Abstract
The wake interactions between two horizontal axis model tidal turbines for two different spacings were investigated by flow measurements using acoustic Doppler velocimetry (ADV). The analysis of the velocity time-series data showed that a velocity sampling time period of at least 1000 integral time scales is required for the convergence of turbulence statistics in the wake of the turbines. The velocity measurement showed that a distinct three-layer (core, inner, and outer) structure was observed in the wake of a single turbine, whereas a simple wake pattern without such a structure was observed downstream of the two turbines. The comparison between the measurements for the different turbine spacings showed that the mean velocity fields, turbulent stress fields, velocity spectra plots, and integral length scales behind the second turbine were nearly independent of the turbine spacing. It was concluded that the increased turbulent momentum transfer across the wake boundary, mainly due to the wake meandering motion by the first turbine, overshadows the effect of the first turbine. This suggests that the ambient turbulence intensity induced by the upstream turbine could be a key factor for the wake characteristics within a tidal turbine array.