453 research outputs found
Towards scaling laws for DDT in obstructed channels
In a coal mine, natural gas can leak through walls and accumulate in enclosed regions that are no longer being mined or ventilated. If there is an accidental spark in the region containing this gas, it can ignite a flame that may transition to detonation (DDT). An important problem is to assess if, when, and where DDT can occur, and thus provide information needed to design strong enough barriers to protect active mining areas. We describe results of numerical simulations of flame acceleration and DDT in obstacle-laden channels to find a scaling law for DDT, the distance the deflagration travels before a detonation forms as a function of channel size d. The scaling law is derived for a stoichiometric natural-gas air mixture in a channel with blockage ratio 0.3 and channel sizes ranging from 0.17 to 3.0 m
Convection-reaction equation based magnetic resonance electrical properties tomography (cr-MREPT)
Cataloged from PDF version of article.Images of electrical conductivity and permittivity
of tissues may be used for diagnostic purposes as well as for
estimating local specific absorption rate distributions. Magnetic
resonance electrical properties tomography (MREPT) aims at
noninvasively obtaining conductivity and permittivity images
at radio-frequency frequencies of magnetic resonance imaging
systems. MREPT algorithms are based on measuring the B1 field
which is perturbed by the electrical properties of the imaged
object. In this study, the relation between the electrical properties
and the measured B1 field is formulated for the first time as a
well-known convection-reaction equation. The suggested novel
algorithm, called “cr-MREPT,” is based on the solution of this
equation on a triangular mesh, and in contrast to previously
proposed algorithms, it is applicable in practice not only for
regions where electrical properties are relatively constant but also
for regions where they vary. The convective field of the convection-reaction
equation depends on the spatial derivatives of the
B1 field, and in the regions where its magnitude is low, a spot-like
artifact is observed in the reconstructed electrical properties
images. For eliminating this artifact, two different methods are
developed, namely “constrained cr-MREPT” and “double-excitation
cr-MREPT.” Successful reconstructions are obtained using
noisy and noise-free simulated data, and experimental data from
phantoms
Thermonuclear Supernovae: Simulations of the Deflagration Stage and Their Implications
Large-scale three-dimensional numerical simulations of the deflagration stage
of a thermonuclear supernova explosion show the formation and evolution of a
highly convoluted turbulent flame in a gravitational field of an expanding
carbon-oxygen white dwarf. The flame dynamics is dominated by the
gravity-induced Rayleigh-Taylor instability that controls the burning rate. The
thermonuclear deflagration releases enough energy to produce a healthy
explosion. The turbulent flame, however, leaves large amounts of unburnt and
partially burnt material near the star center, whereas observations imply these
materials only in outer layers. This disagreement could be resolved if the
deflagration triggers a detonation.Comment: 17 pages, 5 figures. To appear in Science, January 200
Finite Element Simulation of Dense Wire Packings
A finite element program is presented to simulate the process of packing and
coiling elastic wires in two- and three-dimensional confining cavities. The
wire is represented by third order beam elements and embedded into a
corotational formulation to capture the geometric nonlinearity resulting from
large rotations and deformations. The hyperbolic equations of motion are
integrated in time using two different integration methods from the Newmark
family: an implicit iterative Newton-Raphson line search solver, and an
explicit predictor-corrector scheme, both with adaptive time stepping. These
two approaches reveal fundamentally different suitability for the problem of
strongly self-interacting bodies found in densely packed cavities. Generalizing
the spherical confinement symmetry investigated in recent studies, the packing
of a wire in hard ellipsoidal cavities is simulated in the frictionless elastic
limit. Evidence is given that packings in oblate spheroids and scalene
ellipsoids are energetically preferred to spheres.Comment: 17 pages, 7 figures, 1 tabl
Academic Communities of Practice as a Response to Institutional Tensions
Our study aims to contribute to the higher education literature by unpacking how academic Communities of Practice (CoP) support development of teaching as a professional skill. In this study, we focus is on a series of case teaching and writing workshops conducted in Turkey for business and management scholars. The purpose of the workshops was to enhance the usage of case studies in business and management education at the university level in Turkey. The data for this study were collected via semi-structured interviews from a theoretical sample among participants of case workshops who succeeded in completing the authorship of at least one teaching case and its associated teaching note. Our findings reveal that these workshops sowed the seeds towards the development of an academic CoP, where social learning and knowledge transfer occurred through group interaction
Spontaneous Transition of Turbulent Flames to Detonations in Unconfined Media
Deflagration-to-detonation transition (DDT) can occur in environments ranging
from experimental and industrial systems to astrophysical thermonuclear (type
Ia) supernovae explosions. Substantial progress has been made in explaining the
nature of DDT in confined systems with walls, internal obstacles, or
pre-existing shocks. It remains unclear, however, whether DDT can occur in
unconfined media. Here we use direct numerical simulations (DNS) to show that
for high enough turbulent intensities unconfined, subsonic, premixed, turbulent
flames are inherently unstable to DDT. The associated mechanism, based on the
nonsteady evolution of flames faster than the Chapman-Jouguet deflagrations, is
qualitatively different from the traditionally suggested spontaneous reaction
wave model, and thus does not require the formation of distributed flames.
Critical turbulent flame speeds, predicted by this mechanism for the onset of
DDT, are in agreement with DNS results.Comment: 4 pages, 3 figures; accepted to Physical Review Letter
Numerical simulation of detonation reignition in H-O mixtures in area expansions
Time-dependent, two-dimensional, numerical simulations of a transmitted detonation show reignition occuring by one of two mechanisms. The first mechanism involves the collision of triple points as they expand along a decaying shock front. In the second mechanism ignition results from the coalescence of a number of small, relatively high pressure regions left over from the decay of weakened transverse waves. The simulations were performed using an improved chemical kinetic model for stoichiometric H -O mixtures. The initial conditions were a propagating, two-dimensional detonation resolved enough to show transverse wave structure. The calculations provide clarification of the reignition mechanism seen in previous H -O -Ar simulations, and again demonstrate that the transverse wave structure of the detonation front is critical to the reignition process.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41914/1/193-10-1-33_00100033.pd
Effect of Initial Disturbance on The Detonation Front Structure of a Narrow Duct
The effect of an initial disturbance on the detonation front structure in a
narrow duct is studied by three-dimensional numerical simulation. The numerical
method used includes a high resolution fifth-order weighted essentially
non-oscillatory scheme for spatial discretization, coupled with a third order
total variation diminishing Runge-Kutta time stepping method. Two types of
disturbances are used for the initial perturbation. One is a random disturbance
which is imposed on the whole area of the detonation front, and the other is a
symmetrical disturbance imposed within a band along the diagonal direction on
the front. The results show that the two types of disturbances lead to
different processes. For the random disturbance, the detonation front evolves
into a stable spinning detonation. For the symmetrical diagonal disturbance,
the detonation front displays a diagonal pattern at an early stage, but this
pattern is unstable. It breaks down after a short while and it finally evolves
into a spinning detonation. The spinning detonation structure ultimately formed
due to the two types of disturbances is the same. This means that spinning
detonation is the most stable mode for the simulated narrow duct. Therefore, in
a narrow duct, triggering a spinning detonation can be an effective way to
produce a stable detonation as well as to speed up the deflagration to
detonation transition process.Comment: 30 pages and 11 figure
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