Measurements of the wake characteristics of co- and counter-rotating twin H-rotor vertical axis wind turbines

Increasing the efficiency of individual vertical-axis wind turbines (VAWTs) has been the subject of much research. However, wind farms that comprise individual VAWTs are not necessarily the most efficient configuration in accordance with VAWTs' signature wake pattern. This study aims to identify efficient array configurations and thereby increase the packing density and power output by grouping individual VAWTs as a unit. Systematic measurements of wake aerodynamics of co-rotating and counter-rotating (forward- and backward-rotating) twin VAWTs were taken in a wind tunnel. The wake was measured both along the blade mid-span plane up to 10 turbine diameters (10D) and along the vertical plane. The wake of co-rotating twin VAWTs showed great asymmetry, similar to that of a single VAWT. Interestingly, the wake of counter-rotating twin VAWTs demonstrated elegant symmetric patterns. Two pairs of stationary counter-rotating vortices were found to evolve in the wake. The turbulence intensity downstream was observed to experience drastic changes and slow recovery compared with its velocity counterpart. Finally, two types of array units are proposed for applications in wind farms in accordance with the measurements.