Development of Micro-Flap for Active Control of Separated Flows

本项目的目的是开发用于分离流主动控制的微型襟翼。主动控制的目标是防止流从诸如翼型或翼型叶栅的固体表面分离，或者如果流分离是不可避免的，则尽可能地减小分离流的区域。主要结果如下。(1)如果强迫幅值固定，则沿分离边缘沿着布置的微襟翼阵列可以有效地减小特定强迫频率下的前缘分离区的空间范围。如果微襟翼引起的速度脉动小于来流速度的20%，则对于更高的强迫振幅，分离区的减小更大。一个流动模型的构建来解释这个最有效的频率和相关的再附着长度。(2)对前缘分离区进行了交互式控制，通过改变速度脉动与微襟翼输入之间的相位滞后，将分离区代表位置的速度脉动反馈给微襟翼输入。首次发现相位滞后的改变使再附着长度减小了50%以上。(3)一系列涡环从外部引入失速翼型的分离剪切层。这种主动控制可以显著降低翼型吸力侧分离区域的高度。对于产生涡环的较高频率的微襟翼，减少更大，在足够高的涡环产生频率下饱和。

The purpose of this project is to develop micro-flaps for active control of separated flows. The target of the active control is to prevent separation of flow from the surface of solid bodies such as airfoils or airfoils cascades or to reduce as much as possible the region of separated flows if the flow separation is unavoidable. Main results are as follows.(1) An array of micro-flaps arranged along the separation edge can efficiently reduce the spatial extent of a leading-edge separation region at a particular forcing frequency if the forcing amplitude is fixed. The reduction of the separation zone is greater for higher forcing amplitude if the velocity fluctuation induced by the micro-flap is less than 20% of the oncoming-flow velocity. A flow model is constructed to interpret this most effective frequency and the associated reattachment length.(2) An interactive control of the leading-edge separation region is made, in which velocity fluctuation at a representative position in the separation region is fedbeck to the input of the micro-flaps by changing the phase lag between the velocity fluctuation and the input of the micro-flaps. The reattachment length is found, for the first time, to reduce by more than 50% by the change of the phase lag.(3) A chain of vortex rings is introduced from outside into the separated shear layer of a stalled airfoil. This active control can significantly reduce the height of the separated region on the suction side of the airfoil. The reduction is greater for higher frequency of micro-flaps to generate the vortex rings, saturating at sufficiently high frequencies of the vortex-ring generation.