An experimental study was made to quantitatively examine the tip vortex roll-up in the near-field of an oscillating wing. A NACA 0012 rectangular planform wing with an aspect ratio of 4.0 was mounted on the bottom of the wind tunnel test section, and the wing was sinusoidally pitched about the quarter chord. Both the mean incidence angle and the amplitude of oscillation were set at 15°, which made the angle of attack vary from 0° to 30°. Visualized flow patterns both in streamwise and in cross-sectional planes with smoke wire and light sheet arrangement are presented. The flow visualization and measurements were made for the case of K=0.09, and $R_N=3.4\times10^4$. Measurements by using the triple hot-film (55R91) probe were carried out at two downstream stations: X/C=0.5 and 1.5. The streamwise flows for the pitch-up motion remained attached and resulted in well-defined circular vortex cores, while those for the pitch-down motion were separated to yield irregular and turbulent cores. The phase-averaged mean velocity, vorticity and turbulence intensity profiles were distinctly different from one another at the same instantaneous angle of attack for pitch-up and pitch-down motions. The flow field of pitch-down motion was strongly influenced by massive separation, and hence the differences among various flow quantities for pitch-up motion and those for potch-down motion were severer. During pitch-up motion, the data indicated that the qualitative nature of the oscillating vortex in the near field was nearly the same as for a stationary wing. It was found that the axial velocity deficit was present within the vortex region in all the cases of the present work. The axial velocity deficit during pitch-up motion increased with the angle of attack. The area of the axial velocity defect during pitch-down motion was much larger and the magnitude of the deficit was considerably less. The axial vorticity contours for pitch-up case were densely concentrated around the vortex core while those for pitch-down case were dispersed widely. Phase-lag compensated variation of the circulation of the tip vortex during the cycle of oscillation is included. The circulation curve clearly demonstrates the hysteretic property between pitch-up and pitch-down phases. The turbulence intensity around the wing tip vortex for the pitch-down case was much higher than that for the pitch-up case. Stabilizing effect of swirl motion could be ascertained by realizing that the high turbulence level of the inboard wake diminished as the angle of attack increased for pitch up case.
진동하는 3차원 날개의 근접후류에서 Tip vortex Roll-up 현상을 조사하기 위한 정량적인 실험연구가 수행되었다. 가로세로비가 4.0인 NACA 0012 직사각형 날개가 풍동바닥에 설치되었으며, $\frac{1}{4}$ 시위를 중심으로 조화적으로 피칭운동을 한다. 평균받음각과 날개의 진폭은 15°로 정하였으며, 이것은 순간받음각 0°~ 30°까지 진동시킨다. Smoke-wire와 light sheet를 적절히 배열하여 획득한 가시화 흐름 형태가 제시되었다. 흐름 가시화와 정량적인 측정은 무차원 진동수 K=0.09와 레이놀즈수 $3.4\times10^4$에서 수행되었다. 3축 hot-film 프로브에 의한 정량적인 측정은 두 단면(X/C=0.5, 1.5)에서 수행되었다. 날개가 pitch-up중일 때 측면흐름은 날개표면에 부착되어 잘 조직화된 원형의 와류중심을 형성하지만 pitch-down중일 때의 흐름은 날개표면에서 분리되어 비규칙적인 난류중심을 형성한다. 위상평균 속도와 와류강도, 난류강도 등은 날개가 pitch-up과 pitch-down 운동하는 동안에 순간받음각이 같다하더라도 서로 다른 흐름특성을 나타낸다. 날개가 pitch-down중일 때 유동장은 아주 강하게 massive separation영향을 받으므로 날개가 pitch-up과 pitch-down 중일 때 흐름특성의 차이가 더 심하게 된다. 날개가 pitch up중일 때 진동하는 와류의 근접후류에서의 정성적인 성질은 정지된 날개와 거의 유사한 것으로 나타났다. 와류영역내에서의 축방향속도는 본 실험의 모든 경우에 속도결손으로 나타났다. 날개가 pitch-up중일 때 축방향 속도결손은 순간받음각이 증가함에 따라 증가하였다. 날개가 pitch-down중일 때 축방향 속도결손영역은 상당히 커지고 그 크기는 감소한다. Tip vortex의 축방향 와류강도 분포는 pitch-up중일 때 집중되지만, pitch-down중일 때 넓게 흩어진다. 진동사이클 동안에 위상지연이 보상된 tip vortex의 순환데이터가 제시되었다. 순환곡선은 pitch-up과 pitch-down운동 사이에 히스테리시스 성질이 있음을 명확히 나타낸다. 날개가 pitch-down중일 때 날개끝 근처의 난류강도는 pitch-up중일 때의 난류강도에 비하여 상당히 높다. 날개가 pitch-up중일 때 와류의 회전운동에 의한 안정성효과는 순간받음각이 증가함에 따라 날개안쪽의 높은 난류강도를 줄여주고 있다.
