At moderate to high angle-of-attack, an Unmanned Combat Air Vehicle (UCAV) exhibits complex flow structure due to vortex interaction/breakdown, and the onset of separation and stall. These features of the flow patterns have received little attention. The present investigation addresses the degree of interaction of vortices, the onset of vortex breakdown, and the occurrence of a separation as a function of both Reynolds number and angle-of-attack, via dye visualization and quantitative imaging.; The Reynolds number dependence of the near-surface flow structure and topology on a representative UCAV planform is characterized using a technique of high-image-density particle image velocimetry (DPIV), as a complement to classical dye visualization. This technique provides a sequence of instantaneous states, as well as the corresponding time-averaged state. Patterns of streamline topology, including bifurcation lines, contours of streamwise and transverse velocity, surface-normal vorticity and Reynolds stress correlation, all immediately adjacent to the surface of the planform, provide quantitative interpretations.; At low angle-of-attack, these quantitative patterns show significant alterations with Reynolds number, as represented by: large variations of patterns of vortex breakdown and vortex interaction visualized by dye; and substantial alterations of flow patterns in the crossflow plane, including reattachment phenomena, which are interpreted with patterns of velocity and streamwise vorticity. On the other hand, at moderate angle-of-attack, the near-surface quantitative patterns show much less sensitivity to Reynolds number, which is in accord with weak variations of the onset of vortex breakdown with changes in Reynolds number.; Perturbations of the planform at a small amplitude and high frequency can substantially alter both the instantaneous and time-averaged flow structure immediately adjacent to its surface, relative to the case of a stationary planform. A ramp-like pitch up motion at different rates allows examination of the relaxation process of the flow structure adjacent to the surface after cessation of the motion.
展开▼
