1,720,984 research outputs found
A new solver for incompressible non-isothermal flows in natural and mixed convection over unstructured grids
Full text linkIn the present paper we propose a new numerical methodology for the solution of 2D non-isothermal incompressible flows for natural and mixed convection in irregular geometries. The governing equations are the Incompressible Navier-Stokes equations and the energy conservation equation. Fluid velocity and temperature are coupled in the buoyancy term of the momentum equations according to the Oberbeck-Boussinesq approximation. The governing equations are discretized over unstructured triangular meshes satisfying the Delaunay property. Thanks to the Oberbeck-Boussinesq hypothesis, the flow and energy problems are solved in an uncoupled way, and two fractional time step procedures are sequentially applied to solve each problem. The prediction steps of both procedures are solved applying a Marching in Space and Time (MAST) numerical Eulerian scheme, which explicitly handles the non-linear terms in the momentum equations of the fluid problem, and allows numerical stability for Courant numbers greater than one. An analytical solution is applied for the prediction thermal problem. The correction steps of the two fractional time step procedures involve the solution of large linear systems, whose matrices are sparse and symmetric and have the “M” property if the mesh satisfies the Delaunay condition. This allows a well performing condition number. The matrix coefficients are constant in time, so that they are calculated and factorized only once, before the simulations loop starts, saving a lot of computational time. We present four numerical applications. For the first test, an analytical solution is available, and this makes it possible to analyze the spatial convergence order of the numerical solver. In the other applications, we investigate the capability of the proposed algorithm to handle irregular geometries, and we compare the computed results with experimental data and the outputs of literature models. A study of the required computational costs is also presente
Numerical analysis of a new cross-flow type hydraulic turbine for high head and low flow rate
Full text linkCross-flow turbines have recently been proposed for energy recovery in aqueducts when the outlet pressure is greater than zero, owing to their constructive simplicity and good efficiency within a large range of flow rates and head drops. In the case of high head drop (higher than 150 m) and relatively small discharge (lower than 0.2 m3/s), the traditional design of these turbines leads to very small widths of the nozzle and the runner; as a consequence, friction losses grow dramatically and efficiency drops down to very low values. Standard Pelton turbines require zero outlet pressure and cannot be used as alternatives. A new counter-pressure hydraulic turbine for high head and low flow rate, called the High Power Recovery System (H-PRS) is proposed. H-PRS presents a different geometry to reduce friction losses inside the nozzle and the runner by widening the two external walls. Several curved baffles are proposed to guide the fluid particles inside the nozzle and to guarantee the right velocity direction at the inlet surface of the runner. Computational Fluid Dynamics (CFD) 3D transient analyses are carried out to measure H-PRS efficiency for different operating conditions and to compute its characteristic curve for different positions of the regulating flap
MAST-RT0 SOLUTION OF 3D NAVIER STOKES EQUATIONS ON UNSTRUCTURED MESHS. PRELIMINARY RESULTS IN THE LAMINAR CASE
Full text linkDesign of Reliable and Efficient Banki-Type Turbines
Full text linkA new shape for the external surface of the Crossflow turbine blades is proposed, which
allows for the preservation of hydraulic efficiency in spite of a significant maximum blade
thickness providing mechanic robustness and reliability. The final shape of the blades is assessed
using an iterative solution for two uncoupled models: a 2D computational fluid dynamic (CFD)
and a structural 3D finite element method (FEM) analysis of a single blade. Application of the
proposed methodology to the design of a power recovery system (PRS) turbine, a new
backpressure Crossflow-type inline turbine for pressure regulation, and energy production in a
real Sicilian site follows
MAST-RT0 solution of 3D Navier Stokes equations in very irregular domains. Preliminary results in the laminar case
No full textA new numerical methodology to solve the 3D Navier-Stokes equations for incompressible fluids within complex boundaries and unstructured body-fitted tetrahedral mesh is presented and validated with three literature and one real-case tests. We apply a fractional time step procedure where a predictor and a corrector problem are sequentially solved. The predictor step is solved applying the MAST (Marching in Space and Time) procedure, which explicitly handles the non-linear terms in the momentum equations, allowing numerical stability for Courant number greater than one. Correction steps are solved by a Mixed Hybrid Finite Elements discretization that assumes positive distances among tetrahedrons circumcentres. In 3D problems, non-Delaunay meshes are provided by most of the mesh generators. To maintain good matrix properties for non-Delaunay meshes, a continuity equation is integrated over each tetrahedron, but the momentum equations are integrated over clusters of tetrahedrons, such that each external face shared by two clusters belongs to two tetrahedrons whose circumcentres have positive distance. A numerical procedure is proposed to compute the velocities inside clusters with more than one tetrahedron. Model preserves mass balance at the machine error and there is no need to compute pressure at each time iteration, but only at target simulation times
EXTERNAL RECIRCULATION IN PRS TYPE TURBINE: EXPERIMENTAL AND NUMERICAL RESULTS.
No full textCavitation is a relevant phenomenon for structural safety and noise level in hydraulic turbines, occurring when water pressure falls below the vapor value at a given temperature. In this case bubbles of vapor grow inside the liquid and move along with it. When the pressure returns above the vapor value the bubble collapses, and the pressure can locally achieve very high values, up to 7000 bars (Kumar & Saini, 2010). Moreover, if the bubble was confined also by the solid wall of a blade, the solid particles suspended in the fluid can be transported by the fluid ones and hit the solid wall at very high velocity, generating erosion. Cavitation is also the source of high frequency noise, very disturbing for humans.
Cavitation has been extensively studied mainly in reaction turbines, especially Francis and Kaplan types (Luo &Tsujimoto, 2016, Turi et al., 2019; Alligne et al., 2014). Adhikari et al. (2016) carried on a numerical analysis occurring at the tip of the blades of a Crossflow turbine. The conclusion was that, at least in the case study, cavitation occurred only at rotational velocity greater than the design one.
Power Recovery System (PRS) turbine is a reaction turbine with a rotor like the Crossflow one, that can be used instead of the Crossflow in the case of outlet pressure greater than zero. PRS has a very simple design and can provide hydraulic regulation with the use of a mobile flap that can reduce the rotor inlet area. Due to its simplicity and small size, PRS can be easily installed by replacing a short part of the pipeline where the hydraulic jump is available.
In the following paper it is first shown, by means of CFD simulations, that low pressure and cavitation can easily show up in PRS turbines, especially when the downstream outlet pressure is very low, and that pressure attains the minimum value immediately after the tip of the nozzle, according to the rotational direction. The physical explanation is that the momentum Пout of the water volume trapped in the blade channel, immediately after the tip of the nozzle, cannot be balanced by the Пin, inlet one and this leads to strong negative pressure gradients.
A possible countermeasure is the use of external recirculation by means of a small pipe connecting the outlet pipe with a rectangular opening of the case immediately after the tip of the nozzle. The connection leads to a small recirculation flow and to a pressure rise in the target area. The length of the arc between the end of the nozzle and the rectangular opening is a fraction of the arc of the blade channel, so that an inlet momentum is always provided to the blade channel up to the end of the rectangular opening.
The proposed change has been numerically and experimentally tested on a 5kW PRS prototype, installed in the experimental loop of the hydraulic lab of the University of Palermo
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
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