• International Journal of Aeronautical and Space Sciences
• /
• 제18권1호
• /
• pp.8-16
• /
• 2017

Most of small air vehicles with moving wing fly at low Reynolds number condition and the reduced frequency of the moving wing ranges from 0.0 to 1.0. The physical phenomena over the wing dramatically vary with the reduced frequency. This study examines experimentally the effect of the reduced frequency at low Reynolds number. The NACA0012 airfoil performs sinusoidal pitching motion with respect to the quarter chord with the four reduced frequencies of 0.1, 0.2, 0.4 and 0.76 at the Reynolds number $2.3{\times}10^4$. Smoke-wire flow visualization, unsteady surface pressure measurement, and unsteady force calculation are conducted. At the reduced frequency of 0.1 and 0.2, various boundary layer events such as reverse flow, discrete vortices, separation and reattachment change the amplitude and the rotation direction of the unsteady force hysteresis. However, the boundary layer events abruptly disappear at the reduced frequency of 0.4 and 0.76. Especially at the reduced frequency of 0.76, the local variation of the unsteady force with respect to the angle of attack completely vanishes. These results lead us to the conclusion that the unsteady aerodynamic characteristics of the reduced frequency of 0.2 and 0.4 are clearly distinguishable and the unsteady aerodynamic characteristics below the reduced frequency of 0.2 are governed by the boundary layer events.

• 대한기계학회논문집
• /
• 제11권2호
• /
• pp.214-221
• /
• 1987

본 논문에서는 2차원충돌분류에서 사각리브(rib)를 갖는 요철형전열면에 대한 실험에 관한 것이며, 리브의 높이를 (ｅ=5nm) 일정하게 하고 리브의 피치(ｐ=25,35,50 nm) 변화, 노즐출구와 전열면간 거이(H/B=1~14) 및 노즐출구유속(ｕ$_{e}$=12.30~ 18.07m/s)의 변화에 따른 전열특성을 실험적으로 구명하고 평판전열면의 경우와 비교 검토한다. 또한 스모크와이어(smoke-wire)법으로 평판 및 리브판에서의 유체의 거동 을 가시화하여 전열기구의 정성적 특성을 검토하는데 목적이 있다.

• Journal of Advanced Marine Engineering and Technology
• /
• 제29권8호
• /
• pp.915-922
• /
• 2005

This experimental study is conducted to investigate effects of a splitter plate, which is set on the back side of a square prism in the uniform flow. The Reynolds number is $1.44{\times}10^{4}$ based on the width of the square prism. The measurement of velocity vector and pressure distribution are carried out 4 cases of length in the range of 0.5L to 2.0L with 0.5L interval and 3 cases of Position at 0L, 0.25L, 0.5L, Flow visualization is also executed by smoke-wire method to understand the mechanism of vortex formation The results show the strong vortex shedding patterns and drags are decreased effectively, when the position of splitter plate is 0L. And the drag reduction rate is in inverse proportion to the splitter plate length

• Journal of Advanced Marine Engineering and Technology
• /
• 제27권6호
• /
• pp.736-744
• /
• 2003

This study is conducted to investigate aerodynamic forces and wake structures about the pressure interference of a circular cylinder with slits. An experimental investigation of a circular cylinder with slits is carried out in uniform flow in the range of Reynolds number from 8,000 to 32,000 using X-type hot wire. Flow visualization is executed by smoke-wire method to understand the mechanism of these vortex formation process. Inspection in the wake at X/D=5.5 of the cylinder with the slits suggested that a strong vortex-shedding pattern for these cylinders is revealed compare with a circular cylinder without slits. It is found that the rolling up position of shear layer of the cylinder with slits is shorten compare with a circular cylinder without slits.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2000년도 춘계학술대회논문집B
• /
• pp.508-513
• /
• 2000

An experimental investigation has been carried out for two circular cylinders having different groove configurations(U and V-shape). The results were compared with those for the smooth circular cylinder. The drag force, mean velocity and turbulent intensity profiles of wake behind the cylinders were measured with varying the Reynolds number $Re=8000{\sim}14,000$ based on the cylinder diameter. As a results, the U-groove circular cylinder was found to be most effective riblet shape with reducing the drag up to 21%. As the Reynolds number increases, the vortex shedding frequency of the grooved cylinders becomes a little larger, compared to the smooth cylinder. The flow visualization using the smoke-wire technique was also carried out to see the flow structure qualitatively.

• 한국가시화정보학회지
• /
• 제13권1호
• /
• pp.9-14
• /
• 2015

There have been many attempts to reduce the cost of transportation. Especially, drag reduction of heavy vehicles has enormous influence on energy saving by reducing the driving power of the vehicles. In this study, the effects of drag-reducing additive devices such as side skirt, boat tail and cab-roof fairing on the drag reduction of a 5 ton truck model were experimentally investigated. The aerodynamic performance of these flow-control devices attached to heavy vehicle was evaluated through wind tunnel test. In addition, flow patterns around the truck model were visualized by using smoke tube method. The drag coefficient is reduced by up to 5.7%, 7.16% and 22.2% by the side skirt, boat tail and cab-roof fairing, respectively. The interactive effect of the side skirt and boat tail was also investigated.

• 한국추진공학회지
• /
• 제15권3호
• /
• pp.31-39
• /
• 2011

핀/슬롯 그레인 및 내삽노즐을 가진 고체로켓모터 내부와 동일한 기하학적 형상을 가진 표면분사 시험모델을 사용하여 연소유동장을 모사하고, 스모크 와이어를 이용하여 유동장을 가시화하였다. 그레인 전방부 투영창을 통해 촬영하는 기법 등에 이용하여 획득된 내삽노즐 선단 인접부의 반경방향 평면상에서의 유동가시화 이미지 분석을 통해, 슬롯출구 반경방향유동, 핀베이스 축방향유동 및 상류그레인포트 축방향유동의 상호 전단작용에 의한 반경방향 운동량 전달이 노즐 인접부에서의 선회류 유동 및 와류튜브 구조를 발생시키는 것으로 나타났다.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2003년도 춘계학술대회
• /
• pp.2089-2094
• /
• 2003

The flow around a circular cylinder was controlled by attaching O-rings to reduce drag force acting on the cylinder. Four experimental models were tested in this study; one smooth cylinder of diameter D (D=60mm) and three cylinders fitted with O-rings of diameters d=0.0167D, 0.05D and 0.067D with pitches of PPD=1D, 0.5D and 0.25D. The drag force, mean velocity and turbulent intensity profiles in the near wake behind the cylinders were measured for Reynolds numbers based on the cylinder diameter in the range of $Re_D=7.8{\times}10^3{\sim}1.2{\times}10^5$. At $Re_D=1.2{\times}10^5$, the cylinder fitted with O-rings of d=0.0167D in a pitch interval of 0.25D shows the maximum drag reduction of about 5.4%, compared with the smooth cylinder. The drag reduction effect of O-rings of d=0.067D is not so high. For O-ring circulars, as the Reynolds number increases, the peak location of turbulence intensity shifts downstream and the peak magnitude is decreased. Flow field around the cylinders was visualized using a smoke-wire technique to see the flow structure qualitatively. The size of vortices and vortex formation region formed behind the O-ring cylinders are smaller, compared with the smooth cylinder.

• 대한조선학회논문집
• /
• 제59권4호
• /
• pp.225-234
• /
• 2022

The flow field measurement above whole area of the flight deck on 'Landing Platform Helicopter (LPH)' was performed by using PIV system in wind tunnel. In various heading angle conditions (0deg, -30deg, -45deg, -60deg, -75deg and ±90deg), the velocity fields such as U velocity & V velocity were measured at three different height above flight deck. Due to the geometrical characteristics of several bodies like deck, crane and super-structure, various vortex were generated. When the heading angle is 0deg, the deck edge vortex by flight deck and massive separation by super-structure were clearly observed by visualization with smoke and PIV, respectively. In other heading angles, the acceleration of flow in space between crane and super-structure were detected. And area with flow separation by super-structure is directly related to the heading angle of vessel.

• 한국화재소방학회논문지
• /
• 제15권1호
• /
• pp.1-6
• /
• 2001

본 연구는 터널내 화재발생시 자연 배기에 의한 연기의 거동을 해석하기 위하여 축소모형 실험을 수행하였다. 터널내 연기의 유동은 부력에 의해 지배되므로 Froude scaling에 의해 실물 터널을 1/20로 축소한 모형터널에서 실험을 수행하였으며 배기구의 위치변화에 따른 영향을 평가하기 위하여 화원으로부터 각각 1m, 2m, 3m 떨어진 곳에 배기구를 대칭으로 위치시켜 세가지 경우에 대해 실험을 실시하였다. 지름이 4.36 cm인 화원의 경우, 배기구의 위치에 관계없이 배기구 통과 후 연층의 온도가 배기를 실시하지 않은 경우와 비교하여 약 7~$8^{\circ}$ 낮게 유지되었다. 지름이 5.23cm인 화원의 경우, 배기구가 화원에 가까울수록 배기구 통과 후 연기의 평균속도가 감소하였으며 최대 3.86초가 지연되었다. CASE 1에서는 배기구 통과 후 천장에서 약 $8^{\circ}c$, 수직온도가 약 $7^{\circ}c$ 감소하였으며, CASE 2에서는 천장과 수직온도가 약 $3^{\circ}c$ 감소하였으며, CASE 3에서는 각각 약 $2^{\circ}c$가 감소하였다. 레이저와 디지털 캠코더를 이용하여 배기구 주위의 연기 유동을 가시화하여 화재발생 약 1분 후부터 연층의 두께가 터널높이의 25%이하로 일정하게 유지되는 것을 확인할 수 있었다.