低雷诺数小展弦比机翼在大迎角下自诱导滚转振动控制的实验研究

The lateral instability of low-aspect-ratio wing was suggested as an inherent issue due to the design configuration and low moment of inertia around its longitudinal axis. Especially at high angle of attack, when low-aspect-ratio wing is under gust, the asymmetrical flow structure over the suction surface can initiate the self-induced roll oscillations. In order to overcome such a flight instability problem, flow control methods are introduced, which include natural and forcing bleed methods with deflected bleed and periodical bleed excitation. The natural bleed method introduces the tip slot deflection angle that can orientate the jet towards the root of tip-vortex shear layer, and therefore enhance the high-frequency interactions between shear layers shed from the tip and the slot which attenuate the low-frequency roll oscillations; As a more advanced solution, forcing bleed method considers time-lag effect of natural flow shedding from the tip. By activating periodical bleed excitation at an optimum frequency it is expected that natural shedding at tip can be energized and results the restoration of symmetrical structure over the suction surface, thus suppress roll oscillations effectively. Wind tunnel investigations are conducted with a series of rectangular flat plate wings with AR = 1 – 4 at various Reynolds numbers. The aim of this study is to promote the understanding of attenuation of self-induced roll oscillations of low-aspect-ratio wings using flow control methods, and illustrates the control mechanism of using high-frequency interactions of micro vortices to overcome the low-frequency wondering of large vortical structures.

由于小展弦比机翼所具有的低横向转动惯量甚低，随之而来的横滚非稳定性几乎是其所固有的物理特性。特别是在大迎角的情况下，当小展弦比机翼受到阵风的影响时，机翼将进入吸力面不对称涡结构所诱导的滚转振动。为了解决这一小展弦比翼机在低雷诺数下的非稳定性问题，申请者采取了自然和强迫引气这两种流动控制方法对一组不同展弦比的矩形平板机翼在不同雷诺数下进行了风洞试验研究。在自然引气法中，通过使用引气偏转角激励翼尖自然剪切层和射流剪切层之间的高频交互作用，从而抑制翼尖大涡结构的摇摆，进而消除滚转振动；强迫引气针对翼尖剪切层的迟滞现象引入了激励频率参数，通过射流的周期性进一步有效地激励剪切层之间的高频相互作用，从而还原机翼吸力面对称涡结构，以实现最有效地抑制滚转振动的目的。此项目研究意在通过高频小涡结构的相互作用控制大涡结构的低频摇摆，从而为流动控制抑制矩形平板机翼的自诱导滚转振动提供出科学依据和理论指导。

由于固定翼微型无人机的小展弦比机翼具有质量轻、翼载低等特性，在大迎角飞行时容易受到不稳定气流影响，造成吸力面非对称的流动结构，进而逐步引发大振幅的滚转振动，影响其飞行性能和操控性能。围绕这一气动问题，在北京航空航天大学风洞实验室展开流动控制方法抑制小展弦比机翼自诱导滚转振动的实验研究。实验采用展弦比为2的矩形机翼作为基准机翼，实验雷诺数为1.14E5，分别采用斜槽引气和正弦引气两种流动控制方法研究在大迎角下对于机翼滚转振动的抑制效果。研究表明，在翼尖斜槽引气实验中发现气槽角度与产生的抑制效果相关，其中以45°抑制效果最佳。但是，斜槽引气方法会导致机翼损失最大升力系数，影响气动性能。相关的PIV实验表明气槽产生的气流会形成反向涡，与翼尖分离剪切层产生交互作用，起到抑制滚转振动的效果。此外，在正弦引气的研究中发现在波长相同的情况下，增大正弦振幅可以有效抑制机翼滚转振动，并且延缓机翼滚转振动的产生；在振幅相同的条件下，较大波长的正弦较好的抑制滚转振动。测力实验结果发现合理的选择正弦的振幅和波长有利于提高机翼的最大升力系数，同时延缓机翼失速。PIV实验揭示了正弦机翼能够在机翼前缘位置产生对旋涡对，受到对旋涡下沉作用的影响，减弱了分离剪切层的高度，从而抑制机翼滚转振动。