Molecular tagging velocimetry in nitrogen with trace water vapor

We report a new molecular tagging velocimetry method for use in pure nitrogen gas with a small impurity of water vapor on the order of 0.1%. This two laser method can produce a 25 mm long tag line of NH (imidogen) radicals at a standoff of 1 m which is imaged by laser-induced fluorescence after a delay of a few microseconds. The signal-to-noise ratio of the tag image can exceed 50 at standard temperature and pressure, allowing excellent spatial resolution. This new method called imidogen tagging velocimetry is useful for measuring spatially resolved velocity in flow facilities which run pure nitrogen rather than air, contain or can be seeded with trace water vapor, and in which other seeded or intrusive velocimetry methods are impractical.