仿生学覆羽厚度对机翼失速控制效果的实验研究

Experimental study on the thickness dependence of bionics coverts for the wing stall control

受鸟类翼面覆羽结构特点的启发,在前期仿生流动控制工作基础上,本文设计了不同厚度的柔性锯齿型人工覆羽,将其分别安装于NACA0018平直机翼上翼面不同弦长位置,通过实验考察大迎角条件下流动分离控制效果。实验在天津大学低湍流度风洞进行,采用坐标架对机翼尾流区进行扫掠测量,使用热线风速仪获取尾流区的平均速度和脉动速度信息,并使用高速相机拍摄人工覆羽的运动情况。通过平均速度分布、脉动速度均方根曲线、功率谱密度、小波能谱和小波等值云图等对不同厚度覆羽的流动分离控制效果进行对比分析。实验结果表明,对于小厚度覆羽:安装在机翼前缘附近时,能有效减小前缘剪切层和机翼上表面之间的距离,这是由于覆羽自适应振动促进了低频大尺度相干结构向高频小尺度相干结构的转化;安装在机翼尾缘附近时,机翼周围流场无明显变化。相反,对于大厚度覆羽,覆羽阻碍分离回流区沿机翼表面向前缘发展,在靠近机翼尾缘时,流动分离控制效果较好。此外,本文还结合粒子图像测速技术绘制了覆羽的运动情况和周围流场流动示意图,验证了不同工况下人工覆羽的流动分离控制效果,对比分析了不同厚度覆羽实现流动分离控制的机理。

Bio-inspired by the covert feathers on bird wings,the artificial coverts of different thicknesses were designed by using flexible materials,and they were installed at different locations on the upper side of a NACA0018 wing model at a high angle of attack.In the wind tunnel experiments,the hot-wire anemometer was used to measure the velocity distributions in the wake flow,and therefore the time-averaged and turbulent fluctuation velocities were obtained.The flow separation control effectiveness of the different thicknesses were analyzed by the time-averaged velocity profiles,the root-mean-square velocity distributions of the turbulent fluctuations as well as their Power Spectral Density(PSD).The results show that,the thin coverts near the leading-edge effectively reduce the distance between the leading-edge shear layer and the upper surface,whereas installed near the trailing-edge,the flow field around the airfoil has mere change.On the other hand,for the thick coverts,the flow separation control effectiveness is better than those near the leading-edge.Based on the multi-scale wavelet analysis,the artificial coverts improve the transformation of low-frequency large-scale coherent structures to high-frequency small-scale ones by adaptively fluttering and flapping motions,which is highly effective for flow separation control.