An experimental study was conducted to investigate the effects of relative rotation direction on the wake interferences among two tandemwind turbines models.While the oncoming flow conditions were kept in constant during the experiments,turbine power outputs,wind loads acting on the turbines,and wake characteristics behind the turbines were compared quantitatively with turbine models in either co-rotating or counter-rotating configuration.The measurement results reveal that the turbines in counter-rotating would harvest more wind energy from the same oncoming wind,compared with the co-rotating case.While the recovery of the streamwise velocity deficits in the wake flows was found to be almost identical with the turbines operated in either co-rotating or counter-rotating,the significant azimuthal velocity generated in the wake flow behind the upstream turbine is believed to be the reason why the counter-rotating turbines would have a better power production performance.Since the azimuthal flow velocity in the wake flow was found to decrease monotonically with the increasing downstream distance,the benefits of the counter-rotating configuration were found to decrease gradually as the spacing between the tandem turbines increases.While the counter-rotating downstream turbine was found to produce up to 20%more power compared with that of co-rotating configuration with the turbine spacing being about 0.7D,the advantage was found to become almost negligible when the turbine spacing becomes greater than 6.5D.It suggests that the counter-rotating configuration design would be more beneficial to turbines in onshore wind farms due to the smaller turbine spacing(i.e.,~3 rotor diameters for onshore wind farms vs.~7 rotor diameters for offshore wind farms in the prevailing wind direction),especially for those turbines sited over complex terrains with the turbine spacing only about 1–2 rotor diameters. An experimental study was conducted to investigate the effects of relative rotation direction on the wake interferences among two tandemwind turbines models. Whilst the oncoming flow conditions were kept in during during experiments, turbine power outputs, wind loads acting on the turbines, and wake characteristics behind the turbines were compared quantitatively with turbine models in either co-rotating or counter-rotating configuration. The measurement results reveal that the turbines in counter-rotating would harvest more wind energy from the same oncoming wind, compared with the co-rotating case. While the recovery of the streamwise velocity deficits in the wake of was found to be almost identical with the turbines operated either either co-rotating or counter-rotating, the significant azimuthal velocity generated in the wake flow behind the upstream turbine is believed to be the reason why the counter-rotating turbines would have a better power production performance.Since the azimuth al flow velocity in the wake flow was found to decrease monotonically with the increasing downstream distance, the benefits of the counter-rotating configuration were found to decrease gradually as the spacing between the tandem turbines increases. Whilst the counter-rotating downstream turbine was found to produce up to 20% more power compared with that of co-rotating configuration with the turbine spacing being about 0.7D, the advantage was found to become almost negligible when the turbine spacing becomes greater than 6.5D.It suggests that the counter-rotating configuration design would be more beneficial to turbines in onshore wind farms due to the smaller turbine spacing (ie, ~ 3 rotor diameters for onshore wind farms vs. ~ 7 rotor diameters for offshore wind farms in the prevailing wind direction), especially for those turbines sited over complex terrains with the turbine spacing only about 1-2 rotor diameters.