643 research outputs found
Energy-stable discretization of the one-dimensional two-fluid model
In this paper we present a complete framework for the energy-stable
simulation of stratified incompressible flow in channels, using the
one-dimensional two-fluid model. Building on earlier energy-conserving work on
the basic two-fluid model, our new framework includes diffusion, friction, and
surface tension. We show that surface tension can be added in an
energy-conserving manner, and that diffusion and friction have a strictly
dissipative effect on the energy.
We then propose spatial discretizations for these terms such that a
semi-discrete model is obtained that has the same conservation properties as
the continuous model. Additionally, we propose a new energy-stable advective
flux scheme that is energy-conserving in smooth regions of the flow and
strictly dissipative where sharp gradients appear. This is obtained by
combining, using flux limiters, a previously developed energy-conserving
advective flux with a novel first-order upwind scheme that is shown to be
strictly dissipative.
The complete framework, with diffusion, surface tension, and a bounded
energy, is linearly stable to short wavelength perturbations, and exhibits
nonlinear damping near shocks. The model yields smoothly converging numerical
solutions, even under conditions for which the basic two-fluid model is
ill-posed. With our explicit expressions for the dissipation rates, we are able
to attribute the nonlinear damping to the different dissipation mechanisms, and
compare their effects
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Spring 1974
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Performance of the Electromagnetic Calorimeter of the HERMES Experiment
The performance of the electromagnetic calorimeter of the HERMES experiment
is described. The calorimeter consists of 840 radiation resistant F101
lead-glass counters. The response to positrons up to 27.5 GeV, the comparison
between the measured energy and the momentum reconstructed from tracking,
long-term stability, hadron rejection and neutral meson invariant mass
reconstruction are shown.Comment: 22 pages, 13 figures, LaTeX, accepted by NI
Baryon-Baryon Interactions
After a short survey of some topics of interest in the study of baryon-baryon
scattering, the recent Nijmegen energy dependent partial wave analysis (PWA) of
the nucleon-nucleon data is reviewed. In this PWA the energy range for both pp
and np is now 0 < Tlab < 350 MeV and a chi^2_{d.o.f.}=1.08 was reached. The
implications for the pion-nucleon coupling constants are discussed. Comments
are made with respect to recent discussions around this coupling constant in
the literature. In the second part, we briefly sketch the picture of the baryon
in several, more or less QCD-based, quark-models that have been rather
prominent in the literature. Inspired by these pictures we constructed a new
soft-core model for the nucleon-nucleon interaction and present the first
results of this model in a chi^2 -fit to the new multi-energy Nijmegen PWA.
With this new model we succeeded in narrowing the gap between theory and
experiment at low energies. For the energies Tlab = 25-320 MeV we reached a
record low chi^2_{p.d.p.} = 1.16. We finish the paper with some conclusions and
an outlook describing the extension of the new model to baryon-baryon
scattering.Comment: 12 pages LaTeX and one postscript figure included. Invited talk
presented at the XIVth European Conference of Few-Body Problems in Physics,
Amsterdam, August 23-28, 199
Active Tension Network model suggests an exotic mechanical state realized in epithelial tissues.
Mechanical interactions play a crucial role in epithelial morphogenesis, yet understanding the complex mechanisms through which stress and deformation affect cell behavior remains an open problem. Here we formulate and analyze the Active Tension Network (ATN) model, which assumes that the mechanical balance of cells within a tissue is dominated by cortical tension and introduces tension-dependent active remodeling of the cortex. We find that ATNs exhibit unusual mechanical properties. Specifically, an ATN behaves as a fluid at short times, but at long times supports external tension like a solid. Furthermore, an ATN has an extensively degenerate equilibrium mechanical state associated with a discrete conformal - "isogonal" - deformation of cells. The ATN model predicts a constraint on equilibrium cell geometries, which we demonstrate to approximately hold in certain epithelial tissues. We further show that isogonal modes are observed in the fruit y embryo, accounting for the striking variability of apical areas of ventral cells and helping understand the early phase of gastrulation. Living matter realizes new and exotic mechanical states, the study of which helps to understand biological phenomena
Measurement of eâșeâ»-->eâșeâ» and eâșeâ»-->gammagamma at energies up to 36.7 GeV
e+e- +- +- ... + e e und e e + yy wurden bel Energlen zwischen 33.0 und 36.7 GeV gemessen. Die Ergebnisse stimmen mit den Vorhersagen der Quantenelektrodynamik ĂŒberein. Ein Vergleich mit dem Standardmodell der elektroschwachen Wechselwirkung liefert sin 20w= 0.25 ± 0.13
Alignment of cellular motility forces with tissue flow as a mechanism for efficient wound healing
Recent experiments have shown that spreading epithelial sheets
exhibit a long-range coordination of motility forces that leads to
a buildup of tension in the tissue, which may enhance cell division
and the speed of wound healing. Furthermore, the edges of these
epithelial sheets commonly show finger-like protrusions whereas the
bulk often displays spontaneous swirls of motile cells. To explain
these experimental observations, we propose a simple flocking-type
mechanism, in which cells tend to align their motility forceswith their
velocity. Implementing this idea in amechanical tissue simulation, the
proposed model gives rise to efficient spreading and can explain the
experimentally observed long-range alignment of motility forces in
highly disordered patterns, as well as the buildup of tensile stress
throughout the tissue. Our model also qualitatively reproduces the
dependence of swirl size and swirl velocity on cell density reported in
experiments and exhibits an undulation instability at the edge of the
spreading tissue commonly observed in vivo. Finally, we study the
dependence of colony spreading speed on important physical and
biological parameters and derive simple scaling relations that show
that coordination of motility forces leads to an improvement of the
wound healing process for realistic tissue parameters
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