1,720,995 research outputs found
An assessment of growth scenarios and implications for ocean energy industries in Europe
No full textComparing energy yields from fixed and yawing horizontal axis marine current turbines in the English channel
No full textAt many locations with high tidal stream velocities – and potential for tidal stream energy generation – the flow is approximately rectilinear, that is to say the flow direction is always 0 degrees or 180 degrees with respect to a particular orientation. At some sites, however, there is an appreciable change in flow direction (‘swing’) away from 180 degrees between the two maxima of flow speed. In order to assess the performance of horizontal axis marine current turbines in non rectilinear currents, measurements of a model rotor have been made in a towing tank. Curve fits have been calculated as a function of the cosine of the yaw angle squared and the thrust as cosine of the yaw angle. The curve fits have been used in a case study to investigate the impact of fixed-orientation or yawing rotor designs on average annual energy output, at three locations in the English Channel. All three sites are of the type where flow is accelerated around a headland or cape, but their tidal streams vary in deviation from rectilinearity. For two of the sites - Portland Bill (Dorset, UK) and Race of Alderney (Alderney, Channel Islands/Normandy, France) - available data consisted of tidal stream diamonds printed on Admiralty navigational charts. These rely on local tidal elevations for interpolation of tidal streams. At the other site – St. Catherine’s Point, Isle of Wight, Hampshire – current meter measurements of duration one month were available from the British Oceanographic Data Centre (BODC), allowing a direct tidal analysis.
Simultaneous wake- and vortex-induced vibrations of a cylinder with two degrees of freedom in each direction
No full textThe flow-induced vibration of one cylinder in the wake of another is the subject of continuing interest in connection with interactions between vertical tension risers in deep water. When one riser is downstream of another it is likely to be subject to wake-induced and vortex-induced excitations at different frequencies simultaneously. Both are complex mechanisms, and it is reasonable to assume that they interact. To begin to understand this complicated process it is desirable that any modelling should incorporate some features of a multi-degree-of-freedom structural response. With this aim, this paper describes experiments in which one cylinder was free to undergo simultaneous wake- and vortex-induced vibrations downstream of a similar but stationary cylinder in a steady flow. The downstream cylinder was mounted on an elastic system that had two natural frequencies in both the in-line and cross-flow directions. Mass ratios were almost the same in all four modes. Measurements are presented of simultaneous wake- and vortex-induced vibrations for cylinder separations of 5 and 10 diameters in the in-line direction, and up to 4 diameters transversely. At a reduced velocity of 83 (based on the cylinder’s lower submerged natural frequency) and a separation of 5 diameters, excursions of wake-induced vibrations peaked at almost 5 diameters, when the downstream cylinder was near the edge of the upstream cylinder’s wake<br/
Experimental verifications of numerical predictions for the hydrodynamic performance of horizontal axis marine current turbines
No full text
Transition from vortex to wall driven turbulence production in the Taylor-Couette system with a rotating inner cylinder
No full textAxisymmetrically stable turbulent Taylor vortices between two concentric cylinders are studied with respect to the transition from vortex to wall driven turbulent production. The outer cylinder is stationary and the inner cylinder rotates. A low Reynolds number turbulence model using the k- formulation, facilitates an analysis of the velocity gradients in the Taylor-Couette flow. For a fixed inner radius, three radius ratios 0.734, 0.941 and 0.985 are employed to identify the Reynolds number range at which this transition occurs. At relatively low Reynolds numbers, turbulent production is shown to be dominated by the outflowing boundary of the Taylor vortex. As the Reynolds number increases, shear driven turbulence (due to the rotating cylinder) becomes the dominating factor. For relatively small gaps turbulent flow is shown to occur at Taylor numbers lower than previously reported
Turbulence generation and its effect in LES approximations of tidal turbines
No full textTidal flows suitable for the installation of tidal stream turbines are highly turbulent with a broad range of turbulent scales, and intensities of around 10%. It is important to understand how turbulent flows affect the performance and wake profiles for the accurate prediction of the performance of single devices and arrays. Previous studies have considered the effects of turbulence intensity, but the effects of turbulent eddy size have not been investigated. This paper presents a new method for generating turbulence in large eddy simulations of uniform actuator disc rotor models. The advantage of the gridded actuator disc is that the generated turbulence may be controlled, so the effects of turbulence length scale and intensity may be investigated. Initial results show the wake recovery is faster behind a rotor generating larger turbulent length scales and intensities, with a corresponding increase in axial induction and thrust. Therefore turbines may be positioned closer together if they generate more turbulence, increasing the power output of an array for a given area, although the individual device performance may be lower. Future work will consider the effects of free-stream turbulence on the performance and wake profiles of the gridded actuator disc, and validation against experimental data
Experimental verifications of numerical predictions for the hydrodynamic performance of horizontal axis marine current turbines
No full textThe conversion of the kinetic energy presented by ocean or marine currents offers an exciting proposition as it can provide regular and predictable energy resource. The majority of the proposed designs for converting this type of kinetic energy are based on the concept of the horizontal axis turbines, which has common characteristics to those being used in wind energy. Although a lot can be learnt and transferred from wind turbine technology, there are significant differences. These include the effects of the free surface and the occurrence of cavitation. Consequently, any developed numerical methods need to be verified. This study reports on the development and verification of simulation tools based on blade element momentum theory—a commercial code (GH-Tidal Bladed) and an academic in-house code (SERG-Tidal). Validation is derived from experimental measurements conducted on a model 800 mm diameter turbine in a cavitation tunnel and a towing tank. The experimental data includes measurements of shaft power and thrust generated by the turbine for a series of blade pitch settings and speeds. The results derived from the two codes are compared. These indicate that the two developed codes demonstrate similar trends in the results and provide a satisfactory representation of the experimental turbine performance. Such results give the necessary confidence in the developed codes resulting in appropriate tools that can to be utilised by developers of marine current turbines
An investigation of the vortex shedding frequency and natural frequency of a ship propeller
No full text- …
