763 research outputs found
Guidance, flight mechanics and trajectory optimization. Volume 12 - Relative motion, guidance equations for terminal rendezvous
Equations of relative motion and guidance for orbital transfer and docking maneuvers in spacecraft rendezvou
On the generation of internal waves by tidal flow over a sill----a possible nonlinear mechanism
The generation of internal waves in a two layer tidal channel containing a sill is considered. The distorted free surface associated with the locally accelerated tidal flow in the vicinity of the sill is assumed to be the driving force for internal waves. A simplified, nonrigorous model, which incorporates this forcing mechanism, is developed and predicts that internal waves should be generated at twice the tidal frequency as well as at the tidal frequency...
Minimizing Communication in Linear Algebra
In 1981 Hong and Kung proved a lower bound on the amount of communication
needed to perform dense, matrix-multiplication using the conventional
algorithm, where the input matrices were too large to fit in the small, fast
memory. In 2004 Irony, Toledo and Tiskin gave a new proof of this result and
extended it to the parallel case. In both cases the lower bound may be
expressed as (#arithmetic operations / ), where M is the size
of the fast memory (or local memory in the parallel case). Here we generalize
these results to a much wider variety of algorithms, including LU
factorization, Cholesky factorization, factorization, QR factorization,
algorithms for eigenvalues and singular values, i.e., essentially all direct
methods of linear algebra. The proof works for dense or sparse matrices, and
for sequential or parallel algorithms. In addition to lower bounds on the
amount of data moved (bandwidth) we get lower bounds on the number of messages
required to move it (latency). We illustrate how to extend our lower bound
technique to compositions of linear algebra operations (like computing powers
of a matrix), to decide whether it is enough to call a sequence of simpler
optimal algorithms (like matrix multiplication) to minimize communication, or
if we can do better. We give examples of both. We also show how to extend our
lower bounds to certain graph theoretic problems.
We point out recently designed algorithms for dense LU, Cholesky, QR,
eigenvalue and the SVD problems that attain these lower bounds; implementations
of LU and QR show large speedups over conventional linear algebra algorithms in
standard libraries like LAPACK and ScaLAPACK. Many open problems remain.Comment: 27 pages, 2 table
Guidance, flight mechanics and trajectory optimization. Volume 11 - Guidance equations for orbital operations
Mathematical formulation of guidance equations and solutions for orbital space mission
Guidance, flight mechanics and trajectory optimization. Volume 6 - The N-body problem and special perturbation techniques
Analytical formulations and numerical integration methods for many body problem and special perturbative technique
Element orbitals for Kohn-Sham density functional theory
We present a method to discretize the Kohn-Sham Hamiltonian matrix in the
pseudopotential framework by a small set of basis functions automatically
contracted from a uniform basis set such as planewaves. Each basis function is
localized around an element, which is a small part of the global domain
containing multiple atoms. We demonstrate that the resulting basis set achieves
meV accuracy for 3D densely packed systems with a small number of basis
functions per atom. The procedure is applicable to insulating and metallic
systems
Guidance, Flight Mechanics and Trajectory Optimization. Volume 15 - Application of Optimization Techniques
Pontryagin maximum principle, calculus of variations, and dynamic programming optimization techniques applied to trajectory and guidance problem
Monitoring of offshore geological carbon storage integrity: Implications of natural variability in the marine system and the assessment of anomaly detection criteria
The design of efficient monitoring programmes required for the assurance of offshore geological storage requires
an understanding of the variability and heterogeneity of marine carbonate chemistry. In the absence of sufficient
observational data and for extrapolation both spatially and seasonally, models have a significant role to play. In
this study a previously evaluated hydrodynamic-biogeochemical model is used to characterise carbonate
chemistry, in particular pH heterogeneity in the vicinity of the sea floor. Using three contrasting regions, the
seasonal and short term variability are analysed and criteria that could be considered as indicators of anomalous
carbonate chemistry identified. These criteria are then tested by imposing a number of randomised DIC perturbations
on the model data, representing a comprehensive range of leakage scenarios. In conclusion optimal
criteria and general rules for developing monitoring strategies are identified. Detection criteria will be site
specific and vary seasonally and monitoring may be more efficient at periods of low dynamics. Analysis suggests
that by using high frequency, sub-hourly monitoring anomalies as small as 0.01 of a pH unit or less may be
successfully discriminated from natural variability – thereby allowing detection of small leaks or at distance from
a leakage source. Conversely assurance of no leakage would be profound. Detection at deeper sites is likely to be
more efficient than at shallow sites where the near bed system is closely coupled to surface processes. Although
this study is based on North Sea target sites for geological storage, the model and the general conclusions are
relevant to the majority of offshore storage sites lying on the continental shelf
A Review of Scanning Tunneling Microscope and Atomic Force Microscope Imaging of Large Biological Structures: Problems and Prospects
The application of the scanning tunneling microscope (STM) and the atomic force microscope (AFM) to the study of small biological molecules, such as DNA and smaller molecules, has received considerable attention in the literature. This paper reviews STM and AFM studies of larger biological structures such as bacterial membranes, bacteriophages, viruses, antibodies, etc. The problems encountered in these applications are emphasized, with particular reference to the unknown conduction mechanism, tip-sample interaction forces, and tip-sample convolution artifacts in the images. The latter problem is illustrated by new results from IgG antibody complexes attached to a bacterial sheath layer. A new conduction mechanism involving a graphite film overlayer is suggested. The future prospects are discussed, with emphasis on the unique capabilities of these microscopes compared to conventional electron microscopes
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