1,720,972 research outputs found
Unstructured Grid Finite-Volume Algorithm for Shallow-Water Flow and Scalar Transport with Wetting and Drying
No full textA high-resolution, unstructured grid, finite-volume algorithm is developed for unsteady, two-dimensional, shallow-water flow and scalar transport over arbitrary topography with wetting and drying. The algorithm uses a grid of triangular cells to facilitate grid generation and localized refinement when modeling natural waterways. The algorithm uses Roe’s approximate Riemann solver to compute fluxes, a multidimensional limiter for second-order spatial accuracy, and predictor-corrector time stepping for second-order temporal accuracy. The novel aspect of the algorithm is a robust and efficient procedure to consistently track fluid volume and the free surface elevation in partially submerged cells. This leads to perfect conservation of both fluid and dissolved mass, preservation of stationarity, and near elimination of artificial concentration and dilution of scalars at stationary or moving wet/dry interfaces. Multi-dimensional slope limiters, variable reconstruction, and flux evaluation schemes are optimized in the algorithm on the basis of accuracy per computational effort
Wetting and Drying of Triangular Computational Cells
No full textA robust and novel procedure is introduced into a fixed-grid finite volume shallow-water model to consistently track fluid volume and the free surface elevation in partially submerged triangular cells, which leads to excellent flow and scalar transport predictions in the presence of a stationary or moving wet/dry interface. Using this procedure, a Roe-type finite volume scheme is found to perfectly conserve both fluid and dissolved mass, preserve stationarity, and nearly eliminate artificial concentration and dilution of scalars at wet/dry interfaces
Simulation of the St Francis dam-break flood
No full textNumerical simulation of the 1928 St. Francis Dam failure in southern California was accomplished using a 2D Godunov-type finite-volume shallow-water algorithm run on an unstructured grid of triangular cells. The model was found to be accurate based on historical accounts, including flood maps and arrival time data, and sensitivity analysis was performed to determine factors that control the predictability of flooded area and flood arrival times. Results show that predictions of flood arrival times are sensitive to both mesh resolution and Manning coefficient (used to scale flow resistance), while predictions of flooded area were found to be relatively insensitive to the Manning coefficient. These results suggest that bed resistance controlled the speed of the St. Francis flood while flooded area was controlled by topography and the volume of released water. The study also revealed two types of previously unreported oscillatory surging in the dam-break flood. The first is due to a standing wave that develops in a tortuous reach of channel downstream of the dam. The wave is excited by reflections off canyon walls and accounts for a 30% fluctuation in discharge. The second is due to a mode-two standing wave in the reservoir. This wave is caused by the reflection of dam-break rarefaction waves off reservoir walls, and accounts for only 2-3% fluctuation in discharge. Both oscillations are therefore shown to be physically based and should not beinterpreted as spurious oscillations common to many numerical wave models
Conservative Wetting and Drying Methodology for Quadrilateral Grid Finite Volume Models
No full textAlgebraic equations relating fluid volume and the free surface elevation in partially wetted quadrilateral computational cells are derived and incorporated into a Godunov-type, finite-volume, shallow-water model. These equations make it straightforward to reconstruct the free surface elevation based on the volume of fluid in a computational cell, the dependent variable tracked by finite volume models for conservation purposes, regardless of whether the cell is fully or partially wetted. Improvements to the variable reconstruction process streamline the computation of mass and momentum fluxes with approximate Riemann solvers, yielding a model that simulates sub-, super-, and transcritical flows over irregular topography with wetting and drying fronts. Furthermore, the model is free from fluid and scalar mass conservation errors and it eliminates nonphysical distributions of scalars by avoiding artificial concentration and/or dilution at wet/dry interfaces. Use of this wetting and drying methodology adds roughly 10% to the execution time of flow simulations
A balanced treatment of secondary currents, turbulence and dispersion in a depth-integrated hydrodynamic and bed deformation model for channel bends
No full textThis work deals with the formulation and numerical implementation of a two-dimensional mathematical and numerical model describing open channel hydrodynamics, sediment and/or scalar transport and riverbed evolution in curved channels. It is shown that a well balanced 2D model can predict flow features, sediment and scalar concentration, and bed elevation with an accuracy that is suitable for practical river engineering. The term “balanced” implies that important physical processes are modeled with a similar degree of complexity and exhaustiveness. The starting point of the model formulation is the assumption of self-similarity of vertical velocity profiles (horizontal velocities in the longitudinal and transverse directions), that are scaled by shear velocity and streamline curvature, both resolved by the model. The former is scaled by a bed-resistance coefficient that must be estimated or calibrated – as usual – on a application-specific basis, and the latter is computed by a new, grid-based but grid orientation independent, scheme that acts on the discrete solution. All processes, including bottom shear, momentum dispersion, scalar dispersion, turbulent diffusion, bed load, and suspended load, are modeled using physically based, averaged values of empirical or semi-empirical constants, and consistently with the assumed velocity profiles (and bed-generated turbulence). Bed deformation modeling can be implemented with either an equilibrium or non-equilibrium formulation of the Exner equation, depending on the adaptation length scale, which must be taken under consideration if significantly larger than the length scale of the spatial discretization. The governing equations are solved by high-resolution, unstructured-grid Godunov method, which is elsewhere tested and shown to be reliable and second-order accurate. Application of the model to laboratory test cases, using standard parameter values and previously reported bed-resistance coefficients, gives results comparable to many 2D and 3D models previously applied to the same cases, most part of which benefit from case-specific parameter tuning. There are obviously intrinsic limits to the descriptive ability of 2D models in river modeling, but the results of this study point to the utility and cost-effectiveness of a well-designed 2D model
Identification of a robust, efficient and practical finite-volume flood inundation simulator.
No full textSchemes to support a robust and efficient two-dimensional flood inundation simulator based on an unstructured grid finite volume method are compared in the context of steady state and unsteady flow problems of practical importance where terrain is defined by high-resolution Digital Elevation Models (DEMs). The schemes adopt a Godunov-type approach and use Roe’s method to compute mass and momentum fluxes, but differ in formal accuracy and depending on whether conservative variables (CV) or primitive variables (PV) are used for variable reconstruction at cell faces. The CV scheme is found to be best suited to subcritical applications but unstable is cases where a supercritical wetting front advances over highly irregular topography, as in a dam-break application. The PV scheme is more robust, working in all practical applications considered, achieving greater accuracy than the CV scheme in supercritical flow applications, but yielding an order of magnitude more error in steady flow applications involving subcritical flow. A hybrid CV/PV scheme is therefore proposed
Adaptive Godunov-Based Model for Flood Simulation
No full textGodunov-based shallow-water models utilize a discontinuous reconstruction of data at cell faces even for smooth flow, which can cause energy dissipation and degrade accuracy. Analysis of discrete equations shows that jumps (and therefore error) can be minimized by adaptively selecting either primitive or conservative variables for slope limiting and reconstruction according to the local Froude number. Therefore, a Godunov-based model with an adaptive scheme of slope limiting and variable reconstruction is presented. Two practical flood modeling applications are used to compare the performance of the adaptive scheme against two non-adaptive schemes. In addition, performance of second-order accurate schemes is compared to first-order schemes that utilize a second-order accurate description of terrain. Results show that the first-order adaptive scheme possesses the best combination of robustness, efficiency, and accuracy of the tested models
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
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