The Experts below are selected from a list of 294 Experts worldwide ranked by ideXlab platform
Robert J Poole - One of the best experts on this subject based on the ideXlab platform.
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the influence of blade pitch angle on the performance of a model horizontal axis tidal stream turbine operating under wave current Interaction
Energy, 2016Co-Authors: T A De Jesus Henriques, T S Hedges, I Owen, Robert J PooleAbstract:Tidal stream turbines offer a promising means of producing renewable energy at foreseeable times and of predictable quantity. However, the turbines may have to operate under Wave-Current conditions that cause high velocity fluctuations in the flow, leading to unsteady power output and structural loading and, potentially, to premature structural failure. Consequently, it is important to understand the effects that wave-induced velocities may have on tidal devices and how their design could be optimised to reduce the additional unsteady loading. This paper describes an experimental investigation into the performance of a scale-model three-bladed HATT (horizontal axis tidal stream turbine) operating under different Wave-Current combinations and it shows how changes in the blade pitch angle can reduce wave loading. Tests were carried out in the recirculating water channel at the University of Liverpool, with a paddle wavemaker installed upstream of the working section to induce surface waves travelling in the same direction as the current. Three wave conditions were employed in a current-dominated flow. The wave kinematics were measured using an acoustic Doppler velocimeter and there was generally good agreement with Stokes' second-order wave theory. Power and thrust measurements were taken under both current-alone and Wave-Current conditions for different blade pitch angles. It was observed that, as the blade pitch angle was increased from optimum, both the mean power and thrust on the turbine decreased and the reductions in thrust were always greater than in power. The fluctuations in power and thrust also decreased with an increase in the blade pitch angle. Therefore, changes in blade pitch angle can be used as a mechanism for reducing the loading on a HATT when operating with excessive wave-induced loads, while still enabling a significant amount of the available power in the unsteady tidal stream to be extracted.
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the effects of wave current Interaction on the performance of a model horizontal axis tidal turbine
International Journal of Marine Energy, 2014Co-Authors: T A De Jesus Henriques, S C Tedds, A Botsari, G Najafian, T S Hedges, C J Sutcliffe, I Owen, Robert J PooleAbstract:This experimental study investigates the performance of a marine current turbine in the presence of surface waves. The tests were carried out in the high-speed recirculating water channel at the University of Liverpool. A three-bladed model of a horizontal axis tidal turbine with a rotor diameter of 0.5 m was exposed to water flow with a steady uniform upstream velocity of 0.5 m/s. Regular surface waves were generated using a paddle wavemaker capable of producing a wide range of wave conditions in a current dominated flow (i.e. wave-induced velocities lower than current velocity). To determine the fluctuations in the current velocities caused by the surface waves, an acoustic Doppler velocimeter was used to measure the three-dimensional velocity components at various depths beneath two different waveforms. The measured kinematics of the waves showed excellent agreement with linear wave theory. Thrust and power measurements were taken from the turbine under flow conditions with the same two waveforms and compared with results taken in steady uniform flow to evaluate the effect of the wave-induced velocities on the turbine performance. The mean coefficients of power and thrust in the unsteady flow conditions were found to be very similar to those obtained in steady flow, but there were significant cyclic variations in the power and thrust which occurred at the frequency of the waves.
Nicholas C Kraus - One of the best experts on this subject based on the ideXlab platform.
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two dimensional depth averaged circulation model m2d version 2 0 report 1 technical documentation and user s guide
This Digital Resource was created from scans of the Print Resource., 2004Co-Authors: Adele Militello, Christopher W Reed, Alan K Zundel, Nicholas C KrausAbstract:Abstract : The two-dimensional (2-D) circulation model M2D, developed under the Coastal Inlets Research Program conducted at the U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, has been designed for local applications, primarily at inlets, the nearshore, and bays. M2D is computationally efficient, easy to set up, and has features required for many coastal engineering applications including robust flooding and drying, wind-speed dependent (time-varying) wind-drag coefficient, variably- spaced boftom-fiction coefficient, time- and space-varying wave-stress forcing, efficient grid storage in memory, two hot-start options, and the convenience, through control statements, of independently turning on or off the advective terms and mixing terms. If wave information is available, such as through coupling with the STeady state spectral WAVE model STwAVE, M2D will calculate wave friction and wave mixing. M2D can be coupled to regional circulation models through boundary conditions providing flexibility for large-scale applications and connectivity between models. A graphical interface for M2D has been implemented within the Surface-Water Modeling System (SMS) Versions 8.1 and higher. Features of the M2D interface are grid development, control file specification, model runs, post-processing of results, and visualization. M2D can be driven by larger-domain circulation models, such as ADCIRC, through boundary specification capabilities contained within the SMS. The Steering Module in SMS provides an automated means of coupling of M2D with STWAVE, which is convenient for projects that require wave-stress forcing for M2D as well as wave friction and mixing owing to breaking waves. The Steering Module allows the user to choose from seven possible coupling combinations, providing flexibility in conducting simulations of wave-driven currents and Wave-Current Interaction.
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two dimensional depth averaged circulation model m2d version 2 0 report 1 technical documentation and user s guide
This Digital Resource was created from scans of the Print Resource., 2004Co-Authors: Adele Militello, Christopher W Reed, Alan K Zundel, Nicholas C KrausAbstract:Abstract : The two-dimensional (2-D) circulation model M2D, developed under the Coastal Inlets Research Program conducted at the U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, has been designed for local applications, primarily at inlets, the nearshore, and bays. M2D is computationally efficient, easy to set up, and has features required for many coastal engineering applications including robust flooding and drying, wind-speed dependent (time-varying) wind-drag coefficient, variably- spaced boftom-fiction coefficient, time- and space-varying wave-stress forcing, efficient grid storage in memory, two hot-start options, and the convenience, through control statements, of independently turning on or off the advective terms and mixing terms. If wave information is available, such as through coupling with the STeady state spectral WAVE model STwAVE, M2D will calculate wave friction and wave mixing. M2D can be coupled to regional circulation models through boundary conditions providing flexibility for large-scale applications and connectivity between models. A graphical interface for M2D has been implemented within the Surface-Water Modeling System (SMS) Versions 8.1 and higher. Features of the M2D interface are grid development, control file specification, model runs, post-processing of results, and visualization. M2D can be driven by larger-domain circulation models, such as ADCIRC, through boundary specification capabilities contained within the SMS. The Steering Module in SMS provides an automated means of coupling of M2D with STWAVE, which is convenient for projects that require wave-stress forcing for M2D as well as wave friction and mixing owing to breaking waves. The Steering Module allows the user to choose from seven possible coupling combinations, providing flexibility in conducting simulations of wave-driven currents and Wave-Current Interaction.
Yusuke Uchiyama - One of the best experts on this subject based on the ideXlab platform.
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wave current Interaction in an oceanic circulation model with a vortex force formalism application to the surf zone
Ocean Modelling, 2010Co-Authors: Yusuke Uchiyama, James C Mcwilliams, Alexander F ShchepetkinAbstract:Abstract A vortex-force formalism for the Interaction of surface gravity waves and currents is implemented in a three-dimensional (3D), terrain-following, hydrostatic, oceanic circulation model (Regional Oceanic Modeling System: ROMS; Shchepetkin and McWilliams, 2005 ). Eulerian wave-averaged current equations for mass, momentum, and tracers are included in ROMS based on an asymptotic theory by McWilliams et al. (2004) plus non-conservative wave effects due to wave breaking, associated surface roller waves, bottom streaming, and wave-enhanced vertical mixing and bottom drag especially for coastal and nearshore applications. The currents are coupled with a spectrum-peak WKB wave-refraction model that includes the effect of currents on waves, or, alternatively, a spectrum-resolving wave model (e.g., SWAN) is used. The coupled system is applied to the nearshore surf zone during the DUCK94 field measurement campaign. Model results are compared to the observations and effects of parameter choices are investigated with emphasis on simulating and interpreting the vertical profiles for alongshore and cross-shore currents. The model is further compared to another ROMS-based 3D coupled model by Warner et al. (2008) with depth-dependent radiation stresses on a plane beach. In both tests the present model manifests an onshore surface flow and compensating offshore near-bed undertow near the shoreline and around the breaking point. In contrast, the radiation-stress prescription yields significantly weaker vertical shear. The currents’ cross-shore and vertical structure is significantly shaped by the wave effects of near-surface breaker acceleration, vertical component of vortex force, and wave-enhanced pressure force and bottom drag.
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wave current Interaction in nearshore shear instability analyzed with a vortex force formalism
Journal of Geophysical Research, 2009Co-Authors: Yusuke Uchiyama, James C Mcwilliams, Juan M RestrepoAbstract:[1] We examine Wave-Current Interactions in littoral current shear instabilities above a single-barred beach using a fully coupled wave and barotropic current model based on the multiscale asymptotic theory derived by McWilliams et al. (2004). This Eulerian wave-averaged model captures wave effects on currents (WEC) in a manner that leads to simple interpretations. The dynamically conservative WEC are the vortex force and material transport by Stokes drift and the sea level adjustment by wave set-down and setup. In the setting considered here there are also important current effects on the waves (CEW): induction of a Doppler shift by currents and surface elevation variation in the wave dispersion relation. Nonconservative effects, due to wave breaking and bottom drag, also play a prominent role in generating and equilibrating the mean alongshore current, consistent with prior studies. High bottom drag stabilizes the currents, while a drag reduction below a critical threshold value leads to shear instability with nearly periodic, alongshore-fluctuating eddies. An even smaller bottom drag yields irregular eddy motions with intermittent offshore eruption of vortex pairs from the meandering alongshore current. Several alternative parameterizations of the bottom drag are contrasted here. Including CEW in the model leads to a delay in the onset of the instability, a suppression of fluctuations in cross-shore velocity and lateral Reynolds stress, and an enhancement of the mean alongshore velocity. The WEC increase the Reynolds stress in the offshore region, and the conservative vortex force and mean advection are comparable in magnitude to the breaking acceleration and bottom drag. Conversely, the CEW reduce the Reynolds stress and attenuate the breaking acceleration through refractive focusing by current shear. Overall, the WEC enhance the instantaneous cross-shore momentum flux to induce more energetic eddy motions and retard the mean alongshore current, while the CEW stabilize the fluctuations and help maintain a strong mean current.
Till J J Hanebuth - One of the best experts on this subject based on the ideXlab platform.
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storm driven bottom sediment transport on a high energy narrow shelf nw iberia and development of mud depocenters
Journal of Geophysical Research, 2016Co-Authors: Wenyan Zhang, Ana Santos, Till J J HanebuthAbstract:Bottom sediment transport on the NW Iberian shelf was monitored during a downwelling storm in September 2014. Collected data was analyzed and fed into a 3D coastal ocean model to understand storm-driven sediment transport on the shelf and its impact on mid-shelf mud depocenters (MDCs). A significantly enhanced level of bottom sediment resuspension, nearly two orders of magnitude higher than that in the pre-storm period, was recorded at the mooring site. Field data analysis reveals that it was induced by a short-lasting strong bottom current in combination with enhanced Wave-Current Interaction. Simulation results indicate that this strong current was part of a coastal jet resulted from downwelling. An across-shelf horizontal density gradient as high as 0.32 g/m4 occurred at the interface between the downwelling and the bottom waters, forming a remarkable front. Due to buoyancy effect, the downwelling water was mostly confined to the coast with a depth limit of 80 m in the south and 120 m in the north of the region, resulting in a northward-directed coastal jet. Simulation results suggest that during the storm, local near-bottom sediment suspensions with concentrations on the order of 10 kg/m3 would be triggered by Wave-Current Interaction and flow convergence associated with the front. Direct impact on the development of MDCs by transport and deposition of concentrated sediment suspensions is indicated by model results. The seaward limit of the front coincided with the shoreward edge of the MDC nucleus, suggesting the front as a primary control on the deposition of fine-grained sediment. This article is protected by copyright. All rights reserved.
T A De Jesus Henriques - One of the best experts on this subject based on the ideXlab platform.
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the influence of blade pitch angle on the performance of a model horizontal axis tidal stream turbine operating under wave current Interaction
Energy, 2016Co-Authors: T A De Jesus Henriques, T S Hedges, I Owen, Robert J PooleAbstract:Tidal stream turbines offer a promising means of producing renewable energy at foreseeable times and of predictable quantity. However, the turbines may have to operate under Wave-Current conditions that cause high velocity fluctuations in the flow, leading to unsteady power output and structural loading and, potentially, to premature structural failure. Consequently, it is important to understand the effects that wave-induced velocities may have on tidal devices and how their design could be optimised to reduce the additional unsteady loading. This paper describes an experimental investigation into the performance of a scale-model three-bladed HATT (horizontal axis tidal stream turbine) operating under different Wave-Current combinations and it shows how changes in the blade pitch angle can reduce wave loading. Tests were carried out in the recirculating water channel at the University of Liverpool, with a paddle wavemaker installed upstream of the working section to induce surface waves travelling in the same direction as the current. Three wave conditions were employed in a current-dominated flow. The wave kinematics were measured using an acoustic Doppler velocimeter and there was generally good agreement with Stokes' second-order wave theory. Power and thrust measurements were taken under both current-alone and Wave-Current conditions for different blade pitch angles. It was observed that, as the blade pitch angle was increased from optimum, both the mean power and thrust on the turbine decreased and the reductions in thrust were always greater than in power. The fluctuations in power and thrust also decreased with an increase in the blade pitch angle. Therefore, changes in blade pitch angle can be used as a mechanism for reducing the loading on a HATT when operating with excessive wave-induced loads, while still enabling a significant amount of the available power in the unsteady tidal stream to be extracted.
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the effects of wave current Interaction on the performance of a model horizontal axis tidal turbine
International Journal of Marine Energy, 2014Co-Authors: T A De Jesus Henriques, S C Tedds, A Botsari, G Najafian, T S Hedges, C J Sutcliffe, I Owen, Robert J PooleAbstract:This experimental study investigates the performance of a marine current turbine in the presence of surface waves. The tests were carried out in the high-speed recirculating water channel at the University of Liverpool. A three-bladed model of a horizontal axis tidal turbine with a rotor diameter of 0.5 m was exposed to water flow with a steady uniform upstream velocity of 0.5 m/s. Regular surface waves were generated using a paddle wavemaker capable of producing a wide range of wave conditions in a current dominated flow (i.e. wave-induced velocities lower than current velocity). To determine the fluctuations in the current velocities caused by the surface waves, an acoustic Doppler velocimeter was used to measure the three-dimensional velocity components at various depths beneath two different waveforms. The measured kinematics of the waves showed excellent agreement with linear wave theory. Thrust and power measurements were taken from the turbine under flow conditions with the same two waveforms and compared with results taken in steady uniform flow to evaluate the effect of the wave-induced velocities on the turbine performance. The mean coefficients of power and thrust in the unsteady flow conditions were found to be very similar to those obtained in steady flow, but there were significant cyclic variations in the power and thrust which occurred at the frequency of the waves.
