Publications

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Four conventional damage plasticity models for concrete, the Karagozian and Case model (K&C), the Riedel-Hiermaier-Thoma model (RHT), the Brannon-Fossum model (BF1), and the Continuous Surface Cap Model (CSCM) are compared. The K&C and RHT models have been used in commercial finite element programs many years, whereas the BF1 and CSCM models are relatively new. All four models are essentially isotropic plasticity models for which 'plasticity' is regarded as any form of inelasticity. All of the models support nonlinear elasticity, but with different formulations. All four models employ three shear strength surfaces. The 'yield surface' bounds an evolving set of elastically obtainable stress states. The 'limit surface' bounds stress states that can be reached by any means (elastic or plastic). To model softening, it is recognized that some stress states might be reached once, but, because of irreversible damage, might not be achievable again. In other words, softening is the process of collapse of the limit surface, ultimately down to a final 'residual surface' for fully failed material. The four models being compared differ in their softening evolution equations, as well as in their equations used to degrade the elastic stiffness. For all four models, the strength surfaces are cast in stress space. For all four models, it is recognized that scale effects are important for softening, but the models differ significantly in their approaches. The K&C documentation, for example, mentions that a particular material parameter affecting the damage evolution rate must be set by the user according to the mesh size to preserve energy to failure. Similarly, the BF1 model presumes that all material parameters are set to values appropriate to the scale of the element, and automated assignment of scale-appropriate values is available only through an enhanced implementation of BF1 (called BFS) that regards scale effects to be coupled to statistical variability of material properties. The RHT model appears to similarly support optional uncertainty and automated settings for scale-dependent material parameters. The K&C, RHT, and CSCM models support rate dependence by allowing the strength to be a function of strain rate, whereas the BF1 model uses Duvaut-Lion viscoplasticity theory to give a smoother prediction of transient effects. During softening, all four models require a certain amount of strain to develop before allowing significant damage accumulation. For the K&C, RHT, and CSCM models, the strain-to-failure is tied to fracture energy release, whereas a similar effect is achieved indirectly in the BF1 model by a time-based criterion that is tied to crack propagation speed.

A new method is introduced for real-time detection of transient change in scenes observed by staring sensors that are subject to platform jitter, pixel defects, variable focus, and other real-world challenges. The approach uses flexible statistical models for the scene background and its variability, which are continually updated to track gradual drift in the sensor's performance and the scene under observation. Two separate models represent temporal and spatial variations in pixel intensity. For the temporal model, each new frame is projected into a low-dimensional subspace designed to capture the behavior of the frame data over a recent observation window. Per-pixel temporal standard deviation estimates are based on projection residuals. The second approach employs a simple representation of jitter to generate pixelwise moment estimates from a single frame. These estimates rely on spatial characteristics of the scene, and are used gauge each pixel's susceptibility to jitter. The temporal model handles pixels that are naturally variable due to sensor noise or moving scene elements, along with jitter displacements comparable to those observed in the recent past. The spatial model captures jitter-induced changes that may not have been seen previously. Change is declared in pixels whose current values are inconsistent with both models.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

High resolution radar systems generally require combining fast analog to digital converters and digital to analog converters with very high performance digital signal processing logic. These mixed analog and digital printed circuit boards present special challenges with respect to electromagnetic interference. This document first describes the mechanisms of interference on such boards then follows up with a discussion of prevention techniques and finally provides a checklist for designers to help avoid common mistakes.

Results from an experimental study of the aerodynamic and aeroacoustic properties of a ftatback version of the TU Delft DU97-W-300 airfoil are presented. Measurements were made for both the original DU97-W-300 and the flatback version. The chord Reynolds number varied from l.6 x 10⁶ to 3.2 x 10⁶. The data were gathered in the Virginia Tech Stability Wind Tunnel, which includes a special aeroacoustic test section to enable measurements of airfoil self-noise. Corrected wind tunnel aerodynamic measurements for the DU97-W-300 are compared to previous solid wall wind tunnel data and are shown to give good agreement. Force coefficient and surface pressure distributions are compared for the flatback and the original airfoil for both free-transition and tripped boundary layer configurations. Aeroacoustic data are presented for the flatback airfoil, with a focus on the amplitude and frequency of noise associated with the vortex-shedding tone from the blunt trailing edge wake. The effect of a splitter plate trailing edge attachment on both drag and noise of the ftacback airfoil is also investigated.

A new model is developed that accounts for multiple phonon processes on interface transmission between two solids. By considering conservation of energy and phonon population, the decay of a high energy phonon in one material into several lower energy phonons in another material is modeled assuming diffuse scattering. The individual contributions of each of the higher order inelastic phonon processes to thermal boundary conductance are calculated and compared to the elastic contribution. The overall thermal boundary conductance from elastic and inelastic (three or more phonon processes) scattering is calculated and compared to experimental data on five different interfaces. Improvement in value and trend is observed by taking into account multiple phonon inelastic scattering. Three phonon interfacial processes are predicted to dominate the inelastic contribution to thermal boundary conductance. © 2009 American Institute of Physics.

Computational science is paramount to the understanding of underlying processes in internal combustion engines of the future that will utilize non-petroleum-based alternative fuels, including carbon-neutral biofuels, and burn in new combustion regimes that will attain high efficiency while minimizing emissions of particulates and nitrogen oxides. Next-generation engines will likely operate at higher pressures, with greater amounts of dilution and utilize alternative fuels that exhibit a wide range of chemical and physical properties. Therefore, there is a significant role for high-fidelity simulations, direct numerical simulations (DNS), specifically designed to capture key turbulence-chemistry interactions in these relatively uncharted combustion regimes, and in particular, that can discriminate the effects of differences in fuel properties. In DNS, all of the relevant turbulence and flame scales are resolved numerically using high-order accurate numerical algorithms. As a consequence terascale DNS are computationally intensive, require massive amounts of computing power and generate tens of terabytes of data. Recent results from terascale DNS of turbulent flames are presented here, illustrating its role in elucidating flame stabilization mechanisms in a lifted turbulent hydrogen/air jet flame in a hot air coflow, and the flame structure of a fuel-lean turbulent premixed jet flame. Computing at this scale requires close collaborations between computer and combustion scientists to provide optimized scaleable algorithms and software for terascale simulations, efficient collective parallel I/O, tools for volume visualization of multiscale, multivariate data and automating the combustion workflow. The enabling computer science, applied to combustion science, is also required in many other terascale physics and engineering simulations. In particular, performance monitoring is used to identify the performance of key kernels in the DNS code, S3D and especially memory intensive loops in the code. Through the careful application of loop transformations, data reuse in cache is exploited thereby reducing memory bandwidth needs, and hence, improving S3D's nodal performance. To enhance collective parallel I/O in S3D, an MPI-I/O caching design is used to construct a two-stage write-behind method for improving the performance of write-only operations. The simulations generate tens of terabytes of data requiring analysis. Interactive exploration of the simulation data is enabled by multivariate time-varying volume visualization. The visualization highlights spatial and temporal correlations between multiple reactive scalar fields using an intuitive user interface based on parallel coordinates and time histogram. Finally, an automated combustion workflow is designed using Kepler to manage large-scale data movement, data morphing, and archival and to provide a graphical display of run-time diagnostics. © 2009 IOP Publishing Ltd.

Methods are developed for finding an optimal model for a non-Gaussian stationary stochastic process or homogeneous random field under limited information. The available information consists of: (i) one or more finite length samples of the process or field; and (ii) knowledge that the process or field takes values in a bounded interval of the real line whose ends may or may not be known. The methods are developed and applied to the special case of non-Gaussian processes or fields belonging to the class of beta translation processes. Beta translation processes provide a flexible model for representing physical phenomena taking values in a bounded range, and are therefore useful for many applications. Numerical examples are presented to illustrate the utility of beta translation processes and the proposed methods for model selection.

The experimental characterization of single barrier heterostructure thermionic cooling devices at cryogenic temperatures is reported. The device studied was a cylindrical InGaAs microrefrigerator, in which the active layer was a 1 μm thick In 0.527Al 0.218Ga 0.255As heterostructure barrier with n-type doping concentration of 6.68 × 10 16 cm -3 and an In 0.53Ga 0.47As emitter/collector of 5 × 10 18 cm -3 n-doping. A full field thermoreflectance imaging technique was used to measure the distribution of temperature change on the device's top surface when different current excitation values were applied. By reversing the current direction, we studied the device's behavior in both cooling and heating regimes. At an ambient temperature of 100 K, a maximum cooling of 0.6 K was measured. This value was approximately one-third of the measured maximum cooling value at room temperature (1.8 K). The paper describes the device's structure and the first reported thermal imaging at cryogenic temperatures using the thermoreflectance technique. © 2009 The Author(s).

Understanding and predicting sintering, which have been goals since the first attempts to mathematically describe the sintering process in the 1950s, are necessary to eliminate machining after sintering and to reliably predict and control the sintered microstructure and the resultant mechanical and other desired properties. In this study, four different sintering models are evaluated relative to one another and the experimental data, revealing their attributes, deficiencies, and modifications/improvements in order to facilitate their application, including the following: (i) a microstructure-based model for solid state sintering, mainly developed by Riedel and Svoboda (RS); (ii) a viscous sintering (SOVS) model developed by Skorohod and advanced by Olevsky; (iii) a Kinetic Monte Carlo (KMC) model provided by Tikare; and (iv) the master sintering curve (MSC) approach introduced by Johnson et al. For different reasons, all four models have deficiencies that preclude achieving the most challenging goal of being able to comprehensively understand and predict sintering behavior: (i) the RS and the KMC models are complicated and difficult to use; (ii) the SOVS model cannot predict microstructure evolution; and (iii) the KMC model and the MSC have no stresses in their mathematical description, so they cannot simulate the effects of external forces. Each model also has attributes: (i) the KMC model allows one to follow the evolution of mesostructure; (ii) the MSC concept and the RS model are suitable for predicting densification curves for a wide variety of temperature-time profiles; and (iii) the SOVS and the RS models, which are implemented into finite element codes, can be used to predict density gradients and the warping of complex shape parts. Individually and together, the MSC, KMC, SOVS, and RS models can be useful tools to advance the fundamental understanding and improve the control of sintering. © 2009 The American Ceramic Society.

Abstract not provided.

Abstract not provided.

Abstract not provided.