Wake of horizontal axis tidal-current turbine and its effects on scour / Chen Long

Many large scale tidal current turbines (TCTs) have been tested and deployed around the world. It is foreseeable that tidal current will be a vital natural resource in future energy supplies. The wake generated from the TCT amplifies the scour process around the support structure. It causes sediment transport at the seabed and it may result in severe environmental impacts. The study aims to investigate the generated wake and its effects on the scour process around the support structure of the TCT. An analytical wake model is proposed to predict the initial velocity and its lateral distribution downstream of the TCT. The analytical wake model consists of several equations derived from the theoretical works of ship propeller jets. Axial momentum theory is used to predict the minimum velocity at the immediate plane of the wake and followed by recovery equation to determine the minimum velocity at lateral sections along the downstream of the wake. Gaussian distribution is applied to predict the lateral velocity distribution in a wake. The proposed model is also able to predict wake structure under various ambient turbulence conditions (TI= 3%, 5%, 8% and 15%). The proposed wake model is validated by comparing the results with well-accepted experimental measurements. Goodness-of-fit analysis has been conducted by using the estimator of R-square (R2) and Mean Square Error (MSE). The R2 and MSE are in the range of 0.1684–0.9305 and 0.004–0.0331, respectively. A TCT model was incorporated in OpenFOAM to simulate the flow between rotor and seabed due to the fact that the flow is responsible for the sediment transport. The axial component of velocity is the dominating velocity of the flow below the TCT. The maximum axial velocity under the turbine blades is around 1.07 times of the initial incoming flow. The maximum radial and tangential velocity components of the investigated layer are approximately 4.12% and 0.22% of the maximum axial velocity. The acceleration of flow under the rotor changes seabed boundary layer profile. The geometry of the turbine also affects the flow condition. Results showed that the velocity increases with the number of blades. Both the axial and radial velocities were significantly influenced by the number of blades, the tangential velocity was found to be insignificant. A physical model of TCT is placed in a hydraulic flume for scour test. The scour rate of the fabricated model was investigated. The decrease of tip clearance increases the scour depth. The shortest tip clearance results in the fastest and most sediment transport. The maximum scour depth reached approximately 18.5% of rotor diameter. Experimental results indicated that regions susceptible to scour typically persist up to 1.0Dt downstream and up to 0.5Dt to either side of the turbine support centre. The majority of the scour occurred in the first 3.5 hr. The maximum scour depth reaches equilibrium after 24 hr test. The study correlated scour depth of the TCT with the tip clearance. An empirical formula has been proposed to predict the time-dependent scour depth of the pile-supported TCT