Experimental investigation of noise from a wall-mounted swept tip blade in a wind tunnel

A wall-mounted swept tip segment of a wind turbine blade (tip model) is tested in an acoustic Kevlar-walled wind tunnel at free stream velocities ranging from 20 to 80 m/s (corresponding to chord-based Reynolds numbers 4.9 · 105 to 2.0 · 106). The tip model used is the result of a design optimization focused on tip extensions for wind turbine blade upscaling. The trailing edge and tip vortex noise spectra are determined by integration of acoustic images generated with a microphone array using beamforming techniques. Aerodynamic lift and drag coefficients are determined from 128 surface pressure tabs on the model and related to the acoustic results. The results indicate, that tip vortex noise is dominant at high angles of attack (corresponding to high lift coefficients) and low flow-speeds. At higher flow speeds, trailing edge noise is the dominant source of acoustic output. This suggests, that tip vortex noise is important to take into account when wind turbines are operating in low wind speeds or noise curtailment. Additionally, the acoustic spectra dependence on velocity is estimated for trailing edge and tip vortex noise. The results indicate, that trailing edge noise scales with a power between 5 and 6, and similar for tip vortex noise, but only at high lift coefficients. Despite the special model used, the presented methodology clearly shows the benefit of using acoustic imaging techniques to distinguish noise sources in a wind tunnel, and can pave way for improved tip vortex noise models in the future.