94,783 research outputs found
Modelling failure mechanisms of soft cliff profiles
A large proportion of the 11,000 km coastline of the United Kingdom is backed by soft cliffs. These cliffs are subject to frequent slumping and landslip events, particularly where sea and ground water percolates into the soil and rock. Many of these cliffs are formed from glaciogenic sediments, which experience severe erosion and rapid recession with long-term horizontal recession rates typically up to 2-3 m/year. A series of scaled physical model tests have been conducted using a large centrifuge facility with two-dimensional cliff models. These were tested in a wave flume container located on the centrifuge. Wave loading was created using a quasi-flap paddle system that was located at the opposite end of the centrifuge box. A number of tests were conducted using different cliff materials (i.e. combinations of sand and Portland cement). A parametric study was carried out to assess the influence of variations in cliff geometry and height, soil properties, wave amplitude and period. From these tests, it has been found that generally, failures occurred by progressive undercutting of the cliff toe, followed by global failure of the cliff mass
Self Similar Spherical Collapse Revisited: a Comparison between Gas and Dark Matter Dynamics
We reconsider the collapse of cosmic structures in an Einstein-de Sitter
Universe, using the self similar initial conditions of Fillmore & Goldreich
(1984). We first derive a new approximation to describe the dark matter
dynamics in spherical geometry, that we refer to the "fluid approach". This
method enables us to recover the self-similarity solutions of Fillmore &
Goldreich for dark matter. We derive also new self-similarity solutions for the
gas. We thus compare directly gas and dark matter dynamics, focusing on the
differences due to their different dimensionalities in velocity space. This
work may have interesting consequences for gas and dark matter distributions in
large galaxy clusters, allowing to explain why the total mass profile is always
steeper than the X-ray gas profile. We discuss also the shape of the dark
matter density profile found in N-body simulations in terms of a change of
dimensionality in the dark matter velocity space. The stable clustering
hypothesis has been finally considered in the light of this analytical
approach.Comment: 14 pages, 2 figures, accepted for publication in The Astrophysical
Journa
Ultraviolet photonic crystal laser
We fabricated two dimensional photonic crystal structures in zinc oxide films
with focused ion beam etching. Lasing is realized in the near ultraviolet
frequency at room temperature under optical pumping. From the measurement of
lasing frequency and spatial profile of the lasing modes, as well as the
photonic band structure calculation, we conclude that lasing occurs in the
strongly localized defect modes near the edges of photonic band gap. These
defect modes originate from the structure disorder unintentionally introduced
during the fabrication process.Comment: 4 pages, 4 figure
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