Modeling of radiation from neutral collisional excitations in hypersonic flows

The role of neutral collisional excitation mechanisms in populating electronic as well as vibrational and rotational states in hypersonic flows has only recently been appreciated. For speeds less than about 8 km/set, the electron densities are sufficiently low such that neutral excitations become competitive with those of electrons. From the continuum through the highly rarefied flows, flowfield computational techniques have been developed to model these excitations. All methods have to be efficient to potentially handle large numbers of vibrational and rotational states. Specific algorithms are developed for the situation where collisions between the radiation source species and the majorant flow species are infrequent occurrences. The paper gives examples of how the flow and radiation techniques have evolved as well as comparisons with experimental data.