132,879 research outputs found
Numerical investigation of transitional supersonic axisymmetric wakes
No full textTransitional supersonic axisymmetric wakes are investigated by
conducting various numerical experiments. The main objective is to identify hydrodynamic instability mechanisms in the flow at M=2.46 for several Reynolds numbers, and relating these to coherent structures that are found from various visualization techniques. The premise for this approach is the assumption that flow instabilities lead to the formation of coherent structures. Three high-order accurate compressible codes were developed in cylindrical coordinates for this research: a spatial Navier-Stokes (N-S) code to conduct Direct Numerical Simulations (DNS), a linearized N-S code for linear stability investigations using axisymmetric basic states, and a temporal N-S code for performing local stability analyses. The ability of numerical simulations to deliberately exclude physical effects is exploited. This includes intentionally eliminating certain azimuthal/helical modes by employing DNS for various circumferential domain-sizes. With this approach, the impact of structures associated with certain modes on the global wake-behavior can be scrutinized. Complementary spatial and temporal calculations are carried out to investigate whether instabilities are of local or global nature. Circumstantial evidence is presented that absolutely unstable global modes within the recirculation region co-exist with convectively unstable shear-layer modes. The flow is found to be absolutely unstable with respect to modes k>0 for ReD>5,000 and with respect to the axisymmetric mode k=0 for ReD>100,000. It is concluded that azimuthal modes k=2 and k=4 are the dominant modes in the trailing wake, producing a four-lobe wake pattern. Two possible mechanisms responsible for the generation of longitudinal structures within the recirculation region are suggested
Motivic Approach to Enumerating Vector Bundles
No full textDans ce travail, nous utilisons la théorie de l’obstruction en théorie homotopique des schémas pour obtenir certains résultats d’énumération de fibrés vectoriels sur des algèbres lisses de dimension d sur un corps k fixé. Dans un premier temps, nous énumérons les fibrés vectoriels de rang d sur ces algèbres, obtenant au passage de nouvelles preuves de certains théorèmes de Suslin et Bhatwadekar. Nous étudions ensuite les fibrés de rang d-1, prouvant au passage une conjecture de Suslin en admettant une conjecture de Asok et Fasel. Finalement, nous utilisons des méthodes similaires pour prouver des résultats de simplification pour des fibrés symplectiques de rang critique.In this thesis, we establish, via obstruction theory in motivic homotopy theory, some enumeration results on vector bundles of rank dover a smooth affine k-algebra A of dimension d for a base field k, in analogy with some results of James-Thomas. In the rank d case, we recover in particular results of Suslin and Bhatwadekar on cancellation of such vector bundles. Admitting a conjecture of Asok and Fasel, we prove cancellation of such modules of rank d-1 if the base field k is algebraically closed. Using similar methods, we also obtain cancellation results for symplectic vector bundles of critical rank
Investigation of supersonic wakes using conventional and hybrid turbulence models
No full textTransitional and turbulent supersonic wakes behind axisymmetric bodies with a blunt base are investigated numerically using state-of-the-art RANS models and the Flow Simulation Methodology (FSM). The centerpiece of the FSM is a strategy to provide the proper amount of modelling of the subgrid scales. This is accomplished by a contribution function which locally and instantaneously compares the smallest relevant scales to the local grid size. The underlying compressible Navier-Stokes code in cylindrical coordinates developed for this research employs high-order accurate finite differences and a high-order accurate axis treatment.The turbulence closures chosen are a state-of-the-art wall-distance free explicit Algebraic Stress Model (EASMalpha), or a standard K-epsilon model (STKE) for comparison. Axisymmetric RANS and fully three-dimensional FSM calculations are performed on various computational grids for wakes at M=2.46 for several Reynolds numbers. The data obtained from all simulation strategies are compared to available DNS results for the transitional cases and to experimental results at the highest Reynolds number investigated. Of particular interest is the performance of commonly used compressibility corrections and modifications to closure-coefficients specifically derived for high-Reynolds number flows. The ability of FSM to reproduce flow structures found in DNS is scrutinized and a reason for the failure of RANS calculations to correctly predict the base pressure distribution is given
Une approche motivique de l'énumération de fibrés vectoriels
No full textIn this thesis, we establish, via obstruction theory in motivic homotopy theory, some enumeration results on vector bundles of rank dover a smooth affine k-algebra A of dimension d for a base field k, in analogy with some results of James-Thomas. In the rank d case, we recover in particular results of Suslin and Bhatwadekar on cancellation of such vector bundles. Admitting a conjecture of Asok and Fasel, we prove cancellation of such modules of rank d-1 if the base field k is algebraically closed. Using similar methods, we also obtain cancellation results for symplectic vector bundles of critical rank.Dans ce travail, nous utilisons la théorie de l’obstruction en théorie homotopique des schémas pour obtenir certains résultats d’énumération de fibrés vectoriels sur des algèbres lisses de dimension d sur un corps k fixé. Dans un premier temps, nous énumérons les fibrés vectoriels de rang d sur ces algèbres, obtenant au passage de nouvelles preuves de certains théorèmes de Suslin et Bhatwadekar. Nous étudions ensuite les fibrés de rang d-1, prouvant au passage une conjecture de Suslin en admettant une conjecture de Asok et Fasel. Finalement, nous utilisons des méthodes similaires pour prouver des résultats de simplification pour des fibrés symplectiques de rang critique
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
A methodology for simulation of complex turbulent flows
No full textA flow simulation Methodology (FSM) is presented for computing the time-dependent behavior of complex compressible turbulent flows. The development of FSM was initiated in close collaboration with C. Speziale (then at Boston University). The objective of FSM is to provide the proper amount of turbulence modeling for the unresolved scales while directly computing the largest scales. The strategy is implemented by using state-of-the-art turbulence models (as developed for Reynolds averaged Navier-Stokes (RANS)) and scaling of the model terms with a "contribution function." The contribution function is dependent on the local and instantaneous "physical" resolution in the computation. This physical resolution is determined during the actual simulation by comparing the size of the smallest relevant scales to the local grid size used in the computation. The contribution function is designed such that it provides no modeling if the computation is locally well resolved so that it approaches direct numerical simulations (DNS) in the fine-grid limit and such that it provides modeling of all scales in the coarse-grid limit and thus approaches a RANS calculation. In between these resolution limits, the contribution function adjusts the necessary modeling for the unresolved scales while the larger (resolved) scales are computed as in large eddy simulation (LES). However, FSM is distinctly different from LES in that it allows for a consistent transition between RANS, LES, and DNS within the same simulation depending on the local flow behavior and "physical" resolution. As a consequence, FSM should require considerably fewer grid points for a given calculation than would be necessary for a LES. This conjecture is substantiated by employing FSM to calculate the flow over a backward-facing step and a plane wake behind a bluff body, both at low Mach number, and supersonic axisymmetric wakes. These examples were chosen such that they expose, on the one hand, the inherent difficulties of simulating (physically) complex flows, and, on the other hand, demonstrate the potential of the FSM approach for simulations of turbulent compressible flows for complex geometries
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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