Increment of the rotation frequency of a downstream passive rotor in an array of three in-line rotors

Abstract

The installation of wind farms has increased recently. Despite the interest in obtaining electrical energy from these setups, certain physical phenomena need attention in order to increase efficiency. Specifically, the processes in the wakes of turbines are not completely represented in most analytical models. For that reason, we examine the rotation frequency of three passive, horizontal-axis rotors that are lined up in a water channel. By using an imaging method, we analyzed the rotors’ rotation period. We found that if the distance between each rotor is lower than three and a half rotor diameters, the third rotor gets a higher rotation rate than itself if the first in-line rotor is absent. However, if the distance is greater, no improvement in the frequency rotation of the third rotor is observed. An analysis of the temporal mean of the axial velocity allowed to conclude that the rotation improvement is related to a contraction of the second in-line wake.