扑翼尾流脱落涡特性的PIV实验研究

Experimental Investigation with PIV on Characteristics of Shedding Vortices in Wake of Flapping Wings

通过色流实验和粒子成像测速技术(particle image velocimetry,PIV)对扑翼近场尾流脱落涡的结构轨迹和能量进行了定性及定量研究.结果表明:因展向流动充分性的不同,存在两种牛角型涡系结构;上下扑时翅翼交替产生顺时针和逆时针脱落涡,两涡运动轨迹呈近似弧形对称,对称轴的仰角略大于攻角;脱落涡的涡心涡量在上下扑极点达到最大值,环量最大值出现在到达极点前的1/5~2/5周期之间;产生脱落涡的半周期内,涡的平均环量都随减缩频率的增大而增大,减缩频率较低时,下扑平均环量大于上扑平均环量,减缩频率较高时则相反;振幅对涡能量影响明显,减缩频率为2~2.5时,振幅±40°时的涡平均环量约是振幅±30°时的两倍,减缩频率越大振幅影响越明显.

Qualitative and quantitative studies on the vortices structure, motion path and energy in the near-f ield wake of flapping wings were conducted through dye-flow tests and particle image velocimetry （ PIV） in tank. Results indicate that two kinds of horn-shaped vortices structures exist during f lapping under insuf f icient or suf iicient spanwise flow. Ant iclock-wise and clockwise vortices arise alternately during downward and upward flapping, whose traces are approximately symmet-rical. The elevation angle of symmetry-axis is slight ly larger than the attack angle of flapping wings. The vort icity of vortex core reaches its maximum at the top or bottom point, while the maximum circulat ion occurs at 1/5 ~ 2/5 period before the top or bottom point. The average circulations of both vortices during half period of producing them ascend with the increase of reduced frequency. At small reduced frequency, the average circulat ion during downward f lapping is larger than that dur-ing upward flapping. Contrary result is observed in case of large reduced frequency. Flapping amplitude signif icant ly inf lu-ence the vortex intensities, e.g. when reduced frequency is 2-2.5, the average circulat ion of vortices with an amplitude of ±40° is about twice that of amplitude of ±30°,and the influence grows with the increase of reduced frequency.