1,721,026 research outputs found
Flow fields in reflected waves at a sloped sea wall
No full textWave run-up, and flow visualization experiments were conducted with a 1:2 sloped sea wall model. The visualization experiments gave an overview of flow fields in reflected, non-breaking conditions. Maximum particle velocities were found to be significantly smaller than suggested in the literature. Downrush produced a fast sheet flow, extending down to the toe of the sea wall. This created a ‘reverse’ breaker during the retreat of the initially non-breaking wave, which explains the high energy dissipation rates for non-breaking waves reported in the literature. Embankments may therefore be exposed to wave impact pressures in areas up to 1.18 H0 below MW
Wellendruckschlagbelastungen auf historische Küstenbauwerke (Wave impact loads on historical coastal structures)
No full textDuring the 19th and the early 20th Century, many coastal structures were built from block-work, i.e. large granite or concrete blocks. A large number of these structures are still in use. The continuous damage to blockwork structures, as well as a possible increase in wave loading due to increased storm activities, mean that coastal engineers occasionally have to develop repair and maintenance strategies or conduct risk assessments. The available information about this type of structures is however limited, and the interaction between waves and structure is not fully understood. A number of observed cases of damage indicate that wave impact induced pressure pulses may propagate into water or air filled joints or cracks, generating high pressure fluctuations inside of the structure. These pressures then push individual blocks out of their position into the sea. Several cases of damage to existing breakwaters illustrate this mechanism. Model tests showed that wave impacts can generate pressure pulses which then propagate into water and air filled cracks, whereby the speed of propagation increases with increasing crack width. Large scale tests and field measurements gave additional information about the characteristics of the pressure pulses. A similar damage mechanism is thought to be responsible for damage to sea dykes. photoelastic and numerical models of blockwork walls showed that the stress distribution inside of such a wall deviates considerably from standard assumptions. The analysis of damage events and model tests showed that not only the stability but also the integrity of the structure should be considered for the risk-assessment of blockwork structure
Sagebien and Zuppinger water wheels for very low head hydropower applications
No full textIn rivers and at old mill sites there is a substantial amount of very low-head hydropower with head differences from 0.3 to 1.5 m. Much of this potential is unused since in these conditions modern hydropower technology is not cost effective. In the mid-nineteenth century, the Sagebien and Zuppinger wheels were developed for such situations. Recent model tests showed the Zuppinger wheel has a maximum efficiency of 85%. No such data are however available for the Sagebien wheel. At Southampton University, 1:10 scale model tests with Sagebien and Zuppinger wheels were conducted. Both wheels had maximum efficiency of 84%. The Sagebien wheel’s performance curve was less affected by the flow rate. Contrary to the Zuppinger wheel, the Sagebien wheel’s geometry avoids noise generation at the upstream blade entry. The results indicate that the Sagebien wheel is an attractive, although so far underestimated, hydropower converter for very low-head situations.</p
Experimental investigation of the atmospheric steam engine with forced expansion
Get PDFLow and medium temperature thermal energy with temperatures of 100°–150 °C is available from renewable energy sources such as solar thermal or geothermal energy. Recent progress in flat plate solar thermal collector technology indicates that economical solutions for this temperature range are now becoming possible. Current technologies to generate mechanical energy from this temperature bracket such as Organic Rankin Cycle machines are however complex, and therefore only economical for larger units. There is a need for a simple, cost-effective medium temperature thermal engine for small-scale applications. Recently, the atmospheric steam engine was re-evaluated for this application. The theory was extended to include a forced expansion stroke. This can increase the theoretical efficiency of the ideal engine from 6.5% to 20%. In order to assess this theory, a series of experiments was conducted at Southampton University. It was found that the isothermal expansion of steam, and its subsequent condensation, is possible. The experiments showed a maximum efficiency of 10.2% for an expansion ratio of 1:4, indicating the validity of the theory. A further increase of efficiency to approximately 17% appears possible. It was concluded that the atmospheric engine with forced expansion has development potentia
Review of current developments in low head, small hydropower
No full textIn Europe, the development of hydropower with installed capacities of more than 1 MW and head differences of more than 2 – 3 m is nearly completed. A promising corner of the future hydropower market remains the economical and ecologically acceptable exploitation of smaller hydropower from 100 – 1000 kW with very low head differences of 0.80 up to 2.00 m, where conventional turbines are neither economical nor ecologically undisputed. In response to this situation, a number of novel energy converters, some of which reliant on rather unusual principles, have been proposed and developed. The high efficiencies stated by the inventors are however often neither confirmed by theoretical considerations nor by measurements. Often a positive view of the innovator will valuate the efficiency too high, as well as the specific investment costs as too low, leaving potential investors with a large margin of risk. The most important new technologies were analysed from the view of underlying technical principles and possible efficiency, economy and ecological impact. It was found that most proposed technologies suffer from low efficiencies as well as high investment costs. Only the Rotary Hydraulic Pressure Machine appears to have development potential. The article aims to give an overview over an emerging market in order to give engineers a background for the rational assessment of a variety of new technologies
Theoretische und physikalische Grundlagen für die Wasserkraftnutzung
No full textFor the planning, construction and operation of hydropower plants, knowledge of the theoretical and physical principles is important to enable a determination of the hydropower potential available at the site and a design of the hydropower machine and the hydropower plant. The basic physical principles required are presented in the following paper.</p
Hydropower converters with head differences below 2·5 m
Get PDFHydropower with very low head (VLH) differences constitutes a largely unused potential source of renewable energy. A literature review of hydropower converters for VLH differences covering the engineering literature from the 1830s onwards was conducted to assess the potential of historic, current and emerging technologies. The results showed that a large number of different interesting technologies were used/developed, and that several novel and promising concepts are currently under development. Technologies emerged for different market segments: water wheels for low flow rates, turbines for large flow volumes. To take account of increased ecological demands and the desire for cost reduction, further improvements of existing technologies and new converter technologies have been developed. Other concepts described in the literature were apparently never applied although they promise development potential. The information gathered allowed for a comparison of available technologies with respect to area of application, performance, ecological impact and cost level
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
Mapping of bridge pier scour with projection Moiré
No full textDuring the construction works for an extension of an existing bridge pier severe scouring occurred after the placement of a sheet pile cofferdam. A detailed investigation of the situation was conducted at the Bundesanstalt für Wasserbau (Federal Hydraulic Research Institute) in Karlsruhe, Germany. For the measurement of the scour development without protective layer, an optical measurement method - out-of-plane projection Moiré - was employed in order to investigate its suitability. This technique uses the Moiré effect to map surface contours which are indicated by superposition lines or fringes. The Moiré fringe patterns generated by the measurements were then digitised for further analysis. The contour model revealed that, although the scouring depth did not increase significantly with time after the initial scour had formed, scour size did in fact do so. In addition, the contour model allows to map the scour extension and to determine the maximum deposition height. A 3D model of bridge pier and scour showed, in combination with photographs, the validity of the measurement technique and pointed towards the generation of a virtual environment for the representation of experimental results
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