RESEARCH INTERESTS

IN CHEMICALLY-REACTING HYPERSONIC FLOWS

Rockets and spacecraft moving at speeds greater than Mach 5 produce hypersonic shocks that create large amounts of vehicle heating. In the popular vein this was shown in the movie, Apollo 13, during the simulated reentry of that mission. Today's space mission goals are more aggressive and require a better understanding of these phenomena than was available in the seventies when Apollo 13 was launched. Hence the modeling of high temperature gas dynamic flows remains an active, cross disciplinary research area. The measurement of optical (ultraviolet through infrared) spectral emissions from these flows provides the most sensitive diagnostic of a very complex chemically reacting system. Moreover, the simulation of the spectral radiation from three-dimensional flows with reacting chemistry is computationally intensive, requiring the use of high-speed Beowulf clusters that enable low-cost, large scale parallel computing.

My research in the simulation of high temperature flows combines topics in aerospace engineering and chemical physics. I have been fortunate to be able to apply my research to the planning and post-mission data analyses of small rocket space experiments. I have also participated in the detailed planning and design of optical instrumentation for space experiments. These flights have involved inter-disciplinary collaborations with other researchers at various university and government institutions as is reflected in the authorship on some of the publications listed below.

The majority of my research involves the modeling of radiation from rarefied flows. At sufficiently low gas densities the continuum approximation for fluid flow is no longer valid. This transition occurs when the gas mean free path approaches a characteristic dimension such as the radius of curvature of the front surface of a space vehicle or the nozzle exit radius of a small thruster. In this area of research, the Direct Simulation Monte Carlo (DSMC) method is used to model diffuse, chemically reacting gases and gas-surface interactions. The numerical challenge is to perform the simulations with enough fidelity so that rare events (collisions) which are actually responsible for the radiation can be resolved. The model has been successfully applied to quantitatively explain the mechanism of spacecraft glow which was shown to be due to visible emissions from excited state NO 2 formed from the interactions of atomic oxygen with weakly chemisorbed NO. The technique has been extended to model, in three dimensions, the shuttle IR glow using the same gas-surface model. Similar modeling and simulation methods have been applied to analyze imagery obtained from the MIR spacecraft of hydrazine rocket retro-firings. The counter-flow geometry provides a space laboratory of high energy plume exhaust species colliding with ambient atomic oxygen at relative collisional speeds of 11 km/sec.

An important input to the simulations are the cross sections used to model internal energy exchange and chemical reactions. Using quasi-classical trajectory methods we are using sophisticated techniques to provide this fundamental data on neutral-neutral collisions. The modeling of NO and OH ro-vibronic IR radiation provides an assessment of our understanding of internal energy exchange processes in a strong normal shock. In support of a recently flown rocket experiment, vibrational state-specific predictions of modeling the CO 2 IR shocklayer radiance have been performed. Predictions will now be tested against recently obtained, in-flight data. Finally, the modeling of contamination due to small attitude control thrusters for onboard missile optical sensors is an important aerodynamic problem. Research is being performed to model the complex aerodynamic structure of multiple density small jets expanding into a vacuum, the radiation fields produced by free stream atomic oxygen with plume effluents in highly energetic collisions, and the two-phase flow.

Rarefied and transitional modeling and simulation techniques are not only restricted to spacecraft applications. For example, a 1-µm sized MEMs device or a 1-µm sized spore-cell also fulfill the transitional Knudsen number requirement. A new research program in the area of DSMC modeling of MEMS microthruster propulsion device performance was initiated about three years ago. The first 3-D simulations of cold and hot MEMS micro thruster gas flows showed that the gas-surface interaction model dominates the physics of these device flows which cannot be accurately modeled by continuum flow methods. Recent calculations allow one to analyze the thermal material response coupled to the nozzle gas flow. Eventually a design tool based on these rigorous calculations will enable the systems designer to evaluate thrust performance subject to a specific thermal boundary condition, such as, liquid or convection cooling, or insulated and to compare the length of time a silicon micro-thruster material can survive with the desired burn time.