The present work studies the complex dynamics of an oscillating shock impinging on a laminar/transitional supersonic boundary layer, with emphasis on the radiated post-shock waves and a coherent wave structure induced in the turbulent boundary layer (TBL) downstream of the separation bubble. Fully resolved direct numerical simulations (DNS) have been carried out at Mach 5, with imposed shock-oscillation frequency matching that predicted by earlier direct simulation Monte Carlo (DSMC) studies of the internal shock structure. Shock oscillations are found to produce a field of post-shock waves efficiently transmitted through the reattachment shock into the downstream TBL. The flow response consists of two-dimensional amplified planar waves propagating downstream with sustained amplitude. Increasing shock-oscillation amplitudes progressively enhance this phenomenon, while increasing frequencies, within the DSMC-predicted range, are found to promote a greater disturbance amplification, with amplitudes larger by 50% compared to lower frequencies. This indicates a high susceptibility of the wave transmission mechanism to the shock-oscillation frequencies. Conversely, the region between separation and reattachment shock is found to be sensitive to frequencies different from those of the shock oscillations. This previously unknown generation mechanism of a two-dimensional planar wave system within the TBL is altogether absent when the impinging shock is steady.

Laser-produced plasmas (LPP) are used for numerous applications, including analytical techniques, as surrogates for high explosions, and in nanoparticle generation. LPP is a dynamic system where temperature and density vary with space and time. Temperature of LPP can exceed 10,000 K at the earliest times, producing strong spatial gradients and variable optical thickness that complicate radiative transfer modeling. Conventional plane-parallel approximations are limited to simplified geometries and cannot capture multidirectional photon transport in expanding plumes. A photon Monte Carlo (PMC) solver is employed to solve the radiative transport equation (RTE) in spatially varying plasma domains by stochastically tracing photon bundles.

Magnetic nozzles (MNs) represent an innovative approach to electric propulsion [1]. Unlike conventional nozzles with solid walls, MNs employ magnetic fields to control plasma flow, generating thrust by accelerating ions using thermal electrons (typically in the tens of eV range). Plasma lenses (PL) can be incorporated into MN designs to focus and shape the magnetic field lines, enhancing the ion beam collimation through magnetized electrons [2]–[3]. These lenses, implemented through carefully positioned electromagnetic coils with adjustable currents or permanent magnets, can effectively control the plasma expansion rate that increase the conversion of electron thermal energy into ion kinetic energy. The lens-modified magnetic topology keeps electrons magnetized for extended distances, generating ambipolar electric fields that guide and focus the heavier, partially magnetized ions along the desired trajectory, creating a more collimated plasma plume, which can potentially increase the thrust gain.

Ionic liquids are stable propellants that are candidates for use with electrospray propulsion systems. Ionic liquids such as [EMIM] [BF4] can be ionized by electric fields to produce emissions through various mechanisms including fragmentation. Processes like fragmentation can lead to reduced efficiency or erosion and need to be modeled at high-fidelity to develop a relationship between operating parameters such as the strength of the applied electric field and the resulting plume composition. By introducing a nanopore that allows us to maintain a curved liquid-vacuum interface, we are able to replicate the meniscus that forms at the tip of an electrospray emitter where ion emissions are produced when the applied electric field is able to overcome the surface tension of the liquid. Using ensembles of molecular dynamics simulations, it is possible to quantify the emissions produced by applying uniform electric fields of 1, 2, 4, or 6 V/nm for over 2 ns. These simulations reveal that the number of emissions and cumulative charge increase with the applied electric field. As the magnitude of the electric field increases from values of 1 or 2 V/nm to 4 or 6 V/nm, the predominant emission mechanism changes from heavier species such as dimers to lighter monomers. This atomistic model shows that as the electric field increases, it becomes easier to overcome the surface tension forces on the oscillating chains of ions that emerge at the liquid-vacuum interface to produce ion emissions. Lighter monomers can be cleaved from these chains more easily with stronger applied electric fields leading to fewer dimer and trimer emissions overall. The emissions counts and cumulative charge profiles computed from these simulations reveal that by controlling the applied electric field, it is possible to bias the ion emissions toward fewer dimer or trimer emissions. This is crucial to preventing the emission of heavier ion species that may fragment to create charged products that move at reduced velocities or neutral [EMIM] [BF4] that cannot be accelerated further by the electric field.

This paper presents an overlay-based one-way coupled Eulerian–Lagrangian computational approach designed to investigate the dynamics of particulate phases in extreme high-speed, high-altitude flight conditions characterized by very low particulate mass loading. Utilizing the Direct Simulation Monte Carlo method to generate accurate gas flow fields, this study explores two canonical hypersonic flow systems. First we focus on the hypersonic flow over a sphere-cone, revealing the formation of dust-free zones for small particulate diameters and describing the particulate interaction with gas shocks. As particulate diameter and flight speed increase, the characteristics of the particulate phase evolve, leading to the emergence of distinctive features such as high particulate concentration bands or regions void of particulates. Subsequently, the investigation considers flow over a double-cone, emphasizing the behavior of particulate phases in separated vortex-dominated systems where particulate-inertia-driven interactions with vortices result in unique particulate-free zones in the vicinity of the primary and secondary vortices. Additionally, the paper addresses the importance of using realistic fractal-like particulate shapes and demonstrates that the shape effect tends to decelerate the fractal aggregates and trap them along the boundaries of the primary vortex. This research contributes to a deeper understanding of particulate phase dynamics in extreme flight conditions, offering insights relevant to aerospace and aerodynamic applications.

Three-dimensional (3-D), fully kinetic particle-in-cell (PIC) simulations using the CHAOS code are conducted to investigate the expansion dynamics of magnetized mesothermal plasma plumes under various magnetic field topologies. The simulations incorporate static magnetic fields, including divergent and axially uniform configurations, mapped onto the Electric Forest of Trees (E-FOT) grids in CHAOS to explore their effects on plasma plume collimation. This study examines the influence of field strength and topology on plume behavior by tracking the plume front's time evolution, analyzing macroparameters across cross-sections, evaluating electron thermal energy losses, and characterizing non-Maxwellian electron velocity distribution functions (EVDFs). The study demonstrates that uniform magnetic fields with strengths exceeding 150 G significantly enhance plume collimation, resulting in plume front speeds at least one ion acoustic speed (1 cs(0)) higher than those observed in unmagnetized or diverging field cases. Additionally, improved collimation in uniform fields reduces electron thermal energy losses by 50% and 60% compared to the losses observed in diverging field and unmagnetized cases, respectively.

Recent measurements of nitric oxide (NO) infrared emission from a hypersonic shock suggest that this spectral region may provide important information about nonequilibrium flow chemistry. This work considers a number of fundamental aspects related to the modeling of the spatial distributions of NO vibrational states in the ground electronic state that need to be considered in the interpretation of such experiments. The hypersonic steady state stagnation, expansion, and wake flow regions over a cylinder, a test article that can be employed in ground based measurements, is examined using the direct simulation Monte Carlo (DSMC) approach. Using quasi-classical trajectory derived relaxation cross sections for the most important vibrational relaxation mechanism of NO–O, we observe that the faster rates, compared to [Formula: see text], lead to vibrationally colder NO molecules in the expansion regions of the flow with the maximum decrease in NO vibrational temperatures close to 400 K. We propose a new collisional radiative model to characterize the state-to-state transitions of vibrational states of NO through collisional and radiative processes and compare the difference in NO vibrational state populations obtained when it is implemented directly into the DSMC versus an overlay, quasi-steady state, and Boltzmann approaches. Finally, using those NO vibrational state populations, we perform emission simulations to quantify the differences in emission spectra resulting from the use of Boltzmann and non-Boltzmann distributions for vibrational state populations of NO.

This work investigates the influence of slip boundary conditions on modal stability analysis of compressible Couette flow at two distinct Mach numbers. Slip boundary conditions are extracted directly from kinetic gas theory, using Direct Simulation Monte Carlo, and are applied to the ordinary differential equations governing compressible Couette flow. Modal stability analysis shows only qualitative differences between least stable eigensolutions for the respective cases with and without slip boundary conditions. This is in good agreement with findings of a similar study conducted for hypersonic boundary layers by Klothakis et al. (Theoretical and Computational Fluid Dynamics 36, 117-139, 2022).

In the study of gas-particulate multiphase systems, the flow of high-speed gas through a distribution of solid particulates is of utmost importance. While these aerodynamically interacting systems have been extensively studied for low-speed gas flows in the gas continuum regime, less attention has been given to high-speed systems where non-continuum effects are significant due to the high flow gradients. To address this, the flow of rarefied gas through an aerodynamically interacting monodisperse spherical particle system is studied using the Direct Simulation Monte Carlo (DSMC) gas-kinetic approach. Since the method provides the best resolution of shocks at supersonic Mach numbers it is used to classify the weak separated shocks and strong collective shocks in these systems based on particle spacing in a two-particulate system at different orientation angles. The study used the two-particle system to help analyze more complex particle distributions of volume fractions, 1%, 5%, and 15%, exposed to gas flows in the slip and transitional gas regime for a free-stream Mach number range of 0.2 <Ma(infinity)< 2.0. We observe that the weak separated shocks in the 1% distribution allow a higher degree of gas penetration and shock-particle interactions or "hypersonic-surfing", exposing a major fraction of the particulates to higher force magnitudes. In contrast, the strong collective shock in the 5% and 15% distributions only generates high particulate forces on the flow-facing particles. Finally, a simple stochastic model is proposed for use in large-scale Eulerian-Lagrangian simulations that captures the non-monotonic behavior of average drag and force variability generated by the complicated gas particulate interactions in the compressible gas regime.

Gridded ion thrusters are tested in ground vacuum chambers to verify their performance when deployed in space. However, the presence of high background pressure and conductive walls in the chamber leads to facility effects that increase uncertainty in the performance of the thruster in space. To address this issue, this study utilizes a fully kinetic simulation to investigate the facility effects on the thruster plume. The in-chamber condition shows a downstream neutral particle density 100 times larger than the in-space case due to ion neutralization at the wall and limited vacuum pump capability, resulting in a significant difference in the density and distribution of charge-exchange ions. The flux, energy, and angle of charge-exchange ions incident on the chamber wall are found to be altered by the electron sheath, which can only be simulated by the fully kinetic approach, as opposed to the conventionally used quasi-neutral Boltzmann approach. We also examine the effect of back sputtering, another important facility effect, and find that it does not necessarily require a fully kinetic simulation as the incident flux and energy of the sampled charge-exchange ion are negligibly small. Finally, we demonstrate that the carbon deposition rate on the thruster is significantly influenced by the angular dependence of the sputtered carbon, with a nearly 50% effect.

An overlay-based one-way coupled Eulerian-Lagrangian solver is used to simulate dilute hypersonic gas-solid flows characterized by low particulate mass loading. Utilizing the Direct Simulation Monte Carlo (DSMC) method to generate accurate gas fields, this study explores flows over a canonical sphere-cone geometry. The general gas-solid phase coupling phenomena in the multiphase flow systems are identified and the effects of flight altitude and speed on the system are evaluated. This study improves understanding of the complex solid-phase dynamics in extreme multiphase flow environments, providing valuable insights to aerospace and aerodynamic applications.

Direct Simulation Monte Carlo (DSMC) was utilized to calculate hypersonic flow over an isolated roughness element. Two separate data-driven methods were employed to identify the least-stable global flow eigenmode downstream of the roughness element, revealing a connection between oscillations present in the shock layer, detached shear layer, and vortical structures behind the roughness. Furthermore, significant oscillations were observed in the cross-flow velocity component of the eigenmode. The analysis also demonstrated the impact of the shock layer on the least stable eigenmode of the flow.

Through an all-atom molecular dynamics approach, a model of a bulk ionic liquid in the presence of an applied electric field is developed to study the relationship between the pressure associated with a mass flow through an emitter, the applied electric field strength, and the resulting ion emission rates for 1-Ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]). In these simulations, a force is applied to a rigid wall located below the bulk ionic liquid placed in a domain that is laterally periodic and fixed in the Z-direction. A nanopore placed above the bulk ionic liquid allows molecules to form a convex meniscus due to the intrinsic surface tension of the ionic liquid. When an electric field is applied to the ionic liquid, ion emissions are produced from the meniscus as long as the force on the liquid due to the field is able to overcome the surface tension of the [EMIM][BF4]. As a pressure differential is applied and the electric field strength is varied, the emission rate changes, and a relationship is developed for the adjustable parameters and the resulting flow of ions. Experimental work on secondary mass flux was carried out in conjunction with the simulations. A diagnostic for measuring ejected mass at a polar angle from a bombarded target was developed and measurements were made to validate the methodology and characterize the energy dependence and angular profile. Mass flux yields from a high voltage primary plume dropped from 7.304x10(-14) ng/ion-sr at a polar angle of 35 degrees to 3.645x10(-14) ng/ion-sr at 80 degrees. When spraying a low voltage plume, the yields were relatively constant across the set of polar angles, with values ranging from 7.391x10(-14) ng/ion-sr to 8.828x10(-14) ng/ion-sr. Results indicate that the high energy plume produces a secondary plume with less mass than atomic carbon sputtering nickel, but more mass than if the carbon atoms simply bounced off the surface of the target.

A quantitative assessment of carbon transport during ground-based testing of high-power electric propulsion (EP) thrusters is essential for accurate lifetime assessment. Novel experimental carbon tracking techniques based on isotopic labeling of 13C, in concert with Direct Simulation Monte Carlo (DSMC) plasma simulation techniques, have added the capability to elucidate the effects of carbon contamination on critical EP surfaces. Here, a multiscale sputtering simulation framework is used to inform and validate these carbon transport experimental-simulation models. Specifically, molecular dynamics (MD) simulations, upscaled to Monte Carlo (MC) simulations are adopted to quantify the relevant macroscopic sputtering properties (sputter yield, angle, and energy) of carbon under krypton ion bombardment at 300 eV. The multiscale sputtering simulation is able to capture the smoothening behavior observed from post-sputtered surface analysis of a 13C pellet from a hall thruster. The sputtering properties from our MD-MC models will be used to inform the boundary conditions of the DSMC-based carbon transport model.

Supersonic and hypersonic flow over several compression ramp configurations were considered in this work. The scaled angle of triple deck theory was used as the main parameter to characterize the different flowfields under varying free stream conditions. It was shown that with increasing scaled angle, the separation length and recirculation strength increase, as expected. Nevertheless, the stability of the single recirculation is not characterized solely by the magnitude of the scaled angle since sometimes smaller scaled angle values caused single recirculation regions to break down into smaller recirculation regions in the separation bubble whereas separation bubbles with higher scaled angles seem to be able to sustain a single recirculation. We show that the two-dimensional separation bubble is more sensitive to Reynolds and Mach numbers than the scaled angle value.