2,473 research outputs found
The application of large amplitude oscillatory stress in a study of fully formed fibrin clots
The suitability of controlled stress large amplitude oscillatory shear (LAOStress) for the characterisation of the nonlinear viscoelastic properties of fully formed fibrin clots is investigated. Capturing the rich nonlinear viscoelastic behaviour of the fibrin network is important for understanding the structural behaviour of clots formed in blood vessels which are exposed to a wide range of shear stresses. We report, for the first time, that artefacts due to ringing exist in both the sample stress and strain waveforms of a LAOStress measurement which will lead to errors in the calculation of nonlinear viscoelastic properties. The process of smoothing the waveforms eliminates these artefacts whilst retaining essential rheological information. Furthermore, we demonstrate the potential of LAOStress for characterising the nonlinear viscoelastic properties of fibrin clots in response to incremental increases of applied stress up to the point of fracture. Alternating LAOStress and small amplitude oscillatory shear measurements provide detailed information of reversible and irreversible structural changes of the fibrin clot as a consequence of elevated levels of stress. We relate these findings to previous studies involving large scale deformations of fibrin clots. The LAOStress technique may provide useful information to help understand why some blood clots formed in vessels are stable (such as in deep vein thrombosis) and others break off (leading to a life threatening pulmonary embolism)
On Renormalized Strong-Coupling Quenched QED in Four Dimensions
We study renormalized quenched strong-coupling QED in four dimensions in
arbitrary covariant gauge. Above the critical coupling leading to dynamical
chiral symmetry breaking, we show that there is no finite chiral limit. This
behaviour is found to be independent of the detailed choice of photon-fermion
proper vertex in the Dyson-Schwinger equation formalism, provided that the
vertex is consistent with the Ward-Takahashi identity and multiplicative
renormalizability. We show that the finite solutions previously reported lie in
an unphysical regime of the theory with multiple solutions and ultraviolet
oscillations in the mass functions. This study supports the assertion that in
four dimensions strong coupling QED does not have a continuum limit in the
conventional sense.Comment: REVTEX 3.0, 15 pages,including 4 eps files comprising 3 figures.
Submitted to Phys. Rev.
Chiral Symmetry Breaking in Quenched Massive Strong-Coupling QED
We present results from a study of subtractive renormalization of the fermion
propagator Dyson-Schwinger equation (DSE) in massive strong-coupling quenched
QED. Results are compared for three different fermion-photon proper vertex
{\it Ans\"{a}tze\/}: bare , minimal Ball-Chiu, and
Curtis-Pennington. The procedure is straightforward to implement and
numerically stable. This is the first study in which this technique is used and
it should prove useful in future DSE studies, whenever renormalization is
required in numerical work.Comment: REVTEX 3.0, 15 pages plus 7 uuencoded PostScript figure
Chiral symmetry breaking in dimensionally regularized nonperturbative quenched QED
In this paper we study dynamical chiral symmetry breaking in dimensionally
regularized quenched QED within the context of Dyson-Schwinger equations. In D
< 4 dimensions the theory has solutions which exhibit chiral symmetry breaking
for all values of the coupling. To begin with, we study this phenomenon both
numerically and, with some approximations, analytically within the rainbow
approximation in the Landau gauge. In particular, we discuss how to extract the
critical coupling alpha_c = pi/3 relevant in four dimensions from the D
dimensional theory. We further present analytic results for the chirally
symmetric solution obtained with the Curtis-Pennington vertex as well as
numerical results for solutions exhibiting chiral symmetry breaking. For these
we demonstrate that, using dimensional regularization, the extraction of the
critical coupling relevant for this vertex is feasible. Initial results for
this critical coupling are in agreement with cut-off based work within the
currently achievable numerical precision.Comment: 24 pages, including 5 figures; submitted to Phys. Rev.
Dynamical chiral symmetry breaking and confinement with an infrared-vanishing gluon propagator?
We study a model Dyson-Schwinger equation for the quark propagator closed
using an {\it Ansatz} for the gluon propagator of the form \mbox{} and two {\it Ans\"{a}tze} for the quark-gluon vertex: the
minimal Ball-Chiu and the modified form suggested by Curtis and Pennington.
Using the quark condensate as an order parameter, we find that there is a
critical value of such that the model does not support dynamical chiral
symmetry breaking for . We discuss and apply a confinement test which
suggests that, for all values of , the quark propagator in the model {\bf is
not} confining. Together these results suggest that this Ansatz for the gluon
propagator is inadequate as a model since it does not yield the expected
behaviour of QCD.Comment: 21 Pages including 4 PostScript figures uuencoded at the end of the
file. Replacement: slight changes of wording and emphasis. ADP-93-215/T133,
ANL-PHY-7599-TH-93, FSU-SCRI-93-108, REVTEX 3.
Effects of High Flavanol Dark Chocolate on Cardiovascular Function and Platelet Aggregation.
Regular consumption of chocolate and cocoa products has been linked to reduced cardiovascular mortality. This study compared the effects of high flavanol dark chocolate (HFDC; 1064mg flavanols/day for 6 weeks) and low flavanol dark chocolate (LFDC; 88mg flavanols/day for 6 weeks) on blood pressure, heart rate, vascular function and platelet aggregation in men with pre-hypertension or mild hypertension. Vascular function was assessed by pulse wave analysis using radial artery applanation tonometry in combination with inhaled salbutamol (0.4 mg) to assess changes due to endothelium-dependent vasodilatation. HFDC did not significantly reduce blood pressure compared to baseline or LFDC. Heart rate was increased by LFDC compared to baseline, but not by HFDC. Vascular responses to salbutamol tended to be greater after HFDC. Platelet aggregation induced by collagen or the thromboxane analogue U46619 was unchanged after LFDC or HFDC, whereas both chocolates reduced responses to ADP and the thrombin receptor activator peptide, SFLLRNamide (TRAP6), relative to baseline. Pre-incubation of platelets with theobromine also attenuated platelet aggregation induced by ADP or TRAP6. We conclude that consumption of HFDC confers modest improvements in cardiovascular function. Platelet aggregation is modulated by a flavanol-independent mechanism that is likely due to theobromine.This study was supported by a grant (to R. Corder) from Barry Callebaut Belgium N
Electromagnetic Pion Form Factor and Neutral Pion Decay Width
The electromagnetic pion form factor, , is calculated for
spacelike- in impulse approximation using a confining quark propagator,
, and a dressed quark-photon vertex, , obtained from realistic,
nonperturbative Dyson-Schwinger equation studies. Good agreement with the
available data is obtained for and other pion observables,
including the decay . This calculation
suggests that soft, nonperturbative contributions dominate at
presently accessible~.Comment: 25 pages, LaTeX, elsart.sty, 5 figures, To appear in Nucl. Phys.
Tradeoffs in jet inlet design: a historical perspective
The design of the inlet(s) is one of the most demanding tasks of the development process of any gas turbine-powered aircraft. This is mainly due to the multi-objective and multidisciplinary nature of the exercise. The solution is generally a compromise between a number of conflicting goals and these conflicts are the subject of the present paper. We look into how these design tradeoffs have been reflected in the actual inlet designs over the years and how the emphasis has shifted from one driver to another. We also review some of the relevant developments of the jet age in aerodynamics and design and manufacturing technology and we examine how they have influenced and informed inlet design decision
HarvardX and MITx: Two Years of Open Online Courses Fall 2012-Summer 2014
What happens when well-known universities offer online courses, assessments, and certificates of completion for free? Early descriptions of Massive Open Online Courses (MOOCs) have emphasized large enrollments, low certification rates, and highly educated registrants. We use data from two years and 68 open online courses offered by Harvard University (via HarvardX) and MIT (via MITx) to broaden the scope of answers to this question. We describe trends over this two-year span, depict participant intent using comprehensive survey instruments, and chart course participation pathways using network analysis. We find that overall participation in our MOOCs remains substantial and that the average growth has been steady. We explore how diverse audiences â including explorers, teachers-as-learners, and residential students â provide opportunities to advance the principles on which HarvardX and MITx were founded: access, research, and residential education
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