The Increase of the Airfoil Trailing Edge Noise and Unsteady Surface Pressure due to High Inflow Turbulence

Abstract

Human factors, specifically visual impact and noise production, are the current main limitations for broader urban wind energy exploitation. Trailing edge noise, caused by the turbulent boundary layer interacting with the airfoil surface, is the primary source of noise of modern horizontal and vertical axis wind turbines. Low inflow turbulence levels do not affect the trailing edge noise. However, an inflow of high turbulence intensity has flow structures that can penetrate the boundary layer, increasing the velocity fluctuations inside the boundary layer and, consequently, the wall pressure fluctuations and trailing edge noise. This research investigates the effect of high free-stream turbulence, observed in the atmospheric boundary layer of urban zones, in the trailing edge noise generation. This was performed by measuring the increment of the turbulence inside of the boundary layer and surface pressure fluctuations near the trailing edge when an airfoil is submitted to high inflow turbulence. Experimental measurements were performed in the Aeroacoustic Wind Tunnel of the University of Twente on a NACA 0012 airfoil subjected to a turbulent inflow. The chord-based Reynolds number of the experiments ranged from 170 x 10^3 to 500 x 10^3. Results showed that high inflow turbulence significantly increases the velocity fluctuations and the integral length scale along the entire boundary layer, resulting in an increment of the surface pressure spectrum more than 6 dB and 10 dB in the entire frequency range for 10% and 20% of free-stream turbulence, respectively. The 10% free-stream turbulence increases the velocity fluctuations just in the low--frequency range, whereas the 20% inflow turbulence influences the velocity spectrum in the entire frequency range, increasing the size of the smallest structures of the turbulence. Amiet's theory is applied to predict the trailing edge far-field noise.