Numerical Simulations of Granular Physics in the Solar System

Granular physics is a sub-discipline of physics that attempts to combine principles that have been developed for both solid-state physics and engineering (such as soil mechanics) with fluid dynamics in order to formulate a coherent theory for the description of granular materials, which are found in both terrestrial (e.g., earthquakes, landslides, and pharmaceuticals) and extra-terrestrial settings (e.g., asteroids surfaces, asteroid interiors, and planetary ring systems). In the case of our solar system, the growth of this sub-discipline has been key in helping to interpret the formation, structure, and evolution of both asteroids and planetary rings. It is difficult to develop a deterministic theory for granular materials due to the fact that granular systems are composed of a large number of elements that interact through a non-linear combination of various forces (mechanical, gravitational, and electrostatic, for example) leading to a high degree of stochasticity. Hence, we study these environments using an N-body code, pkdgrav, that is able to simulate the gravitational, collisional, and cohesive interactions of grains. Using pkdgrav, I have studied the size segregation on asteroid surfaces due to seismic shaking (the Brazil-nut effect), the interaction of the OSIRIS-REx asteroid sample-return mission sampling head, TAGSAM, with the surface of the asteroid Bennu, the collisional disruptions of rubble-pile asteroids, and the formation of structure in Saturn's rings. In all of these scenarios, I have found that the evolution of a granular system depends sensitively on the intrinsic properties of the individual grains (size, shape, sand surface roughness). For example, through our simulations, we have been able to determine relationships between regolith properties and the amount of surface penetration a spacecraft achieves upon landing. Furthermore, we have demonstrated that this relationship also depends on the strength of the local gravity. By comparing our numerical results to laboratory experiments and observations by spacecraft we can begin to understand which microscopic properties (i.e., grain properties) control the macroscopic properties of the system. For example, we can compare the mechanical response of a spacecraft to landing or Cassini observations of Saturn's ring to understand how the penetration depth of a spacecraft or the complex optical depth structure of a ring system depends on the size and surface properties of the grains in those systems

Production and Characterization of Cut Resistant Acrylic/Copolyaramid Fibers Via Bicomponent Wet Spinning

A composite fiber system consisting of a sheath core bicomponent polymer fiber loaded with hard ceramic particles was developed and characterized for use in cut protective clothing. The core component was comprised of a copolyaramid in order to provide high base cut resistance. An acrylic-copolyaramid polymer blend was used for the sheath component to improve processability and provide potential benefits such as dyeability. Lastly, aluminum oxide particles were incorporated into the fiber core to deflect and deform the cutting edge, further improving cut resistance. A series of designed experiments was used to explore the effects of the wet spinning and heat treatment processes on the structure and properties of the bicomponent fiber. Cut strength of the as-spun fibers was highest when the coagulation rate was slow, promoting the formation of a dense, macrovoid free fiber structure. Upon drawing, fibrillar domains developed within the fiber, further improving cut performance. Cut strength was greatly improved by the heat treatment process despite the fibers becoming highly anisotropic. Addition of the hard particle fillers to the bicomponent fibers showed a decrease in cut strength at the fiber level but nearly doubled the cut strength of resulting fabrics. Finally, the processability of the particle loaded bicomponent fibers was evaluated

The Development and Implementation of Numerical Tools for Investigation into the Granular Dynamics of Solid Solar System Bodies

The work advanced in this thesis joins together the disciplines of planetary science and granular physics. Grain dynamics have played a prominent role in the evolution of our Solar System from planetesimal formation billions of years ago to the surface processes that take place today on terrestrial planets, moons, and small bodies. Recent spacecraft images of small Solar System bodies provide strong evidence that the majority of these bodies are covered in regolith. This regolith ranges in size from the fine powder found on the Moon to large rocks and boulders, like the 27 m Yoshinodai boulder on the small asteroid, Itokawa. Accordingly, the processes that take place on the solid bodies of the Solar System vary widely based upon the material properties of the regolith and the gravitational environments on their surfaces. An understanding of granular dynamics is also critical for the design and operations of landers, sampling devices and rovers to be included in space missions. Part of my research is concerned with the development of numerical tools that have the ability to provide explanations for the types of processes that our spacecraft have observed. Granular processes on Earth are incredibly complex and varied, and constitute an enormous field of study on their own, with input taken from across the broad disciplines of engineering and the physical sciences. In micro-gravity, additional forces, which on Earth are relevant only to micron-size particles or smaller, are expected to become important for material up to the size of large rocks, adding further complexity. The numerical tools developed in this work allow for the simulation of grains using an adaptation of the Soft-Sphere Discrete Element Method (SSDEM) along with implementations of cohesive forces between particles into an existing parallel gravity tree code

A Physical Model of Wind-Blown Sand Transport

Eolian saltation, the transport of sand by the wind, involves a variety of physical processes. A fundamental understanding of saltation requires an analysis starting from the level of the individual sand grain. The complexity of this nonlinear dynamical system compels us to divide the problem into more easily handled decoupled components: the saltating grain-bed impact process, the force of the wind on individual grains, the determination of the wind profile from the spatially averaged force of the moving grains on the air, and the formation of small-scale bedforms: ripples. The impact of a moving sand grain with a bed of sand is studied with two-dimensional dynamical computer simulations and an experiment propelling single grains onto a sand bed. We find that the result of the impact may be described in terms of the rebound of the incident particle and the ejection of bed grains. The bed grain ejections originate from a localized area around the impact point, and at steps in the surface (elevation changes of one grain diameter) which are more widely distributed; these surface steps we term brinks (downstream-facing) and anti-brinks (upstream-facing). A model for steady-state saltation is proposed which incorporates both aerodynamics and the mechanics of the grain-bed impacts, and balances the losses of saltating particles on impact with the bed by gains due to impact generated bed grain ejections. This model does not require data on blowing sand. Results are obtained which qualitatively agree with existing data. Quantitative tests will require new experiments. We argue that grain-bed impacts, not fluid stresses, are the means for entraining grains in steady-state eolian saltation. The development of sand surface topography is viewed as a result of surface grain transport (reptation) driven by the impact of high-energy saltating grains onto the bed. The collision and merger of small collections of sand, proto-ripples, lead to the asymptotic development of uniform ripples from an initially smoothed surface. The limiting wavelength is pictured as being determined by statistical fluctuations in the saltating impact flux and/or the shortening of the saltation shadow zone below the mean reptation length during a collision between two ripples. Field observations of developing ripple cross-sectional shapes confirm these ideas qualitatively, and rough calculations of limiting wavelengths agree with existing data.</p

Mixing and Demixing Processes in Multiphase Flows With Application to Propulsion Systems

A workshop on transport processes in multiphase flow was held at the Marshall Space Flight Center on February 25 and 26, 1988. The program, abstracts and text of the presentations at this workshop are presented. The objective of the workshop was to enhance our understanding of mass, momentum, and energy transport processes in laminar and turbulent multiphase shear flows in combustion and propulsion environments

FAILURE MECHANISMS OF ULTRA HIGH MOLAR MASS POLYETHYLENE SINGLE FIBERS AT EXTREME TEMPERATURES AND STRAIN-RATES

The effects of temperature and strain-rate on the mechanical properties of Ultra High Molar Mass Polyethylene (UHMMPE) single fibers was investigated at eleven temperatures from room temperature (20 °C) to the orthorhombic-hexagonal phase transition (148 °C) and at six strain-rates from quasi-static (10-3 s-1) to dynamic (103 s-1). Dimensional analysis of ballistic limit tests using has shown an underperformance of materials comprised of UHMMPE fibers. A possible explanation is the relatively low melting temperature of UHMMPE fibers (~150 °C) in comparison to other fiber materials, such as poly-aramids (~450 °C). The mechanical properties of UHMMPE single fibers were investigated through a series of 437 tensile tests at 66 temperature-strain-rate combinations. Changes in stress-strain curve shapes were observed with respect to temperature and strain-rate. The transition of stress-curve shape with increasing temperature was observed to be pseudo-brittle, plateauing, necking, and non-failure and transitions between these phases were observed within a strain-rate dependent temperature range. For low and intermediate strain rates, a temperature and strain-rate equivalence is observed: a decadal increase of strain-rate is mechanically equivalent to a ~20 °C decrease in temperature. Strain to failure for dynamic strain rates was invariant over the temperature range of this study. Strength and modulus properties were observed to decrease with increasing temperature and increase with increasing strain-rate. An orthorhombic to hexagonal phase transition occurs between 145 °C and 148 °C and a sudden decrease in strength and moduli was observed. The change in dominant stress-relieving mechanism is proposed. Chain slippage is dominant for the majority of conditions in this study except where scission and straightening are the dominant mechanism. At high temperatures for constrained fibers in the hexagonal phase, chain slippage occurs more frequently due to the trans to gauche conformation. Chain scission is only dominant moments before fiber failure and near the failure surface. Chain straightening is dominant at low strain (0 % to 0.5 %) and at temperatures greater than or equal to the necking temperatures for the quasi-static and intermediate strain-rates and at all temperatures for the dynamic strain-rates

LDEF: 69 Months in Space. Third Post-Retrieval Symposium, part 1

This volume (Part 1 of 3) is a compilation of papers presented at the Third Long Duration Exposure Facility (LDEF) Post-Retrieval Symposium. The papers represent the data analysis of the 57 experiments flown on the LDEF. The experiments include materials, coatings, thermal systems, power and propulsion, science (cosmic ray, interstellar gas, heavy ions, micrometeoroid, etc.), electronics, optics, and life science. In addition, papers on preliminary data analysis of EURECA, EOIM-3, and other spacecraft are included

NASA thesaurus. Volume 3: Definitions

Publication of NASA Thesaurus definitions began with Supplement 1 to the 1985 NASA Thesaurus. The definitions given here represent the complete file of over 3,200 definitions, complimented by nearly 1,000 use references. Definitions of more common or general scientific terms are given a NASA slant if one exists. Certain terms are not defined as a matter of policy: common names, chemical elements, specific models of computers, and nontechnical terms. The NASA Thesaurus predates by a number of years the systematic effort to define terms, therefore not all Thesaurus terms have been defined. Nevertheless, definitions of older terms are continually being added. The following data are provided for each entry: term in uppercase/lowercase form, definition, source, and year the term (not the definition) was added to the NASA Thesaurus. The NASA History Office is the authority for capitalization in satellite and spacecraft names. Definitions with no source given were constructed by lexicographers at the NASA Scientific and Technical Information (STI) Facility who rely on the following sources for their information: experts in the field, literature searches from the NASA STI database, and specialized references

Laboratory Studies of Hypervelocity Impacts on Solar System Analogues

Impact cratering and asteroid collisions are major processes throughout the Solar System. Although previous collision-related impact investigations exist (Flynn et al. 2015, Holsapple et al. 2002 and Burchell et al. 1998 are good examples), in the works covering this broad range of investigation, the targets are non-rotating (for the purposes of catastrophic disruption) and different temperature conditions are not considered (for impact cratering). Accordingly, I present experimental processes and data, regarding hypervelocity impact experiments into analogues of (1) rotating asteroids and (2) temperature dependant terrestrial planetary rock. During the course of this work, it was necessary to develop new apparatus and new experimental techniques such as three separate target holders to aid in both catastrophic disruption and heated impact projects, a 3-dimensional model analysis of craters and a completely new, statistically robust, technique to determine a complete crater profile called the KDM method where KDM is Kinnear-Deller-Morris. The main result from this work showed that during an asteroid impact collision where the asteroid is not rotating, the impact energy density for catastrophic disruption is Q*static = 1442 ± 90 J kg-1. However, when the target asteroid was rotating, the condition Q*rotation = 1097 ± 296 J kg-1. The mean value of Q* had thus reduced, but the spread in the data on individual experiments was larger. This leads to two conclusions. The mean value for Q*, based on measurements of many impacts, falls, due to the internal forces acting in the body which are associated with the rotation. This energy term reduction means that the amount of energy to instigate catastrophic disruption is lower and that a rotating asteroid is effectively weaker upon impact than a stationary asteroid. However, the spread in the results indicates that this is not a uniform process, and an individual result for Q* for a rotating or spinning target may be spread over a large range. For the temperature related impacts, as the targets were heated to approximately 1000 K, the target rocks showed an impact dependence more similar to a plastic phase-state than to solidus, due to being held close to temperatures associated with semi-plastic phases. Basalt impact craters displayed this relationship greatest with crater sizes becoming smaller at the higher temperature ranges but larger in the colder brittle solidus temperatures, partly explained in experiments by increased spallation

Nonequilibrium phase transition in binary complex plasmas

Komplexe Plasmen sind Systeme bestehend aus schwach ionisierten Gasen und mesoskopischen Partikeln. Partikel in einem Plasma erhalten ihre Ladung hauptsächlich durch den Fluß von Ionen und Elektronen auf denen Oberflächen. Abhängig von der Teilchengröße und den Plasmabedinungen kann die Ladung pro Teilchen mehrere tausend Elementarladungen betragen. Da das Hintergrundgas sehr dünn ist, können Partikelsysteme unabhängig von dem Plasma betrachtet werden. In vielen Fällen kann das Partikelwechselwirkungspotential als Yukawapotential angenähert werden, welches im Wesentlichen ein abgeschirmtes Coulombpotential ist. Kapitel 1 ist eine kurze Einleitung in die theoretischen Konzepte komplexer Plasmen. Aufgrund der Bedeutung des Mechanismus, beginne ich diese Arbeit mit der Diskussion der Teilchenladung für zwei verschiedene Situationen in Kapitel 2. Zunächst beschreibe ich ein einzigartiges Experiment, die "Coulomb-Explosion", zur Messung der Teilchenladung tief in der Plasmarandschicht. Ein Hybrid-Analyseverfahren, bestehend aus Teilchenverfolgung, MD und PIC Simulationen, wurde angewendet um die Ladung im Anfangsstadium der Explosion abzuschätzen. Dieses wird mit einer theoretische Methode zur Bestimmung der Partikelladung im Bulk-Plasma bei verschiedenen Entladungsfrequenzen ergänzt. Die Abhängigkeit der Partikelladung von der Entladungsfrequenz wird bei drei verschiedenen Drücken gezeigt. Das verwendete Modell ist hilfreich um die Veränderung der Teilchenladung in Abhänigkeit der Entladungsfrequenz abzuschätzen. Die hohe Teilchenladung und die damit verbundene abstoßende Teilchenwechselwirkung verhindern Partikelagglomeration. In Kapitel 3 stelle ich ein Experiment vor, in dem Partikelagglomeration durch selbst-angeregte Wellen induziert wird. Innerhalb der Wellen werden die Teilchen derart beschleunigt, dass das abstoßende Potential durch die erhöhte kinetische Energie überwunden werden kann. Die resultierenden Agglomerate werden mit einem "Long-Distance" Mikroskop überprüft. Im Folgenden stelle ich ein System binärer komplexer Plasmen vor. Unter bestimmten Bedingungen können monodisperse Partikel in einer Monolage eingefangen werden. Die Teilchen ordnen sich in einem Dreiecksgitter mit hexagonaler Symmetrie an. Dies ist als 2D Plasmakristall bekannt. Wenn ein sich bewegendes, einzelnes Teilchen einer anderen Spezies in das System eingeführt wird, verursacht es eine Störung des Kristallgitters. In Kapitel 4 werden die Untersuchungen der Wechselwirkung des Kristallgitters mit einem sich oberhalb des Gitters (stromaufwärts des Ionenflusses) befindlichen Teilchens diskutiert. Dieses zusätzliche Partikel erzeugt einen Mach-Kegel, da es sich mit einer Geschwindigkeit, schneller als der Schall in dem System bewegt. Das stromaufwärts befindliche Teilchen neigt dazu sich zwischen Reihen von Teilchen in dem Gitter zu bewegen, was als "Channeling-Effekt" bekannt ist. Wenn Teilchen einer Spezies eine Partikelwolke einer anderen durchdringen, bilden sowohl die durchfliessende als auch die durchflossene Teilchenwolke Kettenstrukturen ("Lanes") aus. In komplexen Plasmen ist die Wechselwirkung verschiedener Partikel immer stärker abstoßend als das geometrische Mittel der Wechselwirkung gleicher Partikel. Diese Asymmetrie in der gegenseitigen Wechselwirkung heißt "Positive nicht-Additivität". Deren Grad wird von dem nicht-Additivitäts Parameter bestimmt. In Kapitel 5 beschreibe ich zuerst die Ergebnisse von Langevin-Simulationen, um die Abhängigkeit der "Lane - Formation" von dem nicht-Additivitäts Parameters zu studieren. Weiterhin wurde die Rolle des Anfangszustands numerisch untersucht. Zusätzlich wurde eine Reihe umfassender Experimente zur "Lane - Formation" an Bord der Internationalen Raumstation (ISS) durchgeführt. Die Auswertung der Experimente konzentrierte sich auf die Struktur der durchflossenen Teilchen. Der Einfluss der Partikeldichten und -größe wurden untersucht. Das Studium zweier aufeinanderfolgenden Durchdringungen offenbarte einen "Memory-Effekt" in der Kettenstruktur. Zusätzlich wurde ein Übergang von freier "Lane-Formation" zu einem, von Entmischung dominierten, Zustand des Nichtgleichgewichtsystems innerhalb einer Experimentreihe beobachtet. Schließlich stelle ich einen ergänzenden Versuch zur "Lane-Formation" in erdgebundenen Experimenten vor. Die Schwerkraft wurde hier durch thermophoretische Kräfte kompensiert. In dieser Versuchsreihe konnten die durch unregelmässige Teilchengeschwindigkeiten und Inhomgenitäten in der durchflossenen Teilchenwolke entstehenden Nachteile erfolgreich überwunden werden. Mit diesem Modell-System kann die "Lane-Formation" im Detail untersucht werden und die Ergebnisse mit denen numerischer Simulationen und denen aus Experimenten in Kolloiden verglichen werden.Complex plasma is a system composed of weakly ionized gas and mesoscopic particles. The particles are charged mainly by absorbing ions and electrons in the plasma. The charge can reach several thousands of elementary charges depending on the particle size and plasma conditions. Since the background gas is dilute, we can treat the charged particles as a system independent from the plasma. In many cases, the interparticle interaction can be approximated as Yukawa potential, which is essentially a screened Coulomb interaction. A brief introduction covering some basic theoretical aspects of complex plasma is given in Chapter 1. Because of the importance of the particle charging mechanism, we start this thesis with studying particle charge under two situations in Chapter 2. First we introduce a unique experiment of Coulomb "explosion" to measure the particle charge deep in the sheath. A hybrid analysis method composed of particle tracking, molecular dynamics simulation and particle-in-cell simulation is applied to estimate the particle charge at the initial stage of the "explosion". Second we develop a theoretical method to calculate the particle charge in the bulk plasma with different discharge frequencies. We show the dependence of particle charge on the discharge frequency at three different gas pressures. The model is rather simple and thus we can use it for preliminary estimation of the change of particle charge as a result of changing the discharge frequency. Since the particles are heavily charged, the repulsive interparticle interactions prevent particle agglomeration. In Chapter 3 we introduce an experiment to realize particle agglomeration, which is induced by self-excited waves. In the waves the microparticles are accelerated so that the repulsive potential can be overcome by the kinetic energy. We verify the results by using a long-distance microscope. We then start to study a system of binary complex plasmas. Under certain conditions, monodisperse microparticles can be confined in a monolayer, self-organizing in a triangular lattice with hexagonal symmetry. This is known as 2D plasma crystal. If a moving single particle of another species is introduced into the system, it creates a disturbance on the crystal lattice. In Chapter 4, we study the interaction of the crystal lattice with a particle moving above it (upstream of the ion flow). This extra particle generates a Mach cone as it moves faster than the sound speed. It turns out that the upstream particle tends to move between rows of particles in the lattice layer, which is known as "channeling" effect. As particles of one species penetrate into a particle cloud of another species, both the penetrating particles and background particles form lane structure. For microparticles of two species, the interspecific interaction is always more repulsive than the geometric mean of two intraspecific interactions. This asymmetry in the mutual interaction is called "positive non-additivity" and its degree is measured by the non-additivity parameter. In Chapter 5 we first employ Langevin dynamics simulation to study the dependence of lane formation on positive non-additivity parameter. The role of initial condition has been also studied numerically. Second we comprehensively investigate a series of experiments on lane formation performed on board the International Space Station. In the experiments, we focus on the lanes formed by the background particles. The influence of the number density ratio and size ratio between two particle types has been studied. By investigating two consecutive penetrations, a "memory" effect of the first penetration on the second is revealed. In addition, we observe a crossover from free lane formation to a demixing dominated mode of the nonequilibrium system in the same set of experiments. Finally, we introduce an supplementary experiment to investigate the lane formation of the penetrating particles on the ground. The gravity is compensated by the thermophoretic force. The experiment successfully overcomes the drawbacks in terms of non-uniformity of the speed of the penetrating particles and inhomogeneity of the background particles. Using it as a model system we can investigate the process of lane formation in great detail and directly compare it with numerical simulations and experiments in colloidal suspensions