On the impact of layout in the dynamics of wind turbine arrays under passive oscillations

Abstract

Laboratory experiments were performed to explore the layout effect on multiscale motions of wind turbines that are able to oscillate passively and the impact on power output fluctuations, which is instrumental toward understanding the dynamics of floating turbine arrays. We studied 3 × 3 and 3 × 5 turbine arrays in aligned and staggered configurations with inter-row separations of Sx/dT=5 and ten sharing transverse spacing of Sy/dT=2.5. A three-axis accelerometer characterized turbines' oscillations, whereas the power output was obtained directly. Particle image velocimetry was used to monitor eventual flow irregularities. The standard deviation of pitch angle, Ap(°) about the equilibrium, obtained from direct integration of instantaneous angular velocity, shows that the turbines underwent relatively small-amplitude pitch motions with maximum intensity that monotonically decreasing with increasing row location in the aligned layout. However, this was not the case in the staggered configurations; indeed, the second row of turbines underwent larger pitch amplitude in the Sx/dT=10 case. Flow channeling and larger turbine spacing promote the development and entrainment of large coherent motions producing larger unsteady forcing and triggering enhanced turbine motions. The instantaneous pitching angle density distribution exhibited Gaussian-like distribution irrespective of the units' location. A formulation for turbine pitching motions based on the balance between wind load restoring force and gravity shows that the bulk natural frequency modulates the turbine pitching angular velocity. The variation in turbine pitching amplitudes was similar to those of the mean power output. The power output spectra evidenced modulation of the local turbulence and turbine pitching natural frequency due to the flow-induced turbine pitching motions.