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Aluie H.,Los Alamos National Laboratory | Aluie H.,Center for Nonlinear Studies | Li S.,Los Alamos National Laboratory | Li H.,Los Alamos National Laboratory
Astrophysical Journal Letters | Year: 2012

The physical nature of compressible turbulence is of fundamental importance in a variety of astrophysical settings. We investigate the question: "At what scales does the mechanism of pressure-dilatation operate?" and present the first direct evidence that mean kinetic energy cascades conservatively beyond a transitional "conversion" scale range despite not being an invariant of the dynamics. We use high-resolution 10243 subsonic and transonic simulations. The key quantity we measure is the pressure-dilatation cospectrum, E PD(k), where we show that it decays at a rate faster than k -1 in wavenumber in at least the subsonic and transonic regimes. This is sufficient to imply that mean pressure-dilatation acts primarily at large scales and that kinetic and internal energy budgets statistically decouple beyond a transitional scale range. However, we observe that small-scale dynamics remains highly compressible locally in space and that the statistical decoupling in the energy budgets is unrelated to the existence of a subsonic scale range. Our results suggest that an extension of Kolmogorov's inertial-range theory to compressible turbulence is possible. © 2012. The American Astronomical Society. All rights reserved.. Source


Dorfler F.,University of California at Los Angeles | Jovanovic M.R.,University of Minnesota | Chertkov M.,Center for Nonlinear Studies | Bullo F.,University of California at Santa Barbara
IEEE Transactions on Power Systems | Year: 2014

Inter-area oscillations in bulk power systems are typically poorly controllable by means of local decentralized control. Recent research efforts have been aimed at developing wide-area control strategies that involve communication of remote signals. In conventional wide-area control, the control structure is fixed a priori typically based on modal criteria. In contrast, here we employ the recently-introduced paradigm of sparsity-promoting optimal control to simultaneously identify the optimal control structure and optimize the closed-loop performance. To induce a sparse control architecture, we regularize the standard quadratic performance index with an ℓ1- penalty on the feedback matrix. The quadratic objective functions are inspired by the classic slow coherency theory and are aimed at imitating homogeneous networks without inter-area oscillations. We use the New England power grid model to demonstrate that the proposed combination of the sparsity-promoting control design with the slow coherency objectives performs almost as well as the optimal centralized control while only making use of a single wide-area communication link. In addition to this nominal performance, we also demonstrate that our control strategy yields favorable robustness margins and that it can be used to identify a sparse control architecture for control design via alternative means. © 2014 IEEE. Source


Li H.,Wayne State University | Wu C.,University of Notre Dame | Malinin S.V.,Wayne State University | Tretiak S.,Center for Nonlinear Studies | Chernyak V.Y.,Wayne State University
Journal of Physical Chemistry B | Year: 2011

The capability of the exciton scattering approach, an efficient methodology for excited states in branched conjugated molecules, is extended to include symmetric triple and ! quadruple joints that connect linear segments on the basis of the phenylacetylene backbone. The obtained scattering matrices that characterize these vertices are used in application of our approach ! to several test structures, where we find excellent agreement with the transition energies computed by the reference quantum chemistry. We introduce topological charges, associated with the scattering matrices, which help to formulate useful relations between the number of excitations in the exciton band and the number of repeat units. The obtained features of the scattering phases are analyzed in terms of the observed excited state electronic structure. © 2010 American Chemical Society. Source


Hinnell A.C.,University of Arizona | Ferr T.P.A.,University of Arizona | Vrugt J.A.,Center for Nonlinear Studies | Vrugt J.A.,University of California at Irvine | And 4 more authors.
Water Resources Research | Year: 2010

There is increasing interest in the use of multiple measurement types, including indirect (geophysical) methods, to constrain hydrologic interpretations. To date, most examples integrating geophysical measurements in hydrology have followed a three-step, uncoupled inverse approach. This approach begins with independent geophysical inversion to infer the spatial and/or temporal distribution of a geophysical property (e.g., electrical conductivity). The geophysical property is then converted to a hydrologic property (e.g., water content) through a petrophysical relation. The inferred hydrologic property is then used either independently or together with direct hydrologic observations to constrain a hydrologic inversion. We present an alternative approach, coupled inversion, which relies on direct coupling of hydrologic models and geophysical models during inversion. We compare the abilities of coupled and uncoupled inversion using a synthetic example where surface-based electrical conductivity surveys are used to monitor one-dimensional infiltration and redistribution. Through this illustrative example, we show that the coupled approach can provide significant reductions in uncertainty for hydrologic properties and associated predictions if the underlying model is a faithful representation of the hydrologic processes. However, if the hydrologic model exhibits structural errors, the coupled inversion may not improve the hydrologic interpretation. Despite this limitation, our results support the use of coupled hydrogeophysical inversion both for the direct benefits of reduced errors during inversion and because of the secondary benefits that accrue because of the extensive communication and sharing of data necessary to produce a coupled model, which will likely lead to more thoughtful use of geophysical data in hydrologic studies. © 2010 by the American Geophysical Union. Source


Nayyar I.H.,Center for Nonlinear Studies | Nayyar I.H.,National Center for Nanosciences and Technology of China | Nayyar I.H.,University of Central Florida | Batista E.R.,Center for Nonlinear Studies | And 5 more authors.
Journal of Chemical Theory and Computation | Year: 2013

Five different Density Functional Theory (DFT) models (ranging from pure GGA to long-range-corrected hybrid functionals) were used to study computationally the nature of the self-trapped electronic states in oligophenylene vinylenes. The electronic excitations in question include the lowest singlet (S1) and triplet (T1 †) excitons (calculated using Time Dependent DFT (TD-DFT) method), positive (P +) and negative (P-) polarons, and the lowest triplet (T1) states (computed with the Self-Consistent Field (SCF) scheme). The polaron formation (spatial localization of excitations) is observed only with the use of range-corrected hybrid DFT models including long-range electronic exchange interactions. The extent of localization for all studied excitations is found to be invariant with respect to the size of the oligomer chain in their corresponding optimal geometries. We have analyzed the interdependence between the extent of the geometrical distortion and the localization of the orbital and spin density, and have observed that the localization of the P+ and P- charged species is quite sensitive to solvent polarization effects and the character of the DFT functional used, rather than the structural deformations. In contrast, the localization of neutral states, S1 and T1 †, is found to follow the structural distortions. Notably, T1 excitation obtained with the mean field SCF approach is always strongly localized in range-corrected hybrid DFT models. The molecular orbital energetics of these excitations was further investigated to identify the relationship between state localization and the corresponding orbital structure. A characteristic stabilization (destabilization) of occupied (virtual) orbitals is observed in hybrid DFT models, compared to tight-binding model-like orbital filling in semilocal GGA functionals. The molecular and natural orbital representation allows visualization of the spatial extent of the underlying electronic states. In terms of stabilization energies, neutral excitons have higher binding energies compared to charged excitations. In contrast, the polaronic species exhibit the highest solvation energies among all electronic states studied. © 2012 American Chemical Society. Source

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