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.

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.

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.

Oscillating wings can extract energy from an oncoming water or air stream, and first large-scale marine demonstrators are being tested. Oscillating wing hydrodynamics is highly unsteady, may feature dynamic stall and leading edge vortex shedding, and is significantly three-dimensional due to finite-wing effects. Understanding the interaction of these phenomena is essential for maximizing power generation efficiency. Much of the knowledge on oscillating wing hydrodynamics stemmed from two-dimensional low-Reynolds number computational fluid dynamics studies and laboratory testing; real installations, however, will feature Reynolds numbers higher than 1 million and unavoidable finite-wing-induced losses. This study investigates the impact of flow three-dimensionality on the hydrodynamics and the efficiency of a realistic aspect ratio 10 device in a stream with Reynolds number of 1.5 million. The improvements achievable by using endplates to reduce finite-wing-induced losses are also analyzed. Three-dimensional time-dependent Navier–Stokes simulations using the shear stress transport turbulence model and a 30-million-cell grid are performed. Detailed comparative hydrodynamic analyses of the finite and the infinite wings reveal that flow three-dimensionality reduces the power generation efficiency of the finite wing with sharp tips and that with endplates by about 17% and 12% respectively. Presented analyses suggest approaches to further reducing these power losses.

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.