Wind turbine wake intermittency dependence on turbulence intensity and pitch motion

Abstract

Turbulence intermittency characteristics of the flow behind pitching and fixed wind turbines are assessed via hot-wire anemometry in a wind tunnel experiment. The pitching wind turbine model is free to oscillate in the streamwise direction to simulate pitch motion. Two inflow conditions are considered: 15% and 1.8% turbulent intensities. Empirical mode decomposition and Hilbert Huang transform are employed and validated by comparing the Hilbert energy spectrum with the Fourier energy spectrum. The extended self-similarity model indicates that pitching effects are more pronounced at locations where the flow is less turbulent due to its effect of being overshadowed by intermittency caused by tip vortex shedding. This agrees with arbitrary order Hilbert spectrum analysis (HSA) results. HSA is proven to be more accurate for scaling exponent estimation than structure functions as the latter results are significantly affected by the energetic scales. Premultiplied energy spectra show that pitch motion affects preferably large scales 0.1 D - 0.5 D and the same amount of energy is contained on smaller scales compared to the fixed turbine, suggesting potential of higher power production. This work considers offshore wind turbine wakes by examining the pitch motion effects on the flow. Hence, results have direct implications on power production and quantification of fatigue loads due to pitch cyclic motion.