Instantaneous velocity fields in a hypersonic wake*

The cold wake behind a slender object in hypersonic flow offers a challenging environment for off-body measurements. Femtosecond Laser Electronic Excitation Tagging (FLEET) is a simple (single-laser, single imaging system) diagnostic that permits flowfield visualization without seeding or physical probes. FLEET optical and imaging parameters are tailored for measurements in the wake of a sharp cone in Mach 8 nitrogen flow, over freestream Reynolds numbers from 4*10$^{\mathrm{6}}$/m to 14*10$^{\mathrm{6}}$/m. Fluorescing FLEET lines for 1D velocity measurements and crosses for 2D measurements are written into the flow. The signal-to-noise ratio and tagging efficiency are maximized by using third harmonic FLEET at 267 nm, which has never before been implemented in a hypersonic test case. Flow tagging captures prominent wake features such as the separation shear layer, wake turbulence and two-dimensional velocity components.