Study Of Tip-bed Velocity And Scour Depth Of Horizontal-axis Tidal Turbine

The research of horizontal-axis tidal turbine scour based on the combination of theory and experiment is important for the offshore installation and maintenance of the turbine.This thesis summarizes the main research of tidal turbine wake and the scour in the field of cylinder and turbine from the aspects of experiment and numerical simulation.In this thesis,the mechanism of seabed scour caused by horizontal-axis tidal turbine is systematically described through the scour experiment and the turbine scour is predicted by the empirical formula.The study proposed a recirculating flume associated with 3D printed turbine system to investigate the seabed scour induced by horizontal-axis tidal current turbine.The cost and spatial effective experiments were successfully used to investigate the impact of the rotating turbine to the seabed scour with a monopile-supported system.The internal flow field environment of this recirculating flume is measured by an acoustic Doppler velocimetry to discuss the mean velocity and turbulent intensity and illustrate the flow condition at various sections in the entire flume for the accuracy purposes.Existence of turbine obstructs the tidal flow to divert the flow passing through the narrow channel in between the blades and seabed.Shadowing effect of tidal turbine resulting in flow suppression is the main cause to induce tidal turbine scour and its accelerated velocity is being termed as tip-bed velocity.Tip-bed velocity is greater than the undisturbed free flow due to the blockage effects to maintain its mass conservation.In this thesis,theoretical equation is proposed to predict the tip-bed velocity based on the axial momentum theory and conservation of mass.The tidal scour results are compared with the bridge pier scour and propeller jet scour resulting in that the tip-bed velocity is the key role to influence the turbine on seabed scour.This thesis also discusses the influence of the location of the turbine,the number of blades and the free flow velocity on the tidal turbine seabed scour.Based on the above experimental scour results,turbine coefficient K_t is proposed to estimate the influence of tidal turbine on the maximum scour depth.An empirical equation is proposed to estimate the maximum tidal turbine scour depth based on the cylinder equation.In addition,an empirical equation is proposed to predict the tidal turbine scour development with time based on the previous study.Besides the research on single tidal turbine scour,twin tidal turbines in tandem arrangement has been experimentally studied in this thesis.The maximum scour depth and scour development of single and twin turbine are compared to illustrate the scouring mechanism of twin turbines scour and the scour development of the twin turbines is divided into four stages.The influence of the distance between two turbines on the scour results is studied in this study.The study provide a fundamental understanding on the scour nature of two tidal turbines in tandem arrangement.