21,899 research outputs found
Axisymmetric buckling of a spherical shell embedded in an elastic medium under uniaxial stress at infinity
The problem of a thin spherical linearly-elastic shell, perfectly bonded to an infinite linearly-elastic medium is considered. A constant axisymmetric stress field is applied at infinity in the matrix, and the displacement and stress fields in the shell and matrix are evaluated by means of harmonic potential functions. In order to examine the stability of this solution, the buckling problem of a shell which experiences this deformation is considered. Using Koiter's nonlinear shallow shell theory, restricting buckling patterns to those which are axisymmetric, and using the Rayleigh–Ritz method by expanding the buckling patterns in an infinite series of Legendre functions, an eigenvalue problem for the coefficients in the infinite series is determined. This system is truncated and solved numerically in order to analyse the behaviour of the shell as it undergoes buckling, and to identify the critical buckling stress in two cases — namely where the shell is hollow and the stress at infinity is either uniaxial or radial
High temperature glass coatings for superalloys and refractory metals
New glasses are used as protective coatings on metals and alloys susceptible to oxidation at high temperatures in oxidizing atmospheres. Glasses are stable and solid at temperatures up to 1000 deg C, adhere well to metal surfaces, and are usable for metals with broad range of expansion coefficients
Development and evaluation of controlled viscosity coatings for superalloys
Controlled viscosity glass based protective coatings for superalloys for turbine blade application
Detailed Structure and Dynamics in Particle-in-Cell Simulations of the Lunar Wake
The solar wind plasma from the Sun interacts with the Moon, generating a wake
structure behind it, since the Moon is to a good approximation an insulator,
has no intrinsic magnetic field and a very thin atmosphere. The lunar wake in
simplified geometry has been simulated via a 1-1/2-D electromagnetic
particle-in-cell code, with high resolution in order to resolve the full phase
space dynamics of both electrons and ions. The simulation begins immediately
downstream of the moon, before the solar wind has infilled the wake region,
then evolves in the solar wind rest frame. An ambipolar electric field and a
potential well are generated by the electrons, which subsequently create a
counter-streaming beam distribution, causing a two-stream instability which
confines the electrons. This also creates a number of electron phase space
holes. Ion beams are accelerated into the wake by the ambipolar electric field,
generating a two stream distribution with phase space mixing that is strongly
influenced by the potentials created by the electron two-stream instability.
The simulations compare favourably with WIND observations.Comment: 10 pages, 13 figures, to be published in Physics of Plasma
Homogeneous Relaxation at Strong Coupling from Gravity
Homogeneous relaxation is a ubiquitous phenomenon in semiclassical kinetic
theories where the quasiparticles are distributed uniformly in space, and the
equilibration involves only their velocity distribution. For such solutions,
the hydrodynamic variables remain constant. We construct asymptotically AdS
solutions of Einstein's gravity dual to such processes at strong coupling,
perturbatively in the amplitude expansion, where the expansion parameter is the
ratio of the amplitude of the non-hydrodynamic shear-stress tensor to the
pressure. At each order, we sum over all time derivatives through exact
recursion relations. We argue that the metric has a regular future horizon,
order by order in the amplitude expansion, provided the shear-stress tensor
follows an equation of motion. At the linear order, this equation of motion
implies that the metric perturbations are composed of zero wavelength
quasinormal modes. Our method allows us to calculate the non-linear corrections
to this equation perturbatively in the amplitude expansion. We thus derive a
special case of our previous conjecture on the regularity condition on the
boundary stress tensor that endows the bulk metric with a regular future
horizon, and also refine it further. We also propose a new outlook for
heavy-ion phenomenology at RHIC and ALICE.Comment: 60 pages, a section titled "Outlook for RHIC and ALICE" has been
added, accepted for publication in Physical Review
Examples of mathematical modeling tales from the crypt
Mathematical modeling is being increasingly recognized within the biomedical sciences as an important tool that can aid the understanding of biological systems. The heavily regulated cell renewal cycle in the colonic crypt provides a good example of how modeling can be used to find out key features of the system kinetics, and help to explain both the breakdown of homeostasis and the initiation of tumorigenesis. We use the cell population model by Johnston et al. (2007) Proc. Natl. Acad. Sci. USA 104, 4008-4013, to illustrate the power of mathematical modeling by considering two key questions about the cell population dynamics in the colonic crypt. We ask: how can a model describe both homeostasis and unregulated growth in tumorigenesis; and to which parameters in the system is the model most sensitive? In order to address these questions, we discuss what type of modeling approach is most appropriate in the crypt. We use the model to argue why tumorigenesis is observed to occur in stages with long lag phases between periods of rapid growth, and we identify the key parameters
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