6,724 research outputs found
Dynamical behavior of a complex fluid near an out-of-equilibrium transition: approaching simple rheological chaos
We report here an extensive study of sustained oscillations of the viscosity
of a complex fluid near an out-of-equilibrium transition. Using well defined
protocols, we perform rheological measurements of the onion texture near a
layering transition in a Couette flow. This complex fluid exhibits sustained
oscillations of the viscosity, on a large time scale (500s) at controlled
stress. These oscillations are directly correlated to an oscillating
microstrutural change of the texture of the fluid. We observe a great diversity
of dynamical behavior and we show that there is a coupling with spatial effects
in the gradient v direction. This is in agreement with a carefull analysis of
the temporal series of the viscosity with the dynamical system theory. This
analysis indicates that the observed dynamical responses do not strictly
correspond to 3-dimensional chaotic states, probably because some
spatio-temporal effects are present and are likely to play an important role.Comment: submitted to Phys. Rev.
3D printing dimensional calibration shape: Clebsch Cubic
3D printing and other layer manufacturing processes are challenged by
dimensional accuracy. Several techniques are used to validate and calibrate
dimensional accuracy through the complete building envelope. The validation
process involves the growing and measuring of a shape with known parameters.
The measured result is compared with the intended digital model. Processes with
the risk of deformation after time or post processing may find this technique
beneficial. We propose to use objects from algebraic geometry as test shapes. A
cubic surface is given as the zero set of a 3rd degree polynomial with 3
variables. A class of cubics in real 3D space contains exactly 27 real lines.
We provide a library for the computer algebra system Singular which, from 6
given points in the plane, constructs a cubic and the lines on it. A surface
shape derived from a cubic offers simplicity to the dimensional comparison
process, in that the straight lines and many other features can be analytically
determined and easily measured using non-digital equipment. For example, the
surface contains so-called Eckardt points, in each of which three of the lines
intersect, and also other intersection points of pairs of lines. Distances
between these intersection points can easily be measured, since the points are
connected by straight lines. At all intersection points of lines, angles can be
verified. Hence, many features distributed over the build volume are known
analytically, and can be used for the validation process. Due to the thin shape
geometry the material required to produce an algebraic surface is minimal. This
paper is the first in a series that proposes the process chain to first define
a cubic with a configuration of lines in a given print volume and then to
develop the point cloud for the final manufacturing. Simple measuring
techniques are recommended.Comment: 8 pages, 1 figure, 1 tabl
Intermediate range chemical ordering of cations in simple molten alkali halides
The presence of first sharp diffraction peaks in the partial structure
factors is investigated in computer simulations of molten mixtures of alkali
halides. An intermediate range ordering appears for the Li+ ions only, which is
associated with clustering of this species and is not reflected in the
arrangement of other ions. This ordering is surprising in view of the
simplicity of the interionic interactions in alkali halides. The clustering
reflects an incomplete mixing of the various species on a local length scale,
which can be demonstrated by studying the complementary sub-space of cations in
the corresponding pure alkali halides by means of a void analysis.Comment: 5 pages, 5 figure
Recommended from our members
The cytoplasmic domain of CD4 promotes the development of CD4 lineage T cells.
Thymocytes must bind major histocompatibility complex (MHC) proteins on thymic epithelial cells in order to mature into either CD8+ cytotoxic T cells or CD4+ helper T cells. Thymic precursors express both CD8 and CD4, and it has been suggested that the intracellular signals generated by CD8 or CD4 binding to class I or II MHC, respectively, might influence the fate of uncommitted cells. Here we test the notion that intracellular signaling by CD4 directs the development of thymocytes to a CD4 lineage. A hybrid protein consisting of the CD8 extracellular and transmembrane domains and the cytoplasmic domain of CD4 (CD884) should bind class I MHC but deliver a CD4 intracellular signal. We find that expression of a hybrid CD884 protein in thymocytes of transgenic mice leads to the development of large numbers of class I MHC-specific, CD4 lineage T cells. We discuss these results in terms of current models for CD4 and CD8 lineage commitment
Are the stars of a new class of variability detected in NGC~3766 fast rotating SPB stars?
A recent photometric survey in the NGC~3766 cluster led to the detection of
stars presenting an unexpected variability. They lie in a region of the
Hertzsprung-Russell (HR) diagram where no pulsation are theoretically expected,
in between the Scuti and slowly pulsating B (SPB) star instability
domains. Their variability periods, between 0.1--0.7~d, are outside the
expected domains of these well-known pulsators. The NCG~3766 cluster is known
to host fast rotating stars. Rotation can significantly affect the pulsation
properties of stars and alter their apparent luminosity through gravity
darkening. Therefore we inspect if the new variable stars could correspond to
fast rotating SPB stars. We carry out instability and visibility analysis of
SPB pulsation modes within the frame of the traditional approximation. The
effects of gravity darkening on typical SPB models are next studied. We find
that at the red border of the SPB instability strip, prograde sectoral (PS)
modes are preferentially excited, with periods shifted in the 0.2--0.5~d range
due to the Coriolis effect. These modes are best seen when the star is seen
equator-on. For such inclinations, low-mass SPB models can appear fainter due
to gravity darkening and as if they were located between the ~Scuti and
SPB instability strips.Comment: 6 pages, 2 figures, to appear in the proceedings of the IAU Symposium
307, New windows on massive stars: asteroseismology, interferometry, and
spectropolarimetr
The ontology of causal process theories
There is a widespread belief that the so-called process theories of causation developed by Wesley Salmon and Phil Dowe have given us an original account of what causation really is. In this paper, I show that this is a misconception. The notion of "causal process" does not offer us a new ontological account of causation. I make this argument by explicating the implicit ontological commitments in Salmon and Dowe's theories. From this, it is clear that Salmon's Mark Transmission Theory collapses to a counterfactual theory of causation, while the Conserved Quantity Theory collapses to David Fair's phsyicalist reduction of causation
Determining the metallicity of the solar envelope using seismic inversion techniques
The solar metallicity issue is a long-lasting problem of astrophysics,
impacting multi- ple fields and still subject to debate and uncertainties.
While spectroscopy has mostly been used to determine the solar heavy elements
abundance, helioseismologists at- tempted providing a seismic determination of
the metallicity in the solar convective enveloppe. However, the puzzle remains
since two independent groups prodived two radically different values for this
crucial astrophysical parameter. We aim at provid- ing an independent seismic
measurement of the solar metallicity in the convective enveloppe. Our main goal
is to help provide new information to break the current stalemate amongst
seismic determinations of the solar heavy element abundance. We start by
presenting the kernels, the inversion technique and the target function of the
inversion we have developed. We then test our approach in multiple
hare-and-hounds exercises to assess its reliability and accuracy. We then apply
our technique to solar data using calibrated solar models and determine an
interval of seismic measurements for the solar metallicity. We show that our
inversion can indeed be used to estimate the solar metallicity thanks to our
hare-and-hounds exercises. However, we also show that further dependencies in
the physical ingredients of solar models lead to a low accuracy. Nevertheless,
using various physical ingredients for our solar models, we determine
metallicity values between 0.008 and 0.014.Comment: Accepted for publication in MNRA
Adoption and Influence: Industry Evaluation of the GrassGro\u3csup\u3eTM\u3c/sup\u3e Decision Support Tool
The GrassGro Decision Support tool (DS tool) (Donnelly and Moore, 1999) was released for commercial use in Australia with a training package in late 1997. An assessment of its adoption was made by a survey of 53 registered users in March 1999. An evaluation of the software and training package was made at training workshops by 92 users between 1997 and December 1999. The response rates to the survey and the evaluation were 62% and 82% respectively. Seventy six percent of survey respondents had analysed at least one problem with GrassGro and 27% had applied GrassGro to five or more problems encountered in the management of grazing enterprises. Users valued training highly and on completion were confident in their use of GrassGro; 63% of repondents had extrapolated the results of their analyses to a district or regional level
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