1,720,988 research outputs found
On the bi-stabilities of vortex shedding flows behind a pair of square solids
No full text[[abstract]]Low Reynolds number flows behind two side-by-side square solids with a gap were studied by both numerical simulation and experimental flow-visualization methods. The numerical simulations were conducted by solving the two-dimensional incompressible Navier-Stokes equations. The spatial discretization consists of a seventh-order accurate upwind-biased method for the convective terms and a sixth-order central difference method for the diffusive terms. Arranging the shedding vortices from the upper and lower solids in a synchronal phase and non-synchronal phase as the initial conditions, the stationary in-phase and anti-phase vortex shedding flows take place, respectively. The anti-phase vortex streets remain stable and proceed to the far wake zone. While the in-phase vortex pair, during downstream motion, is observed to rotate around one another. The process eventually leads to the development of a single large-scale wake. The vortex shedding patterns are experimentally studied in a water tunnel by using a Particle Tracing Visualization (PTV) method. The developed in-phase and anti-phase vortex shedding flows along with time-velocity histories are presented and analyzed with the help of signal analysis, i.e., Fourier power spectrum analysis of the velocity evolution and the phase-space trajectory of the velocities. Interestingly, spatially inverse period-doubling phenomena are observed for the in-phase vortex shedding flows.[[note]]SC
Transition in a 2-D lid-driven cavity flow
No full text[[abstract]]Direct numerical simulations about the transition process from laminar to chaotic flow in square lid-driven cavity flows are considered in this paper. The chaotic flow regime is reached after a sequence of successive supercritical Hopf bifurcations to periodic, quasi-periodic, inverse period-doubling, period-doubling, and chaotic self-sustained flow regimes. The numerical experiments are conducted by solving the 2-D incompressible Navier-Stokes equations with increasing Reynolds numbers (Re). The spatial discretization consists of a seventh-order upwind-biased method for the convection term and a sixth-order central method for the diffusive term. The numerical experiments reveal that the first Hopf bifurcation takes place at Re equal to 7402 +/- 4%, and a consequent periodic flow with the frequency equal to 0.59 is obtained. As Re is increased to 10,300, a new fundamental frequency (FF) is added to the velocity spectrum and a quasi-periodic flow regime is reached. For slightly higher Re (10,325), the new FF disappears and the flow returns to a periodic regime. Furthermore, the flow experiences an inverse period doubling at 10,325 < Re < 10,700 and a period-doubling regime at 10,600 < Re < 10,900. Eventually, for flows with Re greater than 11,000, a scenario for the onset of chaotic flow is obtained. The transition processes are illustrated by increasing Re using time-velocity histories, Fourier power spectra, and the phase-space trajectories. In view of the conducted grid independent study, the values of the critical Re presented above are estimated to be accurate within 4%. (C) 2002 Elsevier Science Ltd. All rights reserved.[[note]]SC
Flow and Heat Transfer for Three-Dimensional Flow over an Exponentially Stretching Surface
No full text[[abstract]]This study analyzes the laminar boundary-layer flow and heat transfer characteristics of a steady, three-dimensional viscous fluid driven by a horizontal surface stretched exponentially in two lateral directions. The simulations in this study assume that the surface temperature is also distributed exponentially and reduce the governing equations to a set of ordinary differential equations using a similarity transformation. This study develops a numerical procedure that combines the Ackroyd method and Runge-Kutta integration scheme to solve the transformed equations. Results show that heat transfer characteristics depend strongly on the stretching ratio, temperature exponent, and Prandtl number.[[note]]SC
Nested Cartesian grid method in incompressible viscous fluid flow
No full text[[abstract]]In this work, the local grid refinement procedure is focused by using a nested Cartesian grid formulation. The method is developed for simulating unsteady viscous incompressible flows with complex immersed boundaries. A finite-volume formulation based on globally second-order accurate central-difference schemes is adopted here in conjunction with a two-step fractional-step procedure. The key aspects that needed to be considered in developing such a nested grid solver are proper imposition of interface conditions on the nested-block boundaries, and accurate discretization of the governing equations in cells that are with block-interface as a control-surface. The interpolation procedure adopted in the study allows systematic development of a discretization scheme that preserves global second-order spatial accuracy of the underlying solver, and as a result high efficiency/accuracy nested grid discretization method is developed. Herein the proposed nested grid method has been widely tested through effective simulation of four different classes of unsteady incompressible viscous flows, thereby demonstrating its performance in the solution of various complex flow-structure interactions. The numerical examples include a lid-driven cavity flow and Pearson vortex problems, flow past a circular cylinder symmetrically installed in a channel, flow past an elliptic cylinder at an angle of attack, and flow past two tandem circular cylinders of unequal diameters. For the numerical simulations of flows past bluff bodies an immersed boundary (IB) method has been implemented in which the solid object is represented by a distributed body force in the Navier-Stokes equations. The main advantages of the implemented immersed boundary method are that the simulations could be performed on a regular Cartesian grid and applied to multiple nested-block (Cartesian) structured grids without any difficulty. Through the numerical experiments the strength of the solver in effectively/accurately simulating various complex flows past different forms of immersed boundaries is extensively demonstrated, in which the nested Cartesian grid method was suitably combined together with the fractional-step algorithm to speed up the solution procedure. (C) 2010 Elsevier Inc. All rights reserved.[[note]]SC
Criticality of flow transition behind two side-by-side elliptic cylinders
No full text[[abstract]]In this study, near-critical bifurcations of low Reynolds number (Re) flows past a pair of elliptic cylinders in the side-by-side arrangement are numerically investigated, and onsets of several distinct transition scenarios are addressed. A nested Cartesian-grid formulation, in combination with an effective immersed boundary method and a two-step fractional-step procedure, has been adopted to simulate the flows. The transition scenarios associated with various periodic, quasi-periodic, and biased flows, their bifurcation characteristics, corresponding critical Reynolds numbers, and phase-portraits are exploited to better understand the governing physics. From the global point of view, there appear variety of flow patterns within the investigated parameter space, 40 <= Re <= 300, 0.2 <= G <= 3.0 (G being the gap-ratio of the cylinders), and 1.5 <= A <= 3 (A is the cylinder aspect-ratio), which include, symmetric vortex shedding mode, semi-single/twin vortex street formations, asymmetric/deflected flows, stationary/biased flip-flopped-type vortex shedding, weakly-chaotic flows, and in-phase/anti-phase vortex synchronizations. We numerically present these flows by tuning Re quasi-stationary, and provide a broader understanding of the entire transition process. A comprehensive analysis of effects of Reynolds number, the gap-ratio, and the angle of incidence on different flow-induced forces on the cylinders is included in this regard. On the other hand, our simulated wakes with various non-zero incidence-angles are found to reveal a rich variety of instability induced weakly synchronized physical evolution characteristics, which remained virtually unexplored. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3687450][[note]]SC
Effects of strong ground motion on the susceptibility of gully type debris flows
No full text[[abstract]]Typhoon Herb in 1996 caused widespread debris flows in central Taiwan. The 7.3 Chi-Chi earthquake on September 21,1999, which also took place in central Taiwan, induced many landslides in the region. These landslides turned into debris flows when Typhoon Toraji struck Taiwan in 2001. This research selects three regions which suffered a ground motion class of 5, 6, and 7 on the Richter scale during the Chi-Chi earthquake as study areas. Air photos from 1997 and 2001 of these regions are used to map the gully-type debris flows that took place after Typhoons Herb and Toraji, respectively. The gullies adjacent to the debris flow, but without a trace of debris flows, are also mapped as the non-debris flow data. The topography, hydrogeology, and rainfall factors - where debris flow occurred and in which there was no occurrence of debris flows in these gullies were retrieved from DTM, geological maps, and iso-countour maps, and of rainfall through GIS processing. These characteristic are introduced into a probabilistic neural network to build a predicting model for the probability of the occurrence of debris flows. Three series of cross analyses are conducted to compare the probability of the occurrence of debris flows of the same dataset predicted by different prediction models. The results reveal that the susceptibility of debris flows was elevated after the Chi-Chi earthquake struck. The upsurge of susceptibility was more obvious for the regions that received a higher class of ground motion. (C) 2008 Elsevier B.V. All rights reserved.[[note]]SC
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
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