Experimental investigation of the wake characteristics behind twin vertical axis turbines

Highlights: • Twin-turbine wakes depended more on rotational direction than lateral spacing. • Turbines in a counter-rotating forward setup attained the fastest wake recovery. • Wake recovery was delayed when devices operated with the same rotational direction. • Wake merging and evolution were highly three dimensional even up to 10D downstream. • Linear superposition of a single turbine wake represented well twin-turbine wakes. Vertical axis wind and tidal turbines are a promising technology, well suited to harness kinetic energy from highly turbulent environments such as urban areas or rivers. The power density per occupied land area of two or three vertical axis rotors deployed in close proximity can notably exceed that of their horizontal axis counterparts. Using acoustic Doppler velocimetry, the three-dimensional wake developed downstream of standalone and twin vertical axis turbines of various shaft-to-shaft distances and rotational direction combinations was characterised in terms of mean velocity and turbulence statistics, with their impact on momentum recovery quantified. Results show that the wake hydrodynamics were more impacted by turbine rotational direction than lateral distance between devices for the range of lateral spacing considered. In the cases with turbines operating in a counter-rotating forward configuration, the wake mostly expanded laterally and attained the largest velocities that exceeded those in the single turbine case, with full momentum recovery at 5 turbine diameters downstream. The wake developed by the counter-rotating backward setup notably extended over the vertical direction, whilst devices rotating in the same direction featured the greatest lateral wake expansion with reduced velocities. Linear wake superposition of the single turbine wake provided a good representation of the mean velocity field behind twin-turbine setups. The presented results indicate that, in the design of twin-turbine arrays moving in counter-rotating forward direction, a lateral spacing of, at least, two turbine diameters should be kept as this allows the kinetic energy in the wake to be fully recovered by five turbine diameters downstream.