A fluid dynamic study of free-surface proximity and inertia effects on tidal turbines

A combination of theoretical, computational and experimental work has been conducted to investigate both free-surface proximity effects and inertia effects on tidal turbines. The free-surface proximity effects include transverse blockage in linear arrays of rotors. The inertia (added mass) effects investigated are such as a turbine may be subjected to due to passing waves. Free-surface proximity effects on the flow field around tidal-stream turbines have been modelled using actuator disc theory. Theoretical results are presented for a blocked configuration of tidal-stream turbines. As a first step to simulate the flow field a porous disc and a strip were tested in a water flume and measurements of axial thrust were taken using a cantilevered strain gauge arrangement. These results were then complemented by more detailed measurements on a rig which was designed and built for taking steady measurements of both power and thrust on a horizontal-axis rotor. The measurements of power were obtained using a friction brake (torque) and an infra-red tachometer (rotation rate). Measurements were carried out in both a water flume and a wind tunnel representing highly blocked and effectively unblocked cases, respectively. The results of these investigations into free-surface proximity have been incorporated into a blade element momentum code as used in the industry and agreement is found to be good for a range of turbine operating states. Where agreement weakens alternative corrections are suggested. A second rig was designed and built for investigation of inertia effects on horizontal- axis rotors. This rig employed an electromagnetic brake to provide a constant torque and was subjected to planar oscillations in a water flume. Instantaneous measurements of axial thrust and rotation rate were taken using a strain gauge arrangement and a shaft encoder respectively, in order to assess the magnitude of the out-of- plane (axial) and in-plane (rotational) added inertia. The experiments conducted spanned a range of ...