Observed Effects of a Changing Step-Edge Density on Thin-Film Growth Dynamics

We grew SrTiO3 on SrTiO3 [001] by pulsed laser deposition, while observing x-ray diffraction at the (0 0 .5) position. The drop dI in the x-ray intensity following a laser pulse contains information about plume-surface interactions. Kinematic theory predicts dI/I = -4sigma(1-sigma), so that dI/I depends only on the amount of deposited material sigma. In contrast, we observed experimentally that |dI/I| < 4sigma(1-sigma), and that dI/I depends on the phase of x-ray growth oscillations. The combined results suggest a fast smoothing mechanism that depends on surface step-edge density.Comment: 4 figure

Lunar plume-surface interactions using rarefiedMultiphaseFoam

Understanding plume-surface interactions is essential to the design of lander modules and potential bases on bodies such as the Moon, as it is important to predict erosion patterns on the surface and the transport of the displaced regolith material. Experimentally, it is difficult to replicate the extra-terrestrial conditions (e.g. the effects of reduced gravity). Existing numerical tools have limited accessibility and different levels of sophistication in the modelling of regolith entrainment and subsequent transport. In this work, a fully transient open source code for solving rarefied multiphase flows, rarefiedMultiphaseFoam, is updated with models to account for solid-solid interactions and applied to rocket exhaust plume-lunar regolith interactions. Two different models to account for the solid-solid collisions are considered; at relatively low volume fractions, a stochastic collision model, and at higher volume fractions the higher fidelity multiphase particle-in-cell (MPPIC) method. Both methods are applied to a scaled down version of the Apollo era lunar module descent engine and comparisons are drawn between the transient simulation results. It is found that the transient effects are important for the gas phase, with the shock structure and stand-off height changing as the regolith is eroded by the plume. Both models predict cratering at early times and similar dispersion characteristics as the viscous erosion becomes dominant. In general, the erosion processes are slower with the multiphase particle-in-cell method because it accounts for more physical effects, such as enduring contacts and a maximum packing limit. It is found that even if the initial volume fraction is low, the stochastic collision method can become unreliable as the plume impinges on the surface and compresses the regolith particles, invalidating the method’s assumption of only binary collisions. Additionally, it is shown that the breakdown of the locally free-molecular flow assumption that is used to calculate the drag and heat transfer on the solid particles has a strong influence on the temperatures that the solid particles obtain

Underexpanded Supersonic Plume Surface Interactions: Applications for Spacecraft Landings on Planetary Bodies

Numerical and experimental investigations of both far-field and near-field supersonic steady jet interactions with a flat surface at various atmospheric pressures are presented in this paper. These studies were done in assessing the landing hazards of both the NASA Mars Science Laboratory and Phoenix Mars spacecrafts. Temporal and spatial ground pressure measurements in conjunction with numerical solutions at altitudes of approx.35 nozzle exit diameters and jet expansion ratios (e) between 0.02 and 100 are used. Data from steady nitrogen jets are compared to both pulsed jets and rocket exhaust plumes at Mach approx.5. Due to engine cycling, overpressures and the plate shock dynamics are different between pulsed and steady supersonic impinging jets. In contrast to highly over-expanded (e 5 (lunar atmospheric regime), the ground pressure is minimal due to the development of a highly expansive shock structure. We show this is dependent on the stability of the plate shock, the length of the supersonic core and plume decay due to shear layer instability which are all a function of the jet expansion ratio. Asymmetry and large gradients in the spatial ground pressure profile and large transient overpressures are predominantly linked to the dynamics of the plate shock. More importantly, this study shows that thruster plumes exhausting into martian environments possess the largest surface pressure loads and can occur at high spacecraft altitudes in contrast to the jet interactions at terrestrial and lunar atmospheres. Theoretical and analytical results also show that subscale supersonic cold gas jets adequately simulate the flow field and loads due to rocket plume impingement provided important scaling parameters are in agreement. These studies indicate the critical importance of testing and modeling plume-surface interactions for descent and ascent of spacecraft and launch vehicles

Experimental and Computational Investigation of Plume Surface Interactions in Vacuum Microgravity

Plume surface interactions (PSI) are caused by rocket exhaust impinging on planetary surfaces. PSI-induced environmental changes pose hazards to spacecraft and astronauts; thus, it is crucial to understand the gas-particle dynamics of these systems. We have conducted novel experimental and computational work to study PSI effects in relevant vacuum microgravity environments. To study flow effects and regolith instability we developed a computational model that describes the gas flow through a porous medium based on Darcy\u27s Law. This flow depends on regolith properties, and the resulting subsurface pressure distribution is used to estimate ejecta mass. We find flow behaviors and the resulting ejecta are significantly affected by the surface pressure distribution, pulse duration, and material properties. We have also developed an experimental apparatus, the Gas Regolith Interaction Testbed (GRIT), for studying PSI in vacuum microgravity in the UCF Center for Microgravity Research Drop Tower. It consists of a small, cylindrical vacuum chamber in which a cold gas jet interacts with a bed of regolith simulant. Video data is analyzed to determine PSI trends based on gravity level, nozzle distance, simulant type, and plume duration. We observe PSI effects ranging from perturbation of the granular media to ejection of the entire simulant mass. Phenomena are significantly more pronounced for experiments conducted at microgravity than at Earth gravity (1g). We measure peak ejecta velocities up to tens of m/s, and note how particle properties, jet distance, and pulse duration affect ejecta angle and cratering depth. Our numerical and experimental results have implications for the validity of existing studies of PSI that are conducted in 1g and under ambient conditions, and can be used to inform modeling, lander design, and risk assessment for future missions that will aim to land on or interact with planetary surfaces

Modeling Particle-Laden Compressible Flows with an Application to Plume-Surface Interactions

During planetary descent, rocket plumes fluidize and eject surface granular matter. Consequently, ejected matter has been shown to obscure the landing site and even collide with the lander, causing serious damage. Given the high risk and cost of space exploration, the challenges associated with plume-surface interactions (PSI) are capable of jeopardizing future missions. The erosion, fluidization, and ejecta of granular matter during PSI occurs under transonic/supersonic, high Reynolds number conditions. These flow conditions pose significant challenges in both experimental and numerical analyses. To date, accurate and predictive physics-based models of PSI at relevant landing conditions do not exist. The objective of this project is to develop high-fidelity simulation capabilities to model compressible gas-particle flows at conditions relevant to PSI. To start, a rigorous derivation of the volume-filtered (locally averaged) compressible Navier--Stokes equations is presented for the first time. This derivation reveals many unclosed terms, for which models are either non-existent or not valid under the regimes of interest. To this end, key terms including pseudo-turbulent kinetic energy and pseudo-turbulent Reynolds stresses, are isolated and modeled via a transport equation in a new high-order finite difference Eulerian-Lagrangian framework. A new immersed boundary method is introduced to generate highly resolved, multi-particle simulations for model closure development. Using the proposed immersed boundary method and the Eulerian--Lagrangian framework, high-fidelity PSI simulations are performed. Single-phase jet impingement on flat surfaces is first shown for validation of the flow conditions. The work is then extended to PSI over a granular bed. For this case, it is shown that that ejected particles can exceed sonic speeds at high particle Reynolds numbers while the majority of the granular bed experiences subsonic particle Mach numbers. In addition, granular temperature is found to be most prevalent in region of high shear during crater formation. The uniqueness of this work lies in the combination of first principles physics and numerics to generate a modeling framework to improve predictions of plume-surface interactions for future missions involving entry, descent, and landing on planetary and satellite bodies.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/169969/1/grshall_1.pd

Plume-Surface Interactions due to Spacecraft Landings and The Discovery of Water on Mars.

Pulsed supersonic jets or rocket plumes have different surface flow physics than steady jets, in particular in tenuous atmospheres such as that of Mars where jets are collimated over large distances compared to their diameters. We show that plate shock formation and collapse during each cycle of pulsed jets impinging on a surface causes large pressure fluctuations capable of producing extensive erosion during spacecraft landings. Here, we study the pressure loads and erosion caused by pulsed jets of the Phoenix spacecraft on the surface of Mars and its implications to engineering and science. While steady thruster jets caused only modest surface erosion during the landings of previous spacecraft on the moon and Mars, the pulsed jets from Phoenix led to extensive alteration of its landing site on the martian arctic, exposed a large fraction of the subsurface water ice under the lander, and led to the discovery of evidence for liquid saline water on Mars. We report the discovery of the ‘explosive erosion’ process that led to this extensive erosion and evidence for liquid water. We show that the impingement of supersonic pulsed jets fluidizes porous soils and forms cyclic shock waves that propagate through the soil producing erosion rates more than an order of magnitude larger than that of other jet-induced processes. The understanding of ‘explosive erosion’ allows the calculation of bulk physical properties of the soils altered by it, provides new insights into the behavior of granular flow at extreme conditions, and explains the alteration of the Phoenix landing site at the northern arctic plains of Mars. We then show new photometric evidence that the Phoenix spacecraft imaged liquid saline water in the arctic, and that deliquescence causes liquid water to sporadically flow in the polar region. This finding also corroborates the hypothesis that the thermodynamics of freezing/thaw cycles leads to the seasonal formation of liquid saline water where ice and salts exist near the surface. Finally, we show broadband spectral signature of liquid brines in flow-like and pond-like features on defrosting polar dunes. This has important implications for geology, geochemistry and the habitability of Mars.Ph.D.Atmospheric and Space SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/78975/1/manishm_1.pd

Study of supersonic nozzle flows in low-pressure environments: starting jets and lunar plume-surface interactions

Supersonic nozzle flows play an important role in aerospace engineering, e.g. controlling motions, attitudes, and orbits of space vehicles using various propulsion systems. Supersonic nozzle flows include free nozzle flows and restricted nozzle flows, such as plume-surface interactions if a surface obstructs the flow propagation. When compressed gas is discharged from a nozzle into a low-pressure environment in the case of free nozzle flows, the shock wave diffracts around the nozzle lip and a vortex loop forms. These phenomena have attracted much attention in the continuum flow regime, but how the shock diffraction and vortex behave under rarefied flow conditions has received less attention. Understanding transient flow in rarefied conditions is helpful for increasing thrust vector control and avoiding potential contamination and erosion of spacecraft surfaces. Furthermore, comprehending plume-surface interactions is critical for the design of lander modules and future bases on bodies such as the moon, as it is necessary to anticipate surface erosion patterns and the transport of displaced regolith material. Extraterrestrial conditions are difficult to recreate experimentally (e.g. the effects of low gravity, strong radiation and extreme temperature difference). Available numerical techniques for modelling regolith entrainment and subsequent movement suffer from limited accessibility and different levels of sophistication. In this thesis, a design for an open-ended shock tube connected to a vacuum chamber is presented. This is used to release a shockwave into a low-pressure environment and study the subsequent vortex ring formation as the gas diffracts around the shocktube exit. Schlieren visualisation and pressure measurements of the vortex ring formation are conducted. The flow structure degenerates through a decrease in the strength of the embedded shock waves and an increase in their thickness, and the counter-rotating vortex ring when the environmental pressure decreases. The existence of the vortex ring is confirmed through spectral analysis when the environmental pressure is as low as 1.0kPa. Due to limitations with experimental measurement equipment and techniques, the shock wave diffraction problem should be complemented with numerical techniques. A program to generate ensemble-averaged direct simulation Monte Carlo (DSMC) results is designed. Computational fluid dynamics (CFD) and ensemble-averaged DSMC methods are implemented to simulate the formation of a two-dimensional vortex loop due to shock wave diffraction around a 90◦ corner. The influence of the Mach number and rarefaction on the development and growth of the vortex loop are studied. A concept, called rorticity, was used to investigate the transient structures of vortex loops. The simplification of the internal structure of vortex loops and postponement of the vortex loop formation due to the increase of the rarefaction level are confirmed. Two properties from the decomposition equation of vorticity to quantify the vortex strength; rorticity flux (i.e. representing the vortex rotational strength), and the shear vector flux (i.e. representing the vortex shear movement strength), are derived. A mutual transformation relationship between the rorticity and shear vectors has been identified, suggesting that this concept can be employed to better explain vortex flow phenomena. It is found that the increase of the Knudsen number thickens the Knudsen layer, causing the failure of the generation of the vortex sheet and the subsequent formation of vortex loops. A new solver based on dsmcFoamPlus – rarefiedMultiphaseFoam, is developed for solving rarefied multiphase flows. The solver is extended to include a two-way coupling model and a particle phase change model. Additionally, the solid stochastic collison model and the multiphase nparticle-in-cell (MPPIC) method for solving dilute and dense granular flows, respectively, have been implemented in the new solver. The models mentioned are rigorously benchmarked against analytical solutions and previous results in the literature. The benchmarking results of the two-way coupling method show excellent agreement with analytical results. The results of a reproduced uniform gas-solid flow and a purely gravity-controlled granular flow sedimentation agree well with previous numerical results in the literature. A solid particle is allowed to experience a physical and continuous phase change and diameter variation using the updated phase change model. Finally, the rarefiedMultiphaseFoam solver is used to simulate two lunar plume-surface interaction (PSI) cases using the stochastic collision model and the MPPIC method, respectively. Both methods are applied to a scaled down version of the Apollo era lunar module descent engine and comparisons are made between the two simulation results. The results show that the transient effects are essential to both the gas and solid phase evolution and the entrained dust particles significantly influence the evolution of the gas flow. In the PSI simulations, the MPPIC method is more reliable than the stochastic collision method because it takes enduring contacts and the close-packing limit into account. Furthermore, it is identified that the breakdown of the locally free-molecular flow assumption has a significant impact on the solid particle temperatures

Rocket Plume Interactions for NASA Landing Systems

No abstract availabl

Gas-Granular Simulation Framework for Spacecraft Landing Plume-Surface Interaction and Debris Transport Analysis

The Gas-Granular Flow Solver (GGFS) multi-phase flow computational framework has been developed to enable simulations of particle flows complex extra-terrestrial regolith materials. Particle flows of interest include the damage of unprepared landing sites from rocket plume impingement on Moon, Mars, and asteroids. The flow solver implements an Eulerian-Eulerian two-fluid model with fluid representation of the gas phase and granular phase to avoid the need to model billions of particle interactions. The granular phase is modeled as an Eulerian fluid with constituent physics closure models derived from first-principle Discrete Element Model (DEM) particle interaction simulations that capture the complex, non-linear granular particle interaction effects. Granular phase constituent models have been developed and integrated that address the complex, non-linear granular material mechanics complexities resulting from both: the irregular, jagged particle shapes and poly-disperse mixture effects encountered in extra-terrestrial regolith, with lunar regolith as the extreme. The GGFS capabilities are being integrated into a proven NASA plume-surface interaction and debris transport simulation framework featuring the Loci/CHEM CFD program and Debris Transport Analysis (DTA) post-processing tools for applications in robotic and human Moon and Mars lander development. Integration of the three simulation tool components. Loci/CHEM, GGFS, and DTA, into a coordinated simulation framework will enable time-accurate spacecraft landing simulations that account for the alteration of the landing surface through plume-induced cratering and the resulting redirection of plume impingement flow and debris transport. Initial implementation of this simulation framework and application examples will be presented