Effect of Transport Coefficients on RAM-C-II Plasma Density

The first study comparing the effect of transport coefficients on the plasma density around the RAM-C-II vehicle is presented using two commonly used transport coefficients in hypersonics: the Gupta-Yos model and the Dixon-Lewis model. The Dixon-Lewis model is seen to result in as much as a twofold increase in peak electron density compared to Gupta-Yos. This discrepancy is found to be mostly due to differences in ambipolar diffusion and, hence, in ion mobilities. It is not clear which model has the most accurate ion mobilities in the temperature range under consideration. While the Gupta-Yos model is known to provide good agreement with experimental data of reactive conductivity (which depends on ion mobilities) at very high temperatures, it does not agree well with experimental data of ion drift velocity at lower temperatures. The opposite is observed for the Dixon-Lewis model which is reliable at lower temperatures but has considerable error at higher temperatures. Improved models that work equally well at low and high gas temperatures are needed to cover the whole temperature range present in the RAM-C-II flow field.