The development of marine current turbine arrays depends on the understanding of the interaction effects that exist between turbines in close proximity. Moreover, the ambient turbulence intensity also plays a major role in the behaviour of tidal turbines. Thus it is necessary to take ambient turbulence into account when studying interaction effects between several turbines. In order to highlight these interaction effects, experiments have been carried out in the IFREMER flume tank. These experiments focus on interactions between three horizontal axis turbines. This paper presents the experimental results obtained for three configurations with two ambient turbulence intensity rates.

The results are presented in terms of turbine wakes and performance. The wake characterisation presents complex features for the three configurations and the lowest ambient turbulence rate: upstream turbines wakes are still present at the location of the downstream turbine and their wakes can interact or merge, depending on the tested configurations. On the contrary, for the highest turbulence rate, the downstream turbine wake is not affected in his shape by the two upstream ones which are not visible any more. In fact, as already observed in the previous studies of Mycek et al. [1, 2], the wake shape rapidly spreads out in the stream-wise direction behind the turbines. However, the velocity deficit and the turbulence intensity are higher for the downstream turbine comparing to the upstream ones. In terms of performance, one tested case presents an increase of the downstream turbine power production: when this turbine is exactly in the centre of the two upstream turbines and for the lowest turbulence rate only. A small misalignment of the layout axis with respect to the tidal current may result in a decrease of performance at the end. An analysis of the power spectral density functions of the downstream turbine torque and thrust shows that no signature of the upstream turbines can be found in these answers. Furthermore, the same spectral analysis carried out on the velocity measurements shows no signature of the upstream turbines either, from 3 diameters distance. This result is noticeable for the highest and the lowest tested turbulence cases and whatever the turbines configuration is.