Modeling of Unsteady Shock Tube Flows Using Direct Simulation Monte Carlo

Abstract

The simulation of the UV spectra of moderate Mach number flows is the strongest test possible of the complex thermochemical models in hypersonic flows. To take advantage of low-density ground-based measurements, however, one needs to be able to model unsteady flows using kinetic particle approaches. The direct simulation Monte Carlo approach is used in this paper to model the thermal relaxation in nonreacting nitrogen shocks corresponding to the data and the higher Mach number air shocks and radiation of previous research. In the first case, it is found that better agreement with experimental data can be achieved when assuming a constant rotational collision number instead of a temperature-dependent one. For the second data set, the nonequilibrium NO UV radiation from unsteady reflected air shocks are modeled and compared with experimental data. The Larsen–Borgnakke and total collision energy models are used to simulate the internal energy relaxation and chemistry of five chemical species (, , N, O, and NO), respectively. Unlike steady shock layer flows, the long period of equilibration behind the shock wave occurs and potential factors that prevent complete thermochemical equilibrium are explored. The main goal of this paper, however, is to compare the spectrally averaged peak shock radiation with data. Because there are insufficient collisions to assume a Boltzmann distribution of the NO electronic states, a set of excitation/deexcitation mechanisms are proposed and the quasi-steady-state assumption is used in calculating the neutral-impact excitation of NO and the associated radiation. The calculated spectrally averaged peak radiations in the shock are in good agreement with the experimental data of Gorelov for shock speeds below , but are lower at higher shock speeds.