1,713 research outputs found
Enhanced Laboratory X-ray Particle Tracking Velocimetry With Newly Developed Tungsten-Coated O(50 m) Tracers
Tracer particles designed specifically for X-ray particle tracking and
imaging velocimetry (XPTV and XPIV) are necessary to widen the range of flows
that can be studied with these techniques. In this study, we demonstrate in-lab
XPTV using new, custom-designed (50 m) diameter tungsten-coated hollow
carbon spheres and a single energy threshold photon counting detector. To
explore the measurement quality enhancement enabled by the new tracer particles
and photon counting detector, a well understood Poiseulle pipe flow is
measured. The data show agreement with the analytical solution for the
depth-averaged velocity profile. The experiment also shows that the
tungsten-coated particles achieve higher contrast and are better localized than
previously available silver-coated particles, making faster and more precise
measurements attainable. The particles are manufactured with a readily scalable
chemical vapor deposition process.
We further show that laboratory XPTV is practical with currently available
energy-resolving photon counting detectors (PCDs), despite their presently
lower spatiotemporal resolution compared to scintillating detectors. This
finding suggests that energy-thresholding identification of different classes
of tracers is feasible, further motivating the exploration of the X-ray tracer
particle design space. The latest generation of PCDs are incorporating multiple
energy thresholds, and have higher count rate limits. In the near future one
could potentially expand on the work presented and track multiple tracer
species and scalar fields simultaneously.Comment: Submitted to Experiments in Fluids for consideration for publicatio
(1 kHz) In-Lab X-ray Particle Velocimetry for Multiphase Flows
We combine X-ray-specific tracer particles, a photon counting detector, and a
liquid metal anode X-ray source to achieve (1 kHz) X-ray imaging speeds in
the laboratory, 15 faster than previous comparable studies. To
demonstrate the capabilities of these imaging speeds, we conduct three
experiments: 2D and 3D X-ray particle tracking velocimetry (XPTV) of Poiseuille
pipe flow, 3D XPTV of flow around a Taylor bubble, and 3D scalar mixing with a
laminar jet. These experiments demonstrate the performance improvement of
combining the aforementioned elements, the applicability to multiphase flows
and deforming systems, and the potential to capture scalar and vector
quantities simultaneously. Most importantly, these experiments are conducted in
the laboratory, showing that in-lab X-ray particle velocimetry techniques are
now usable for a wider range of flows of interest
National Art Library: Services and Expansion
The goal of this report is to assist the National Art Library with their upcoming expansion and refurbishment. We performed case studies of comparable institutions, conducted interviews with Library Staff, and researched products related to library management and technological displays. We were then able to provide our sponsor with a number of recommendations to improve the quality of service to the current customers and to broaden the user base during the re-opening the West Room
Ultraviolet and Optical Observations of OB Associations and Field Stars in the Southwest Region of the Large Magellanic Cloud
Using photometry from the Ultraviolet Imaging Telescope (UIT) and photometry
and spectroscopy from three ground-based optical datasets we have analyzed the
stellar content of OB associations and field areas in and around the regions N
79, N 81, N 83, and N 94 in the LMC. We compare data for the OB association
Lucke-Hodge 2 (LH 2) to determine how strongly the initial mass function (IMF)
may depend on different photometric reductions and calibrations. We also
correct for the background contribution of field stars, showing the importance
of correcting for field star contamination in determinations of the IMF of star
formation regions. It is possible that even in the case of an universal IMF,
the variability of the density of background stars could be the dominant factor
creating the differences between calculated IMFs for OB associations.
We have also combined the UIT data with the Magellanic Cloud Photometric
Survey to study the distribution of the candidate O-type stars in the field. We
find a significant fraction, roughly half, of the candidate O-type stars are
found in field regions, far from any obvious OB associations. These stars are
greater than 2 arcmin (30 pc) from the boundaries of existing OB associations
in the region, which is a distance greater than most O-type stars with typical
dispersion velocities will travel in their lifetimes. The origin of these
massive field stars (either as runaways, members of low-density star-forming
regions, or examples of isolated massive star formation) will have to be
determined by further observations and analysis.Comment: 16 pages, 10 figures (19 PostScript files), tabular data + header
file for Table 1 (2 ASCII files). File format is LaTeX/AASTeX v.502 using the
emulateapj5 preprint style (included). Also available at
http://www.boulder.swri.edu/~joel/papers.html . To appear in the February
2001 issue of the Astronomical Journa
Compressive Sensing under Matrix Uncertainties: An Approximate Message Passing Approach
In this work, we consider a general form of noisy compressive sensing (CS) when there is uncertainty in the measurement matrix as well as in the measurements. Matrix uncertainty is motivated by practical cases in which there are imperfections or unknown calibration parameters in the signal acquisition hardware. While previous work has focused on analyzing and extending classical CS algorithms like the LASSO and Dantzig selector for this problem setting, we propose a new algorithm whose goal is either minimization of mean-squared error or maximization of posterior probability in the presence of these uncertainties. In particular, we extend the Approximate Message Passing (AMP) approach originally proposed by Donoho, Maleki, and Montanari, and recently generalized by Rangan, to the case of probabilistic uncertainties in the elements of the measurement matrix. Empirically, we show that our approach performs near oracle bounds. We then show that our matrix-uncertain AMP can be applied in an alternating fashion to learn both the unknown measurement matrix and signal vector. We also present a simple analysis showing that, for suitably large systems, it suffices to treat uniform matrix uncertainty as additive white Gaussian noise
A Protocol for the Administration of Real-Time fMRI Neurofeedback Training
Neurologic disorders are characterized by abnormal cellular-, molecular-, and circuit-level functions in the brain. New methods to induce and control neuroplastic processes and correct abnormal function, or even shift functions from damaged tissue to physiologically healthy brain regions, hold the potential to dramatically improve overall health. Of the current neuroplastic interventions in development, neurofeedback training (NFT) from functional Magnetic Resonance Imaging (fMRI) has the advantages of being completely non-invasive, non-pharmacologic, and spatially localized to target brain regions, as well as having no known side effects. Furthermore, NFT techniques, initially developed using fMRI, can often be translated to exercises that can be performed outside of the scanner without the aid of medical professionals or sophisticated medical equipment. In fMRI NFT, the fMRI signal is measured from specific regions of the brain, processed, and presented to the participant in real-time. Through training, self-directed mental processing techniques, that regulate this signal and its underlying neurophysiologic correlates, are developed. FMRI NFT has been used to train volitional control over a wide range of brain regions with implications for several different cognitive, behavioral, and motor systems. Additionally, fMRI NFT has shown promise in a broad range of applications such as the treatment of neurologic disorders and the augmentation of baseline human performance. In this article, we present an fMRI NFT protocol developed at our institution for modulation of both healthy and abnormal brain function, as well as examples of using the method to target both cognitive and auditory regions of the brain
Measuring and Correcting Wind-Induced Pointing Errors of the Green Bank Telescope Using an Optical Quadrant Detector
Wind-induced pointing errors are a serious concern for large-aperture
high-frequency radio telescopes. In this paper, we describe the implementation
of an optical quadrant detector instrument that can detect and provide a
correction signal for wind-induced pointing errors on the 100m diameter Green
Bank Telescope (GBT). The instrument was calibrated using a combination of
astronomical measurements and metrology. We find that the main wind-induced
pointing errors on time scales of minutes are caused by the feedarm being blown
along the direction of the wind vector. We also find that wind-induced
structural excitation is virtually non-existent. We have implemented offline
software to apply pointing corrections to the data from imaging instruments
such as the MUSTANG 3.3 mm bolometer array, which can recover ~70% of
sensitivity lost due to wind-induced pointing errors. We have also performed
preliminary tests that show great promise for correcting these pointing errors
in real-time using the telescope's subreflector servo system in combination
with the quadrant detector signal.Comment: 17 pages, 11 figures; accepted for publication in PAS
New Insights into Amino Acid Preservation in the Early Oceans Using Modern Analytical Techniques
Protein- and non-protein-amino acids likely occupied the oceans at the time of the origin and evolution of life. Primordial soup-, hydrothermal vent-, and meteoritic-processes likely contributed to this early chemical inventory. Prebiotic synthesis and carbonaceous meteorite studies suggest that non-protein amino acids were likely more abundant than their protein-counterparts. Amino acid preservation before abiotic and biotic destruction is key to biomarker availability in paleoenvironments and remains an important uncertainty. To constrain primitive amino acid lifetimes, a 1992 archived seawater/beach sand mixture was spiked with D,L-alanine, D,L-valine (Val), alpha-aminoisobutyric acid (alpha-AIB), D,L-isovaline (Iva), and glycine (Gly). Analysis by high performance liquid chromatography with fluorescence detection (HPLC-FD) showed that only D-Val and non-protein amino acids were abundant after 2250 days. The mixture was re-analyzed in 2012 using HPLC-FD and a triple quadrupole mass spectrometer (QqQ-MS). The analytical results 20 years after the inception of the experiment were strikingly similar to those after 2250 days. To confirm that viable microorganisms were still present, the mixture was re-spiked with Gly in 2012. Aliquots were collected immediately after spiking, and at 5- and 9-month intervals thereafter. Final HPLC-FD/QqQ-MS analyses were performed in 2014. The 2014 analyses revealed that only alpha-AIB, D,L-Iva, and D-Val remained abundant. The disappearance of Gly indicated that microorganisms still lived in the mixture and were capable of consuming protein amino acids. These findings demonstrate that non-protein amino acids are minimally impacted by biological degradation and thus have very long lifetimes under these conditions. Primitive non-protein amino acids from terrestrial synthesis, or meteorite in-fall, likely experienced great-er preservation than protein amino acids in paleo-oceanic environments. Such robust molecules may have reached a steady state concentration dependent on ocean circulation through hydrothermal systems and synthetic input processes. We are presently trying to estimate this concentration
Prebiotic Synthesis of Methionine and Other Sulfur-Containing Organic Compounds on the Primitive Earth: A Contemporary Reassessment Based on an Unpublished 1958 Stanley Miller Experiment
Original extracts from an unpublished 1958 experiment conducted by the late Stanley L. Miller were recently found and analyzed using modern state-of-the-art analytical methods. The extracts were produced by the action of an electric discharge on a mixture of methane (CH4), hydrogen sulfide (H2S), ammonia (NH3), and carbon dioxide (CO2). Racemic methionine was formed in significant yields, together with other sulfur-bearing organic compounds. The formation of methionine and other compounds from a model prebiotic atmosphere that contained H2S suggests that this type of synthesis is robust under reducing conditions, which may have existed either in the global primitive atmosphere or in localized volcanic environments on the early Earth. The presence of a wide array of sulfur-containing organic compounds produced by the decomposition of methionine and cysteine indicates that in addition to abiotic synthetic processes, degradation of organic compounds on the primordial Earth could have been important in diversifying the inventory of molecules of biochemical significance not readily formed from other abiotic reactions, or derived from extraterrestrial delivery
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