Graham V. Candler
Department of Aerospace Engineering and Mechanics
University of Minnesota, Minneapolis, MN 55455
Deborah A. Levin
Science and Technology Division
Institute for Defense Analyses, Alexandria, VA 22311
Vibration-dissociation coupling in low-density, hypersonic flows
of air is investigated. Radiative emission data for nitric oxide and
for atomic oxygen measured by a re-entry flight
experiment are used to assess the modeling of this phenomenon. Flow
field computations are performed using the direct simulation Monte
Carlo method. Due to the relatively small number of collisions
under high altitude, low-density flow conditions, an overlay approach
is used to simulate changes in chemical composition of trace species
including both nitric oxide and atomic oxygen. Radiative emission is
calculated using a nonequilibrium radiation method. It is found that
the strong degree of thermal nonequilibrium that occurs in
high-altitude, hypersonic flows makes the chemistry very sensitive to
the vibration-dissociation coupling model. A number of such models
based on continuum and particle representations of the flow are
assessed. A variation in dissociation rate of up to nine orders of
magnitude among these models is found for the lowest density
flight conditions. By using a sophisticated dissociation model, the
emission calculated at the highest altitude for which measurements are
available is improved from a factor of 220 too low to within
a factor of 4 too low. With the same model, improvement by a factor of
50 is also obtained for the computation of emission from atomic oxygen.
This is the first time that the observed dependence of the flight data
on the free-stream density has been predicted correctly.