A wind tunnel study of adverse pressure gradient impact on wind turbine wake dynamics

Abstract

Wind tunnel experiments were conducted to investigate the impact of an adverse streamwise pressure gradient on the wake dynamics of a small-scale horizontal-axis wind turbine. The study used high-frequency constant temperature hotwire anemometry to measure wake velocities up to 7 rotor diameters (dT) downstream. Two cases were studied: zero pressure gradient (ZPG) and adverse pressure gradient (APG). Experimental conditions included a Reynolds number based on the rotor diameter of Re≈6.7×104 and an inflow velocity of 7 m/s. The results show significant differences between the ZPG and APG cases as the wake progresses downstream. The APG case exhibits a higher maximum velocity deficit in the far wake and a more distorted, oval-shaped wake than the ZPG case. Turbulence intensity trends also differ, with a decreasing trend in the ZPG case and an increasing trend in the APG case. Energy spectra analysis at specific wake locations offers insights into flow structure development. In both cases, small-scale (high-frequency) energy decreases as the wake advances, while large-scale (low-frequency) energy increases. Specifically, the adverse pressure gradient attenuates certain large-scale flow structures λx/dT∈[0.05,0.2], corresponding to f∈[20,100]Hz near the rotor tip height and narrows downstream. Additionally, in the APG case, energy increases in the lower half of the wake for flow structures ranging from λx/dT∈[0.04,0.5], or f∈[10,100]Hz near the rotor, narrowing further downstream. The results suggest that an adverse pressure gradient enhances turbulence and hinders velocity recovery toward freestream conditions in the wake of a wind turbine.