• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.687-690
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• 2002

Multimode boundary-layer transition on a NACA0012 airfoil is experimentally investigated under periodically passing wakes and the moderate level of free-stream turbulence. The periodic wakes are generated by rotating circular cylinders clockwise or counterclockwise around the airfoil. The free-stream turbulence is produced by a grid upstream of the rotating cylinder, and its intensity(Tu) at the leading edge of the airfoil is $0.5\;or\;3.5\;{\%}$. The Reynolds number ($Re_c$) based on chord length (C) of the alrfoil is $2.0{\times}10^5$, and Strouhal number ($St_c$) of the passing wake is about 0.7. Time- and phase-averaged streamwise mean velocities and turbulence fluctuations are measured with a single hot-wire probe, and especially, the corresponding wall skin friction is evaluated using a computational Preston tube method. The wake-passing orientation changes pressure distribution on the airfoil in a different manner irrespective of the free-stream turbulence. Regardless of free-stream turbulence level, turbulent patches for the receding wakes propagate more rapidly than those for the approaching wake because adverse pressure gradient becomes larger. The patch under the high free-stream turbulence ($Tu=3.5{\%}$) grows more greatly in laminar-like regions compared with that under the low background turbulence ($Tu=0.5{\%}$) in laminar regions. The former, however, does not greatly change the original turbulence level in the very near-wall region while the latter does it. At further downstream, the former interacts vigorously with high environmental turbulence inside the pre-existing transitional boundary layer and gradually lose his identification, whereas the latter keep growing in the laminar boundary layer. The calmed region is more clearly observed under the lower free-stream turbulence level and for the receding wakes. The calmed region delays the breakdown further downstream and stabilizes more the boundary layer.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2001년도 Proceedings of 2001 Korea-Japan Joint Seminar on Particle Image Velocimetry
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• pp.86-99
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• 2001

Vortex lock-on or resonance in the flow behind a circular cylinder is investigated from a time-resolved PIV when a single frequency oscillation is superimposed on the mean incident velocity. Measurements are made of the $K\acute{a}rm\acute{a}n$ and streamwise vortices in the wake-transition regime at the Reynolds number 360. Streamwise vortices at the lock-on and natural shedding states are observed, as well as the changes in the wake region with the change of the shedding frequency of lock-on state. 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 $K\acute{a}rm\acute{a}n$ vortices are observed to be more disordered by the increased strength and spanwise wavelength of the streamwise vortices, which leads to a strong three-dimensional motion. Recirculation and vortex formation region at the lock-on state is reduced as the oscillating frequency is increased. By comparing the Reynolds stresses at the lock-on and natural shedding states, $\bar{u'u'}\;and \;\bar{u'u'}$ at the lock-on state are concentrated on the shear layer around the cylinder. The $\bar{u'u'}\;at\;f_o/f_n=2.0$ has a large value near the centerline, compared with that of other cases. Considering the traces of maximum of u', in the wake region near the cylinder, wake width at the lock-on state is wider than that at the natural shedding state.

• 한국항공우주학회지
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• 제32권10호
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• pp.1-11
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• 2004

평판의 근접 후류에서 상류 경계층의 영향을 조사하기 위한 실험적 연구가 수행되었다. 트리핑 와이어(tripping wire)의 여러 부착 위치가 경계층의 유동조건을 유발시키기 위하여 선택되었다. 평판 앞전에서부터 0.98C 위치에서의 층류, 천이, 난류경계층이 대칭, 비대칭 후류의 발달과정을 조사하기 위하여 부과되었다. X형태의 열선 프로브(55P61)를 이용한 측정은 근접 후류의 8위치에서 수행되었고, 측정된 평균속도분포는 기존의 후류 상사변수로 무차원화 되어 제시되었다. 대칭 후류는 후류의 중앙부분에서 기존의 상사곡선과 일치하지만, 비대칭 후류는 측정된 범위에서 기존의 상사 곡선과 일치하지 않았다.

• Wind and Structures
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• 제15권1호
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• pp.43-64
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• 2012

Many potential small wind turbine locations are near obstacles such as buildings and shelterbelts, which can have a significant, detrimental effect on the local wind climate. A neural network-based model has been developed which predicts mean wind speed and turbulence intensity at points in an obstacle's region of influence, relative to unsheltered conditions. The neural network was trained using measurements collected in the wakes of 18 scale building models exposed to a simulated rural atmospheric boundary layer in a wind tunnel. The model obstacles covered a range of heights, widths, depths, and roof pitches typical of rural buildings. A field experiment was conducted using three unique full scale obstacles to validate model predictions and wind tunnel measurements. The accuracy of the neural network model varies with the quantity predicted and position in the obstacle wake. In general, predictions of mean velocity deficit in the far wake region are most accurate. The overall estimated mean uncertainties associated with model predictions of normalized mean wind speed and turbulence intensity are 4.9% and 12.8%, respectively.

• 한국가시화정보학회지
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• 제19권3호
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• pp.46-53
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• 2021

In the present study, wind tunnel experiments were performed to reduce the drag of a paragliding helmet in the range of Reynolds numbers from 46,000 to 155,000. The drag force of the helmet model with dimples and deflectors installed was measured by varying the dimple depth and the slant angle of the deflector. The dimples were effective in reducing the drag at low Reynolds numbers, but no significant drag reduction was found in the Reynolds number range in which an actual paraglider flight takes place. On the other hand, the deflector installed tangentially to the side outline of the helmet showed an average drag reduction of 7% in the flight Reynolds number range of real paragliding. This was because the deflector shrunk the size of the wake region and moved the wake region downstream of the deflector.

• 대한기계학회논문집B
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• 제23권3호
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• pp.320-329
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• 1999

An experimental study has been performed to investigate the heat transfer characteristics of impinging jets with corrugated nozzle and wake generation plate. Three different shapes of corrugated nozzle and five different shapes of wake generation plate were tested to improve the heat transfer characteristics of impinging jet. Heat transfer coefficients were obtained by using transient method based on the liquid crystal thermography. The effects of corrugated nozzle and wake generation plate on the heat transfer characteristics of impinging jets were discussed in detail. The results showed that both the corrugated nozzle and the wake generation plate improved the heat transfer characteristics of impinging jet. Especially, heat transfer coefficients around stagnation region of impinging jets were highly increased.

• 한국항공운항학회지
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• 제12권3호
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• pp.25-39
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• 2004

An experimental study was quantitatively carried out in order to investigate the influence of flow conditions on a boundary layer in the near-wake of a flat plate. Tripping wires attached at various positions were selected to change flow conditions of a boundary layer in the vicinity of trailing edge. The flows such as laminar, transitional, and turbulent boundary layer at 0.98C from the leading edge are imposed to investigate the evolution of symmetric and asymmetric wake. Measurements were made at freestream velocity of 6.0m/s, and the corresponding Reynolds number is $2.8{\times}10^5$. An x-type hot-wire probe(55P61) was employed to measure at 8 stations in the near-wake region. Test results show that the near-wake of the flat plate for the case of a laminar and transitional boundary layer is sensitive to mean flow shear generated after separation but for the case of turbulent boundary layer is insensitive.

• 대한기계학회논문집
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• 제18권10호
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• pp.2779-2786
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• 1994

This study presents an experimental investigation of the effect of the wake on mass transfer about a circular cylinder in crossflow. The flat-plate wake was generated by merging two mirror images of turbulent boundary layers that were well developed along the both sides of flat plate with a sharp trailing edge. The velocity field was measured by a hot-wire system and the mass transfer rate by a naphthalene sublimation method. The mixing and developing stages of the wake were addressed to identify flow conditions. The mass transfer effects of different developing stages of the wake was discussed in detail. It is noted that a local maximum appears not at the front stagnation point but at a point a little downstream when the cylinder is located in the nearwake region and much more elevated mass transfer rate is obtained compared to effect of free-stream turbulence.

• 대한조선학회논문집
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• 제36권3호
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• pp.15-21
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• 1999

KRISO 300K VLCC 이중모형선 주위의 유동특성을 풍동실험을 통해 연구하였다. 선체 선미 주위유동과 후류유동의 평균속도 성분, 난류강도, 레이놀즈 전단응력 및 난류 운동에너지 분포를 열선풍속계를 이용하여 측정하였다. 실험은 선미와 후류의 횡단면에서 수행하였으며, 선체 표면에서의 유동 패턴을 정성적으로 조사하기 위하여 유막법을 이용한 유동가시화도 수행하였다. 선미와 근접 후류영역은 매우 복잡한 3차원의 유동특성을 가지고 있으며, 특히 종방향 와류영역에서 고리 모양의 후류 구조를 볼 수 있었다. 그리고 중앙평행부에서의 얇은 경계층은 선미 영역을 지나며 점차 두꺼워지고 복잡한 3차원 난류후류로 발전하였다.

• 대한기계학회논문집B
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• 제23권5호
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• pp.603-609
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• 1999

The effect of an external acoustic excitation on the wake structure behind a circular cylinder was experimentally investigated. The sound wave was excited in the frequency range of the shear layer instability and two sound pressure levels of 114 and 120dB were used in this study. As a result, the acoustic excitation modified the wake structure by increasing the velocity fluctuation energy without changing the vortex shedding frequency. The acoustic excitation enhanced the vortex shedding process and promoted the shear layer instability. Consequently, the acoustic excitation reduced the length of the vortex formation region and decreased the base pressure. In addition, the vortex strength of vortices was increased and the width of the wake was spread out due to the acoustic excitation. When the excitation frequency was identical to the shear layer instability frequency, the effect of the external flow control on the cylinder wake was maximized. In addition, with increasing the sound pressure level, the effect of the external acoustic excitation on the wake structure increased.