26,095 research outputs found
A numerical investigation into the aerodynamic characteristics and aeroelastic stability of a footbridge
The results of a numerical investigation into the aerodynamic characteristics and aeroelastic stability of a proposed footbridge across a highway in the north of England are presented. The longer than usual span, along with the unusual nature of the pedestrian barriers, indicated that the deck configuration was likely to be beyond the reliable limits of the British design code BD 49/01. The calculations were performed using the discrete vortex method, DIVEX, developed at the Universities of Glasgow and Strathclyde. DIVEX has been successfully validated on a wide range of problems, including the aeroelastic response of bridge deck sections. In particular, the investigation focussed on the effects of non-standard pedestrian barriers on the structural integrity of the bridge. The proposed deck configuration incorporated a barrier comprised of angled flat plates, and the bridge was found to be unstable at low wind speeds with the plates having a strong turning effect on the flow at the leading edge of the deck. These effects are highlighted in both a static and dynamic analysis of the bridge deck, along with modifications to the design that aim to improve the aeroelastic stability of the deck. Proper orthogonal decomposition (POD) was also used to investigate the unsteady pressure field on the upper surface of the static bridge deck. The results of the flutter investigation and the POD analysis highlight the strong influence of the pedestrian barriers on the overall aerodynamic characteristics and aeroelastic stability of the bridge
Laser assisted decay of quasistationary states
The effects of intense electromagnetic fields on the decay of quasistationary
states are investigated theoretically. We focus on the parameter regime of
strong laser fields and nonlinear effects where an essentially nonperturbative
description is required. Our approach is based on the imaginary time method
previously introduced in the theory of strong-field ionization. Spectra and
total decay rates are presented for a test case and the results are compared
with exact numerical calculations. The potential of this method is confirmed by
good quantitative agreement with the numerical results.Comment: 24 pages, 5 figure
Spectrophotometric calibration of low-resolution spectra
Low-resolution spectroscopy is a frequently used technique. Aperture prism
spectroscopy in particular is an important tool for large-scale survey
observations. The ongoing ESA space mission Gaia is the currently most relevant
example. In this work we analyse the fundamental limitations of the calibration
of low-resolution spectrophotometric observations and introduce a calibration
method that avoids simplifying assumptions on the smearing effects of the line
spread functions. To this aim, we developed a functional analytic mathematical
formulation of the problem of spectrophotometric calibration. In this
formulation, the calibration process can be described as a linear mapping
between two suitably constructed Hilbert spaces, independently of the
resolution of the spectrophotometric instrument. The presented calibration
method can provide a formally unusual but precise calibration of low-resolution
spectrophotometry with non-negligible widths of line spread functions. We used
the Gaia spectrophotometric instruments to demonstrate that the calibration
method of this work can potentially provide a significantly better calibration
than methods neglecting the smearing effects of the line spread functions.Comment: Final versio
A reliable and efficient implicit a posteriori error estimation technique for the time harmonic Maxwell equations
We analyze an implicit a posteriori error indicator for the time harmonic Maxwell equations and prove that it is both reliable and locally efficient. For the derivation, we generalize some recent results concerning explicit a posteriori error estimates. In particular, we relax the divergence free constraint for the source term. We also justify the complexity of the obtained estimator
A fast semi-direct least squares algorithm for hierarchically block separable matrices
We present a fast algorithm for linear least squares problems governed by
hierarchically block separable (HBS) matrices. Such matrices are generally
dense but data-sparse and can describe many important operators including those
derived from asymptotically smooth radial kernels that are not too oscillatory.
The algorithm is based on a recursive skeletonization procedure that exposes
this sparsity and solves the dense least squares problem as a larger,
equality-constrained, sparse one. It relies on a sparse QR factorization
coupled with iterative weighted least squares methods. In essence, our scheme
consists of a direct component, comprised of matrix compression and
factorization, followed by an iterative component to enforce certain equality
constraints. At most two iterations are typically required for problems that
are not too ill-conditioned. For an HBS matrix with
having bounded off-diagonal block rank, the algorithm has optimal complexity. If the rank increases with the spatial dimension as is
common for operators that are singular at the origin, then this becomes
in 1D, in 2D, and
in 3D. We illustrate the performance of the method on
both over- and underdetermined systems in a variety of settings, with an
emphasis on radial basis function approximation and efficient updating and
downdating.Comment: 24 pages, 8 figures, 6 tables; to appear in SIAM J. Matrix Anal. App
Local renormalization method for random systems
In this paper, we introduce a real-space renormalization transformation for
random spin systems on 2D lattices. The general method is formulated for random
systems and results from merging two well known real space renormalization
techniques, namely the strong disorder renormalization technique (SDRT) and the
contractor renormalization (CORE). We analyze the performance of the method on
the 2D random transverse field Ising model (RTFIM).Comment: 12 pages, 13 figures. Submitted to the Special Issue on "Quantum
Information and Many-Body Theory", New Journal of Physics. Editors: M.B.
Plenio, J. Eiser
Contour integral method for obtaining the self-energy matrices of electrodes in electron transport calculations
We propose an efficient computational method for evaluating the self-energy
matrices of electrodes to study ballistic electron transport properties in
nanoscale systems. To reduce the high computational cost incurred in large
systems, a contour integral eigensolver based on the Sakurai-Sugiura method
combined with the shifted biconjugate gradient method is developed to solve
exponential-type eigenvalue problem for complex wave vectors. A remarkable
feature of the proposed algorithm is that the numerical procedure is very
similar to that of conventional band structure calculations. We implement the
developed method in the framework of the real-space higher-order finite
difference scheme with nonlocal pseudopotentials. Numerical tests for a wide
variety of materials validate the robustness, accuracy, and efficiency of the
proposed method. As an illustration of the method, we present the electron
transport property of the free-standing silicene with the line defect
originating from the reversed buckled phases.Comment: 36 pages, 13 figures, 2 table
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