Diagnostics of Unseeded Air and Nitrogen Flows by Molecular Tagging.

摘要

This research effort has focused on the development of Femtosecond Laser Electronic Excitation Tagging (FLEET), a new molecular tagging diagnostic for subsonic, supersonic and hypersonic flows. A femtosecond laser is focused into a nitrogen containing flow of interest and creates a line of dissociated nitrogen molecules through the focal zone. The subsequent recombination of those nitrogen atoms occurs over tens of microseconds through a fluorescing upper electronic state, so the displacement and distortion of the line with the flow can be imaged with a time-gated camera. No seeding is required. The use of point tagging for the acquisition of full three dimensional velocity and acceleration data, line tagging the measurement of cross stream correlations and structure functions in free jets, and the tracking of cross patterns for the measurement of velocity and vorticity have all been examined in this effort. Megahertz rate imaging of patterns tagged at kilohertz rates have been demonstrated. The research has also addressed the perturbation that FLEET creates to the flow through the tagging mechanism. Collaborations have been undertaken with the Air Force Research Lab, NASA, the Arnold Engineering Development Center and with international partners.