8,103 research outputs found
Geophysics and the great escape
In August 2011, the Centre for Battlefield Archaeology at the University of Glasgow undertook excavations at the prisoner of war camp of Stalag Luft III at Zagań, Poland. This was the site of the famous “Great Escape” in March 1944, when 76 officers escaped the camp through Harry, one of four tunnels dug by the prisoners during their incarceration. Of the escapers, 73 were recaptured and 50 of them were executed by the Gestapo, and the camp stands as a memorial to them. The tunnels are an important part of the memorial, testifying to the ingenuity and superhuman effort made by the prisoners in their attempt to escape and disrupt the German war machine
Evaporation of Schwarzschild Black Holes in Matrix Theory
Recently, in collaboration with Susskind, we proposed a model of
Schwarzschild black holes in Matrix theory. A large Schwarzschild black hole is
described by a metastable bound state of a large number of D0-branes which are
held together by a background, whose structure has so far been understood only
in 8 and 11 dimensions. The Hawking radiation proceeds by emission of small
clusters of D0-branes. We estimate the Hawking rate in the Matrix theory model
of Schwarzschild black holes and find agreement with the semiclassical rate up
to an undetermined numerical coefficient of order 1.Comment: 9 pages, harvma
Approximation techniques for parameter estimation and feedback control for distributed models of large flexible structures
Approximation ideas are discussed that can be used in parameter estimation and feedback control for Euler-Bernoulli models of elastic systems. Focusing on parameter estimation problems, ways by which one can obtain convergence results for cubic spline based schemes for hybrid models involving an elastic cantilevered beam with tip mass and base acceleration are outlined. Sample numerical findings are also presented
Computational methods for the identification of spatially varying stiffness and damping in beams
A numerical approximation scheme for the estimation of functional parameters in Euler-Bernoulli models for the transverse vibration of flexible beams with tip bodies is developed. The method permits the identification of spatially varying flexural stiffness and Voigt-Kelvin viscoelastic damping coefficients which appear in the hybrid system of ordinary and partial differential equations and boundary conditions describing the dynamics of such structures. An inverse problem is formulated as a least squares fit to data subject to constraints in the form of a vector system of abstract first order evolution equations. Spline-based finite element approximations are used to finite dimensionalize the problem. Theoretical convergence results are given and numerical studies carried out on both conventional (serial) and vector computers are discussed
New developments in the representation of Saharan dust sources in the aerosol–climate model ECHAM6-HAM2
In the aerosol–climate model ECHAM6-HAM2, dust source activation (DSA)
observations from Meteosat Second Generation (MSG) satellite are proposed to
replace the original source area parameterization over the Sahara Desert.
The new setup is tested in nudged simulations for the period 2007 to 2008.
The evaluation is based on comparisons to dust emission events inferred from
MSG dust index imagery, Aerosol Robotic Network (AERONET) sun photometer observations, and satellite
retrievals of aerosol optical thickness (AOT).<br><br>The model results agree well with AERONET measurements especially in terms
of seasonal variability, and a good spatial correlation was found between
model results and MSG-SEVIRI (Spinning-Enhanced Visible and InfraRed Imager) dust AOT as well as Multi-angle Imaging SpectroRadiometer (MISR) AOT. ECHAM6-HAM2 computes a more realistic
geographical distribution and up to 20 % higher annual Saharan dust
emissions, using the MSG-based source map. The representation of dust AOT is
partly improved in the southern Sahara and Sahel. In addition, the spatial
variability is increased towards a better agreement with observations
depending on the season. Thus, using the MSG DSA map can help to circumvent
the issue of uncertain soil input parameters.<br><br>An important issue remains the need to improve the model representation of
moist convection and stable nighttime conditions. Compared to sub-daily DSA
information from MSG-SEVIRI and results from a regional model, ECHAM6-HAM2
notably underestimates the important fraction of morning dust events by the
breakdown of the nocturnal low-level jet, while a major contribution is from
afternoon-to-evening emissions
Multiple core hole formation by free-electron laser radiation in molecular nitrogen
We investigate the formation of multiple-core-hole states of molecular
nitrogen interacting with a free-electron laser pulse. We obtain bound and
continuum molecular orbitals in the single-center expansion scheme and use
these orbitals to calculate photo-ionization and Auger decay rates. Using these
rates, we compute the atomic ion yields generated in this interaction. We track
the population of all states throughout this interaction and compute the
proportion of the population which accesses different core-hole states. We also
investigate the pulse parameters that favor the formation of these core-hole
states for 525 eV and 1100 eV photons
Clarifying the generic limits of Talbotiella and Hymenostegia (Detarieae, Caesalpinioideae, Leguminosae)
The formal description of four species of Cameroonian forest legume trees new to science has been hampered by uncertainty as to whether their correct generic placement is within Hymenostegia Harms or Talbotiella Baker f. As there has been doubt as to whether these two genera differ from one another, an investigation was undertaken so that the new species could be correctly assigned to genus. Using morphological, molecular and pollen data, our study supports the recognition of Hymenostegia and Talbotiella as distinct genera, consequently the new species are correctly placed in Talbotiella. In addition, our data reveal the extensive heterogeneous nature of Hymenostegia as currently circumscribed and the need to transfer H. breteleri to Talbotiella
An approximation theory for the identification of nonlinear distributed parameter systems
An abstract approximation framework for the identification of nonlinear distributed parameter systems is developed. Inverse problems for nonlinear systems governed by strongly maximal monotone operators (satisfying a mild continuous dependence condition with respect to the unknown parameters to be identified) are treated. Convergence of Galerkin approximations and the corresponding solutions of finite dimensional approximating identification problems to a solution of the original finite dimensional identification problem is demonstrated using the theory of nonlinear evolution systems and a nonlinear analog of the Trotter-Kato approximation result for semigroups of bounded linear operators. The nonlinear theory developed here is shown to subsume an existing linear theory as a special case. It is also shown to be applicable to a broad class of nonlinear elliptic operators and the corresponding nonlinear parabolic partial differential equations to which they lead. An application of the theory to a quasilinear model for heat conduction or mass transfer is discussed
Vertex Operators in 2K Dimensions
A formula is proposed which expresses free fermion fields in 2K dimensions in
terms of the Cartan currents of the free fermion current algebra. This leads,
in an obvious manner, to a vertex operator construction of nonabelian free
fermion current algebras in arbitrary even dimension. It is conjectured that
these ideas may generalize to a wide class of conformal field theories.Comment: Minor change in notation. Change in references
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