3,658 research outputs found
Equilibrium and eigenfunctions estimates in the semi-classical regime
We establish eigenfunctions estimates, in the semi-classical regime, for
critical energy levels associated to an isolated singularity. For Schr\"odinger
operators, the asymptotic repartition of eigenvectors is the same as in the
regular case, excepted in dimension 1 where a concentration at the critical
point occurs. This principle extends to pseudo-differential operators and the
limit measure is the Liouville measure as long as the singularity remains
integrable.Comment: 13 pages, 1 figure, perhaps to be revise
Interaction of multiple particles with a solidification front : from compacted particle layer to particle trapping
The interaction of solidification fronts with objects such as particles,
droplets, cells, or bubbles is a phenomenon with many natural and technological
occurrences. For an object facing the front, it may yield various fates, from
trapping to rejection, with large implications regarding the solidification
pattern. However, whereas most situations involve multiple particles
interacting with each other and the front, attention has focused almost
exclusively on the interaction of a single, isolated object with the front.
Here we address experimentally the interaction of multiple particles with a
solidification front by performing solidification experiments of a monodisperse
particle suspension in a Hele-Shaw cell, with precise control of growth
conditions and real-time visualization. We evidence the growth of a particle
layer ahead of the front at a close-packing volume fraction and we document its
steady state value at various solidification velocities. We then extend single
particle models to the situation of multiple particles by taking into account
the additional force induced on an entering particle by viscous friction in the
compacted particle layer. By a force balance model, this provides an indirect
measure of the repelling mean thermomolecular pressure over a particle entering
the front. The presence of multiple particles is found to increase it following
a reduction of the thickness of the thin liquid film that separates particles
and front. We anticipate the findings reported here to provide a relevant basis
to understand many complex solidification situations in geophysics,
engineering, biology, or food engineering, where multiple objects interact with
the front and control the resulting solidification patterns.Comment: 13 pages, 10 figures, submitted to Langmui
Unintended Consequences: How Qualification Constrains Innovation
The development and implementation of new materials and manufacturing processes for aerospace application is often hindered by the high cost and long time span associated with current qualification procedures. The data requirements necessary for material and process qualification are extensive and often require millions of dollars and multiple years to complete. Furthermore, these qualification data can become obsolete for even minor changes to the processing route. This burden is a serious impediment to the pursuit of revolutionary new materials and more affordable processing methods for air vehicle structures. The application of integrated computational materials engineering methods to this problem can help to reduce the barriers to rapid insertion of new materials and processes. By establishing predictive capability for the development of microstructural features in relation to processing and relating this to critical property characteristics, a streamlined approach to qualification is possible. This paper critically examines the advantages and challenges to a modeling-assisted qualification approach for aerospace structural materials. An example of how this approach might apply towards the emerging field of additive manufacturing is discussed in detail
The Lawrentian Woman: Monsters in the Margins of 20th-Century British Literature
Despite his own conservative values, D.H. Lawrence writes sexually liberated female characters. The most subversive female characters in Lawrence’s oeuvre are the Brangwens of The Rainbow. The Brangwens are prototypical models of a form of femininity that connects women to Nature while distancing them from society; his women are cast as monsters, but are strengthened from their link with Nature. They represent what I am calling the Lawrentian-Woman.
The Lawrentian-Woman has proven influential for contemporary British authors. I examine the Lawrentian-Woman’s adoption by later writers and her evolution from modernist frame to postmodern appropriation. First, I look at the Brangwens. They establish the tropes of the Lawrentian-Woman and provide the base from which to compare the model’s subsequent mutations. Next, I examine modern British writers and their appropriation of the Lawrentian-Woman. The Lawrentian-Woman’s attributes remain intact, but are deconstructed in ways that explore women’s continued liminality in patriarchal society
Bi-Metallic Composite Structures With Designed Internal Residual Stress Field
Shape memory alloys (SMA) have a unique ability to recover small amounts of plastic strain through a temperature induced phase change. For these materials, mechanical displacement can be accomplished by heating the structure to induce a phase change, through which some of the plastic strain previously introduced to the structure can be reversed. This paper introduces a concept whereby an SMA phase is incorporated into a conventional alloy matrix in a co-continuous reticulated arrangement forming a bi-metallic composite structure. Through memory activation of the mechanically constrained SMA phase, a controlled residual stress field is developed in the interior of the structure. The presented experimental data show that the memory activation of the SMA composite component significantly changes the residual stress distribution in the overall structure. Designing the structural arrangement of the two phases to produce a controlled residual stress field could be used to create structures that have much improved durability and damage tolerance properties
Functionally Graded Metal-Metal Composite Structures
Methods and devices are disclosed for creating a multiple alloy composite structure by forming a three-dimensional arrangement of a first alloy composition in which the three-dimensional arrangement has a substantially open and continuous porosity. The three-dimensional arrangement of the first alloy composition is infused with at least a second alloy composition, where the second alloy composition comprises a shape memory alloy. The three-dimensional arrangement is consolidated into a fully dense solid structure, and the original shape of the second alloy composition is set for reversible transformation. Strain is applied to the fully dense solid structure, which is treated with heat so that the shape memory alloy composition becomes memory activated to recover the original shape. An interwoven composite of the first alloy composition and the memory-activated second alloy composition is thereby formed in the multiple alloy composite structure
Microlensing of Circumstellar Disks
We investigate the microlensing effects on a source star surrounded by a
circumstellar disk, as a function of wavelength. The microlensing light curve
of the system encodes the geometry and surface brightness profile of the disk.
In the mid- and far-infrared, the emission of the system is dominated by the
thermal emission from the cold dusty disk. For a system located at the Galactic
center, we find typical magnifications to be of order 10-20% or higher,
depending on the disk surface brightness profile, and the event lasts over one
year. At around 20 microns, where the emission for the star and the disk are
comparable, the difference in the emission areas results in a chromatic
microlensing event. Finally, in the near-infrared and visible, where the
emission of the star dominates, the fraction of star light directly reflected
by the disk slightly modifies the light curve of the system which is no longer
that of a point source. In each case, the corresponding light curve can be used
to probe some of the disk properties. A fraction of 0.1% to 1% optical
microlensing events are expected to be associated with circumstellar disk
systems. We show that the lensing signal of the disk can be detected with
sparse follow-up observations of the next generation space telescopes. While
direct imaging studies of circumstellar disks are limited to the solar
neighborhood, this microlensing technique can probe very distant disk systems
living in various environments and has the potential to reveal a larger
diversity of circumstellar disks.Comment: 9 pages, 7 figures. Accepted for publication in Ap
Trees of Unusual Size: Biased Inference of Early Bursts from Large Molecular Phylogenies
An early burst of speciation followed by a subsequent slowdown in the rate of
diversification is commonly inferred from molecular phylogenies. This pattern
is consistent with some verbal theory of ecological opportunity and adaptive
radiations. One often-overlooked source of bias in these studies is that of
sampling at the level of whole clades, as researchers tend to choose large,
speciose clades to study. In this paper, we investigate the performance of
common methods across the distribution of clade sizes that can be generated by
a constant-rate birth-death process. Clades which are larger than expected for
a given constant-rate branching process tend to show a pattern of an early
burst even when both speciation and extinction rates are constant through time.
All methods evaluated were susceptible to detecting this false signature when
extinction was low. Under moderate extinction, both the gamma-statistic and
diversity-dependent models did not detect such a slowdown but only because the
signature of a slowdown was masked by subsequent extinction. Some models which
estimate time-varying speciation rates are able to detect early bursts under
higher extinction rates, but are extremely prone to sampling bias. We suggest
that examining clades in isolation may result in spurious inferences that rates
of diversification have changed through time.Comment: 17 pages, 5 figure
A constitutive material model for nonlinear finite element structural analysis using an iterative matrix approach
A unified numerical method for the integration of stiff time dependent constitutive equations is presented. The solution process is directly applied to a constitutive model proposed by Bodner. The theory confronts time dependent inelastic behavior coupled with both isotropic hardening and directional hardening behaviors. Predicted stress-strain responses from this model are compared to experimental data from cyclic tests on uniaxial specimens. An algorithm is developed for the efficient integration of the Bodner flow equation. A comparison is made with the Euler integration method. An analysis of computational time is presented for the three algorithms
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