This dissertation describes an experimental exploration of structural and mixing characteristics of the unforced as well as forced jet in crossflow (JICF). For unforced experiments, variable jet-to-crossflow density ratios (0.35≤S≤1.00) and momentum flux ratios (5≤J≤41) were explored, for three alternative injectors with a fixed jet Reynolds number (Rej=1900). For forced jet experiments, an equidensity jet emanating from two alternative flush nozzles (Rej=1900; Rej=1500) were explored.

The interplay between scalar and velocity fields for the JICF was studied using simultaneous PLIF/stereo PIV measurements in the jet’s centerplane. POD analysis showed a transition from convective to absolutely instability in the upstream shear layer as J was reduced, consistent with hotwire measurements. Local strain rates were extracted from both PLIF and PIV data via the Howarth transformation.

Mixing characterization with variable mixing scale lengths was explored via centerplane PLIF images. Mixing associated with stirring as well as molecular mixing could be distinguished, apparent especially in vortical structures for flush pipe injection.

Sinusoidal forcing of the convectively unstable JICF demonstrated that relatively low amplitude forcing affected jet structures and mixing, especially close to the fundamental frequency. For the absolutely unstable JICF, a much higher forcing amplitude was required to alter jet characteristics, consistent with lock-in, leading to enhanced molecular mixing.

Single-pulse square wave forcing created deeply-penetrating vortical structures for specific stroke ratios (L/D), even for the absolutely unstable JICF. L/D producing the greatest jet spread and penetration decreased with decreasing J, but L/D for optimal molecular mixing corresponded to different values and trends in J.

Double-pulse square wave excitation, with two variable pulses within each temporal period, enabled the jet's nearfield vortex rings to interact and collide, with attendant effect on molecular mixing.

Energy and space propulsion are two of the largest applied research areas requiring contributions from fundamental physical sciences, due to the growing world-wide demand in energy and continuing interests in expanding the frontier of deep space exploration. One of the common thrust areas in these two disciplines is plasma physics, the study of the motion of charged particles and their interaction with the electromagnetic field. The characterization of these plasma systems requires a comprehensive understanding of the physics of charged particles, collisional and radiative interactions among these particles, and how they interact with the electromagnetic field.

This dissertation presents some advances in the development of hydrodynamic models for plasma modeling and simulations in highly non-equilibrium conditions. Expressed in the form of conversation laws, these governing equations are solved by a finite volume discretization with a high-order reconstruction procedure and a multi-stage time integration method. High-fidelity collisional-radiative (CR) models are constructed by taking into account various elementary processes responsible for the excitation and ionization kinetics. The accuracy of the CR model is benchmarked against different experimental shock tube data, and yields satisfactory agreement for a wide range of flow conditions. A mechanism reduction scheme, based on a level grouping approach, is derived to lower the complexity of the CR kinetics while maintaining sufficient accuracy to capture the non-equilibrium dynamics of the plasma kinetics. The method is shown to be more accurate and efficient than standard level grouping approach, and is suitable for multidimensional flow calculations.

Although the hydrodynamic or fluid approach offers a convenient way to model the system, it requires some assumptions on the time and length scales, which in some case might be violated. Fortunately, small deviations from these assumptions can still be captured by extending the fluid equations to multi-fluid equations, which characterize the plasma species (ions and electrons) via their own set of conservation laws. The extension of the CR model to the multi-fluid regime requires a new derivation for exchange source terms. A model for excitation and deexcitation collisions within the multi-fluid framework is derived, starting from kinetic theory, where the model obeys the principle of detailed balance. The multi-fluid equations developed in the current work are used to study ion acceleration in laser-plasma interaction. The role of the laser parameters and the mechanism of the acceleration are examined in detail, demonstrating the capabilities of this computational framework.

The dissertation addresses the formulation of Large-Eddy Simulations (LES) with direct consideration of a base finite difference scheme, and with the intent of reducing numerical error influences on the closure model and ultimately the solution. As such, spectral characteristics of the explicitly-defined LES filter are considered with respect to the discretization method’s spectral accuracy (i.e., resolvability). Analysis and development of discrete filtering stencils is undertaken, placing emphasis on the ability to specify desired scale-separation (e.g., cut-off wavenumber and scale-discriminant attenuation) relative to the computational grid. Assessment of the LES procedure is preceded by the establishment of a suitable base scheme, comprised of high-order discretizations and the addition of stabilization presented in a filter-based artificial dissipation form. Subsequent robustness and preservation of the overall solution accuracy is achieved by tuning the dissipation according to the dispersion characteristics of the underlying numerical method and seeking to deliberately remove the effects of discretization error. Extension to LES is then established by properly defining the explicit filter in relation to these numerical characteristics. Effectiveness of the procedure is evaluated by means of a priori and a posteriori inspection of turbulence calculations for the Burgers and Navier-Stokes equations, wherein the impacts of discretization and filter cut-off are assessed in light of scale-similarity and “perfect” modeling (i.e., a DNS-based closure). Results demonstrate the benefits of employing mutually tuned high-order discretizations and filters in the limit of the idealized “perfect” model, yet highlight the likely possibility of modeling error overshadowing such gains when actual closures, such as scale-similarity models, are used. In an attempt to enhance the scale-similarity models considered herein, the filter-based artificial dissipation is employed in order to enforce the prescribed LES field, and is shown to reduce overall model error. Meanwhile, its use is shown to be beneficial even in the presence of “perfect” modeling, wherein the dissipation can be tuned to specifically target discretization errors.

Plasma turbulence and the resulting anomalous electron transport due to azimuthal current driven instabilities in Hall-effect thrusters is a promising candidate for developing predictive models for the observed anomalous transport. A theory for anomalous electron transport and current driven instabilities has been recently studied by [Lafluer et al., 2016a]. Due to the extreme cost of fully resolving the Debye length and plasma frequency, hybrid plasma simulations utilizing kinetic ions and quasi-steady state fluid electrons have long been the principle workhorse methodology for Hall-effect thruster modeling. Using a reduced dimension particle in cell simulation implemented in the Thermophysics Universal Research Framework developed by the Air Force Research Lab, we show collective electron-wave scattering due to large amplitude azimuthal fluctuations of the electric field and the plasma density. These high-frequency and short wavelength fluctuations can lead to an effective cross-field mobility many orders of magnitude larger than what is expected from classical electron-neutral momentum collisions in the low neutral density regime. We further adapt the previous study by [Lampe et al., 1971] and [Stringer, 1964] for related current driven instabilities to electric propulsion relevant mass ratios and conditions. Finally, we conduct a preliminary study of resolving this instability with a modified hybrid simulation with the hope of integration with established hybrid Hall-effect thruster simulations.

This experimental study focuses on combustion of liquid fuel droplets subject to external acoustic disturbances in the form of standing waves within an acoustic waveguide. Acoustic perturbations create a mean flame deflection dependent on the droplet location relative to the pressure node (PN) or pressure antinode (PAN) in the waveguide. This flame deflection is consistent with the sign of a theoretical acoustic acceleration acting on the burning system. Yet experimentally derived acoustic accelerations estimated from the degree of flame deflection differ quantitatively from that predicted by the acoustic radiation force theory. Phase locked OH* chemiluminescence imaging reveals a deflected flame which oscillates in position relative to the stationary droplet. Flame luminosity fluctuates with pressure throughout the forcing period, indicating the flame heat release rate is influenced by acoustic excitation. The thermoacoustic instability fostered by the in-phase oscillation of pressure and heat release rate is quantified via the Rayleigh index. Evaluation of the Rayleigh index over a range of acoustic forcing frequencies and droplet locations exposes the frequency sensitive character of the combustion mechanism stemming from dissimilar acoustic and chemical kinetic time scales. The present experimental configuration provides a useful test bed for evaluating the response of different alternative and conventional fuels to an acoustically resonant environment in the context of the well-known Rayleigh criterion.

These experiments examined the reactive processes involving nanoparicle laden liquid droplets, and turbulent jet flames as two separate sets of studies. The first part of this dissertation (Chapters 2 and 3) deals with the combustion of ethanol liquid droplets loaded with nano particulate additives using different droplet formation methods. For this study, an apparatus at the Energy and Propulsion Research Laboratory at UCLA was used to keep the droplet in a quiescent environment. Three different types of droplet combustion experiments were performed, involving: (I) the classic single droplet suspended from a quartz fiber, (II) a single droplet suspended from a quartz capillary, (III) a burning droplet that has continual fuel deliver to sustain the droplet for longer periods of time during the combustion process. Two alternative nanoparticles were explored to demonstrate the effect of energetic additives: reactive nano aluminum (nAl) and inert nano silicon dioxide (nSiO2), each having nominal average diameters of 80 nm. Simultaneous high speed visible and OH* chemiluminescence images were taken to determine the shape of the droplet over time and hence the burning rate constant (K), flame standoff distance, and mean OH* chemiluminescence intensity with varying particulate concentrations. Visible imaging showed particle/vapor ejections and jetting in continuously fed droplet experiments, while rod-suspended burning droplets showed limited particle ejection, usually towards the end of the droplet lifetime. The nSiO2-laden, rod-suspended droplets formed a porous, shell-like structure resembling the shape of a droplet at higher nSiO2 concentrations, in contrast to smaller residue structures left for nAl-laden droplets. A systematic increase in the burning rate constant was observed as the loading concentration of nAl was increased from 1wt%-6wt%. The droplet with continual fuel delivery had the greatest improvement in K of 13% over the pure fuel value. For nSiO2, the continuously fed droplet showed the greatest increase of 5% at 1 wt% loading concentrations, and no consistent trend was observed for nSiO2, likely due to the large shell-like residue structures in the latter stages of combustion. Transmission electron microscopy (TEM) images of particle residue revealed additional insights.The second part of this dissertation (Chapters 4 and 5) studied reacting gaseous turbulent jets in a newly constructed experiment at the Air Force Research Laboratory (AFRL/RQR) located at Edwards Air Force base. This experimental study aimed to characterize the coupling of an acoustic field with a turbulent gaseous methane nonpremixed flame under atmospheric pressure conditions. Two separate injection configurations were examined: one that involved a classic single methane jet surrounded by a minimal velocity oxidizer co-flow and a second coaxial jet configuration with annular oxidizer flow and the same low-velocity co-flow. The different jets were placed within an acoustic waveguide in which standing waves could be created using several speakers. The reacting jets could thus be situated at either a pressure node or a pressure anti-node location. High-speed Schlieren and OH* chemiluminescence images recorded the near field behavior of the flame under both unforced and acoustically forced conditions. High-speed imaging showed two different phenomena associated with these standing waves. When the flame was forced while situated at a pressure node, a sinuous oscillatory response of the flame was observed, in addition to transverse oscillations of the center fuel jet, which shortened the intact fuel core length. The flame “flattened” into an ellipsoidal shape in the direction of the acoustic waves. Conversely, at a pressure anti-node, the coupling of the acoustics and flame gave rise to an axisymmetric response (puff-like oscillations), which prompted the flame to become unstable at the anchoring region. This could lead to periodic liftoff or permanent flame liftoff. A receptivity study for a methane jet at Reynolds number of 5,300 and an ambient oxygen concentration of 40\% showed that the reacting jet was able to respond at the frequency of the unsteady acoustic field for a range of frequencies, but with a diminishing response of the flame for both the pressure node and the pressure anti-node under high frequency excitation. Proper Orthogonal Decomposition (POD) analysis was able to extract mode shapes and frequencies based on pixel intensity fluctuations. For the cases of pressure node forcing, this analysis method illustrated the different modes of flame oscillation, in many cases which were similar to corresponding low Reynolds number fuel jet experiments with pressure node excitation conducted at UCLA. A forcing susceptibility diagram was created to map the three different anchoring stability regimes the flame experienced under pressure anti-node forcing, demonstrating the need for higher amplitude excitation required for the flame to lift off when forced at higher frequency pressure anti-node conditions. As an extension to the single jet, the shear coaxial jet configuration kept the center fuel and surrounding oxidizer co-flow constant. Only the outer annular oxidizer flowrate was varied, with annulus-to-inner jet velocity ratios ranging from R = 0.05 to 0.3, to investigate its impact on the flame's ability to respond to the acoustics. In the absence of acoustic excitation, the coaxial jet did demonstrate natural shear layer/wake like instabilities at higher annular-to-jet velocity ratios, for R = 0.17 and 0.3. The dynamical response of the coaxial jet to pressure node excitation exhibited similar characteristics to that of the single jet for a range of forcing frequencies. But when forced at a pressure anti-node, a notable difference between the two configurations was found. The shear coaxial jet was more responsive to the acoustic forcing at higher forcing frequencies, for example, than the single fuel jet. The susceptibility diagrams for the full range of annular-to-inner jet velocity ratios demonstrated opposite trends when compared to the single jet, that is, that the coaxial jet was more responsive to excitation at a given excitation amplitude when the forcing frequency was higher, and thus closer to the natural coaxial jet instability frequency. Hence evidence suggests that the natural instabilities of the coaxial jet shear layer may be causing the difference in susceptibility diagrams. Both sets of experimental studies here, the nanofuel droplet combustion studies and the acoustically-coupled turbulent fuel jet combustion experiments, provide useful advances to our understanding of reactive flows relevant to liquid rocket engine systems. Enhancement in burning rates with nanoparticulate additives show potential benefits for rocket fuels, and attendant benefits are documented in the presence of acoustic disturbances, studied separately [1]. AFRL-based acoustically coupled turbulent fuel jet studies reveal different dynamical characteristics, depending on the injection system and the acoustic frequency and amplitude range. Different characteristic signatures extracted via POD analysis are both relevant in understanding combustion instabilities and in developing reduced order models underlying control of such instabilities. The present studies contribute to these goals in important ways.

A number of promising alternative rocket propulsion concepts have been developed over the past two decades that take advantage of unsteady combustion waves in order to produce thrust. These concepts include the Pulse Detonation Rocket Engine (PDRE), in which repetitive ignition, propagation, and reflection of detonations and shocks can create a high pressure chamber from which gases may be exhausted in a controlled manner. The Pulse Detonation Rocket Induced Magnetohydrodynamic Ejector (PDRIME) is a modification of the basic PDRE concept, developed by Cambier (1998), which has the potential for performance improvements based on magnetohydrodynamic (MHD) thrust augmentation. The PDRIME has the advantage of both low combustion chamber seeding pressure, per the PDRE concept, and efficient energy distribution in the system, per the rocket-induced MHD ejector (RIME) concept of Cole, et al. (1995).

In the initial part of this thesis, we explore flow and performance characteristics of different configurations of the PDRIME, assuming quasi-one-dimensional transient flow and global representations of the effects of MHD phenomena on the gas dynamics. By utilizing high-order accurate solvers, we thus are able o investigate the fundamental physical processes associated with the PDRIME and PDRE concepts and identify potentially promising operating regimes.

In the second part of this investigation, the detailed coupling of detonations and electric and magnetic fields are explored. First, a one-dimensional spark-ignited detonation with complex reaction kinetics is fully evaluated and the mechanisms for the different instabilities are analyzed. It is found that complex kinetics in addition to sufficient spatial resolution are required to be able to quantify high frequency as well as low frequency detonation instability modes. Armed with this quantitative understanding, we then examine the interaction of a propagating detonation and the applied MHD, both in one-dimensional and two-dimensional transient simulations. The dynamics of the detonation are found to be affected by the application of magnetic and electric fields. We find that the regularity of one-dimensional cesium-seeded detonations can be ignificantly altered by reasonable applied magnetic fields (Bz &le 8T ), but that it takes a stronger applied field (Bz > 16T ) to significantly alter the cellular structure and detonation velocity of a two-dimensional detonation in the time in which these phenomena were observed. This observation is likely attributed to the additional coupling of the two-dimensional detonation with the transverse waves, which are not captured in the one-dimensional simulations. Future studies involving full ionization kinetics including collisional-radiative processes, will be used to examine these processes in further detail.

Understanding the effects of alternative chemical kinetic mechanisms in turbulent reactive flows is critical to the ability to accurately simulate combustion processes, especially in practical systems. Exploring such effects is not a trivial endeavor because turbulent reactive simulations can be costly, especially when Direct Numerical Simulations (DNS) are employed and/or for large parameter studies. In addition, detailed chemical kinetic mechanisms are often too large and impractical for incorporation in multi-dimensional transient flow field simulations. The large number of species and reactions, as well as the wide range of time scales, in the detailed chemical kinetics account for the computational cost in largescale combustion simulations. Currently, reduced mechanisms are developed under specific laminar flow conditions in which selected global properties of a flame (e.g., ignition delay time, laminar flame speed, adiabatic flame temperature) are matched to those of the original detailed mechanism. However, this imposes restrictions on the operating range and applicability of these reduced mechanisms. For example, in addition to the presence of turbulence, it cannot be guaranteed that these specific conditions will be met everywhere in the flowfield for non-premixed combustion. If turbulence is shown to affect the results from reduced models, then use of the model would become flow and regime specific. It may even be necessary to simulate each flow configuration with detailed chemical kinetic mechanisms before reduced models can be developed for that flow configuration. A better understanding of the sensitivities of turbulent reactive flow results is clearly needed to address these issues. The Chemical Explosive Mode Analysis (CEMA) appears to be an efficient computational diagnostic tool that may give insight into the the important species and reactions in a given flowfield, and to help to explain differences that various kinetic mechanisms may produce in a reactive flowfield. Thus, CEMA may have the potential to help in the development of reduced mechanisms.

The objective of this dissertation is to gain insights into the influence of alternative chemical kinetics mechanisms on the results of turbulent combustion simulations and, specifically, the effects of these mechanisms under conditions representative of rocket injector applications. Methane-oxygen combustion simulations of a shear coaxial injection configuration are performed using several chemical kinetic mechanisms ranging from detailed, to skeletal, to reduced mechanisms. Multi-dimensional simulations of rocket injector flowfields are used to establish the underlying issues and motivate the studies. 0D and 1D simulations in concert with the the Chemical Explosive Mode Analysis (CEMA) procedure are then employed to develop insight into the important species and reactions involved to explain differences between the different kinetic mechanisms. Injector results reveal that it is important to establish grid convergence before making comparisons of reaction mechanisms. They also show that the skeletal FFCM1-21 chemical mechanism has time-step and spatial grid sensitivity compared to the detailed GRI-Mech 3.0 mechanism. Given that FFCM1-21 is a skeletal mechanism, the absence of certain species may be responsible for the sensitivity.

The CEMA module is first validated with published hydrogen-air 1D premixed flame results. The CEMA method is then applied to a 0D homogeneous combustion problem to obtain insights about the important species and reactions in methane-oxygen combustion for various chemistry models relevant to the rocket injector problem described earlier. A gaseous methane-oxygen mixture is studied as well as mixtures with the addition of H and/or O radicals to simulate the effects of turbulent mixing of burnt gases with reactants. For these cases, a new detailed mechanism (FFCM-1) and a reduced version (FFCMY-12) are used to study the underlying sensitivities. It is found that there is poor prediction of the ignition delay by the reduced mechanism FFCMY-12 in the presence of radicals as compared with the full FFCM-1 mechanism. Trends seen in 0D results help to identify the important species and reactions necessary for a reduced mechanism to replicate important phenomena such as ignition. Because of this, there is confidence that 0D simulations with the CEMA implementation could also help in pinpointing the pertinent species and reactions and in identifying and determining what to examine in a large and more complex turbulent dataset.