Washington, DC, United States
Washington, DC, United States

Time filter

Source Type

Mensching D.J.,National Research Council Research Associate | Rahbar-Rastegar R.,University of New Hampshire | Underwood B.S.,Arizona State University | Daniel J.S.,University of New Hampshire
Transportation Research Record | Year: 2016

A critical distress in asphalt concrete pavements is fatigue cracking, which results in decreased ride quality and fuel economy, and provides an avenue for water intrusion, which causes a pavement system to deteriorate rapidly. Given the poor state of the infrastructure network, changes are needed in the current mixture design process to promote innovation and alternative approaches to production. This study addressed this need by pursuing the following objectives: (a) relate mixture stiffness, fatigue, and pavement system characteristics for performance-based mixture design; (b) identify a simplified viscoelastic continuum damage (S-VECD) output parameter that most clearly distinguishes between poor and satisfactory performance when combined with dynamic modulus information; and (c) evaluate the impact of recycled materials on performance indicators for fatigue cracking. The results show that a pavement structure selection process related to the S-VECD failure criterion produces better performance predictions than does a stiffness-based approach. Promising correlations with performance exist for the pseudostiffness at failure and storage modulus for an Interstate pavement structure, phase angle for a state highway surface and base course, and model term alpha for the same state highway base course. © 2016, National Research Council. All rights reserved.

Pirlo R.K.,National Research Council Research Associate | Wu P.,Southern Oregon University | Liu J.,Center for Biomolecular Science and Engineering | Ringeisen B.,U.S. Navy
Biotechnology and Bioengineering | Year: 2012

Two major challenges in tissue engineering are mimicking the native cell-cell arrangements of tissues and maintaining viability of three-dimension (3D) tissues thicker than 300μm. Cell printing and prevascularization of engineered tissues are promising approaches to meet these challenges. However, the printing technologies used in biofabrication must balance the competing parameters of resolution, speed, and volume, which limit the resolution of thicker 3D structures. We suggest that high-resolution conformal printing techniques can be used to print 2D patterns of vascular cells onto biopaper substrates which can then be stacked to form a thicker tissue construct. Towards this end we created 1cm×1cm×300μm biopapers to be used as the transferable, stackable substrate for cell printing. 3.6% w/v poly-lactide-co-glycolide was dissolved in chloroform and poured into molds filled with NaCl crystals. The salt was removed with DI water and the scaffolds were dried and loaded with a Collagen Type I or Matrigel™. SEM of the biopapers showed extensive porosity and gel loading throughout. Biological laser printing (BioLP™) was used to deposit human umbilical vein endothelial cells (HUVEC) in a simple intersecting pattern to the surface of the biopapers. The cells differentiated and stretched to form networks preserving the printed pattern. In a separate experiment to demonstrate "stackability," individual biopapers were randomly seeded with HUVECs and cultured for 1 day. The mechanically stable and viable biopapers were then stacked and cultured for 4 days. Three-dimensional confocal microscopy showed cell infiltration and survival in the compound multilayer constructs. These results demonstrate the feasibility of stackable "biopapers" as a scaffold to build 3D vascularized tissues with a 2D cell-printing technique. © 2011 Wiley Periodicals, Inc.

Siemiginowska A.,Harvard - Smithsonian Center for Astrophysics | Burke D.J.,Harvard - Smithsonian Center for Astrophysics | Aldcroft T.L.,Harvard - Smithsonian Center for Astrophysics | Worrall D.M.,University of Bristol | And 4 more authors.
Astrophysical Journal | Year: 2010

We present the first results from a new, deep (200 ks) Chandra observation of the X-ray luminous galaxy cluster surrounding the powerful (L ∼ 10 47 erg s-1), high-redshift (z = 1.067), compact-steep-spectrum radio-loud quasar 3C 186. The diffuse X-ray emission from the cluster has a roughly ellipsoidal shape and extends out to radii of at least ∼60 arcsec (∼500 kpc). The centroid of the diffuse X-ray emission is offset by 0.68±0″ .11 (∼5.5±0.9 kpc) from the position of the quasar. We measure a cluster mass within the radius at which the mean enclosed density is 2500 times the critical density, r2500 = 283+18-13 kpc, of 1.02+0.21 -0.14 ×1014M⊙. The gas-mass fraction within this radius is fgas = 0.129+0.015-0.016. This value is consistent with measurements at lower redshifts and implies minimal evolution in the fgas(z) relation for hot, massive clusters at 0 < z < 1.1. The measured metal abundance of 0.42+0.08 -0.07 Solar is consistent with the abundance observed in other massive, high-redshift clusters. The spatially resolved temperature profile for the cluster shows a drop in temperature, from kT ∼ 8 keV to kT ∼ 3 keV, in its central regions that is characteristic of cooling-core clusters. This is the first spectroscopic identification of a cooling-core cluster atz > 1. We measure cooling times for the X-ray emitting gas at radii of 50 kpc and 25 kpc of 1.7 ± 0.2 × 109 years and 7.5±2.6×10 8 years, as well as a nominal cooling rate (in the absence of heating) of 400±190M⊙ year-1 within the central 100 kpc. In principle, the cooling gas can supply enough fuel to support the growth of the supermassive black hole and to power the luminous quasar. The radiative power of the quasar exceeds by a factor of 10 the kinematic power of the central radio source, suggesting that radiative heating may be important at intermittent intervals in cluster cores. 2010. The American Astronomical Society. All rights reserved.

Ostorero L.,University of Turin | Ostorero L.,National Institute of Nuclear Physics, Italy | Moderski R.,Nicolaus Copernicus Astronomical Center | Moderski R.,Kavli Institute for Particle Astrophysics and Cosmology | And 10 more authors.
Astrophysical Journal | Year: 2010

In a dynamical-radiative model we recently developed to describe the physics of compact, GHz-peaked-spectrum (GPS) sources, the relativistic jets propagate across the inner, kpc-sized region of the host galaxy, while the electron population of the expanding lobes evolves and emits synchrotron and inverse-Compton (IC) radiation. Interstellar-medium gas clouds engulfed by the expanding lobes, and photoionized by the active nucleus, are responsible for the radio spectral turnover through free-free absorption (FFA) of the synchrotron photons. The model provides a description of the evolution of the spectral energy distribution (SED) of GPS sources with their expansion, predicting significant and complex high-energy emission, from the X-ray to the γ-ray frequency domain. Here, we test this model with the broadband SEDs of a sample of 11 X-ray-emitting GPS galaxies with compact-symmetric-object morphology, and show that (1) the shape of the radio continuum at frequencies lower than the spectral turnover is indeed well accounted for by the FFA mechanism and (2) the observed X-ray spectra can be interpreted as non-thermal radiation produced via IC scattering of the local radiation fields off the lobe particles, providing a viable alternative to the thermal, accretion-disk-dominated scenario. We also show that the relation between the hydrogen column densities derived from the X-ray (N H) and radio (N HI) data of the sources is suggestive of a positive correlation, which, if confirmed by future observations, would provide further support to our scenario of high-energy emitting lobes. © 2010. The American Astronomical Society. All rights reserved.

Hodges-Kluck E.J.,University of Maryland University College | Reynolds C.S.,University of Maryland University College | Cheung C.C.,NASA | Cheung C.C.,National Research Council Research Associate | Miller M.C.,University of Maryland University College
Astrophysical Journal | Year: 2010

We present new and archival Chandra X-ray Observatory observations of -shaped radio galaxies (XRGs) within z ∼ 0.1 alongside a comparison sample of normal double-lobed FR I and II radio galaxies. By fitting elliptical distributions to the observed diffuse hot X-ray emitting atmospheres (either the interstellar or intragroup medium), we find that the ellipticity and the position angle of the hot gas follow that of the stellar light distribution for radio galaxy hosts in general. Moreover, compared to the control sample, we find a strong tendency for -shaped morphology to be associated with wings directed along the minor axis of the hot gas distribution. Taken at face value, this result favors the hydrodynamic backflow models for the formation of XRGs which naturally explain the geometry; the merger-induced rapid reorientation models make no obvious prediction about orientation. © 2010 The American Astronomical Society.

Cummings R.M.,United States Air Force Academy | Jirasek A.,United States Air Force Academy | Jirasek A.,National Research Council Research Associate | Petterson K.,Tech Hampshire | Schmidt S.,Defence Science and Technology Organisation, Australia
28th AIAA Applied Aerodynamics Conference | Year: 2010

A comprehensive research program designed to investigate the ability of computational methods to predict stability and control characteristics of realistic flight vehicles has been undertaken. The integrated approach to simulating static and dynamic stability characteristics for a generic UCAV named SACCON (Stability and Control Configuration), was performed by NATO RTO Task Group AVT-161. The static and dynamics stability characteristics of the vehicles were evaluated by a group of researchers using the Cobalt Navier-Stokes flow solver. Comparisons with experimental results show that fine grids coupled with appropriate RANS and hybrid turbulence models yield good predictions for both the static and dynamic aerodynamics of the vehicle, including the estimation of stability derivatives.

Takeuchi Y.,Waseda University | Kataoka J.,Waseda University | Stawarz L.,Japan Aerospace Exploration Agency | Stawarz L.,Jagiellonian University | And 8 more authors.
Astrophysical Journal | Year: 2012

We report the results of a Suzaku X-ray imaging study of NGC6251, a nearby giant radio galaxy with intermediate FRI/II radio properties. Our pointing direction was centered on the γ-ray emission peak recently discovered with the Fermi Large Area Telescope (LAT) around the position of the northwest (NW) radio lobe 15 arcmin offset from the nucleus. After subtracting two "off-source" pointings adjacent to the radio lobe and removing possible contaminants in the X-ray Imaging Spectrometer field of view, we found significant residual X-ray emission most likely diffuse in nature. The spectrum of the excess X-ray emission is well fitted by a power law with a photon index Γ = 1.90 ± 0.15 and a 0.5-8keV flux of 4 × 10 -13ergcm-2s-1. We interpret this diffuse X-ray emission component as being due to inverse Compton upscattering of the cosmic microwave background photons by ultrarelativistic electrons within the lobe, with only a minor contribution from the beamed emission of the large-scale jet. Utilizing archival radio data for the source, we demonstrate by means of broadband spectral modeling that the γ-ray flux of the Fermi-LAT source 2FGLJ1629.4+8236 may well be accounted for by the high-energy tail of the inverse Compton continuum of the lobe. Thus, this claimed association of γ-rays from the NW lobe of NGC6251, together with the recent Fermi-LAT imaging of the extended lobes of CentaurusA, indicates that particles may be efficiently (re-)accelerated up to ultrarelativistic energies within extended radio lobes of nearby radio galaxies in general. © © 2012. The American Astronomical Society. All rights reserved.

Bandilla K.W.,National Research Council Research Associate | Kraemer S.R.,U.S. Environmental Protection Agency | Birkholzer J.T.,Lawrence Berkeley National Laboratory
International Journal of Greenhouse Gas Control | Year: 2012

This study examines using the threshold critical pressure increase and the extent of the carbon dioxide (CO 2) plume to delineate the area of potential impact (AoPI) for geologic CO 2 storage projects. The combined area covering both the CO 2 plume and the region where the pressure is greater than the threshold critical pressure increase is defined as the AoPI. The threshold critical pressure increase is defined as the pressure needed to lift formation brine up the length of an unplugged well to an underground source of drinking water. Using parameter values based on existing and planned CO 2 injection sites, in combination with a simple conceptual model, semi-analytic solutions are used to find the radial pressure response and spread of the CO 2 plume. A set of sensitivity analyses investigates the parameters that have the strongest impact on the size of the AoPI. The sensitivity analyses show that the injection formation salinity and the vertical distance between injection formation and drinking water source have a strong impact on the threshold critical pressure increase. In addition, the formation permeability has a strong impact on the radius at which the threshold critical pressure is reached, as does the amount of diffuse leakage into neighboring formations. The radius of the CO 2 plume is mainly impacted by the available storage space (thickness and porosity), the formation permeability and the injection rate. The radius of the AoPI is determined by the threshold critical pressure increase in about half of 1458 sensitivity cases, the rest are determined by the maximum extent of the free phase CO 2 plume. When brine leakage into and through the cap rock is considered, the size of the AoPI is determined by the threshold critical pressure increase for only about 29% of the cases. © 2012 Elsevier Ltd.

PubMed | U.S. Environmental Protection Agency and National Research Council Research Associate
Type: Journal Article | Journal: Environmental science & technology | Year: 2016

Sodium dodecyl sulfate (SDS) facilitates multiwalled carbon nanotube (MWCNT) debundling and enhances nanotube stability in the aqueous environment by adsorbing on the nanotube surfaces, thereby increasing repulsive electrostatic forces and steric effects. The resulting SDS-wrapped MWCNTs are utilized in industrial applications and have been widely employed in environmental studies. In the present study, MWCNTs adsorbed SDS during ultrasonication to form stable MWCNTs suspensions. Desorption of SDS from MWCNTs surfaces was then investigated as a function of Suwannee River Humic Acid (SRHA) and background electrolyte concentrations. Due to hydrophobic effects and - interactions, MWCNTs exhibit higher affinity for SRHA than SDS. In the presence of SRHA, SDS adsorbed on MWCNTs was displaced. Cations (Na(+), Ca(2+)) decreased SDS desorption from MWCNTs due to charge screening effects. Interestingly, the presence of the divalent calcium cation facilitated multilayered SRHA adsorption on MWCNTs through bridging effects, while monovalent sodium reduced SRHA adsorption. Results of the present study suggest that properties of MWCNTs wrapped with commercial surfactants will be altered when these materials are released to surface waters and the surfactant coating will be displaced by natural organic matter (NOM). Changes on their surfaces will significantly affect MWCNTs fate in aquatic environments.

PubMed | National Research Council Research Associate
Type: Journal Article | Journal: Journal of physics. Condensed matter : an Institute of Physics journal | Year: 2016

We explore the possibility of storing excitons in excitonic dark states in monolayer semiconducting transition-metal dichalcogenides. In addition to being optically inactive, these dark states require the electron and hole to be spatially separated, thus inhibiting electron/hole recombination and allowing exciton lifetimes to be extended. Based on an atomistic exciton model, we derive transition matrix elements and an approximate selection rule showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on a population analysis for different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing.

Loading National Research Council Research Associate collaborators
Loading National Research Council Research Associate collaborators