Unseeded Velocity Measurements Around a Transonic Airfoil Using Femtosecond-Laser Tagging

Femtosecond laser electronic excitation tagging (FLEET) velocimetry was used to study the flowfield around a symmetric, transonic airfoil in the NASA Langley 0.3-m TCT facility. A nominal Mach number of 0.85 was investigated with a total pressure of 125 kPa and total temperature of 280 K. Two-components of velocity were measured along vertical profiles at different locations above, below, and aft of the airfoil at angles of attack of 0°, 3.5°, and 7°. Velocity profiles within the wake showed sufficient accuracy, precision, and sensitivity to resolve both the mean and fluctuating velocities and general flow physics such as shear layer growth. Evidence of flow separation is found at high angles of attack. Velocity measurements were assessed for their accuracy, precision, dynamic range, spatial resolution, and overall measurement uncertainty as they relate to the present experiments. Measurement precisions as low as 1 m/s were observed, while the velocity dynamic range was found to be nearly a factor of 500. The spatial resolution of between 1 mm and 5 mm was found to be primarily limited by the FLEET spot size and advection of the flow. Overall measurement uncertainties ranged from 3 to 4 percent.