1,721,029 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
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
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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
Numerical investigation of transitional supersonic base flows with flow control
No full textDrag reduction by means of flow control is investigated forsupersonic base flows at Mach number M = 2.46 using DirectNumerical Simulations (DNS) and the Flow Simulation Methodology(FSM). The objective of the present work is to understand theevolution of coherent structures in the flow and how flow controltechniques modify these structures. For such investigations,simulation methods that capture the dynamics of the large turbulentstructures are required. DNS are performed for transitional baseflows at Re_D = 30,000. Due to the drastically increasedcomputational cost of DNS at higher Reynolds numbers, a hybridRANS/LES method (FSM) is applied to simulate base flows with flowcontrol at Re_D = 100,000. Active and passive flow controltechniques that alter the near-wake by introducing axisymmetric andlongitudinal perturbations are investigated. A detailed analysis ofthe dynamics of the resulting turbulent (coherent) structures ispresented
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