1,721,037 research outputs found
Experimental Study of the Initiation of Isolated Boulder Motion under Uniform Flow Conditions on a Horizontal Surface
No full textDimensionally reduced shallow moment simulations of a pumped-storage plant tunnel system
No full textSecondary Currents, Turbulence Characteristics, and Bed Shear Stress Variations in Supercritical Narrow Channel Flows
No full textSediment bypass tunnels (SBTs) belong to the active sediment management strategies of combating reservoir sedimentation, which route high-speed sediment-laden flows around reservoirs and primarily experience supercritical narrow channel flow conditions that characterize secondary currents and redistributed primary velocity, turbulence properties, and bed shear stress, which intensified by the presence of in-plan bend. These alternations in the flow characteristics influence the sediment transport, invert abrasion, and hence, the design of SBTs. Although recent laboratory-scaled experiments in straight channels produced two-dimensional flow properties, individual sediment transport, and hydro-abrasion estimate models, three-dimensional investigations are scarce, and therefore, a supplementary and cost-efficient design alternative can be the computational fluid dynamics (CFD) simulation. Furthermore, field investigations at SBT bends demonstrated deeper invert abrasion near the inner wall than toward the outer wall. However, a detailed investigation of such flow characteristics and sediment transport was missing. Under these circumstances, research was initiated at NTNU to bridge the gap between experimental and numerical studies of high-speed flows while emphasizing some of the challenges and phenomena associated with SBT hydraulics.
This research focuses on (a) developing open-source CFD models to simulate the turbulence-driven secondary currents and their interactions with the flow characteristics in straight narrow channels conveying supercritical flows and subsequently applying this model to investigate the effect of cross-sectional geometries found at SBTs and to compare open channel flows with pressurized flows or duct flows, which are sometimes observed at SBTs and (b) experimental investigation of the effect of a channel bend on the flow properties, bed shear stress, and bed load transport those are linked to the abrasion patterns observed at SBT bends and producing experimental data useful in further development of numerical models. Here, computationally economical Reynolds stress models were utilized to compute steady-state results of high-speed flows after modifying the existing model codes in OpenFOAM, as the alternate transient options are still computationally very demanding. Furthermore, a state-of-the-art particle tracking velocimetry system in conjunction with the Shake-The-Box particle tracking algorithm was employed to measure the flow characteristics in a channel bend model of the Solis SBT, Switzerland, and high-speed cameras were utilized to observe the bed load movements.
The modified Reynolds stress models compute promising results of turbulence-driven secondary currents, distributions of longitudinal velocity and turbulence parameters, and bed shear stress undulation and its averaged value for uniform supercritical flows. Channel aspect ratio influences the secondary current formations and their influence on the flow characteristics and bed shear stress. Intermediate vortex develops with decreasing aspect ratio and is fully developed for aspect ratios ≤ 1.05. The bottom vortex guides the near-bed redistribution of flow properties and bed shear stress undulation. Additionally, altering the sidewall curvature influences the bottom vortex and developing intermediate vortex, bottom corner zone of low-momentum fluids, and bed shear stress distribution. A horseshoe cross-section with plane invert is the suitable design choice over the archway and circular geometries, as it helps in shrinking the corner zone of low-momentum fluids while providing a wider invert than a circular cross-section without imposing a stress concentration. Besides, the flow properties in the bottom half flow depth and bed shear stress distribution of open channel flows are comparable to that of duct flows with the same aspect ratios, particularly for a square cross-section due to similar bottom corner configurations and comparable secondary currents. However, duct flows experience greater boundary shear stress than open channel flows. In addition, the experimental investigation indicates that the free surface undulates significantly across and along a channel bend caused by the cross-waves, the curvature-induced secondary currents and their impacts develop over a stretch in the bend, and the bed load movements in the channel bend is not a function of the bed shear stress magnitude but rather of the curvature-induced secondary currents which push and keep the sediments near the inner wall. As a result, deeper invert abrasions are formed toward that wall at SBT bends and further downstream.
This study contributes to the available knowledge of three-dimensional narrow channel flows and the interactions between hydraulic and hydrodynamic phenomena associated with high-speed sediment-laden flows, particularly for aspect ratios ≤ 2. The produced open-source CFD models are useful in computing complex three-dimensional supercritical flow characteristics in uniform narrow channels. In addition, the obtained experimental findings enlighten the hydrodynamics behind the abrasion patterns observed at SBT bends and generate data useful in validating numerical models in the future. Further advancements in computational resources in the years ahead are expected to make the alternate and more accurate transient CFD techniques feasible for supercritical flows in straight and curved channels
A concept for sustainable and digitalized hydropower going beyond the sustainable hydropower sustainability tool
No full textThe EU energy policy has the ambitious objective to become the first carbon-neutral continent in the world. In order to achieve this objective hydropower will have to play an essential role as energy source and energy storage in pump storage facilities. Hydropower is a clean, low carbon, and cost-efficient energy source that can be exploited sustainably if an adequate management system is implemented. Nevertheless, in the past, hydropower operations have led to conflicting interests over water usage, impacts on aquatic flora and fauna, and significant socio-economic implications. In order to avoid and mitigate possible negative consequences of hydropower plants the Hydropower Sustainability Assessment Protocol (HSAP, https://www.hydrosustainability.org/) provides a helpful tool to minimize related impacts. In this presentation we will delineate how the 26 topics of the HSAP could be complemented in order to provide a fully digitalized sustainability framework for hydropower. In particular, we will outline innovative solutions for the most challenging topics of sustainable hydropower plants, including i) energy supply securing with a high share of renewable energies ii) climate change impacts on water resources and hydropower production, ii) altered flow and changed turbidity dynamics in rivers, iii) long-term downstream effects on river beds and groundwater exchange, iv) degradation of river ecology, v) socio-economic impacts on local stakeholders, vi) adequate assessment of the water-energy-food nexus, vii) near real time digitalisation framework to streamline information. Through the digitalization of the HSAP a standardized and transparent flow of information will be guaranteed. Within the presented digitalized framework, all data will be processed to standardize, harmonize and synthesize results and information from all working tasks into a data lake. We may apply this framework to demonstration hydropower plants of different types to improve their sustainability, efficiency and management
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
Stepped Large Scale Spillways in Unlined Rock
No full textWater is an imperative source to human life. The change in climate is affecting the availability of water round the year which combined with increasing water demand has become a global problem. This has rendered water storage as the only option for proper management of water. Construction of dams for the storage helps in the field of, flood control, navigation, power generation, irrigation, drinking water, and recreation. The inability to store all the water in the river streams economically while also maintaining a high level of safety, leads to the problem of conveying the excess water safely to the downstream. Construction of steps on spillway chute, construction of plane chute with stilling basins, flip bucket structures with plunge pool, and construction of energy dissipators like Roberts splitters and baffle blocks are some of the adopted methods for dissipating the energy of the flowing water.
Stepped spillways are efficient at dissipating the flow energy as the water flows down the steps. This design method has been adopted for a long time for its easy to build design, structural stability provided and the energy dissipation ability. In this study, the energy dissipation capability of the stepped spillway was tested in a physical hydraulic model. The effect of discharge rate, step dimension, and surface roughness on the energy dissipation rate has been tested. In addition, the change in energy dissipation with a change in spillway height has been studied. Furthermore, the viability of constructing stepped spillway in an unlined rock has been looked briefly.
The results obtained show a high energy dissipation efficiency for all the steps tested with the dissipation efficiency reaching as high as 93% for a low flow rate. For a constant step height to length ratio and for the same discharge, the dissipation efficiency was found to be higher for the bigger steps. Introduction of surface roughness further improved the dissipation efficiency. The construction of a weir on the steps to create pool of water was found to increase the dissipation efficiency considerably. Dissipation was considerably high for low discharges for which fully developed hydraulic jump was created for the falling jet. An inverse pattern between the discharge and energy dissipation was observed in the test
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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