Modeling of Energy Transfer in Hypersonic Shocks Using High Fidelity Models PDF Download

Author: Tong Zhu
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Languages : en
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Book Description
The spectra of high-temperature, chemically reacting hypersonic flows provides the most powerful diagnostic available for testing thermochemically nonequilibrium models in re-entry conditions. Several shock tube experiments have revealed that conventional phenomenological approach can not accurately predict the internal temperature of the gas and also the corresponding radiation.In particular, large rotational nonequilibrium in strong shocks has been observed in several experiments with high peak translational temperatures. The Direct Simulation Monte Carlo (DSMC) method is a particle-based simulation method that is capable of properly simulating flows with large nonequilibrium. In the experiments above, one dimensional shocks are most widely studied but they are challenging to simulate using the DSMC method due to the unsteady nature of the flows and especially for hypersonic flows with chemical reactions taking place. Therefore, efficient approaches for simulating one-dimensional shocks are developed for use in DSMC simulations.Both a shock stabilization technique and a modified DSMC unsteady sampling approach are used in simulating one dimensional, unsteady shocks. In the latter approach, a moving sampling region is used to obtain an accurate profile of the reflected shock in air. The shock number density and temperature profiles are obtained and used to calculate excitation and radiation. The Quasi-Steady-State (QSS) assumption is made in the excitation calculation where both electron impact and heavy particle impact excitation for the NO(A) and the N2+(B) states are studied. The calculated NO radiation in the wavelength range of lambda = 235+/-7nm for shock speeds below 7km/s are in good agreement with the experiment, but, the predicted radiation is lower than the experiment for shock speeds above 7km/s. In addition, the N2+ radiation in the wavelength range of lambda = 391.4+/-0.2nm are in good agreement with the experimental data for shock speeds above 9km/s. High fidelity models for simulating both the dissociation and relaxation processes in N+N2 and N2+N2 systems are also investigated. Relaxation cross sections are computed and the 99 bin method shows good agreement between the bin-to-bin and state specific relaxation cross sections for both N-N2 and N2-N2 relaxation. These relaxation cross sections are then implemented separately in 0D DSMC isothermal relaxation cases. For both cases, the rotational and vibrational temperatures relax to the equilibrium heat bath temperature. For N-N2 relaxations, the rotational temperature relaxes faster than the vibrational temperature at relatively low translational temperature and at a very similar rate to the vibrational temperature at relatively high temperature. These are in qualitative agreement with the observation of earlier experiments. The one-dimensional binning method and associated cross sections by Parsons et al. are implemented in DSMC simulations and the results are compared with those using the traditional TCE and LB models. For shock conditions similar to those in the experiments of Gorelov, it is found that the MD-QCT chemical reaction model predicts more dissociation and faster relaxation of the vibrational temperature. In the higher speed shock condition of the experiment by Fujita, the use of MD-QCT databases for both chemical reaction and internal energy predicts more dissociation in the downstream of the shock but slower relaxation of the rotational temperature. Also the rotational temperature in the shock region is in somewhat better agreement with the experiment of Fujita.