Experimental Study on Flow around the Low-Aspect-Ratio Wing of Start-Up Motion at Low Reynolds Numbers

Micro air vehicles (MAV) is a popular research topic. Because of the high maneuverability, simulation of flapping wings of birds and insects may have good potential in the MAV development. Micro air vehicles operate in a relatively low Reynolds number regime. In this regime, many complex flow phenomena take place within the boundary layers: separation, transition, and reattachment can all occur within a short distance along the chord of a wing. Under the condition of low-aspect-ratio, the effect of wing-tip vortex and it’s interaction with leading-edge vortex become more important. To simplify the problem, low-aspect-ratio flat plates are used to investigate the formation of leading-edge vortex, wing-tip vortices by means of quantitative flow visualization. The experiment is performed in a tank filled with a Glycerine/water mixer. The low-aspect-ratio rectangular plate is impulsively started and translated at high angles of attack at low Reynolds numbers(Re=100、300). The considered aspect-ratios are 1, 2 and 3, respectively. The angles of attack are 15, 30, 45 and 60 degrees. The plate is rigidly mounted to a six-axis force sensor recording lift and drag force with time. The variations of lift and drag coefficients between nondimensional time T=2 and T=7, and the effect of aspect-ratio to lift and drag coefficients are analyzed. For flow visualization, small particles are released in the fluids illuminated by laser light sheet. The trajectories of the particles are captured by CCD camera. The velocity and vorticity fields can be calculated by PIV method, through which, the formation of leading-edge vortex and wing-tip vortices can be observed. Results show that at the beginning of translation the plate has larger lift and drag coefficients and larger lift to drag ratios, which conform to the effect of delayed stall. In the observation, we find that under the conditions of lower Reynolds number and higher aspect-ratio, the leading-edge vortex can be gathered to form a bigger vortex; otherwise, the leading-edge vortex will be influenced by the wing-tip vortices and dissipate to an irregular and slow motion. In the situation of free development of wing-tip, the Von Kármán Vortex was not observed in the present study.