23,244 research outputs found
Planar Detonation Wave Initiation in Large-Aspect-Ratio Channels
In this study, two initiator designs are presented that are able to form planar detonations with low input energy in large-aspect-ratio channels over distances corresponding to only a few channel heights. The initiators use a single spark and an array of small channels to shape the detonation wave. The first design, referred to as the static initiator, is simple to construct as it consists of straight channels which connect at right angles. However, it is only able to create planar waves using mixtures that can reliably detonate in its small-width channels. An improved design, referred to as the dynamic initiator, is capable of detonating insensitive mixtures using an oxyacetylene gas slug injected into the initiator shortly before ignition, but is more complex to construct. The two versions are presented next, including an overview of their design and operation. Design drawings of each initiator are available elsewhere [7]. Finally, photographs and pressure traces of the resulting planar waves generated by each device are shown
Analytical Model for the Impulse of Single-Cycle Pulse Detonation Tube
An analytical model for the impulse of a single-cycle pulse detonation tube has been developed and validated against experimental data. The model is based on the pressure history at the thrust surface of the detonation tube. The pressure history is modeled by a constant pressure portion, followed by a decay due to gas expansion out of the tube. The duration and amplitude of the constant pressure portion is determined by analyzing the gasdynamics of the self-similar flow behind a steadily moving detonation wave within the tube. The gas expansion process is modeled using dimensional analysis and empirical observations. The model predictions are validated against direct experimental measurements in terms of impulse per unit volume, specific impulse, and thrust. Comparisons are given with estimates of the specific impulse based on numerical simulations. Impulse per unit volume and specific impulse calculations are carried out for a wide range of fuelâoxygenânitrogen mixtures (including aviation fuels) of varying initial pressure, equivalence ratio, and nitrogen dilution. The effect of the initial temperature is also investigated. The trends observed are explained using a simple scaling analysis showing the dependency of the impulse on initial conditions and energy release in the mixture
Chemical structure matching using correlation matrix memories
This paper describes the application of the Relaxation By Elimination (RBE) method to matching the 3D structure of molecules in chemical databases within the frame work of binary correlation matrix memories. The paper illustrates that, when combined with distributed representations, the method maps well onto these networks, allowing high performance implementation in parallel systems. It outlines the motivation, the neural architecture, the RBE method and presents some results of matching small molecules against a database of 100,000 models
On the Sensitivity of Massive Star Nucleosynthesis and Evolution to Solar Abundances and to Uncertainties in Helium Burning Reaction Rates
We explore the dependence of pre-supernova evolution and supernova
nucleosynthesis yields on the uncertainties in helium burning reaction rates.
Using the revised solar abundances of Lodders (2003) for the initial stellar
composition, instead of those of Anders & Grevesse (1989), changes the
supernova yields and limits the constraints that those yields place on the
12C(a,g)16O reaction rate. The production factors of medium-weight elements (A
= 16-40) were found to be in reasonable agreement with observed solar ratios
within the current experimental uncertainties in the triple alpha reaction
rate. Simultaneous variations by the same amount in both reaction rates or in
either of them separately, however, can induce significant changes in the
central 12C abundance at core carbon ignition and in the mass of the supernova
remnant. It therefore remains important to have experimental determinations of
the helium burning rates so that their ratio and absolute values are known with
an accuracy of 10% or better.Comment: Accepted for publication by the Astrophysical Journa
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Unraveling How Candida albicans Forms Sexual Biofilms.
Biofilms, structured and densely packed communities of microbial cells attached to surfaces, are considered to be the natural growth state for a vast majority of microorganisms. The ability to form biofilms is an important virulence factor for most pathogens, including the opportunistic human fungal pathogen Candida albicans. C. albicans is one of the most prevalent fungal species of the human microbiota that asymptomatically colonizes healthy individuals. However, C. albicans can also cause severe and life-threatening infections when host conditions permit (e.g., through alterations in the host immune system, pH, and resident microbiota). Like many other pathogens, this ability to cause infections depends, in part, on the ability to form biofilms. Once formed, C. albicans biofilms are often resistant to antifungal agents and the host immune response, and can act as reservoirs to maintain persistent infections as well as to seed new infections in a host. The majority of C. albicans clinical isolates are heterozygous (a/α) at the mating type-like (MTL) locus, which defines Candida mating types, and are capable of forming robust biofilms when cultured in vitro. These "conventional" biofilms, formed by MTL-heterozygous (a/α) cells, have been the primary focus of C. albicans biofilm research to date. Recent work in the field, however, has uncovered novel mechanisms through which biofilms are generated by C. albicans cells that are homozygous or hemizygous (a/a, a/Î, α/α, or α/Î) at the MTL locus. In these studies, the addition of pheromones of the opposite mating type can induce the formation of specialized "sexual" biofilms, either through the addition of synthetic peptide pheromones to the culture, or in response to co-culturing of cells of the opposite mating types. Although sexual biofilms are generally less robust than conventional biofilms, they could serve as a protective niche to support genetic exchange between mating-competent cells, and thus may represent an adaptive mechanism to increase population diversity in dynamic environments. Although conventional and sexual biofilms appear functionally distinct, both types of biofilms are structurally similar, containing yeast, pseudohyphal, and hyphal cells surrounded by an extracellular matrix. Despite their structural similarities, conventional and sexual biofilms appear to be governed by distinct transcriptional networks and signaling pathways, suggesting that they may be adapted for, and responsive to, distinct environmental conditions. Here we review sexual biofilms and compare and contrast them to conventional biofilms of C. albicans
Improved AURA k-Nearest Neighbour approach
The k-Nearest Neighbour (kNN) approach is a widely-used technique for pattern classification. Ranked distance measurements to a known sample set determine the classification of unknown samples. Though effective, kNN, like most classification methods does not scale well with increased sample size. This is due to their being a relationship between the unknown query and every other sample in the data space. In order to make this operation scalable, we apply AURA to the kNN problem. AURA is a highly-scalable associative-memory based binary neural-network intended for high-speed approximate search and match operations on large unstructured datasets. Previous work has seen AURA methods applied to this problem as a scalable, but approximate kNN classifier. This paper continues this work by using AURA in conjunction with kernel-based input vectors, in order to create a fast scalable kNN classifier, whilst improving recall accuracy to levels similar to standard kNN implementations
Exhaust of Underexpanded Jets from Finite Reservoirs
The response of an underexpanded jet to a depleting finite reservoir is examined with experiments and simulations.
An open-ended shock-tube facility with a variable reservoir length is used to obtain images of nitrogen- and helium-jet
structures at successive instances during the blowdown from initial pressure ratios of up to 250. The reservoir and
ambient pressures are simultaneously measured to obtain the instantaneous pressure ratio. We estimate the time
scales for jet formation and reservoir depletion as a function of the specific heat ratio of the gas and the initial pressure
ratio. The jet structure formation time scale is found to become approximately independent of the pressure ratio for
ratios greater than 50. In the present work, no evidence of time dependence in the Mach disk shock location is
observed for rates of pressure decrease associated with isentropic blowdown of a finite reservoir while the pressure
ratio is greater than 15. The shock location in the finite-reservoir jet can be calculated from an existing empirical fit to
infinite-reservoir jet data evaluated at the instantaneous reservoir pressure. For pressure ratios below 15, however,
the present data deviate from a compilation of data for infinite-reservoir jets. A new fit is obtained to data in the
lower-pressure regime. The self-similarity of the jet structure is quantified, and departure from similarity is noted to
begin at pressure ratios lower than about 15, approximately the same ratio that limits existing empirical fits
Design Considerations and Structural Analysis of the Narrow Channel Facility
The narrow channel (NC) facility design is based on the GALCIT Detonation Tube (GDT) test section design of Mike Kaneshige [3]. The main differences are 1) the design of the longitudinal bolted joint was simplified and 2) the keys for the flanges are not designed to assist with shear
loading. The GDT side windows and PLIF window may be used in the NC facility. 304 SS was chosen for its excellent corrosion resistance. The yield strength is 275 MPa. 304 SS is non-magnetic and is therefore more difficult to grind on a table with magnetic locking. All four plates were blanchard ground (using mechanical locking to the table through some of the bolt holes which were drilled before grinding) by a subcontractor for Hales. All pieces were machined by Hales except the initiator, window sealing plates, end flange sealing plates which were made
by the Aeroshop. Hales also supplied the material and checked the assembly of the pieces.
After delivery from Hales, the internal surfaces of the four channel plates were hand-sanded to
a mirror finish
Two- and three-dimensional simulations of core-collapse supernovae with CHIMERA
Ascertaining the core-collapse supernova mechanism is a complex, and yet
unsolved, problem dependent on the interaction of general relativity,
hydrodynamics, neutrino transport, neutrino-matter interactions, and nuclear
equations of state and reaction kinetics. Ab initio modeling of core-collapse
supernovae and their nucleosynthetic outcomes requires care in the coupling and
approximations of the physical components. We have built our multi-physics
CHIMERA code for supernova modeling in 1-, 2-, and 3-D, using ray-by-ray
neutrino transport, approximate general relativity, and detailed neutrino and
nuclear physics. We discuss some early results from our current series of
exploding 2D simulations and our work to perform computationally tractable
simulations in 3D using the "Yin-Yang" grid.Comment: Proceedings of the 12th Symposium on Nuclei in the Cosmos. 5-12
August 2012. Cairns, Australia. Published online at
http://pos.sissa.it/archive/conferences/146/208/NIC%20XII_208.pdf Corrected
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