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We have begun a comprehensive series of analyses and experiments to study the basic problem of shock wave dynamics in ionized media. Our objective is to isolate the mechanisms that are responsible for the decrease in the shock amplitude and also to determine the relevant plasma parameters that will be required for a drag reduction scheme in an actual high altitude hypersonic flight. Specifically, we have initiated a program of analyses and measurements with the objective of (i) fully characterizing the propagation dynamics in plasmas formed in gases of aerodynamic interest, (ii) isolating the mechanisms responsible for the decreased shock strength and increased shock velocity, (iii) extrapolating the laboratory observations to the technology of supersonic flight.

Fluctuating wall pressure measurements have been made on centerline upstream of a blunt fin in a Mach 5 turbulent boundary layer. By examining the ensemble averaged wall pressure distributions for different separation shock foot positions, it has been shown that local fluctuating wall pressure measurements are due to a distinct pressure distribution, Rho(sub i), which undergoes a stretching and flattening effect as its upstream boundary translates aperiodically between the upstream influence and separation lines. The locations of the maxima and minima in the wall pressure standard deviation can be accurately predicted using this distribution, providing quantitative confirmation of the model. This model also explains the observed cross-correlations and ensemble average measurements within the interaction. Using the Rho(sub i) model, wall pressure signals from under the separated flow region were used to reproduce the position-time history of the separation shock foot. Further, the negative time delay peak in the cross-correlation between the predicted and actual shock foot histories suggests that the separated region fluctuations precede shock foot motion. The unsteady behavior of the primary horseshoe vortex and its relation to the unsteady separation shock are described.

An exact solution of a forced Burgers' equation representing the dynamics of a "dust fluid" in a one-dimensional flow is presented. The test case considered starts with a steady (time independent) two-fluid flow in one dimension, where the two fluid components represent gas and dust. It is then assumed that a shock wave travels through the gas at a constant speed and without radiative energy losses and diffusion. Then, adopting a constant stopping time for the dust particles in the dust fluid (mono-dispersed grain sizes), the equation of motion for the dust fluid can be transformed into a simple ordinary differential equation, which is satisfied by the Wright omega function. Implications for the formation of detached shells around carbon stars are briefly discussed.

We consider the stability problem for shock layers in Slemrod's model of an isentropic gas with capillarity. We show that these traveling waves are monotone in the weak capillarity case, and become highly oscillatory as the capillarity strength increases. Using a spectral energy estimate we prove that small-amplitude monotone shocks are spectrally stable. We also show, through the use of a novel spectral energy estimate, that monotone shocks have no unstable real spectrum regardless of amplitude; this implies that any instabilities of these monotone traveling waves, if they exist, must occur through a Hopf-like bifurcation, where one or more conjugate pairs of eigenvalues cross the imaginary axis. We then conduct a systematic numerical Evans function study, which shows that monotone and mildly oscillatory profiles in an adiabatic gas are spectrally stable for moderate values of shock and capillarity strengths. In particular, we show that the transition from monotone to non-monotone profiles does not appear to trigger any instabilities.

Molecular dynamics simulation is used to study the piston driven shock wave at Mach 1.5, 3, and 10. A shock tube, whose shape is a circular cylinder, is filled with hard sphere molecules having a Maxwellian thermal velocity distribution and zero mean velocity. The piston moves and a shock wave is generated. All collisions are specular, including those between the molecules and the computational boundaries, so that the shock development is entirely causal, with no imposed statistics. The structure of the generated shock is examined in detail, and the wave speed; profiles of density, velocity, and temperature; and shock thickness are determined. The results are compared with published results of other methods, especially the direct simulation Monte-Carlo method. Property profiles are similar to those generated by direct simulation Monte-Carlo method. The shock wave thicknesses are smaller than the direct simulation Monte-Carlo results, but larger than those of the other methods. Simulation of a shock wave, which is one-dimensional, is a severe test of the molecular dynamics method, which is always three-dimensional. A major challenge of the thesis is to examine the capability of the molecular dynamics methods by choosing a difficult task.

An investigation of the dynamics of shock waves in weakly ionized argon plasmas has been performed using a pressure ruptured shock tube. The velocity of the shock is observed to increase when the shock traverses the plasma. The observed increases cannot be accounted for by thermal effects alone. Possible mechanisms that could explain the anomalous behavior include a vibrational/translational relaxation in the nonequilibrium plasma, electron diffusion across the shock front resulting from high electron mobility, and the propagation of ion-acoustic waves generated at the shock front. Using a turbulence model based on reduced kinetic theory, analysis of the observed results suggest a role for turbulence in anomalous shock dynamics in weakly ionized media and plasma-induced hypersonic drag reduction.

Simulations of the Mach 5 compression-corner shock / turbulent boundary layer interaction experimentally mapped by Prof. David Dolling and co-workers have been performed using a hybrid large-eddy / Reynolds-averaged Navier-Stokes (LES/RANS) model. The model captures the mean-flow structure of the interaction reasonably well, with observed deficiencies traced to an under prediction of the displacement effects of the shock-induced separation region. The computational results provide some support for a recent theory relating to the underlying causes of low-frequency shock wave oscillation. The simulation results indicate that the sustained presence of a collection of neighboring streaks of low / high momentum fluid within the boundary layer induces a low frequency undulation of the separation front. Power spectra obtained at various streamwise stations are in good agreement with experimental results, indicating that the LES/RANS method is capable of predicting both the low and high-frequency dynamics of the interaction. Downstream of re-attachment, the simulations capture a three-dimensional mean flow structure, dominated by counter-rotating vortices that produce wide variations in the surface skin friction. Predictions of the structure of the re-attaching boundary layer agree well with experimental pitot pressure measurements. In comparison with Reynolds-averaged model predictions, the LES/RANS model predicts more amplification of Reynolds stresses and a broadening of the Reynolds-stress distribution within the boundary layer that is probably due to re-attachment shock motion.

This research characterizes the blast wave and temperature evolution of an explosion fireball in order to improve the classification of aluminized conventional munitions based on a single explosive type such as RDX. A drag model fit to data shows initial shock velocities of 1.6-2.8 km/s and maximum fireball radii ranging from 4.3-5.8 m with most of the radii reached by 50 ms upon detonation. The Sedov-Taylor point blast model is fitted to data where a constant release (s=1) of energy upon detonation suggests shock energies of 0.5-8.9 MJ with blast dimensionalities indicative of the spherical geometry (n3) observed in visible imagery. An inverse correlation exists between blast wave energy and overall aluminum content in the test articles. Using a radiative cooling term and a secondary combustion term, a physics-based empirical model is able to reduce 82 data points to five fit parameters to describe post-detonation combustion fireballs. The fit-derived heat of combustion has a 96% correlation with the calculated heat of combustion but has a slope of 0.49 suggesting that only half of the theoretical heat of combustion is realized. Initial temperature is not a good discriminator of detonation events but heat of combustion holds promise as a potential variable for event classification.

Fluctuating wall pressure measurements have been made on centerline upstream of blunt fins in a Mach 5 flow. Standard time series analysis and conditional sampling algorithms have been used to examine the effects of leading edge sweep, leading edge shape, and fin root modifications on the RMS level and spectral content of fluctuating pressures. Results show that the fluctuating loads can be reduced significantly by appropriate modification of the fin leading edge. Leading edge sweep considerably reduces the mean and RMS pressure loading at the fin root, the extent of the region of unsteady separation shock motion (i.e. the intermittent region), and the separation length. The spectral content of pressure fluctuations in the intermittent region shifts to higher frequencies with leading edge sweep, while the spectral content of pressure fluctuations in the separated region is virtually unchanged by leading edge sweep. Of the different fin leading edge geometries which induce the same size interaction, the 'blunter' configurations produce smaller intermittent regions and larger separated regions. While the use of a strake at the fin leading edge root has virtually no effect, a swept hemicylindrically blunted root fillet reduces the centerline upstream influence and intermittent region length by 50%, and reduces the mean and RMS pressure loading at the fin root by 75% and 95% respectively.

The goal of this project is to develop a fundamental understanding, at the molecular level, of the initial events occurring during the first picoseconds of impact initiation of energetic materials. It is believed this fundamental research will lead to a clearer picture of the factors which determine the sensitivity of energetic materials to shock impact initiation. Understanding and perhaps controlling sensitivity will result in safer explosives. In a larger sense, the problem of molecular level understanding of chemical and physical phenomena involved in impact processes has much wider and more general implications for such relevant processes as mechanical failure, crack propagation, lubrication and wear, etc. At the present time experimental techniques to study these problems are lacking. By developing such techniques with the goal of understanding impact initiation of energetic materials, these other areas may be addressed as well.

The project focuses on understanding the initial steps of energetic material ignition at the molecular level, with the long range goal of understanding the fundamental mechanisms of the sensitivity of energetic materials. The effort involves three distinct, but ultimately related projects: (1) understanding the detailed behavior of solid-state molecules immediately behind a shock front; (2) developing new diagnostic techniques and using them to investigate the picosecond time scale behavior of insensitive energetic materials under shock loading conditions; and (3) studying molecular mechanical energy transfer in condensed energetic materials. In the past three years, new methods were developed for reproducibly shocking energetic materials and probing the result using vibrational spectroscopy with extremely high time resolution. A new vibrational spectroscopy technique was developed and used to study vibrational energy transfer in a condensed high explosive, nitromethane. Besides providing the first ever view into vibrational energy transfer in a condensed high explosive, some intriguing results were obtained involving the channeling of vibrational energy into the ubiquitous nitro group.

The separation process in cylinder and unswept compression ramp induced shock wave turbulent boundary layer interactions was examined using a conditional-analysis of wall pressure fluctuation measurements. The tests were performed in a Mach 5 blow down tunnel under adiabatic wall temperature conditions. The conditional analysis showed that the instantaneous separation position is at or close to the instantaneous separation shock foot in these interactions. The separation line as indicated by traditional surface tracer methods is at, or close to, the downstream boundary of a region of intermittent separation. The dynamics of the separation bubble in the unswept compression ramp flow field were also examined and initial results suggest that the motion of the separation point and reattachment point locations are correlated. The preliminary results indicate the reattachment point is at its downstream locations when the separation shock is at its upstream locations. Keywords: Flow separation bubble; Turbulent flow; Shock waves; Turbulent boundarylayer; Unsteady flow; Reattached flow; Flowfield interactions.

Eight applications of the Random-Choice Method (RCM) to the solution of problems in shock and detonation-wave dynamics are presented. It is shown that unlike other numerical methods, the RCM yields sharp-fronted shocks and contact surfaces without resorting to artificial and perhaps erroneous means of predicting their locations. It has been shown by examples that the RCM provides exceptionally high computational accuracy for flows involving planar and spherical shock waves, rarefaction waves and contact surfaces, as well as their interactions. In addition, the RCM can handle flows with detonations and chemical reactions involving the above transition fronts.