High Power Demonstration of a 100 kW Nested Hall Thruster System

The XR-100 team successfully completed high power system testing of a Nested Hall Thruster system made up of the X3 Nested Hall Thruster, a modular Power Processing Unit, and a 5 valve Mass Flow Controller as the culmination of work performed under a NASA NextSTEP program. The test campaign attained several key firsts, including highest directly measured thrust of an electric propulsion (EP) string, highest demonstrated current of an EP string, and highest power operation of an EP string at thermal equilibrium published to date. Most importantly, the XR-100 system testing demonstrated that a 100 kW-class Nested Hall Thruster system has comparable performance and behavior to current state-of-the-art mid power Hall Thrusters, validating that the heritage technology can be scaled up to 100+ k

High Power Demonstration of a 100 kW Nested Hall Thruster System

The XR-100 team successfully completed high power system testing of a Nested Hall Thruster system made up of the X3 Nested Hall Thruster, a modular Power Processing Unit, and a 5 valve Mass Flow Controller as the culmination of work performed under a NASA NextSTEP program. The test campaign attained several key firsts, including highest directly measured thrust of an electric propulsion (EP) string, highest demonstrated current of an EP string, and highest power operation of an EP string at thermal equilibrium published to date. Most importantly, the XR-100 system testing demonstrated that a 100 kW-class Nested Hall Thruster system has comparable performance and behavior to current state-of-the-art mid power Hall Thrusters, validating that the heritage technology can be scaled up to 100+ k

Der Übergang von Micro Scale Simulationen zu einer makroskopischen Beschreibung granularer Medien

Sitzt man am Strand und beobachtet Kinder beim Bau von Sandburgen oder Pferde die über den Sand galoppieren, wird sich kaum jemand Gedanken über eine mathematische Beschreibung des Sandes machen. Dennoch lohnen sich diese Gedanken. Sand gehört zu einer Gruppe von Materialien, die als granulares Material oder Schüttgut bezeichnet wird. Im alltäglichen Gebrauch fallen uns granulare Materialien meist nicht auf, obwohl bereits beim Frühstück das Kaffeepulver oder die Cornflakes Beispiele granularer Medien sind. Zucker, Tabletten oder Zahncreme sind weitere Beispiele granularen Materials im Haushalt. Auch im industriellen Umfeld sind Schüttgüter wie Erze, Zement oder auch Plastikgranulate omnipräsent. Aufgrund ihrer Allgegenwärtigkeit erscheinen Granulate häufig als einfach und gut verstanden, allerdings geben einige Phänomene im Verhalten von Schüttgütern bis heute Rätsel auf. Die vorliegende Arbeit beschäftigt sich mit Scherzonen und Dilatanz in einem gescherten Granulat, ihrer Modellierung und theoretischen Beschreibung. Der Aufbau der Arbeit spiegelt diese Ziele wider, indem zunächst ein experimentelles Modellsystem vorgestellt und anschließend mittels einer Molekulardynamik simuliert wird. Um einen Vergleich von Experiment und Simulation zu ermöglichen, wird ein geeigneter Mittelungsformalismus entwickelt, um aus den diskreten "mikroskopischen" Größen der Simulation "makroskopische" Messgrößen zu erhalten. Dieser Formalismus wird verwendet, um kinematische Größen wie Geschwindigkeitsprofile und Rotationen in der Simulation der Scherzelle zu ermitteln und mit den experimentellen Daten zu vergleichen. Aufgrund der gefundenen Vergleichbarkeit von Experiment und Simulation lassen sich dann vertrauenswürdige Aussagen auch über Größen treffen, die im Experiment gar nicht oder nur schwer zugänglich, jedoch für das Verständnis der Vorgänge innerhalb des Granulates hilfreich sind. Im Rahmen eines kontinuumstheoretischen Ansatzes werden die Spannungen und die Deformationen des Granulates bestimmt. Zusätzlich wird der Fabric-, oder Strukturtensor ermittelt, mit dessen Hilfe sich Aussagen über die innere Struktur des Schüttgutes, wie beispielsweise den Grad der Anisotropie, treffen lassen. Die ermittelten Feldgrößen werden dann verwendet, um Materialkenngrößen einer Kontinuumstheorie zu bestimmen. Dazu wird zunächst ein elastisches Materialgesetz nach Hooke verwendet und der Elastizitäts- und Schermodul berechnet. Da es sich zeigt, dass die Rotationen der einzelnen Körner im System eine wichtige Rolle für das Verhalten des Materials insbesondere in der Scherzone spielen, führen wir einen Cosserat-Ansatz ein, in welchem die klassische Kontinuumstheorie um die rotatorischen Freiheitsgrade erweitert wird. Daher müssen die Bilanzrelationen um Gleichungen für Momente und Krümmungen erweitert werden. Diese Größen werden ebenfalls aus den Simulationen bestimmt und eine neue Materialgröße, die Verdrehungssteifigkeit errechnet. Im letzten Teil der vorliegenden Arbeit werden die Ergebnisse der Simulationen mit den Vorhersagen eines elasto-plastischen Cosserat-Modells verglichen. Da experimentelle Daten für diesen Vergleich fehlen, bietet die Simulation hier erstmals die Möglichkeit einen Test des Modells durchzuführen.While sitting on a beach and watching children building sand castles or horses galloping on the sand no one will think of how to describe sand in a mathematical way. But it is worth thinking about. Sand belongs to a group of materials known as granular materials. Most of the time we handle granular materials in everyday life, we do not even notice it. At breakfast, the coffee powder and the cereals are granular materials. Sugar, drugs and tooth paste are other examples of granular media in a household. In industrial environments granular materials are also omnipresent, e.g. cement, ore and plastic pellets. With the abundance of granular materials they often seem particularly ordinary and well understood, yet there are a lot of phenomena which are still not. In the present thesis we will focus on the shear zone and dilatancy in a sheared granular media, its modeling and theoretical description. Therefore, the aim of this thesis is twofold. On the one hand, a discrete element method (DEM) is carried out and compared with an experiment. On the other hand, a micro-macro transition is developed and applied, leading to insights related to constitutive models for continuum theories. These two goals also reflect in the structure of the thesis. After an introductory part the setup of the simulation and the experiment are presented. A motivation for the use of the Couette shear device is given, as well as an overview of the literature on Couette devices. The dimensions of the system and the particles confined in the cell are shown and the way of preparing the system is outlined. Interspersed in this first sections the differences between the physical system and the simulation are pointed out. The molecular dynamics (MD) algorithm used for simulation is briefly outlined and the integration method and a speed up method for the neighborhood search, namely the linked-cell algorithm, are recalled. Since the interaction forces between the particles play a significant role in the simulation of granular media, the necessary laws and their implementation are provided. Forces in the normal direction at a contact point are dealt with as well as tangential forces. In order to compare the results of the simulation to experiments and to move forward towards a continuum description of the system, a consistent way of obtaining various quantities is developed. The use of the averaging formalism is demonstrated by computing the local density profile and the velocity profile in the shear cell. The simulation results are compared to the experimental data. In the simulations and in the experiments an initial, homogeneous density becomes radially non-uniform as a consequence of the shear induced dilatancy. The investigation of this shear zone shows good quantitative agreement between experiment and simulation. Special attention is drawn to the kinematic properties of the device such as radial and angular velocities and the spin of the particles. Profile as well as distribution data are compared and the quantitative agreement/disagreement is discussed and possible reasons are given. Because of the good agreement the simulation is used to gain further insights on quantities not available from the experiment. These quantities are useful in order to explore granular media by means of a continuum theory. In the context of a continuum approach different macroscopic tensorial quantities are obtained. Even if not a quantity of the classical continuum theory the fabric tensor is introduced. The fabric tensor describes the local structure of the granulate to some extent and therefore is a measure for the anisotropy of the system. It is also used in the definition of the stress and strain tensors. Finally, these tensors are used to compute the macroscopic moduli, namely the Young's and the shear modulus which we use to develop a new constitutive model relating the stress with the deformations and the structure inside a granular assembly. Due to the ability of the single grains to rotate freely, the classical continuum theory has to be extended. Therefore, a Cosserat type theory is used in which the balance equations of the classical theory are extended by equations for couple stresses and curvature. The related macroscopic quantities of the theory are calculated from the simulations and a new modulus, the torque resistance is calculated. In the last part of this thesis the results of the simulations are compared with the predictions of a recently presented micropolar continuum model involving the previously discussed ideas of coupled stresses and a flow rule as an additional ingredient. As experimental results are missing for a comparison with this theory the simulations yield a first possibility to test the model

Hi4D-ADSIP 3-D dynamic facial articulation database

The face is an important medium used by humans to communicate, and facial articulation also reflects a person's emotional and awareness states, cognitive activity, personality or wellbeing. With the advances in 3-D imaging technology and ever increasing computing power, automatic analysis of facial articulation using 3-D sequences is becoming viable. This paper describes Hi4D-ADSIP — a comprehensive 3-D dynamic facial articulation database, containing scans with high spatial and temporal resolution. The database is designed not only to facilitate studies on facial expression analysis, but also to aid research into clinical diagnosis of facial dysfunctions. The database currently contains 3360 facial sequences captured from 80 healthy volunteers (control subjects) of various age, gender and ethnicity. The database has been validated using psychophysical experiments used to formally evaluate the accuracy of the recorded expressions. The results of baseline automatic facial expression recognition methods using Eigen- and Fisher-faces are also presented alongside some initial results obtained for clinical cases. This database is believed to be one of the most comprehensive repositories of facial 3-D dynamic articulations to date. The extension of this database is currently under construction aiming at building a comprehensive repository of representative facial dysfunctions exhibited by patients with stroke, Bell's palsy and Parkinson's disease

A study to assess the feasibility of a 3D Dynamic Imaging System in quantifying facial dysfunction

Facial weakness is common after stroke and can cause distress and embarrassment, as well as interfering with speech and swallowing. Currently we lack any evidence based interventions to treat facial weakness, possibly due to the lack of a robust measure. Dynamic 3D optical scanning is a new technology that measures symmetry of the face. It may allow small changes in facial symmetry to be measured accurately, thus detecting the effects of treatments, and rehabilitation