A study of the wake of an isolated tidal turbine with application to its effects on local sediment transport

T Technology (General)

Vybulkova, Lada ; Vezza, Marco

University:University of Glasgow

Department:School of Engineering

关键词: tidal, turbine, wake, sediment, environment;   

Others  :  http://theses.gla.ac.uk/4997/1/2013vybulkovaphd.pdf

来源: University of Glasgow

【 摘 要 】

Tidal energy conversion devices (TECDs) are in development throughout the world tohelp reduce the need for fossil fuels. These devices will generally be mounted on theseabed and remain there over a period of years. Most of the previous research on TECDshas focused on their power extraction capability and efficient design. The handful ofstudies which have focused on the effects of the devices on the marine environment havenot considered small-scale three-dimensional phenomena occurring in the flow near therotor. These phenomena are likely to disturb the marine environment by altering thedynamics of sediment.The accurate prediction of the rapidly changing flow down-stream of a TECD andits influence on the seabed poses a challenge. The nature of the interactions betweensuch a flow and sediment has not been experimentally established. Predictions of theseinteractions, as is necessary for an assessment of the effects of the devices on the seabed,need to account for the depth-dependence of the flow velocity and its changes duringthe tidal cycle. The difference between the typical time-scales of the development of therotor wake and the tidal cycle represents a difficulty for the computational modellingof the interactions between the device and the tidal flow.This dissertation presents an inviscid analysis of the flow down-stream of horizontal-axis, vertical-axis and cross-flow TECDs by means of computer modelling. The Vortic-ity Transport Model, modified to simulate the flow down-stream of a TECD mountedonto the seabed, predicts the shear stress inflicted by the flow on the seabed. Theshear stresses on the seabed, generated by small-scale vortical structures in the wakedown-stream of the devices, cause sediment to uplift. This process along with the sub-sequent motion of the sediment is simulated by a sediment model implemented intothe Vorticity Transport Model. The critical bed shear stress is known as a thresholdfor initiation of sediment motion, therefore the relative difference between the stresson the seabed and the critical bed shear stress, called the excess bed shear stress, ischosen here as an indicator of the impact of the TECDs on the seabed.The evolution of the instantaneous stresses on the seabed is predicted to vary withthe configuration of TECD. The results suggest that the average excess bed shear stressinflicted on the seabed by the horizontal-axis device increases with the inflow velocityduring the flood part of the representative tidal cycle and that the increase can beexpressed by a simple algebraic expression. It is also predicted that the impact ofthis device on the seabed does not monotonically decrease with increasing separationbetween the rotor and the seabed. In addition, the relationship between the excess bedshear stress and the position of the rotor is established. Furthermore, the simulationsindicate that the wake down-stream of the horizontal-axis device is lifted by the flowaway from the seabed, which result in a confinement of its impact to the vicinity ofthe rotor. In contrast with the horizontal-axis configuration, it is concluded that thevertical-axis and cross-flow configurations of the rotor would promote the erosion ofthe seabed further away from the device, at a location where the wake approaches theseabed again and that this location depends on the inflow velocity.The predicted effects of these devices on the marine environment need to be con-sidered in advance of their installation on the seabed.