Publications

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A hybrid closed bomb-strand burner is used to measure the burning behavior of the titanium subhydride potassium perchlorate pyrotechnic with an equivalent hydrogen concentration of 1.65. This experimental facility allows for simultaneous measurement of the closed bomb pressure rise and pyrotechnic burn rate as detected by electrical break wires over a range of pressures. Strands were formed by pressing the pyrotechnic powders to bulk densities between 60% and 90% theoretical maximum density. The burn rate dependance on initial density and vessel pressure are measured. At all initial strand densities, the burn is observed to transition from conductive to convective burning within the strand. The measured vessel pressure history is further analyzed following the closed bomb analysis methods developed for solid propellants.

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We introduce a methodology for optimally locating fixed disinfectant booster stations for response to contamination incidents in water distribution networks. In this work a stochastic programming problem considering uncertainty in both the location and time of the contamination incident is formulated, resulting in a large Mixed Integer Linear Programming problem. While the original full-space problem is intractably large, we show a series of reductions that decrease the size of the problem by up to five orders of magnitude and allow solutions of the optimal placement problem for realistically-sized water network models.

Identifying and removing malicious insiders from a network is a topic of active research. Vehicular ad hoc networks (VANETs) may suffer from insider attacks; that is, an attacker may use authorized vehicles to attack other vehicles. Specifically, attackers may use their vehicles to broadcast specially formed packets that will trigger warnings in target vehicles. This malicious behavior could have a significant detrimental effect on cooperative safety applications (SAs), one of the driving forces behind VANET deployment. We propose modifications to the intersection collision warning (ICW) SA that enable a certificate authority (CA) to be offline and yet to decide to revoke a vehicle's certificates using retransmitted information that cannot repudiated. Our approach differs from previous proposals in that it is SA specific, and it is immune to Sybil attacks. We simulate and measure the resources an attacker requires to attack a vehicle using the ICW SA without our modifications and demonstrate that our additions reduce the false positive rate arising from errors in estimated vehicle dynamics. © 2012 Author.

We present the first demonstration of a travelling wave carrier depletion Mach-Zehnder modulator impedance matched to 50 . This device has a bandwidth of 24 GHz and a halfwave voltage length product of 0.7 V-cm, placing it among the best in its class. © 2012 IEEE.

We demonstrate stable, single-frequency output from single, asfabricated GaN nanowire lasers operating far above lasing threshold. Each laser is a linear, double-facet GaN nanowire functioning as gain medium and optical resonator, fabricated by a top-down technique that exploits a tunable dry etch plus anisotropic wet etch for precise control of the nanowire dimensions and high material gain. A single-mode linewidth of ∼0.12 nm and >18dB side-mode suppression ratio are measured. Numerical simulations indicate that single-mode lasing arises from strong mode competition and narrow gain bandwidth. © 2012 Optical Society of America.

We present simulations and preliminary experimental results of a new method to stabilize the resonant wavelength of an optical resonant modulator using bit error rate measurements from a local receiver to drive an integrated microheater. Optical interconnections have the potential to significantly reduce the power dissipation and greatly increase the aggregate connection bandwidth in high performance multiprocessor digital computers, intra-satellite communications, and data centers. Silicon photonic micro-ring and micro-disk modulators for the transmit side of the links are an active area of research, because of they are compatible with silicon electronics processing and have been demonstrated at data rates above 10 Gb/s and at sub 100fJ/bit switching energies [1-3]. However, a key problem that remains to be solved for these devices is control of their optical wavelength that varies as a function of fabrication tolerances (thicknesses and dimensions) and temperature. In [4], a heater and sensor were integrated with a modulator to stabilize its wavelength, but the method demonstrated suffers the potential drawbacks of aging of the sensor over time and the potential need to pre-calibrate every sensor/modulator. Here, we present a new method of tuning the resonant wavelength of the device to match the incident light's wavelength using independent logic one and logic zero bit errors from a local receiver and simple logic circuitry to drive an integrated micro-heater to adjust the temperature of the device. Simulations of the control loop show it to be robust to the choice of gain, receiver decision threshold, and starting point temperature. Preliminary experimental results using an integrated microresonant heater modulator device [5] operating at 3.125 Gb/s with an FPGA and external reciver driving the control loop show a tuning range of 25C-32C (>2 nm shift). No dithering or calibration is required; the technique is not susceptible to sensor aging, and it can compensate for long-term drift in the characteristics of the modulator. © 2012 IEEE.

Markov chains are convenient means of generating realizations of networks with a given (joint or otherwise) degree distribution, since they simply require a procedure for rewiring edges. The major challenge is to find the right number of steps to run such a chain, so that we generate truly independent samples. Theoretical bounds for mixing times of these Markov chains are too large to be practically useful. Practitioners have no useful guide for choosing the length, and tend to pick numbers fairly arbitrarily. We give a principled mathematical argument showing that it suffices for the length to be proportional to the number of desired number of edges. We also prescribe a method for choosing this proportionality constant. We run a series of experiments showing that the distributions of common graph properties converge in this time, providing empirical evidence for our claims. © 2012 Springer-Verlag.

Graph algorithms tend to suffer poor performance due to the irregularity of access patterns within general graph data structures, arising from poor data locality, which translates to high memory latency. The result is that advances in high-performance solutions for graph algorithms are most likely to come through advances in both architectures and algorithms. Specialized MMT shared memory machines offer a potentially transformative environment in which to approach the problem. Here, we explore the challenges of implementing Subgraph Isomorphism (SI) algorithms based on the Ullmann and VF2 algorithms in the Cray XMT environment, where issues of memory contention, scheduling, and compiler parallelizability must be optimized. Copyright is held by the author/owner(s).

Validation of finite element models using experimental data with unknown boundary conditions proves to be a significant obstacle. For this reason, the boundary conditions of an experiment are often limited to simple approximations such as free or mass loaded. This restriction means that vibration testing and modal analysis testing have typically required separate tests since vibration testing is often conducted on a shaker table with unknown boundary conditions. If modal parameters can be estimated while the test object is attached to a shaker table, it could eliminate the need for a separate modal test and result in a significant time and cost savings. This research focuses on a method to extract fixed base modal parameters for model validation from driven base experimental data. The feasibility of this method was studied on an Unholtz-Dickie T4000 shaker and slip table using a mock payload and compared with results from traditional modal analysis testing methods. © The Society for Experimental Mechanics, Inc. 2012.

Computing the coordinate-wise maxima of a planar point set is a classic and well-studied problem in computational geometry. We give an algorithm for this problem in the self-improving setting. We have n (unknown) independent distributions D 1, D 2,⋯, D n of planar points. An input pointset (p1,p2,⋯,pn) is generated by taking an independent sample pi from each Di, so the input distribution D is the product Π i D i. A self-improving algorithm repeatedly gets input sets from the distribution D (which is a priori unknown) and tries to optimize its running time for D. Our algorithm uses the first few inputs to learn salient features of the distribution, and then becomes an optimal algorithm for distribution D. Let OPTD denote the expected depth of an optimal linear comparison tree computing the maxima for distribution D. Our algorithm eventually has an expected running time of O(OPT D + n), even though it did not know D to begin with. Our result requires new tools to understand linear comparison trees for computing maxima. We show how to convert general linear comparison trees to very restricted versions, which can then be related to the running time of our algorithm. An interesting feature of our algorithm is an interleaved search, where the algorithm tries to determine the likeliest point to be maximal with minimal computation. This allows the running time to be truly optimal for the distribution D. Copyright © 2012 ACM.

Micro-electro-mechanical systems (MEMS) deformable mirrors are known for their ability to correct optical aberrations, particularly when the wavefront is expanded via Zernike polynomials. This capability is combined with adaptive optical zoom to enable diffraction limited performance over broad spectral and zoom ranges. Adaptive optical zoom (AOZ) alters system magnification via variable focal length elements instead of axial translation found in traditional zoom designs. AOZ systems are simulated using an efficient approach to optical design, in which existing theories for telescope objective design and third-order aberration determination are modified to accommodate the additional degrees of freedom found with AOZ. An AOZ system with a 2.7x zoom ratio and 100mm entrance pupil diameter is presented to demonstrate the validity and capability of the theory. © 2012 SPIE.

Microelectronics and microsystems have been part of the key technologies that enabled the incredible pace of development we have seen over the last five to six decades. This paper presents a basic view in terms of technology development supporting new approaches to generation and transfer of knowledge for critical activities, one of the key ones being the ability to capture, store and more efficiently use energy. © 2012 SPIE.

The use of the Transfer Function approach to Model-Assisted Probability of Detection (MAPOD) is explored by inspecting airplane lap joint specimen sets with multiple site fatigue damage. Four specimen sets representing a matrix with structure type and defect types as the two axes were inspected using ultrasonic linear array. Probability of Detection (POD) curves were generated in each quadrant for 4 inspectors. POD curves in the fourth quadrant, which represents actual in-service crack detection in real airplane structures were predicted from other quadrant curves. Predicted POD curves were compared to observed curves. © 2012 American Institute of Physics.

The reaction of O(3P) with propene (C3H6) has been examined using tunable vacuum ultraviolet radiation and time-resolved multiplexed photoionization mass spectrometry at 4 Torr and 298 K. The temporal and isomeric resolution of these experiments allow the separation of primary from secondary reaction products and determination of branching ratios of 1.00, 0.91 ± 0.30, and 0.05 ± 0.04 for the primary product channels CH3 + CH2CHO, C2H5 + HCO, and H2 + CH3CHCO, respectively. The H + CH3CHCHO product channel was not observable for technical reasons in these experiments, so literature values for the branching fraction of this channel were used to convert the measured product branching ratios to branching fractions. The results of the present study, in combination with past experimental and theoretical studies of O(3P) + C3H6, identify important pathways leading to products on the C3H6O potential energy surface (PES). The present results suggest that up to 40% of the total product yield may require intersystem crossing from the initial triplet C3H6O PES to the lower-lying singlet PES. © the Owner Societies.

Recent studies have questioned the separability of the tight and roaming mechanisms to molecular decomposition. We explore this issue for a variety of reactions including MgH2 → Mg + H2, NCN → CNN, H2CO → H2 + CO, CH3CHO → CH 4 + CO, and HNNOH → N2 + H2O. Our analysis focuses on the role of second-order saddle points in defining global dividing surfaces that encompass both tight and roaming first-order saddle points. The second-order saddle points define an energetic criterion for separability of the two mechanisms. Furthermore, plots of the differential contribution to the reactive flux along paths connecting the first- and second-order saddle points provide a dynamic criterion for separability. The minimum in the differential reactive flux in the neighborhood of the second-order saddle point plays the role of a mechanism divider, with the presence of a strong minimum indicating that the roaming and tight mechanisms are dynamically distinct. We show that the mechanism divider is often, but not always, associated with a second-order saddle point. For the formaldehyde and acetaldehyde reactions, we find that the minimum energy geometry on a conical intersection is associated with the mechanism divider for the tight and roaming processes. For HNNOH, we again find that the roaming and tight processes are dynamically separable but we find no intrinsic feature of the potential energy surface associated with the mechanism divider. Overall, our calculations suggest that roaming and tight mechanisms are generally separable over broad ranges of energy covering most kinetically relevant regimes. © 2012 American Chemical Society.

As mobile devices become more widespread and powerful, they store more sensitive data, which includes not only users' personal information but also the data collected via sensors throughout the day. When mobile applications have access to this growing amount of sensitive information, they may leak it carelessly or maliciously. Google's Android operating system provides a permissions-based security model that restricts an application's access to the user's private data. Each application statically declares the sensitive data and functionality that it requires in a manifest, which is presented to the user upon installation. However, it is not clear to the user how sensitive data is used once the application is installed. To combat this problem, we present AndroidLeaks, a static analysis framework for automatically finding potential leaks of sensitive information in Android applications on a massive scale. AndroidLeaks drastically reduces the number of applications and the number of traces that a security auditor has to verify manually. We evaluate the efficacy of AndroidLeaks on 24,350 Android applications from several Android markets. AndroidLeaks found 57,299 potential privacy leaks in 7,414 Android applications, out of which we have manually verified that 2,342 applications leak private data including phone information, GPS location, WiFi data, and audio recorded with the microphone. AndroidLeaks examined these applications in 30 hours, which indicates that it is capable of scaling to the increasingly large set of available applications. © 2012 Springer-Verlag.

We present an algorithm for the fast and efficient solution of integral equations that arise in the analysis of scattering from periodic arrays of PEC objects, such as multiband frequency selective surfaces (FSS) or metamaterial structures. Our approach relies upon the method of Accelerated Cartesian Expansions (ACE) to rapidly evaluate the requisite potential integrals. ACE is analogous to FMM in that it can be used to accelerate the matrix vector product used in the solution of systems discretized using MoM. Here, ACE provides linear scaling in both CPU time and memory. Details regarding the implementation of this method within the context of periodic systems are provided, as well as results that establish error convergence and scalability. In addition, we also demonstrate the applicability of this algorithm by studying several exemplary electrically dense systems.

In recent years pyrolysis of interferometrically-patterned photoresists has produced three-dimensionally nanopatterned, electrically conductive carbon films with applications from energy storage to biological sensing. We investigate here conditions for rapid thermal pyrolysis that drastically reduce film processing time (from hours to minutes) while preserving the films' unique nanoscale morphology, film adhesion, and electrochemical properties. We specifically show that heating rate dramatically affects nanoscale morphology, while reducing atmosphere composition, dwell time, and dwell temperature impact the electrochemical performance of these rapidly pyrolyzed nanostructures. Accelerated processing with rapid thermal pyrolysis may facilitate the expanded applicability and rapid fabrication of these promising nanostructured materials. © 2012 Elsevier Ltd. All rights reserved.

A parametric analysis is used to examine the supply demand interactions between the US light-duty vehicle (LDV) fleet, its fuels, and the corresponding primary energy sources through 2050. The analysis emphasizes competition between conventional internal combustion engine (ICE) vehicles, including hybrids, and electric vehicles (EVs), represented by both plug-in hybrid and battery electric vehicles. We find that EV market penetration could double relative to our baseline case with policies to extend consumers' effective payback period to 7 years. EVs can also reduce per vehicle petroleum consumption by up to 5% with opportunities to increase that fraction at higher adoption rates. However, EVs have limited ability to reduce LDV greenhouse gas (GHG) emissions with the current energy source mix. Alone, EVs cannot drive compliance with the most aggressive GHG emission reduction targets, even if the electricity grid shifts towards natural gas powered sources. Since ICEs will dominate the LDV fleet for up to 40 years, conventional vehicle efficiency improvements have the greatest potential for reductions in LDV GHG emissions and petroleum consumption over this time. Specifically, achieving fleet average efficiencies of 72. mpg or greater can reduce average GHG emissions by 70% and average petroleum consumption by 81%. © 2012 Elsevier Ltd.

Phase diversity algorithms allow a wavefront to be reconstructed from through-focus measurements of a point source or extended scene. These algorithms have traditionally been limited to systems that are Nyquist sampled. Many optical systems for remote sensing applications are designed to be undersampled, however. One approach to phase diversity with undersampled systems is to employ superresolution techniques to first create properly sampled scenes. This is demonstrated experimentally for a point object, but is applicable to extended scenes as well. © 2012 Optical Society of America.

In this article, we develop goal-oriented error indicators to drive adaptive refinement algorithms for the Poisson-Boltzmann equation. Empirical results for the solvation free energy linear functional demonstrate that goal-oriented indicators are not sufficient on their own to lead to a superior refinement algorithm. To remedy this, we propose a problem-specific marking strategy using the solvation free energy computed from the solution of the linear regularized Poisson-Boltzmann equation. The convergence of the solvation free energy using this marking strategy, combined with goal-oriented refinement, compares favorably to adaptive methods using an energy-based error indicator. Due to the use of adaptive mesh refinement, it is critical to use multilevel preconditioning in order to maintain optimal computational complexity. We use variants of the classical multigrid method, which can be viewed as generalizations of the hierarchical basis multigrid and Bramble-Pasciak-Xu (BPX) preconditioners. © 2011 Springer Science+Business Media (outside the USA).

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Single-mode fibers were cleverly embedded into fixtures holding nitromethane, and used in conjunction with a photonic Doppler velocimeter (PDV) to measure the associated detonation velocity. These measurements have aided us in our understanding of energetic materials and enhanced our diagnostic capabilities.

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