159 research outputs found
Particle Streak Velocimetry of Supersonic Nozzle Flows
A novel velocimetry technique to probe the exhaust flow of a laboratory scale combustor is being developed. The technique combines the advantages of standard particle velocimetry techniques and the ultra-fast imaging capabilities of a streak camera to probe high speed flows near continuously with improved spatial and velocity resolution. This "Particle Streak Velocimetry" technique tracks laser illuminated seed particles at up to 236 picosecond temporal resolution allowing time-resolved measurement of one-dimensional flows exceeding 2000 m/s as are found in rocket nozzles and many other applications. Developmental tests with cold nitrogen have been performed to validate and troubleshoot the technique with supersonic flows of much lower velocity and without background noise due to combusting flow. Flow velocities on the order of 500 m/s have been probed with titanium dioxide particles and a continuous-wave laser diode. Single frame images containing multiple streaks are analyzed to find the average slope of all incident particles corresponding to the centerline axial flow velocity. Long term objectives for these tests are correlation of specific impulse to theoretical combustion predictions and direct comparisons between candidate green fuels and the industry standard, monomethylhydrazine, each tested under identical conditions
Molecular dynamics simulations of vibrated granular gases
We present molecular dynamics simulations of mono- or bidisperse inelastic
granular gases driven by vibrating walls, in two dimensions (without gravity).
Because of the energy injection at the boundaries, a situation often met
experimentally, density and temperature fields display heterogeneous profiles
in the direction perpendicular to the walls. A general equation of state for an
arbitrary mixture of fluidized inelastic hard spheres is derived and
successfully tested against numerical data. Single-particle velocity
distribution functions with non-Gaussian features are also obtained, and the
influence of various parameters (inelasticity coefficients, density...)
analyzed. The validity of a recently proposed Random Restitution Coefficient
model is assessed through the study of projected collisions onto the direction
perpendicular to that of energy injection. For the binary mixture, the
non-equipartition of translational kinetic energy is studied and compared both
to experimental data and to the case of homogeneous energy injection
(``stochastic thermostat''). The rescaled velocity distribution functions are
found to be very similar for both species
Genetic basis of thermal nociceptive sensitivity and brain weight in a BALB/c reduced complexity cross
Thermal nociception involves the transmission of temperature-related noxious information from the periphery to the CNS and is a heritable trait that could predict transition to persistent pain. Rodent forward genetics complement human studies by controlling genetic complexity and environmental factors, analysis of end point tissue, and validation of variants on appropriate genetic backgrounds. Reduced complexity crosses between nearly identical inbred substrains with robust trait differences can greatly facilitate unbiased discovery of novel genes and variants. We found BALB/cByJ mice showed enhanced sensitivity on the 53.5°C hot plate and mechanical stimulation in the von Frey test compared to BALB/cJ mice and replicated decreased gross brain weight in BALB/cByJ versus BALB/cJ. We then identified a quantitative trait locus (QTL) on chromosome 13 for hot plate sensitivity (LOD = 10.7; p < 0.001; peak = 56 Mb) and a QTL for brain weight on chromosome 5 (LOD = 8.7; p < 0.001). Expression QTL mapping of brain tissues identified H2afy (56.07 Mb) as the top transcript with the strongest association at the hot plate locus (FDR = 0.0002) and spliceome analysis identified differential exon usage within H2afy associated with the same locus. Whole brain proteomics further supported decreased H2AFY expression could underlie enhanced hot plate sensitivity, and identified ACADS as a candidate for reduced brain weight. To summarize, a BALB/c reduced complexity cross combined with multiple-omics approaches facilitated identification of candidate genes underlying thermal nociception and brain weight. These substrains provide a powerful, reciprocal platform for future validation of candidate variants
The Reverse Brazil Nut Problem: Competition between Percolation and Condensation
In the Brazil nut problem (BNP), hard spheres with larger diameters rise to
the top. There are various explanations (percolation, reorganization,
convection), but a broad understanding or control of this effect is by no means
acheived. A theory is presented for the crossover from the BNP to the reverse
Brazil nut problem (RBNP) based on a competition between the percolation effect
and the condensation of hard spheres. The crossover condition is determined,
and the theoretical predictions are compared to Molecular Dynamics simulations
in two and three dimensions.Comment: 9 pages which includes 7 figure
Diffusion of impurities in a granular gas
Diffusion of impurities in a granular gas undergoing homogeneous cooling
state is studied. The results are obtained by solving the Boltzmann--Lorentz
equation by means of the Chapman--Enskog method. In the first order in the
density gradient of impurities, the diffusion coefficient is determined as
the solution of a linear integral equation which is approximately solved by
making an expansion in Sonine polynomials. In this paper, we evaluate up to
the second order in the Sonine expansion and get explicit expressions for
in terms of the restitution coefficients for the impurity--gas and gas--gas
collisions as well as the ratios of mass and particle sizes. To check the
reliability of the Sonine polynomial solution, analytical results are compared
with those obtained from numerical solutions of the Boltzmann equation by means
of the direct simulation Monte Carlo (DSMC) method. In the simulations, the
diffusion coefficient is measured via the mean square displacement of
impurities. The comparison between theory and simulation shows in general an
excellent agreement, except for the cases in which the gas particles are much
heavier and/or much larger than impurities. In theses cases, the second Sonine
approximation to improves significantly the qualitative predictions made
from the first Sonine approximation. A discussion on the convergence of the
Sonine polynomial expansion is also carried out.Comment: 9 figures. to appear in Phys. Rev.
Mechanisms of Size Control and Polymorphism in Viral Capsid Assembly
We simulate the assembly dynamics of icosahedral capsids from subunits that
interconvert between different conformations (or quasi-equivalent states). The
simulations identify mechanisms by which subunits form empty capsids with only
one morphology but adaptively assemble into different icosahedral morphologies
around nanoparticle cargoes with varying sizes, as seen in recent experiments
with brome mosaic virus (BMV) capsid proteins. Adaptive cargo encapsidation
requires moderate cargo-subunit interaction strengths; stronger interactions
frustrate assembly by stabilizing intermediates with incommensurate curvature.
We compare simulation results to experiments with cowpea chlorotic mottle virus
empty capsids and BMV capsids assembled on functionalized nanoparticles and
suggest new cargo encapsidation experiments. Finally, we find that both empty
and templated capsids maintain the precise spatial ordering of subunit
conformations seen in the crystal structure even if interactions that preserve
this arrangement are favored by as little as the thermal energy, consistent
with experimental observations that different subunit conformations are highly
similar
Pharmacokinetics and metabolism of ifosfamide in relation to DNA damage assessed by the COMET assay in children with cancer
The degree of damage to DNA following ifosfamide (IFO) treatment may be linked to the therapeutic efficacy. The pharmacokinetics and metabolism of IFO were studied in 19 paediatric patients, mostly with rhabdomyosarcoma or Ewings sarcoma. Ifosfamide was dosed either as a continuous infusion or as fractionated doses over 2 or 3 days. Samples of peripheral blood lymphocytes were obtained during and up to 96 h after treatment, and again prior to the next cycle of chemotherapy. DNA damage was measured using the alkaline COMET assay, and quantified as the percentage of highly damaged cells per sample. Samples were also taken for the determination of IFO and metabolites. Pharmacokinetics and metabolism of IFO were comparable with previous studies. Elevations in DNA damage could be determined in all patients after IFO administration. The degree of damage increased to a peak at 72 h, but had returned to pretreatment values prior to the next dose of chemotherapy. There was a good correlation between area under the curve of IFO and the cumulative percentage of cells with DNA damage (r2 = 0.554, P = 0.004), but only in those patients receiving fractionated dosing. The latter patients had more DNA damage (mean ± s.d., 2736 ± 597) than those patients in whom IFO was administered by continuous infusion (1453 ± 730). The COMET assay can be used to quantify DNA damage following IFO therapy. Fractionated dosing causes a greater degree of DNA damage, which may suggest a greater degree of efficacy, with a good correlation between pharmacokinetic and pharmacodynamic data
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