中低雷诺数翼型失速流动特征及气动噪声机理研究

Airfoil stall of wings and propellers at low to moderate Reynolds numbers may occur on unmanned aerial vehicles (UAVs) because of special configurations and operation conditions of UAVs, which grow fast in civil aeronautical engineering. Compared with an airfoil which is operated a low angle of attack, the operation at a high angle of attack would lead to significant drop of lift and noise increase. For the application of lifting surfaces being operated at high angle of attack, the lift theory of attached flow is not enough and the complex flow at high angle of attack must be investigated. This project focuses on airfoil stall at high angle of attack by investigating shear flow at leading and trailing edges, separation vortex at the trailing edge, and secondary vortex at the centre chord. Spatial-temporal-modal analysis is applied for the development of airfoil stall mechanism, which was conducted by probing individual points in the flow by previous researchers. 'Seeing noise' is implemented upon the traditional 'hearing noise' method, which is significant for the noise source investigation. Aeroacoustic mechanism of airfoil stall is investigated by correlation analysis in the time and frequency domains. By tuning the parameters of angle of attack, Reynolds number and turbulence level, the flow spectra, the mode characteristics, and the narrow band of noise are analysed, which are useful for UAV engineering.

小型无人飞行器是当前民用航空增长较快的领域之一，该类飞行器由于其特殊布局及操作形式，机翼及螺旋桨可能出现中低雷诺数翼型失速的流动现象。相较于小攻角附着流动，大攻角翼型失速将导致升力急剧减小以及噪声增大。只有突破传统附着流线升力的概念，理解大攻角失速复杂流动机理，才能扩展升力体可用攻角，更好满足工程需要。本项目以翼型大攻角失速为研究对象，通过对翼型大攻角条件下前缘剪切层、后缘脱落涡、中间二次涡的速度场测量，采用时间-空间-模态相互关联研究补充与发展过去对于局部点流动测量的结论。将传统基于麦克风的“听噪声”方法与当前流行的基于速度场测量的“看噪声”方法结合起来，在频域与时间域对流场与声场进行关联分析，从中分析噪声产生的流动机理。在攻角、雷诺数、来流湍流度这三个常见无人机工况变化的条件下，测量流动频率特征、流动模态特征、以及噪声窄频特征的变化趋势，为工程实践提供理论指导。

对于小型无人机、低速散热设备、小型风力机等设备，其翼型截面的工况比较复杂，既包含了低攻角的附着流，也有包含高攻角的失速流动。为了系统性的探索翼型在中低雷诺数时不同攻角条件下，翼型噪声特征以及流动机理，本研究在雷诺数220,000—450,000范围开展了翼型从零度攻角到深度失速攻角的噪声机理研究。在南方科技大学气动声学风洞，采用粒子图像测速系统与麦克风同步测量，采用速度场正交分解提取流场的主要特征，通过频谱、统计、关联分析方法研究了翼型的噪声辐射机理。研究发现了在翼型三个攻角区间具有不同的噪声特征及机理。接近零度攻角时翼型噪声表现为纯音噪声，噪声由压力面、吸力面声反馈回路共同引起，两侧涡脱落的相位变化引起了幅值调制进而产生了主离散纯音。随着攻角增大，翼型表现出了明显的流动不对称性，此时纯音噪声是由于压力侧流动声反馈回路引起的，分离泡下游的涡脱落间歇性破裂，导致了主离散纯音噪声，而且，本研究第一次观察并解释了次级离散纯音噪声，这类次级离散纯音噪声由分离泡的周期性位移导致。随着攻角进一步增大，翼型噪声表现为宽频噪声，由流动分离引起的湍流引起。在流动的失速区，除了升力系数的迟滞效应，还观察到了总声压级的迟滞效应，攻角下行段的噪声辐射要明显小于上行段的声辐射，在200赫兹左右最为明显，流场显示上行段为附着流，下行段为分离流，通过流场分析显示其声辐射减小的主要原因有可能是分离流的三维效应导致了流动的展向相关性降低，从而导致了声辐射降低。