1,720,967 research outputs found
River re-naturalization: historic and anthropogenic constraints, ecosystems and their interaction
No full textThe propagation of wave impact induced pressures into cracks and fissures
No full textRock cliffs and blockwork coastal structures often suffer a peculiar type of damage, whereby individual blocks are removed out of their location towards the sea. The location of damage suggests that breaking wave action is the main cause. It has been suggested that wave impact pressures travel into the water or air filled cracks and fissures of the structures, leading to large pressures acting inside of the structure or cliff and to the removal of blocks. This assumption was only recently confirmed for water filled cracks with a series of model tests at Queen's University Belfast. Real cracks in rock cliffs are, however, often only partially filled with water. A new experimental study, also conducted at Queen's University Belfast, revealed that wave impact generated pressures can ravel into both fully or partially water filled cracks or joints. In partially submerged cracks the pressure pulse was found to travel in the air, propagating fast and with little attenuation deep into the structure, signifying that partially filled cracks are potentially more dangerous for the integrity of the structure than completely water filled cracks. These pressure pulses may be the main cause for the seaward removal of blockwork in coastal engineering structures or of rock cliff material
The breast shot water wheel: design and model tests
No full textBreastshot waterwheels—that is, waterwheels where the water enters the wheel approximately at the level of the axis—were in widespread use in England and Germany during the nineteenth and early twentieth century. Although this type of wheel even today has the potential for the economical and environmentally acceptable exploitation of small hydropower with low heads from 1·5 to 2·5 m, very little is known about its performance characteristics. In order to assess the breastshot waterwheel for hydropower generation, a study of design methods and a series of model tests were conducted at Queen’s University Belfast. Sample calculations for a 4 m diameter wheel are given to explain the design principles. Tests on a 1:4 scale, 1 m diameter model gave efficiencies of 78·5% over a broad range of flows. Based on these measurements and observations, improved geometries for in- and outflow were developed, resulting in maximum efficiencies of 87·3%. An initial ecological assessment indicated that waterwheels may have a significantly reduced ecological impact when compared with turbines. The breastshot waterwheel was found to be an efficient and ecologically acceptable hydraulic energy converter with the potential for further development.<br/
The development of the rotary hydraulic pressure machine
No full textThere currently exists a demand for economical hydro-electric machinery that operates at low heads and moderate flows; sites where the drop from a higher water level to a lower one is quite small, in the order of 1-3m, with flows between 5m3/s and 15m3/s. The economics which determine a machine’s viability are a function of its size and complexity, its efficiency characteristics and its power output. The author believes that the most promising route lies with the Rotary Hydraulic Pressure Machine (RHPM), a concept developed from the Staudruckmaschine (SDM) by Brinnich (2001). This paper analyses the SDM in detail using scale model tests. These tests found that the SDM had many deficiencies resulting in unnecessarily low efficiencies and flow capacity, which is the volume of water which the wheel can process per second per metre width. The scale tests also aided the development of a new detailed theory of operation for wheels operated by hydraulic pressure. Significant modifications are then proposed for the SDM to address its deficiencies. These revise its channel design, blade shape, and support structure. The result is effectively a new machine, the RHPM, which is expected to have a 20% higher efficiency, greater flow capacity and smoother operation than the SDM. The RHPM can be built at any site; however it is especially suitable for incorporation into existing weir structures with minimal modifications being required to the weir structure itself. Furthermore, the RHPM is unlike most conventional rotodynamic machines as it is symmetrical and therefore fully reversible. This property means it also has potential application in tidal flow environments
Stream wheels for applications in shallow and deep water
No full textThe interest in renewable energies has initiated a re-consideration of hydropower resources. Currently there is no economical energy converter for the kinetic energy of shallow free surface flows available. Stream wheels or impulse type water wheels were employed in this role until the middle of the last century, and this type of water wheel could again be of interest today. Little engineering or performance information is however available for such machines. Following a literature review, three types of stream wheels were identified and theoretical models for performance evaluation were developed. Model tests were employed to verify theoretical predictions. It was found that both in terms of potential power production and number of possible sites the stream wheel in deep water is the most promising candidate for further development
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