The effect of vertical arrangement on performance and wake characteristics of two tandem offshore wind turbines under various operating conditions

Abstract

Apart from streamwise and lateral distances, a vertical distance may also be imposed for adjacent wind turbines to achieve performance improvement of downstream turbines. Previous research was primarily devoted to wind farm layout optimisation using simplified analytical wake models, while impacts of operating conditions were ignored for studies using more advanced Large-Eddy Simulation (LES) methods. In this paper, the effect of vertical arrangement on performance and wake characteristics of two tandem offshore wind turbines under various operating conditions is investigated using an LES approach coupled with an actuator line technique. Results show that vertically arranging turbines can improve downstream turbine performance, but its effects vary with the operating conditions. If turbines typically operate below or at the rated condition, due to deflection of upstream turbine wake associated with shaft tilt, raising the upstream turbine can result in greater improvement in power output, with a considerable rise of 50% for a relatively small height difference of 0.1 times rotor diameter. For above-rated operating conditions, the wind shear effect becomes important and it is preferable to increase the hub height of the downstream turbine to take advantage of higher wind speed. Wake characteristics of both turbines are analysed to support the results.