Experimental study on the evolution of wind turbine wake under streamwise pressure gradients

Abstract

Wind tunnel experiments were conducted to explore how streamwise pressure gradients, commonly encountered in complex terrains, influence the wake behavior of a small-scale horizontal-axis wind turbine. The study involved measuring the wake characteristics up to 8 rotor diameters downstream in three pressure gradient scenarios: adverse (APG), favourable (FPG), and zero (ZPG) pressure gradients, assessed using the pressure coefficient (Cp) and its spatial gradient (ΔCp/Δx). These experiments were conducted at a rotor diameter Reynolds number of 96,000 with an inflow velocity of 10 m/s. The results indicate that while the pressure gradients have a more substantial impact on the wake’s velocity deficit, they have a minor effect on its velocity distribution. Nevertheless, non-zero pressure gradients cause significant deformations in the wake’s shape and vertical displacement of its center. Specifically, the FPG case demonstrates enhanced wake velocity recovery, whereas the APG case exhibits slower recovery than the ZPG case. Turbulent kinetic energy decreases as the wake progresses downstream in all cases, with the APG case consistently showing higher values at all measurement locations. Moreover, a consistent pattern of axial vorticity is observed close to the rotor across different pressure gradient cases, with the FPG case displaying notable organisation and amplification of axial vorticity.