Simultaneous temperature and velocity measurement in unseeded air flows with FLEET

High spatial resolution measurements of local flow velocities and temperature distributions are crucial for understanding turbulent, high speed, and reacting flows. In this paper, we demonstrate that femtosecond laser electronic excitation tagging (FLEET), a recently developed molecular tagging velocimetry method can also be used to simultaneously measure temperature. A 150 femtosecond pulse populates excited molecular nitrogen states, driving significant emission in the nitrogen Second Positive and First Negative systems. Spectroscopy of emission from the excited states allows temperature to be determined from both the distribution of rotational energy and the relative strengths of the Second Positive and First Negative systems. Temperature profiles are measured to an uncertainty of 20 K and 1 mm spatial resolution between 300 and 600 K in a heated turbulent jet. FLEET temperature and velocity measurements involve separate systems and can be conducted simultaneously without interference.