The Quasimonotonicity of Linear Differential Systems - The Complex Spectrum

The article of record as published may be located at http://dx.doi.org/10.1080/00036810108840984The method of vector Lyapunov functions to determine stability in dynamical systems requires that the comparison system be quasimonotone nondecreasing with respect to a cone contained in the nonnegative orthant. For linear comparison systems in Rn with real spectra, Heikkilﾨa solved the problem for n = 2 and gave necessary conditions for n > 2. We previously showed a su_cient condition for n > 2, and here, for systems with complex eigenvalues, we give conditions for which the problem reduces to the nonnegative inverse eigenvalue problem

Towards and FVE-FAC Method for Determining Thermocapillary Effects on Weld Pool Shape

SEE ParentDocumentRecord|Ntt=19970006857 "Seventh Copper Mountain Conference on Multigrid Methods"; p. 147-166; Part 1; NASA-CP-3339Several practical materials processes, e.g., welding, float-zone purification, and Czochralski crystal growth, involve a pool of molten metal with a free surface, with strong temperature gradients along the surface. In some cases, the resulting thermocapillary flow is vigorous enough to convect heat toward the edges of the pool, increasing the driving force in a sort of positive feedback. In this work we examine this mechanism and its effect on the solid-liquid interface through a model problem: a half space of pure substance with concentrated axisymmetric surface heating, where surface tension is strong enough to keep the liquid free surface flat. The numerical method proposed for this problem utilizes a finite volume element (FVE) discretization in cylindrical coordinates. Because of the axisymmetric nature of the model problem, the control volumes used are torroidal prisms, formed by taking a polygonal cross-section in the (r, z) plane and sweeping it completely around the z-axis. Conservation of energy (in the solid), and conservation of energy, momentum, and mass (in the liquid) are enforced globally by integrating these quantities and enforcing conservation over each control volume. Judicious application of the Divergence Theorem and Stokes' Theorem, combined with a Crank-Nicolson time-stepping scheme leads to an implicit algebraic system to be solved at each time step. It is known that near the boundary of the pool, that is, near the solid-liquid interface, the full conduction-convection solution will require extremely fine length scales to resolve the physical behavior of the system. Furthermore, this boundary moves as a function of time. Accordingly, we develop the foundation of an adaptive refinement scheme based on the principles of Fast Adaptive Composite Grid methods (FAC). Implementation of the method and numerical results will appear in a later report.N00014-92-WR-24009Approved for public release; distribution is unlimited

Acoustic Interactions in Arrays of Spherical Elastic Shells

The article of record as published may be located at http://dx.doi.org/10.1121/1.401233The acoustical performance of a submerged linear array of spherical transducers is examined by combining the T-Matrix method of solving for multiple acoustic interactions among separate bodies with a model for the transducers as thin spherical elastic shells. This approach solves the fully coupled problem of the response of the array to internal forcing. The results show that the assumptions giving rise to the Chebyshev criteria for optimal arrays of point sources appear to apply well even for large spheres at low frequencies. However, at frequencies near or above the lowest resonant frequency the directional pattern may be degraded, depending on the material of the shells

A More Compact AES, and More

We reduce the number of bit operations required to implement AES to a new minimum, and also compute improvements to elements of some other ciphers. Exploring the algebra of AES allows choices of basis and streamlining of the nonlinear parts. We also compute a more efficient implementation of the linear part of each round. Similar computational optimizations apply to other cryptographic matrices and S-boxes. This work may be incorporated into a hardware AES implementation using minimal resources, or potentially in a bit-sliced software implementation to increase speed

Space Time Coding Over a Multiple-Channel Free Space Optical Link

Free space optical (FSO) communications in the atmosphere are characterized by additive white Gaussian noise (AWGN) and turbulent fading. We propose a combination of spatial and temporal forward error correction (FEC) coding to instantaneously correct for long-duration fades over a multiple channel link. We simulate this over a turbulent channel and identify the probability of outage of such a channel as the most significant channel parameter

Detecting climate adaptation with mobile network data in Bangladesh: anomalies in communication, mobility and consumption patterns during cyclone Mahasen

Large-scale data from digital infrastructure, like mobile phone networks, provides rich information on the behavior of millions of people in areas affected by climate stress. Using anonymized data on mobility and calling behavior from 5.1 million Grameenphone users in Barisal Division and Chittagong District, Bangladesh, we investigate the effect of Cyclone Mahasen, which struck Barisal and Chittagong in May 2013. We characterize spatiotemporal patterns and anomalies in calling frequency, mobile recharges, and population movements before, during and after the cyclone. While it was originally anticipated that the analysis might detect mass evacuations and displacement from coastal areas in the weeks following the storm, no evidence was found to suggest any permanent changes in population distributions. We detect anomalous patterns of mobility both around the time of early warning messages and the storm’s landfall, showing where and when mobility occurred as well as its characteristics. We find that anomalous patterns of mobility and calling frequency correlate with rainfall intensity (r = .75, p < 0.05) and use calling frequency to construct a spatiotemporal distribution of cyclone impact as the storm moves across the affected region. Likewise, from mobile recharge purchases we show the spatiotemporal patterns in people’s preparation for the storm in vulnerable areas. In addition to demonstrating how anomaly detection can be useful for modeling human adaptation to climate extremes, we also identify several promising avenues for future improvement of disaster planning and response activities

Naval Postgraduate School - Cyber Summit

2009 CyberSecurity Summit, Naval Postgraduate Schoo

Avoid Mask Re-use in Masked Galois Multipliers

The article of record as published may be located at http://eprint.iacr.org/2009/012This work examines a weakness in re-using masks for masked Galois inversion, specifically in the masked Galois multipliers. Here we show that the mask re-use scheme included in our work [1] cannot result in ﾓperfect masking,ﾓ regardless of the order in which the terms are added; explicit distributions are derived for each step. The same problem requires new marks in the subfield calculations, not included in [1]. Hence, for resistance to first-order differential attacks, the masked S-box must use distinct independent masks for input and output bytes of the masked inverterm and new masks in the subfields, resulting in a larger size