International Journal of Marine Energy最新文献

This paper investigates the accuracy of the Computational Fluid Dynamics (CFD) based Immersed Body Force (IBF) turbine modelling method for predicting the flow characteristics of a Momentum-Reversal-Lift type of tidal turbine. This empirically-based CFD model has been developed based on the actuator disc method enhanced with additional features to mimic the effect of the complex blade motion on the downstream wake, without the high computational costs of explicitly modelling the dynamic blade motion. The model has been calibrated against the flow characteristics data obtained from experiment and found to perform well, although there are few inconsistencies in the flow patterns which show some of the limitations of the IBF model compared to a full dynamic blade motion simulation. However, given the complexity and computational cost of modelling the detailed blade motion the limitations of the IBF model are acceptable and will be useful especially for optimisation of arrays of devices where there is a significant computational demand.

The effect of mass ratio (m^∗ = the mass of oscillating body/the mass of displaced fluid) on vortex induced vibration of an elastically mounted rigid circular cylinder over a wide range of Reynolds numbers (1.7 × 10⁴ < Re < 7 × 10⁴) in a high damping system is studied in this paper. The cylinder is limited to a transverse oscillation and is carried inside a 14 m long water channel for constant velocities. The amplitude of response depends on various parameters such as mass ratio, damping ratio, natural frequency of system and Reynolds number. Here we considered three different circular cylinders with the same diameter and length, but distinct masses (m^∗ = 1.6, 2.3 and 3.4). The experiments carried out in a towing-tank water channel and the following results achieved: the peak amplitude of oscillation principally depends on the mass ratio and it increases with decrease of m^∗. For systems with constant mass-damping parameters, by reducing the mass ratio, the maximum amplitude of oscillation remains constant, while the range of synchronization increases considerably. Higher Reynolds numbers in our experiments led to reach the same maximum amplitude of oscillation as some of the previous studies (A^∗ = 1), even with higher mass-damping parameter (m^∗ζ) in our system. The high damping in our experiments, resulted in disappearance of the lower branch in the amplitude response graphs.

Marine current energy conversion can provide significant electrical power from resource-rich sites. However since no large marine current turbine arrays currently exist, validation of methods for simulating energy extraction relies upon scaled down laboratory experiments. We present results from an experiment using porous fences spanning the width of a recirculating flume to simulate flow through large, regular, multi-row marine current turbine arrays. Measurements of fence drag, free surface elevation drop and velocity distribution were obtained to validate a method for parameterising array drag in the distributed drag approach, which is typically implemented in regional scale models. The effect of array density was also investigated by varying the spacing between fences. Two different inflow conditions were used; the first used the flume bed in its natural state, whilst the second used roughness strips on the flume bed to significantly enhance ambient turbulence intensity to levels similar to those recorded at tidal sites. For realistic array densities (<0.07), a depth averaged formulation of effective array drag coefficient agreed within 10% of that derived from experimental results for both inflow conditions. The validity of the distributed drag approach was shown to be dependent on longitudinal row spacing between porous fences and ambient turbulence intensity, two features that determine the level of wake recovery downstream of each porous fence. Finally a force balance analysis quantified the change in bed drag as a result of the presence of porous fence arrays. Adding arrays to the flow gave an increase in bed drag coefficient of up to 95% which was 20% of the total added bed and array drag coefficient. Results have implications for regional scale hydrodynamic modelling, where array layout along with site specific characteristics such as turbulence intensity and bed profile determine the validity of the distributed drag approach for simulating energy extraction.

In a found region of energetic flow in the Agulhas Current, two locations are analysed. It is found that the current core borders on the mid-shelf (91 m depth) location and off-shore (255 m depth) location lies within its predominant path, with mean velocities of 1.34 m/s and 1.59 m/s respectively at a 30 m depth. In the period analysed only one large meander is noted at a depth of 30 m. The percentage current reversals are 4.1% and 3.1% at the mid-self and off-shore locations, respectively.

The most suited developed technology found to potentially deploy in this current is Minesto Deep Green turbine. The found capacity factor and specific yield for one Minesto (DG-12) 500 kW turbine at 30 m depth is estimated to be 62% with (4886 kWh/year)/kW for the mid-shelf location and 76% with 5416 (kWh/year)/kW for the off-shore location. Through a simplified analysis, the capacity credit of a possible 2000 MW array is analysed. It is found that an ocean current power plant can add to the load carrying capacity of the country and outperforms onshore wind power plants in this respect. The largest barriers at present are the cost and mooring challenges which presents a need for a detailed economic assessment to determine if associated costs of working in deeper waters (off-shore site) are justified.

A methodology of tidal flow resource assessment in the Dover Strait is presented. The resource assessment is performed using surface velocity time series recorded by Very High Frequency Radars (VHFR) and ADCP velocity measurements. Following the EMEC guideline, the major parameters of tidal flow conventionally used for tidal energy site screening are estimated and mapped. The combination of two sources of data allowed to characterize the current velocity variation in three spatial dimensions and in time, which increased confidence in hydrokinetic resource assessment from the radar data. Current velocities provided by the radars show strong spatial variation and fortnightly modulation. The most energetic area was found west of the Cape Gris Nez with the peak velocity of 2.5 m/s, mean velocity of 1 m/s, and spring tide average velocity of 1.4 m/s. Velocities exceeding 1 m/s are observed more than 50% of time there. Averaged velocity profiles derived from ADCP data were obtained for different stages of the tidal cycle and then approximated by a power law function. Using velocity time series provided by the radars and the power law velocity profiles, the power density time series in the surface and bottom layers were generated. The analysis of these data show that west of the Cape Gris Nez, the mean power density attains its maximum value 0.9 kW/m² in the surface layer and a peak value 5 kW/m². In the rest of the domain, the mean power density varies from 0.1 to 0.6 kW/m². The power density is found three times lower in the bottom layer. A three dimensional hydrodynamic model MARS-3D is used for comparison with experimental data. The model results are in good agreement with observations thus allowing the use of the model for assessing tidal stream resource in extended area.