribution of notchback road vehicle. For this purpose, the models of test vehicle with four different air dam heights are introduced and PHOENICS, a commercial CFD code, is used to simulate the flow phenomena and to estimate the values of pressure coefficients along the surface of vehicle. The standard $k-{\varepsilon}$ model is adopted for the simulation of turbulence. The numerical results show that the height variation of air dam makes almost no influence on the distribution of the value of pressure coefficient along upper and rear surface but makes strong effects on the bottom surface. That is, the value of pressure coefficient becomes smaller as the height is increased along the bottom surface. Approaching air velocity makes no differences on pressure coefficients. Through the analysis of pressure coefficient on the vehicle surface, one tries to assess aerodynamic drag and lift of vehicle. The pressure distribution on the bottom surface affects more on lift than the pressure distribution on the upper surface of the vehicle does. The increase of air dam height makes positive effects on the lift decrease but no effects on drag reduction.

• Journal of Advanced Research in Ocean Engineering
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• 제3권4호
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• pp.165-173
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• 2017

This study aims at validating simulations of the forced and freely vibrating cylinders at Reynolds number of approximately 500 in order to identify the capability of the CFD code, and to establish the analysis process of the vortex-induced vibration (VIV). The direct numerical and large eddy simulations were employed to resolve the various length scales of the vortices, and the morphing technique was used to consider a motion of the circular cylinder. For the forced vibration case, both in- and anti-phase VIV processes were observed regarding the frequency ratio. Namely, when the frequency ratio approaches to unity, the synchronization/lock-in process occurs, leading to substantial increases in drag and lift coefficients. This is strongly linked with the switch in timing of the vortex formation, and this physical tendency is consistent with that of Blackburn and Henderson (J. Fluid Mech., 1999, 385, 255-286) as well as force coefficients. For the free oscillation case, the mass and damping ratio of 50.8 and 0.0024 were considered based on the study of Blackburn et al. (J. Fluid Struct., 2000, 15, 481-488) to allow the direct comparison of simulation results. The simulation results for a peak amplitude of the cylinder and a shedding mode are reasonably comparable to that of Blackburn et al. (2000). Consequently, based on aforementioned results, it can be concluded that numerical methods were successfully validated and the calculation procedure was well established for VIV analysis with reasonable results.

• 대한기계학회논문집B
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• 제29권5호
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• pp.577-589
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• 2005

The present study has numerically investigated two-dimensional laminar flow over a steadily rotating circular cylinder with a uniform planar shear, where the free-stream velocity varies linearly across the cylinder. It aims to find the combined effect of rotation and shear on the flow. Numerical simulations using the immersed boundary method are performed for the ranges of $-2.5{\le}\alpha{\le}2.5$ and $0{\le}K{\le}0.2$ at a fixed Reynolds number of Re=100, where a and K are respectively the dimensionless rotational speed and velocity gradient. Results show that the positive shear, with the upper side having the higher free-stream velocity than the lower one, favors the effect of the counter-clockwise rotation $(\alpha<0)$ but countervails that of the clockwise rotation $(\alpha>0)$. Accordingly, the absolute critical rotational speed, below which vortex shedding occurs, decreases with increasing K for $(\alpha>0)$, but increases for $\alpha>0$. The vortex shedding frequency increases with increasing \alpha (including the negative) and the variation becomes steeper with increasing K. The mean lift slightly decreases with increasing K regardless of the rotational direction. However, the mean drag and the amplitudes of the lift- and drag-fluctuations strongly depend on the direction. They all decrease with increasing K for $\alpha>0$, but increase for $\alpha<0$. Flow statistics as well as instantaneous flow folds are presented to identify the characteristics of the flow and then to understand the underlying mechanism.

• 대한기계학회논문집B
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• 제38권10호
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• pp.859-864
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• 2014

본 연구는 상류 쪽에 설치된 물체의 직경 변화에 따른 후류에 존재하는 하류 쪽 물체의 영향에 대해 조사한 실험적 연구이다. 풍동의 측정부에서 상류 쪽에는 알파벳 V자 형태의 물체를 삽입하고, 하류 쪽에는 로드셀을 장착한 원기둥을 설치하여 상류 쪽 물체의 직경 변화에 따라 후류의 유속분포가 어떻게 변화하는지를 조사하였다. 그리고 후류에 존재하는 원기둥의 위치를 변화시키면서 원기둥의 변동양력과 카르만 와류 방출 주파수를 조사하므로서 다음과 같은 결론을 얻었다. i) 후류의 유속은 주유속보다 작다. ii) 상류 쪽 물체의 직경이 하류 쪽 물체의 직경보다 클 때 록크-인 현상이 일어난다. iii) 후류에 있는 원기둥의 변동양력이 최대가 되는 위치는 상류 쪽 물체의 직경의 변화에 따라 위치도 함께 변해야 한다.

• Wind and Structures
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• 제35권4호
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• pp.255-268
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• 2022

Light-weight or low-damped structures may encounter the unsteady galloping instability that occurs at low reduced wind speeds, where the classical quasi-steady assumption is invalid. Although this unsteady phenomenon has been widely studied for rectangular cross sections with one side perpendicular to the incidence flow, the effect of the mean wind angle of attack has not been paid enough attention yet. With four sectional models of different side ratios and geometric shapes, the presented research focuses on the effect of the wind angle of attack on unsteady galloping instability. In static tests, comparatively strong vortex shedding force was noticed in the middle of the range of flow incidence where the lift coefficient shows a negative slope. In aeroelastic tests with a low Scruton number, the typical unsteady galloping, which is due to an interaction with vortex-induced vibration and results in unrestricted oscillation initiating at the Kármán vortex resonance wind speed, was observed for the wind angles of attack that characterize relatively strong vortex shedding force. In contrast, for the wind angles of attack with relatively weak shedding force, an "atypical" unsteady galloping was found to occur at a reduced wind speed clearly higher than the Kármán-vortex resonance one. These observations are valid for all four wind tunnel models. One of the wind tunnel models (with a bridge deck cross section) was also tested in a turbulent flow with an intensity about 9%, showing only the atypical unsteady galloping. However, the wind angle of attack with the comparatively strong vortex shedding force remains the most unfavorable one with respect to the instability threshold in low Scruton number conditions.

• 한국가스학회지
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• 제20권2호
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• pp.23-29
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• 2016

배열회수 보일러의 전열관군은 외부에 고온의 배기가스가 흐르면서 유동 유발 진동을 야기 시키며 배열회수 보일러의 전열관군에서 파손을 야기할 수 있어서 열교환기의 구조적 안정성을 위해 열교환기의 전열관군에서 유동 유발 진동 특성을 규명할 필요가 있다. 일반적인 열교환기 전열관군에서 유동 유발 진동에 관한 실험적 연구는 기존에 많이 진행되어 오고 있으며 단일 원관이나 전열관군의 원관들에서 유동 유발 진동에 대한 무차원 PSD(Power Spectral Density) 함수를 무차원 주파수인 Strouhal 수, fD/U의 함수로 도출된 실험적 결과들이 도출되어 있다. 본 연구는 배열회수 보일러에 사용하는 휜튜브 전열관군에서 유동 유발 진동 특성을 규명하는 것을 목적으로 한다. 이러한 것을 위해 단일 휜튜브에서 비정상 상태 유동해석을 수행하여 주기적인 와동 발생 특성과 단일 휜튜브에서의 양력과 항력 변화 특성을 살펴보았다. 또한 단일 휜튜브에서 양력과 항력 변동 특성으로부터 유동 유발 진동에 따른 PSD 특성 결과를 도출하여 기존에 단순 원관에서 이루어졌던 연구들과 비교를 통해 단일 휜튜브 주위의 PSD 특성을 정립하였다.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2009년 추계학술대회논문집
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• pp.27-33
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• 2009

In this paper, the flow past a rotary oscillating circular cylinder is simulated. The high-order and high-resolution numerical schemes with the characteristic boundary conditions are used for the compressible Navier-Stokes equation. The frequencies of rotating oscillation are $0.19\;{\leq}\;S_f\;{\leq}\;0.25$ for the maximum angular $\theta_{max}=10^{\circ}$ and $17^{\circ}$. The flow conditions are Mach number of 0.3 and Reynolds number of 1000. At Lock-on and Non-lock-on region which are defined by the relation between the vortex shedding frequency and the oscillating frequency, the drag and lift coefficient are analyzed.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2009년 춘계학술대회논문집
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• pp.75-80
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• 2009

The present paper focuses on the analysis of aero-acoustics characteristic by several boundary conditions. In this simulation, a high-order and high-resolution numerical schemes are used for the accurate computation of compressible flow with several boundary conditions including characteristic boundary conditions as well as extrapolation and zonal characteristic boundary condition. These boundary conditions are applied to the computation of two dimensional circular cylinder flows with Mach number of 0.3 and Reynolds number of 400. The computation results are validated with measurement datum and other computation results for the Strouhal frequency of vortex shedding, the mean drag coefficient and root-mean-square lift for the unsteady periodic flow regime. Secondary frequency is predicted by three kinds of boundary conditions characteristic.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2006년 제4회 한국유체공학학술대회 논문집
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• pp.37-40
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• 2006

Wind tunnel testings to develope tilt-rotor Smart Unmanned Aerial Vehicle (SUAV) were intensively performed. Small wind tunnel was used to find and evaluate design parameters and to fix general layout of configuration. The application of large tunnel with 40% scaled model is to collect performance and stability related aerodynamic data. During large scale model test wind tunnel is used as a tool to compare Flaperon types, to improve lift characteristics by using different height vortex generators and to alleviate nacelle separated flow effects on the wing.

• 한국전산유체공학회지
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• 제14권3호
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• pp.25-32
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• 2009

The present paper focuses on the analysis of aero-acoustics characteristic by appling different four boundary conditions. The high-order and high-resolution numerical schemes are used for discrete accurate computation of compressible flow. The four boundary conditions include extrapolation, characteristic boundary condition, zonal characteristic boundary condition. These boundary conditions are applied to the computation of two dimensional circular cylinder flows with Mach number of 0.3 and Reynolds number of 400. The computation results are validated against measurement data and other computation results for the Strouhal frequency of vortex shedding, the mean drag coefficient and root-mean-square lift for the unsteady periodic flow regime. The characteristics of secondary frequency is predicted by three kinds of boundary conditions.