772 research outputs found
Frost weathering of chalk
The processes and factors that determine the heave and fracture of frost-susceptible bedrock exposed to temperature cycling above and below 0°C are little known but important to understanding of rock deformation, weathering and ground conditions. To investigate the early stages of heave, settlement and fracture of intact chalk, physical modelling experiments were performed on blocks of Saint Cyr Tuffeau and Totternhoe Clunch. Unidirectional (downward) freezing simulated seasonally frozen bedrock in non-permafrost regions, and bidirectional (upward from permafrost and downward from the surface) simulated an active layer above permafrost.
Heave and settlement of the top of the blocks were monitored in relation to rock temperature and unfrozen water content. Heave and settlement showed complex behavior that varied with moisture content, freezing regime and time. Progressive heave of wet chalk during thaw periods (simulated summers) is attributed to microcracking in near-surface permafrost. Macrocracking was favoured near the rock top during unidirectional freezing and near the permafrost table during bidirectional freezing, producing extensive fracture networks. Four processes, operating singly or in combination, account for the heave and settlement behavior: (1) thermal expansion and contraction in dry chalk; (2) volumetric expansion of freezing water, causing bursts of heave; (3) ice segregation, causing sustained heave and rock fracture; and (4) freezeâthaw cycling, causing initial consolidation and settling of wet chalk during unidirectional freezing.
The experimental data and field observations of chalk weathering profiles elucidate the nature and origin of chalk brecciation. Type 1 brecciation (angular or subangular rock fragments separated by unfilled fractures with matched sides) is attributed primarily to ice segregation. Type 2 brecciation (subangular to rounded lumps of rockâlithorelictsâset in a fine-grained matrix of the same, but softer and remoulded material) probably resulted from frost weathering and limited ground movement, particularly beneath the sides and bottoms of wet (now dry) valleys
An Efficient Numerical Scheme for Simulating Particle Acceleration in Evolving Cosmic-Ray Modified Shocks
We have developed a new, very efficient numerical scheme to solve the CR
diffusion convection equation that can be applied to the study of the nonlinear
time evolution of CR modified shocks for arbitrary spatial diffusion
properties. The efficiency of the scheme derives from its use of coarse-grained
finite momentum volumes. This approach has enabled us, using
momentum bins spanning nine orders of magnitude in momentum, to carry out
simulations that agree well with results from simulations of modified shocks
carried out with our conventional finite difference scheme requiring more than
an order of magnitude more momentum points. The coarse-grained, CGMV scheme
reduces execution times by a factor approximately half the ratio of momentum
bins used in the two methods. Depending on the momentum dependence of the
diffusion, additional economies in required spatial and time resolution can be
utilized in the CGMV scheme, as well. These allow a computational speed-up of
at least an order of magnitude in some cases.Comment: Accepted for publication in Astroparticle Physics; 19 pages and 5
figure
Self-Similar Evolution of Cosmic-Ray-Modified Quasi-Parallel Plane Shocks
Using an improved version of the previously introduced CRASH (Cosmic Ray
Acceleration SHock) code, we have calculated the time evolution of cosmic-ray
(CR) modified quasi-parallel plane shocks for Bohm-like diffusion, including
self-consistent models of Alfven wave drift and dissipation, along with thermal
leakage injection of CRs. The new simulations follow evolution of the CR
distribution to much higher energies than our previous study, providing a
better examination of evolutionary and asymptotic behaviors. The postshock CR
pressure becomes constant after quick initial adjustment, since the evolution
of the CR partial pressure expressed in terms of a momentum similarity variable
is self-similar. The shock precursor, which scales as the diffusion length of
the highest energy CRs, subsequently broadens approximately linearly with time,
independent of diffusion model, so long as CRs continue to be accelerated to
ever-higher energies. This means the nonlinear shock structure can be described
approximately in terms of the similarity variable, x/(u_s t), where u_s is the
shock speed once the postshock pressure reaches an approximate time asymptotic
state. As before, the shock Mach number is the key parameter determining the
evolution and the CR acceleration efficiency, although finite Alfven wave drift
and wave energy dissipation in the shock precursor reduce the effective
velocity change experienced by CRs, so reduce acceleration efficiency
noticeably, thus, providing a second important parameter at low and moderate
Mach numbers.Comment: 29 pages, 8 figure
Exploring the Integration of Environmental Impacts in the Cost Analysis of the Pilot MEL-SELF Trial of Patient-Led Melanoma Surveillance
Aims
Human health is intrinsically linked with planetary health. But planetary resources are currently being degraded and this poses an existential threat to human health and the sustainability of our healthcare systems. The aims of this study were to (1) describe an approach to integrate environmental impacts in a cost analysis; and (2) demonstrate this approach by estimating select environmental impacts alongside traditional health system and other costs using the example of the pilot MEL-SELF randomised controlled trial of patient-led melanoma surveillance.
Methods
Economic costs were calculated alongside a randomised trial using standard cost analysis methodology from a societal perspective. Environmental impacts were calculated using a type of carbon footprinting methodology called process-based life cycle analysis. This method considers three scopes of carbon emissions: Scope 1, which occur directly from the intervention; Scope 2, which occur indirectly from the interventionâs energy use; and Scope 3, which occur indirectly because of the value chain of the intervention. In this study we only included emissions from patient transport to attend their melanoma clinic over the study period of 6 months.
Results
The environmental impact per participant across allocated groups for patient transport to their melanoma clinic was estimated to be 10 kg carbon dioxide equivalent. Economic costs across the allocated groups indicated substantial health system costs, out-of-pocket costs, and productivity losses associated with melanoma surveillance. The largest cost contributor was health system costs, and the most expensive category of health system cost was hospital admission
The impact on human health of car-related air pollution in the UK, 1995-2005
We have analysed the impact on human health of emissions produced by the UK car fleet in the years 1995
and 2005. Calculations were based on reported measurements of pollutant concentration, literature values of
exposure response coefficients and data for mortality and morbidity. A share was attributed to the car fleet
based on emissions data. Although the total distance driven in the UK increased by 16% over this period to
460 billion km, there was a significant fall in engine emissions as increasingly stringent regulations (EURO
standards) were introduced. As a result there was a decrease of some 25% in the number of deaths
attributable to car-related air pollution â down to 5589 in 2005. The estimated number of years of life lost at
65 000 (England and Wales) in 2005, was about half that caused by road accidents involving cars in the same
year. We report further calculations which show the effect of car-related pollution on hospital admissions.
Our method is straightforward, providing acceptable estimates for health impacts on the predominantly urban
population of the UK. There remains a need for more work, particularly cohort studies of morbidity, to
establish the long-term effects of air pollution
Vacuum structure of Toroidal Carbon Nanotubes
Low energy excitations in carbon nanotubes can be described by an effective
field theory of two components spinor. It is pointed out that the chiral
anomaly in 1+1 dimensions should be observed in a metallic toroidal carbon
nanotube on a planar geometry with varying magnetic field. We propose an
experimental setup for studying this quantum effect. We also analyze the vacuum
structure of the metallic toroidal carbon nanotube including the Coulomb
interactions and discuss some effects of external charges on the vacuum.Comment: 10 pages, 11 figure
The Physics of Cluster Mergers
Clusters of galaxies generally form by the gravitational merger of smaller
clusters and groups. Major cluster mergers are the most energetic events in the
Universe since the Big Bang. Some of the basic physical properties of mergers
will be discussed, with an emphasis on simple analytic arguments rather than
numerical simulations. Semi-analytic estimates of merger rates are reviewed,
and a simple treatment of the kinematics of binary mergers is given. Mergers
drive shocks into the intracluster medium, and these shocks heat the gas and
should also accelerate nonthermal relativistic particles. X-ray observations of
shocks can be used to determine the geometry and kinematics of the merger. Many
clusters contain cooling flow cores; the hydrodynamical interactions of these
cores with the hotter, less dense gas during mergers are discussed. As a result
of particle acceleration in shocks, clusters of galaxies should contain very
large populations of relativistic electrons and ions. Electrons with Lorentz
factors gamma~300 (energies E = gamma m_e c^2 ~ 150 MeV) are expected to be
particularly common. Observations and models for the radio, extreme
ultraviolet, hard X-ray, and gamma-ray emission from nonthermal particles
accelerated in these mergers are described.Comment: 38 pages with 9 embedded Postscript figures. To appear in Merging
Processes in Clusters of Galaxies, edited by L. Feretti, I. M. Gioia, and G.
Giovannini (Dordrecht: Kluwer), in press (2001
k=0Magnetic Structure and Absence of Ferroelectricity in SmFeO3
SmFeO3 has attracted considerable attention very recently due to the reported
multiferroic properties above room-temperature. We have performed powder and
single crystal neutron diffraction as well as complementary polarization
dependent soft X-ray absorption spectroscopy measurements on floating-zone
grown SmFeO3 single crystals in order to determine its magnetic structure. We
found a k=0 G-type collinear antiferromagnetic structure that is not compatible
with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While
the structural data reveals a clear sign for magneto-elastic coupling at the
N\'eel-temperature of ~675 K, the dielectric measurements remain silent as far
as ferroelectricity is concerned
Separation of K+ and Bi3+ displacements in a Pb-free, monoclinic piezoelectric at the morphotropic phase boundary
The best piezoelectric properties of any perovskite oxide known are found in the solid solution of the relaxor Pb(Mg1/3Nb2/3)O3 and ferroelectric PbTiO3. Despite its impressive properties, this system has limited analogy. We present the compositional exploration of the Pb-free analogue (1-x)(K1/2Bi1/2)(Mg1/3Nb2/3)O3-x(K1/2Bi1/2)TiO3 (KBMN-KBT). We locate the morphotropic phase boundary between x = 0.86 and 0.88 changing from Cm to Pm symmetry and the optimally performing composition at x = 0.88. We report a piezoelectric figure of merit (d33*) of 192 pm Vâ1 from strain measurements. Diffraction methods reveal disordered displacements of K+ and Bi3+ which persist from the KBMN endmember through multiple changes in symmetry. Rearrangement of the Bi3+ displacements along the uncommon [011]c direction drives the physical response. Ferroelectric, dielectric, and piezoresponse force microscopy are used to study the progression of physical properties through the MPB and attribute the mechanism to a polarization rotation. Taking account for local, short-range, and average structural features yield a balanced perspective on the structure and properties of this system, isolating the driving force within this system to the Bi3+ bonding configuration. This work yields a strong analogy to the Pb-based analogue, and provides strategies for further optimization
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
- âŠ