The effects of boundary proximity upon the wake structure of horizontal axis marine current turbines

Marine current energy conversion technology is presently at the prototype stage where single devices are deployed, or planned for installation, at isolated testing sites. In the medium term, however, devices will be installed in arrays. Understanding the effect devices have on the flow is critical in determining how one device may modify both the performance of and loading experienced by another device in the array. Hence, investigations characterising the wake of horizontal axis turbines are required. It is the aim of this work to identify and investigate the principle parameters that govern the wake structure and its recovery to the free-stream velocity profile. Scale model testing has been conducted to aid the development of an efficient numerical model. Wake characteristics of small-scale mesh disk rotor simulators have been measured in the 21m tilting flume at the Chilworth hydraulics laboratory, University of Southampton. Mean flow velocity and turbulence intensities have been evaluated at locations upstream and downstream of the disks in order to characterise the flow conditions.The results indicate that several parameters impact on the rate of wake velocity recovery and wake expansion and that the interdependencies are complex.This paper discusses the effect of varying the vertical position of a rotor disk within close proximity of both the water surface and sea bed. Results of the experiments are followed by details of the numerical simulation model developed as part of this work. Based on an established wind turbine wake model, the numerical model has been modified to account for the change in fluid and the presence of bounding surfaces. Experimental data presented herein demonstrates that the rate of wake velocity recovery changes for different vertical disk positions and that wake structure does not behave in an axis-symmetric manner requiring a novel numerical solution