title: A study of the wake of an isolated tidal turbine with application to its effects on local sediment transport
Creators: name: Vybulkova, Lada
Publisher: name: University of Glasgow
Year: 2013
Subjects: subject: tidal, turbine, wake, sediment, environment.

description: Tidal energy conversion devices (TECDs) are in development throughout the world to help reduce the need for fossil fuels. These devices will generally be mounted on the seabed and remain there over a period of years. Most of the previous research on TECDs has focused on their power extraction capability and efficient design. The handful of studies which have focused on the effects of the devices on the marine environment have not considered small-scale three-dimensional phenomena occurring in the flow near the rotor. These phenomena are likely to disturb the marine environment by altering the dynamics of sediment. The accurate prediction of the rapidly changing flow down-stream of a TECD and its influence on the seabed poses a challenge. The nature of the interactions between such a flow and sediment has not been experimentally established. Predictions of these interactions, 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 during the tidal cycle. The difference between the typical time-scales of the development of the rotor wake and the tidal cycle represents a difficulty for the computational modelling of 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 Vorticity Transport Model, modified to simulate the flow down-stream of a TECD mounted onto the seabed, predicts the shear stress inflicted by the flow on the seabed. The shear stresses on the seabed, generated by small-scale vortical structures in the wake down-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 into the Vorticity Transport Model. The critical bed shear stress is known as a threshold for initiation of sediment motion, therefore the relative difference between the stress on the seabed and the critical bed shear stress, called the excess bed shear stress, is chosen 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 with the configuration of TECD. The results suggest that the average excess bed shear stress inflicted on the seabed by the horizontal-axis device increases with the inflow velocity during the flood part of the representative tidal cycle and that the increase can be expressed by a simple algebraic expression. It is also predicted that the impact of this device on the seabed does not monotonically decrease with increasing separation between the rotor and the seabed. In addition, the relationship between the excess bed shear stress and the position of the rotor is established. Furthermore, the simulations indicate that the wake down-stream of the horizontal-axis device is lifted by the flow away from the seabed, which result in a confinement of its impact to the vicinity of the rotor. In contrast with the horizontal-axis configuration, it is concluded that the vertical-axis and cross-flow configurations of the rotor would promote the erosion of the seabed further away from the device, at a location where the wake approaches the seabed again and that this location depends on the inflow velocity. The predicted effects of these devices on the marine environment need to be considered in advance of their installation on the seabed.