Experimental analysis of co-rotating and counter-rotating tandem horizontal-axis wind turbine performance and wake dynamics

This experimental study investigates the performance and wake of tandem wind turbines utilising co-rotating and counter-rotating rotor configurations. Measurements for turbine power, tip-speed ratio, and wake velocity were obtained across various arrangements of single and tandem turbines. Conducted at a Reynolds number of $9.6\times 10^4$ based on turbine diameter, the study evaluates in-line configurations with separation distances from 1.25$D_T$ to 8$D_T$, $D_T$ being the turbine diameter, and different tip-speed ratios. Power measurements indicate that the downstream turbine performs better when its rotational direction opposes that of the upstream turbine, showing a 20% increase in performance compared to the co-rotating arrangement at a separation distance of $1.25D_T$. Nevertheless, the results show the tandem wind turbines’ power generation depends on the spacing between the turbines and the upstream turbine’s optimal tip-speed ratio. This indicates that as the distance between the turbines increases, the advantageous impacts of a counter-rotating setup diminish. Velocity measurements behind the downwind turbine reveal negligible effects on the streamwise velocity due to relative rotational directions but a significant impact on turbulent kinetic energy. Specifically, the co-rotating arrangement exhibits 33% higher turbulence levels than the counter-rotating arrangement. These findings hold considerable implications for designing and optimising wind turbine systems in arrays, both onshore and offshore wind farms.