Experimental investigation of the wake replenishment mechanisms of paired counter-rotating vertical-axis wind turbines

The understanding of wake recovery mechanisms is crucial for the design of efficient wind farm layouts and the development of accurate wake models. Recently, placing two vertical-axis wind turbines (VAWTs) in close proximity has demonstrated potential for increased power output. In this study, wind tunnel experiments were conducted to investigate the wake replenishment mechanisms behind paired VAWTs. The experimental campaign included testing an isolated VAWT and paired counter-rotating VAWTs. By combining qualitative observations of key flow field variables with a quantitative analysis based on momentum conservation, this study aims to enhance our understanding of the mixing mechanisms supporting the reintroduction of streamwise momentum into the wake of paired VAWTs. This research also involves a comparison of these mechanisms with those observed in the wake of a standalone VAWT. The results show that the differences between isolated and paired VAWTs in overall wake characteristics are minimal. The increased lateral advection within the wake of the isolated VAWT is offset by the enhanced vertical advection in the paired configuration, as a result of the change in the direction of cross-stream velocity within the gap between the paired VAWTs, which promotes a shift towards vertical flow rather than lateral flow.