Investigation of small-scale flow structures using NO-flow tagging by photodissociation of NO/sub 2/

In the study presented here, air flows were homogeneously seeded with NO/sub 2/ at low concentrations (few hundreds of ppm). By photodissociation of NO/sub 2/ at 308 nm (XeCl excimer laser at approx. 200 MW/cm/sup 2/) NO was formed, which was imaged by LIF at various delays after its generation. Whereas several other tracers disappear rather quickly due to energy transfer processes (from vibrationally excited molecules) or high reactivity (in the case of OH/sup 4/), due to small reaction cross-sections, NO is stable on the timescale of interest. Therefore, using NO, the movements of volume elements can be imaged at longer times and larger distances compared to other techniques. Furthermore, with the molecular weight of NO being between that of N/sub 2/ and O/sub 2/ and diffusion coefficients close to nitrogen, NO turns out to be a good choice to characterize the small-scale behavior of turbulent air flows.