Physical and Numerical Modelling of the Wake Characteristics of the Savonius Tidal Stream Turbine

The past decade has seen a significant rise in the interest of harnessing marine renewable energy. Tidal stream technology is developing rapidly, with the market leaders moving away from a pre-commercial demonstrator stage towards full-scale array deployment. Learning from the more established wind energy sector, developers have generally favoured horizontal axis designs, as these traditionally offer higher peak efficiencies when compared to their vertical axis counterparts. However, valid arguments have been raised that suggest a vertical axis device would be more suited to a tidal regime, particularly since it has omnidirectional characteristics and can operate in shallow waters. In this study, the wake characteristics of a vertical axis tidal stream turbine were investigated, using both physical and numerical modelling techniques. Due to the unsteady nature of a vertical axis device, previous techniques such as the porous disk approach were unsuitable to model the turbine behaviour. Consequently, a number of scaled turbines have been constructed for this series of experiments. Testing was carried out in the 17m recirculating flume at the Hydro-environmental Research Centre's hydraulics laboratory, at Cardiff University. The numerical model CFX was used for this study, and mesh motion techniques were employed to replicate the dynamic behaviour of the turbines in a like for like model of the flume. Two turbine sizes were used to vary the blockage in the flume. The findings showed that the time-averaged wake was 90% recovered between 10-15 diameters downstream of the turbine, which was comparable to a horizontal axis device. The shape of the wake structure was unsymmetrical, however, the unsteady nature of the flow due to vortex shedding as the turbine rotated suggested that turbines could be spaced closer together. The numerical model results showed good agreement with the obtained laboratory data, highlighting the applicability of using this methodology to assess the impact of such devices.