23,837 research outputs found
Investigating a simple model of cutaneous wound healing angiogenesis
A simple model of wound healing angiogenesis is presented, and investigated using numerical and asymptotic techniques. The model captures many key qualitative features of the wound healing angiogenic response, such as the propagation of a structural unit into the wound centre. A detailed perturbative study is pursued, and is shown to capture all features of the model. This enables one to show that the level of the angiogenic response predicted by the model is governed to a good approximation by a small number of parameter groupings. Further investigation leads to predictions concerning how one should select between potential optimal means of stimulating cell proliferation in order to increase the level of the angiogenic response
Covariant derivative expansion of fermionic effective action at high temperatures
We derive the fermionic contribution to the 1-loop effective action for A_4
and A_i fields at high temperatures, assuming that gluon fields are slowly
varying but allowing for an arbitrary amplitude of A_4.Comment: RevTex 4, 11 pages, 3 figures. Version 2: Typos corrected; magnetic
fields restricted to parallel sector. Version accepted for publication in PR
Optimization of the magnetic dynamo
In stars and planets, magnetic fields are believed to originate from the
motion of electrically conducting fluids in their interior, through a process
known as the dynamo mechanism. In this Letter, an optimization procedure is
used to simultaneously address two fundamental questions of dynamo theory:
"Which velocity field leads to the most magnetic energy growth?" and "How large
does the velocity need to be relative to magnetic diffusion?" In general, this
requires optimization over the full space of continuous solenoidal velocity
fields possible within the geometry. Here the case of a periodic box is
considered. Measuring the strength of the flow with the root-mean-square
amplitude, an optimal velocity field is shown to exist, but without limitation
on the strain rate, optimization is prone to divergence. Measuring the flow in
terms of its associated dissipation leads to the identification of a single
optimal at the critical magnetic Reynolds number necessary for a dynamo. This
magnetic Reynolds number is found to be only 15% higher than that necessary for
transient growth of the magnetic field.Comment: Optimal velocity field given approximate analytic form. 4 pages, 4
figure
Pressure of Hot QCD at Large N_f
We compute the pressure and entropy of hot QCD in the limit of large number
of fermions, N_f >> N_c ~ 1, to next to leading order in N_f. At this order the
calculation can be done exactly, up to ambiguities due to the presence of a
Landau pole in the theory; the ambiguities are O(T^8/\Lambda^4_{Landau}) and
remain negligible long after the perturbative series (in g^2 N_f) has broken
down. Our results can be used to test several proposed resummation schemes for
the pressure of full QCD.Comment: 16 pages including 4 figures. Short enough for you to read. Numerical
results corrected after an error was found by Andreas Ipp and Anton Rebha
Centrality dependence of elliptic flow and QGP viscosity
In the Israel-Stewart's theory of second order hydrodynamics, we have
analysed the recent PHENIX data on charged particles elliptic flow in Au+Au
collisions.
PHENIX data demand more viscous fluid in peripheral collisions than in
central collisions. Over a broad range of collision centrality (0-10%- 50-60%),
viscosity to entropy ratio () varies between 0-0.17.Comment: Final version to be publiashed in J. Phys. G. 8 pages, 6 figures and
3 table
Computing Similarity between a Pair of Trajectories
With recent advances in sensing and tracking technology, trajectory data is
becoming increasingly pervasive and analysis of trajectory data is becoming
exceedingly important. A fundamental problem in analyzing trajectory data is
that of identifying common patterns between pairs or among groups of
trajectories. In this paper, we consider the problem of identifying similar
portions between a pair of trajectories, each observed as a sequence of points
sampled from it.
We present new measures of trajectory similarity --- both local and global
--- between a pair of trajectories to distinguish between similar and
dissimilar portions. Our model is robust under noise and outliers, it does not
make any assumptions on the sampling rates on either trajectory, and it works
even if they are partially observed. Additionally, the model also yields a
scalar similarity score which can be used to rank multiple pairs of
trajectories according to similarity, e.g. in clustering applications. We also
present efficient algorithms for computing the similarity under our measures;
the worst-case running time is quadratic in the number of sample points.
Finally, we present an extensive experimental study evaluating the
effectiveness of our approach on real datasets, comparing with it with earlier
approaches, and illustrating many issues that arise in trajectory data. Our
experiments show that our approach is highly accurate in distinguishing similar
and dissimilar portions as compared to earlier methods even with sparse
sampling
A mathematical model for the capillary endothelial cell-extracellular matrix interactions in wound-healing angiogenesis
Angiogenesis, the process by which new blood capillaries grow into a tissue from surrounding parent vessels, is a key event in dermal wound healing, malignant-tumour growth, and other pathologic conditions. In wound healing, new capillaries deliver vital metabolites such as amino acids and oxygen to the cells in the wound which are involved in a complex sequence of repair processes. The key cellular constituents of these new capillaries are endothelial cells: their interactions with soluble biochemical and insoluble extracellular matrix (ECM) proteins have been well documented recently, although the biological mechanisms underlying wound-healing angiogenesis are incompletely understood. Considerable recent research, including some continuum mathematical models, have focused on the interactions between endothelial cells and soluble regulators (such as growth factors). In this work, a similar modelling framework is used to investigate the roles of the insoluble ECM substrate, of which collagen is the predominant macromolecular protein. Our model consists of a partial differential equation for the endothelial-cell density (as a function of position and time) coupled to an ordinary differential equation for the ECM density. The ECM is assumed to regulate cell movement (both random and directed) and proliferation, whereas the cells synthesize and degrade the ECM. Analysis and numerical solutions of these equations highlights the roles of these processes in wound-healing angiogenesis. A nonstandard approximation analysis yields insight into the travel ling-wave structure of the system. The model is extended to two spatial dimensions (parallel and perpendicular to the plane of the skin), for which numerical simulations are presented. The model predicts that ECM-mediated random motility and cell proliferation are key processes which drive angiogenesis and that the details of the functional dependence of these processes on the ECM density, together with the rate of ECM remodelling, determine the qualitative nature of the angiogenic response. These predictions are experimentally testable, and they may lead towards a greater understanding of the biological mechanisms involved in wound-healing angiogenesis
Measurements of long-lived cosmogenic nuclides in returned comet nucleus samples
Measurements of long lived cosmic ray produced radionuclides have given much information on the histories and rates of surface evolution for meteorites, the Moon and the Earth. These nuclides can be equally useful in studying cometary histories and post nebular processing of cometary surfaces. The concentration of these nuclides depends on the orbit of the comet (cosmic ray intensity changes with distance from the sun), the depth of the sampling site in the comet surface, and the rate of continuous evolution of the surface (erosion rate of surface materials). If the orbital parameters and the sampling depth are known, production rates of cosmogenic nuclides can be fairly accurately calculated by theoretical models normalized to measurement on lunar surface materials and meteoritic samples. Due to the continuous evaporation of surface materials, it is expected that the long lived radioactivities will be undersaturated. Accurate measurements of the degree of undersaturation in nuclides of different half-lives allows for the determination of the rate of surface material loss over the last few million years
Covariant derivative expansion of Yang-Mills effective action at high temperatures
Integrating out fast varying quantum fluctuations about Yang--Mills fields
A_i and A_4, we arrive at the effective action for those fields at high
temperatures. Assuming that the fields A_i and A_4 are slowly varying but that
the amplitude of A_4 is arbitrary, we find a non-trivial effective gauge
invariant action both in the electric and magnetic sectors. Our results can be
used for studying correlation functions at high temperatures beyond the
dimensional reduction approximation, as well as for estimating quantum weights
of classical static configurations such as dyons.Comment: Minor changes. References added. Paper accepted for publication in
Phys.Rev.
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