Surendra P. Sharma

Nonequilibrium and equilibrium shock front radiation measurements

The intensities of the radiation emitted behind a normal shock wave in N were measured in an electric-arc driven shock tube at a shock velocity of 6.2 km/sec. Both a time-resolved broad-band radiation intensity measurement and a time-frozen spectral measurement were conducted. The rotational and vibrational temperatures are determined in both the equilibrium and the nonequilibrium regions. The results are compared with the similar data obtained by Allen et al. (1961). The measured rotational temperature seems to be in nonequilibrium, contradicting the two-temperature assumption of Park (1988), but the measured vibrational temperature agrees with Parks model.

Rate parameters for coupled vibration-dissociation in a generalized SSH approximation

A theoretical study of vibrational excitations and dissociations of nitrogen undergoing a nonequilibrium relaxation process upon heating and cooling is reported. The rate coefficients for collisional induced vibrational transitions and transitions from a bound vibrational state into a dissociative state have been calculated using an extension of the theory originally proposed by Schwarz (SSH) et al. (1952). High-lying vibrational states and dissociative states were explicitly included but rotational energy transfer was neglected. The transition probabilities calculated from the SSH theory were fed into the master equation, which was integrated numerically to determine the population distribution of the vibrational states as well as bulk thermodynamic properties. The results show that: (1) the transition rates have a minimum near the middle of the bound vibrational levels, causing a bottleneck in the vibrational relaxation and dissociation rates; (2) high vibrational states are always in equilibrium with the dissociative state; (3) for the heating case, only the low vibrational states relax according to the Landau-Teller theory; (4) for the cooling case, vibrational relaxation cannot be described by a rate equation; (5) Parks (1985, 1988) two-temperature model is approximately valid; and (6) the average vibrational energy removed in dissociation is about 30 percent of themorexa0» dissociation energy. 29 references.«xa0less

Operating Characteristics of a 60- and 10-cm Electric Arc-Driven Shock Tube-Part 11: The Driven Section

This is the second part of a two-part paper describing the operating characteristics of the electric arc-driven shock-tube facilities at NASA Ames Research Center. This part discusses the performance of the driven sections when the facility is used as a tool to produce a low-density nonequilibrium flow and when used as a shock tunnel. Specifically, the paper discusses the cleanliness of the driven flow at low densities, and the deviation from the equilibrium conditions at the test section of the shock-tunnel flow.

Survey of simulation and diagnostic techniques for hypersonic nonequilibrium flows

The possible means of simulating nonequilibrium reacting flows in hypersonic environments, and the required diagnostic techniques, are surveyed in two categories: bulk flow behavior and determination of chemical rate parameters. Flow visualization of shock shapes for validation of computational-fluid dynamic calculations is proposed. The facilities and the operating conditions necessary to produce the required nonequilibrium conditions, the suitable optical techniques, and their sensitivity requirements, are surveyed. Shock-tubes, shock-tunnels, and ballistic ranges in a wide range of sizes and strengths are found to be useful for this purpose, but severe sensitivity requirements are indicated for the optical instruments, which can be met only by using highly-collimated laser sources. Likewise, for the determination of chemical parameters, this paper summarizes the quantities that need to be determined, required facilities and their operating conditions, and the suitable diagnostic techniques and their performance requirements. Shock tubes of various strengths are found to be useful for this purpose. Vacuum ultraviolet absorption and fluorescence spectroscopy and coherent anti-Stokes Raman spectroscopy are found to be the techniques best suited for the measurements of the chemical data.

Rate parameters for coupled rotation-vibration-dissociation phenomena in H2

A theoretical study of the effects of molecular rotation on coupled vibration-dissociation phenomena for H2, undergoing a nonequilibrium relaxation in a heating and cooling environment, is reported. The rate coefficients for the collisional bound-bound and bound-free transitions have been calculated using a quasi-classical trajectory method. These state-to-state transition rates were fed into the master equation, which was numerically solved to determine the rotational and vibrational population densities as well as the bulk thermodynamic properties. Our results show that a) the vibrational transition rates of H2 increase monotonically with the vibrational levels, faster than the Landau-Teller model, b) the rotation of the molecules helps relax the low vibrational levels, serves as a temporary storage of vibrational energy during the relaxation, and thereby delays the dissociation process, c) the dissociation and recombination rates can be approximated within an order of magnitude using equilibrium formulations and Parks two-temperature model for temperature averaging, d) lower vibrational levels tend to over-relax, and e) the average energy loss due to the dissociation is about 80-90% of the dissociation energy.

Operating characteristics of a 60 cm and a 10 cm electric arc-driven shock-tube

This paper describes the current status of the operating characteristics of the electric arc-driven shock-tube facility at Ames Research Center, focusing on its potential usefulness in the current and anticipated future applications. The paper specifically addresses the questions as to: (1) how well the behavior of the arc driver is understood and controlled, (2) how well the facility is equipped to test low-density, very-high-velocity nonequilibrium flow regimes, and (3) how closely the facility is expected to produce an equilibrium hypersonic flow when operated in shock-tunnel modes. For these issues, it is shown that: (1) a plasma kinetics model of the exploding wire closely describes the arc behavior in the driver, (2) the facility can produce a spectroscopically-clean flow in a low density regime with a shock velocity of 13 km/sec in air when used with an aluminum driven tube, and (3) when operated as a shock-tunnel, the high enthalpy flow in the test section is expected to deviate only slightly from the perfect equilibrium flow conditions at enthalpies corresponding to flight speeds of 5 km/sec or less.