Hypersonic Tripped Boundary Layer Measurements using FLEET Velocimetry

Femtosecond laser electronic excitation tagging was applied in a transitioning boundary layer behind an array of turbulence inducing trips. The Texas A&M University Actively Controlled Expansion tunnel in which FLEET was performed operated at Mach 5.7 at a Reynolds number of 5.2 × 10^6/m with a working fluid of air with a freestream static pressure of 3.3 torr and density of 0.026 kg/m^3. A one dimensional mean and root-mean-squared velocity profile was extracted from FLEET emissions captured in a wall-normal configuration through the boundary layer. Mean velocity in the freestream was measured with an uncertainty of 1.5% and was verified to be within 1% of the tunnel measurement system and a Reynolds-Averaged Navier–Stokes CFD simulated freestream mean velocity. Measured root-mean-squared velocity in the freestream was calculated to be in excess of 5% suggesting large precision error due to low fluid density and emission signal. A substantial flow deceleration was observed in the boundary layer which is attributed to either a shock wave originating from the surface of the model or a counter-rotating streamwise vortex pair from the array of turbulence inducing trips. Comparison of the inner variables and near-wall velocity gradient indicate a transitional but not yet fully turbulent flow at the measurement location. Wall-normal FLEET velocimetry improvements are proposed.