1,344 research outputs found
The biomechanics and energetics of human running using an elastic knee exoskeleton
While the effects of series compliance on running biomechanics are well documented, the effects of parallel compliance are known only for the simpler case of hopping. As many practical exoskeletal and orthotic designs act in parallel with the leg, it is desirable to understand the effects of such an intervention. Spring-like forces offer a natural choice of perturbation for running, as they are both biologically motivated and energetically inexpensive to implement. To this end, we investigate the hypothesis that the addition of an external elastic element at the knee during the stance phase of running results in a reduction in knee extensor activation so that total joint quasi-stiffness is maintained. An exoskeletal knee brace consisting of an elastic element engaged by a clutch is used to provide this stance phase extensor torque. Motion capture of five subjects is used to investigate the consequences of running with this device. No significant change in leg stiffness or total knee stiffness is observed due to the activation of the clutched parallel knee spring. However, this pilot data suggests differing responses between casual runners and competitive long-distance runners, whose total knee torque is increased by the device. Such a relationship between past training and effective utilization of an external force is suggestive of limitations on the applicability of assistive devices
Speckle Patterns With Atomic And Molecular De Broglie Waves
We have developed a nozzle source that delivers a continuous beam of atomic helium or molecular hydrogen having a high degree of transverse coherence and with adequate optical brightness to enable new kinds of experiments. Using this source we have measured single slit diffraction patterns and the first ever speckle-diffraction patterns using atomic and molecular de Broglie waves. Our results suggest fruitful application of coherent matter beams in dynamic scattering and diffractive imaging at short wavelength and with extreme surface sensitivity
Primordial formation of major silicates in a protoplanetary disc with homogeneous 26Al/27Al
Understanding the spatial variability of initial 26Al/27Al in the solar system, i.e., (26Al/27Al)0, is of prime importance to meteorite chronology, planetary heat production, and protoplanetary disc mixing dynamics. The (26Al/27Al)0 of calcium-aluminum–rich inclusions (CAIs) in primitive meteorites (~5 × 10−5) is frequently assumed to reflect the (26Al/27Al)0 of the entire protoplanetary disc, and predicts its initial 26Mg/24Mg to be ~35 parts per million (ppm) less radiogenic than modern Earth (i.e., Δ′26Mg0 = −35 ppm). Others argue for spatially heterogeneous (26Al/27Al)0, where the source reservoirs of most primitive meteorite components have lower (26Al/27Al)0 at ~2.7 × 10−5 and Δ′26Mg0 of −16 ppm. We measured the magnesium isotope compositions of primitive meteoritic olivine, which originated outside of the CAI-forming reservoir(s), and report five grains whose Δ′26Mg0 are within uncertainty of −35 ppm. Our data thus affirm a model of a largely homogeneous protoplanetary disc with (26Al/27Al)0 of ~5 × 10−5, supporting the accuracy of the 26Al→26Mg chronometer
Design and Demonstration of a New Small-Scale Jet Noise Experiment
A facility capable of acoustic and velocity field measurements of high-speed jets has
recently been built and tested. The anechoic chamber that houses the jet has a 2.1 m
× 2.3 m × 2.5 m wedge tip to wedge tip working volume. We aim to demonstrate that
useful experiments can be performed in such a relatively small facility for a substantially
lower cost than in larger facility. Rapid prototyping allows for quick manufacturing of both
simple and complex geometry nozzles. Sideline and 30° downstream acoustic measurements
between 400 Hz and 100 kHz agree well with accepted results. Likewise, nozzle exit-plane
data obtained using particle image velocimetry are in good agreement with other studies
Nonthermal hydrogen loss at Mars: Contributions of photochemical mechanisms to escape and identification of key processes
Hydrogen loss to space is a key control on the evolution of the Martian
atmosphere and the desiccation of the red planet. Thermal escape is thought to
be the dominant loss process, but both forward modeling studies and remote
sensing observations have indicated the presence of a second,
higher-temperature "nonthermal" or "hot" hydrogen component, some fraction of
which also escapes. Exothermic reactions and charge/momentum exchange processes
produce hydrogen atoms with energy above the escape energy, but H loss via many
of these mechanisms has never been studied, and the relative importance of
thermal and nonthermal escape at Mars remains uncertain. Here we estimate
hydrogen escape fluxes via 47 mechanisms, using newly-developed escape
probability profiles. We find that HCO dissociative recombination is the
most important of the mechanisms, accounting for 30-50% of the nonthermal
escape. The reaction CO + H is also important, producing roughly as
much escaping H as momentum exchange between hot O and H. Total nonthermal
escape from the mechanisms considered amounts to 39% (27%) of thermal escape,
for low (high) solar activity. Our escape probability profiles are applicable
to any thermospheric hot H production mechanism and can be used to explore
seasonal and longer-term variations, allowing for a deeper understanding of
desiccation drivers over various timescales. We highlight the most important
mechanisms and suggest that some may be important at Venus, where nonthermal
escape dominates and much of the literature centers on charge exchange
reactions, which do not result in significant escape in this study.Comment: 47 pages, 4 figures, 3 tables. Accepted manuscript. An edited version
of this paper was published by AG
Multi-domain analysis and prediction of the light emitted by an inductively coupled plasma jet
Inductively coupled plasma wind tunnels are crucial for replicating
hypersonic flight conditions in ground testing. Achieving the desired
conditions (e.g., stagnation-point heat fluxes and enthalpies during
atmospheric reentry) requires a careful selection of operating inputs, such as
mass flow, gas composition, nozzle geometry, torch power, chamber pressure, and
probing location along the plasma jet. The study presented herein focuses on
the influence of the torch power and chamber pressure on the plasma jet
dynamics within the 350 kW Plasmatron X ICP facility at the University of
Illinois at Urbana-Champaign. A multi-domain analysis of the jet behavior under
selected power-pressure conditions is presented in terms of emitted light
measurements collected using high-speed imaging. We then use Gaussian Process
Regression to develop a data-informed learning framework for predicting
Plasmatron X jet profiles at unseen pressure and power test conditions.
Understanding the physics behind the dynamics of high-enthalpy flows,
particularly plasma jets, is the key to properly design material testing,
perform diagnostics, and develop accurate simulation modelsComment: 22 pages (including figures, appendix, and references); 13 figure
Layer-thickness dependence of the conductive properties of Mo/Si multilayers
We report new measurements of the conductance and superconducting transition temperature of a set of Mo/Si multilayers, as a function of the metal layer thickness (from 7-85 Angstrom) for a constant semiconductor layer thickness of 22 Angstrom. Unlike previously reported measurements, we do not observe oscillations in either the resistivity, resistivity ratio, or the superconducting transition temperature with the metal layer thickness. Rather, we observe monotonic variations in the transport properties as the metal layer thickness increases. The sheet conductance and its change between 10 and 300 K both vary approximately linearly with the metal layer thickness, above a threshold thickness. The conductance starts to grow with metal layer thickness at approximately 10 Angstrom, whereas the temperature coefficient of resistance changes sign at approximately 25 Angstrom, exhibiting a Mooij correlation with a crossover resistivity of 125 mu Omega cm. The observed temperature dependence of the conductance rules out localization as the origin of the negative temperature coefficient of resistance. The conductance data are analyzed using a simple phenomenological model involving transport in interfacial and metallic layers, whose relative contribution to the conductance depends on the metal layer thickness and the temperature. The model is applied to separate two competing contributions that determine the overall temperature dependence of the conductance. We attribute the differences between our measurements and previous measurements to differences in bulk metallic conductivities and interface morphologies, due to differences in thermal evaporation versus sputtering fabrication processes. Our results show that the level and nature of disorder is an important ingredient in any theory that explains the cause of the observed oscillations
Widespread recombination, reassortment, and transmission of unbalanced compound viral genotypes in natural arenavirus infections.
Arenaviruses are one of the largest families of human hemorrhagic fever viruses and are known to infect both mammals and snakes. Arenaviruses package a large (L) and small (S) genome segment in their virions. For segmented RNA viruses like these, novel genotypes can be generated through mutation, recombination, and reassortment. Although it is believed that an ancient recombination event led to the emergence of a new lineage of mammalian arenaviruses, neither recombination nor reassortment has been definitively documented in natural arenavirus infections. Here, we used metagenomic sequencing to survey the viral diversity present in captive arenavirus-infected snakes. From 48 infected animals, we determined the complete or near complete sequence of 210 genome segments that grouped into 23 L and 11 S genotypes. The majority of snakes were multiply infected, with up to 4 distinct S and 11 distinct L segment genotypes in individual animals. This S/L imbalance was typical: in all cases intrahost L segment genotypes outnumbered S genotypes, and a particular S segment genotype dominated in individual animals and at a population level. We corroborated sequencing results by qRT-PCR and virus isolation, and isolates replicated as ensembles in culture. Numerous instances of recombination and reassortment were detected, including recombinant segments with unusual organizations featuring 2 intergenic regions and superfluous content, which were capable of stable replication and transmission despite their atypical structures. Overall, this represents intrahost diversity of an extent and form that goes well beyond what has been observed for arenaviruses or for viruses in general. This diversity can be plausibly attributed to the captive intermingling of sub-clinically infected wild-caught snakes. Thus, beyond providing a unique opportunity to study arenavirus evolution and adaptation, these findings allow the investigation of unintended anthropogenic impacts on viral ecology, diversity, and disease potential
ColabFit Exchange: open-access datasets for data-driven interatomic potentials
Data-driven (DD) interatomic potentials (IPs) trained on large collections of
first principles calculations are rapidly becoming essential tools in the
fields of computational materials science and chemistry for performing
atomic-scale simulations. Despite this, apart from a few notable exceptions,
there is a distinct lack of well-organized, public datasets in common formats
available for use with IP development. This deficiency precludes the research
community from implementing widespread benchmarking, which is essential for
gaining insight into model performance and transferability, while also limiting
the development of more general, or even universal, IPs. To address this issue,
we introduce the ColabFit Exchange, the first database providing open access to
a large collection of systematically organized datasets from multiple domains
that is especially designed for IP development. The ColabFit Exchange is
publicly available at \url{https://colabfit.org/}, providing a web-based
interface for exploring, downloading, and contributing datasets. Composed of
data collected from the literature or provided by community researchers, the
ColabFit Exchange consists of 106 datasets spanning nearly 70,000 unique
chemistries, and is intended to continuously grow. In addition to outlining the
software framework used for constructing and accessing the ColabFit Exchange,
we also provide analyses of data, quantifying the diversity and proposing
metrics for assessing the relative quality and atomic environment coverage of
different datasets. Finally, we demonstrate an end-to-end IP development
pipeline, utilizing datasets from the ColabFit Exchange, fitting tools from the
KLIFF software package, and validation tests provided by the OpenKIM framework
The Sudden Onset of Acoustic Effects with Increasing Nozzle-Chevron Length In a Mach 0.9 Jet
The addition of chevrons to the exit of a jet has been found to reduce far-field noise, making it an effective passive noise reduction technique. The selection of specific designs, however, is challenging since mechanics models provide insufficient guidance and computations remain far too expensive for trial-and-error parameter exploration. To assess the design optimization problem, we have experimentally evaluated parametric dependence of sound on chevron geometry. Specifically, rapid prototyping is used to change the model geometries and study the relationship between chevron length and the far-field jet noise. We show that the effect of the chevron starts relatively abruptly for lengths less than 4 mm, which is well less than the 16 mm length selected as a starting point from previous studies. Thus a small increase in the chevron length can produce a large change in the far-field sound. However, for longer chevrons the sound is insensitive to the length, which has important implications for design optimization. Additionally, other flow diagnostics are performed to better understand the flow characteristics resulting in the observed changes in jet noise
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