基于飞秒激光电子激发标记测速技术的剪切流场速度测量

流场速度测量精度会影响飞行器气动性能的预测精度，常用的基于激光技术的非接触式速度测量方法已不能完全满足流场速度高精度测量需求，飞秒激光电子激发标记（Femtosecond Laser Electronic Excitation Tagging，FLEET）测速技术有望解决这一问题。利用钛蓝宝石飞秒激光器搭建了FLEET测速系统，分析了流场中的N₂分子在飞秒激光激发下的电子荧光光谱；基于FLEET测速系统，在射流剪切装置上开展了剪切流场速度测量实验，通过调节高速通道的流量/压力获得了不同速度分布的流场，开展了不同流场速度（30～170 m/s）下的FLEET测速实验；研究了延迟时间对流场速度测量的影响。结果表明:随着延迟时间增加，荧光图像会由于等离子体的扩散而发生弥散；FLEET荧光信号衰减会使信噪比有所降低，但不同延迟时间下得到的流场速度分布形态基本一致；FLEET技术在有效荧光寿命范围内具有足够的准确性应用于剪切流场速度测量。

Transient velocity measurement of shear flow using Femtosecond Laser Electronic Excitation Tagging

The measurement accuracy of flow velocity affects the prediction accuracy of aerodynamic performance of the aircraft. However, the current laser-spectroscopy-based velocity non-intrusive measurement technology can’t fully satisfy the requirement of high-precision flow velocity measurement, but the Femtosecond Laser Electronic Excitation Tagging （FLEET） measurement technology can. In this work, a FLEET system is developed based on a Ti:sapphire femtosecond laser. The electronic fluorescence of N₂, which is excited by the femtosecond laser, is analyzed. Based on the FLEET system, transient velocities of the shear flow within 30 m/s to 170 m/s, which is adjusted by the gas pressure or volume flow rate in the high-speed channel, are measured. Finally, the effect of delay time on velocity measurement is investigated. With the delay time increasing, the fluorescence image widens due to the diffusion of plasma, and the signal to noise ratio decreases due to the fluorescence intensity decay. However, the velocities measured at different delay times are basically consistent. The results show that FLEET is practicable to measure the velocity of the shear flow.