Simultaneous Temperature and Velocity Measurements in Air with Femtosecond Laser Tagging

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, 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 from the nitrogen second positive and first negative systems. Spectroscopy of this emission allows temperature to be determined from the distribution of rotational molecular energy. Temperature profiles are measured with better than 10% uncertainty and 500 μm spatial resolution between 300 and 650 K in heated jets. This method is expected to be valid up to at least 2000 K. Femtosecond laser electronic excitation tagging temperature and velocity measurements involve separate nitrogen transitions and can be conducted simultaneously without interference.