Femtosecond Laser Electronic Excitation Tagging Velocimetry in a Mach 6 Ludwieg Tube

Nonintrusive velocity measurements using femtosecond laser electronic excitation tagging (FLEET) were conducted at 1 kHz for both air and nitrogen flows in a Mach 6 Ludwieg tube under ∼600 Pa static pressure conditions. The signal from air and nitrogen were compared in freestream conditions and with a blunt cone model (7 deg half-angle). The long-laser time series operation relative to the facility operation time allowed the characterization of the mean velocity over the entire tunnel operation. The measurement uncertainty for both air and nitrogen FLEET were thoroughly examined. Our finding shows that the measured freestream velocity differed by ∼1–2% from that of the compressible flow calculations. Additionally, the velocity near the leading bow shock was measured at six and three locations in nitrogen and air, respectively, to resolve the large velocity gradient at several locations along the shock layer. The velocity results for the freestream and bow shock were averaged in space and time.