Hydroxyl formation mechanisms and models in high-altitude hypersonic flows

The formation of vibrationally hot OH is examined for a rarefield flow about a sphere at 80 and 100 km using the direct simulation Monte Carlo method. Four main processes are considered leading to OH production in the flow, which include water dissociation, exchange reaction between water and atomic oxygen, and exchange reactions between hydrogen (H and H₂) and oxygen (O₂ and O). The principal mechanism of OH production at 100 km is shown to be the H + O₂ → OH + O reaction, with the maximum OH temperature greater than 5000 K. Water dissociation is found to be the most important source of OH at 80 km, with the maximum vibrational temperature of approximately 2500 K. The molecular dynamics results incorporated into the direct simulation Monte Carlo method are used to simulate water dissociation by N₂ and are compared to the results obtained by the conventional total collision energy model. The OH spatial distribution along the stagnation line predicted by the molecular dynamics approach is significantly different than that obtained with the total collisional energy model.