On the Challenges Associated With the Application of FLEET Measurements in the Shock Layer in Arc-Jet Flows

This research study presents the first attempt of applying the Femtosecond Laser Electronic Excitation Tagging (FLEET) technique within the shock layer formed by a graphite test article in the ONR-UTA arc-jet flow at the Aerodynamics Research Center, University of Texas at Arlington. While FLEET has previously been successfully employed to measure hypersonic velocities in the freestream of Mach 4 and Mach 6 flows at this facility, its feasibility within the shock layer environment in arc-jet flows had not been investigated prior to this work. Compared to the freestream, the shock layer in arc-jet flows presents significant challenges for velocimetry, including elevated background radiation, shortened signal lifetime, decreased gas velocity, and steep velocity gradients. This study addresses these challenges and reports the first attempt of FLEET in an arc-jet shock layer. Experiments were conducted using a 1.3 inch flat-nosed graphite test article with two filtering strategies: a 725 nm long-pass filter and a 750 nm band-pass filter. Nor qualitative or quantitative FLEET data was obtained during this first campaign, but the lessons learnt has paved the way for a new campaign of measurements.