Air Flow Velocity/Density Measurement Using Femtosecond Laser

Abstract

This paper describes Femtosecond Laser Electronic Excitation Tagging (FLEET) for air flow velocity and density measurement. FLEET uses the emitted light which is obtained by focusing a femtosecond laser on nitrogen molecules as a tag for fluid measurement. Because FLEET uses nitrogen molecules in the air, air flow information can be obtained using only a laser system and an imaging system. Therefore, it is suitable for measuring flow fields that include the center of vortices, the wake of objects, and large accelerations and decelerations caused by expansion waves and shock waves. These flow fields are generally not suitable for measurements with particle image velocimetry. In this paper, we first provide an overview of the emission of nitrogen molecules in FLEET. Next, the measurements of underexpansion jet and uniform flow are described as examples of velocity measurement. The uniform flow measurement was performed in large supersonic wind tunnel. Finally, we discuss density measurement using FLEET emission lifetime. As an example of density measurement, we introduce the measurement of an underexpanded jet.