Study of the Inflow Turbulence Distortion for Airfoils and Cylinders

The distortion of the turbulent inflow upstream of an airfoil affects the leading-edge (LE) radiated noise. This distortion is due to the finite airfoil thickness and is usually accounted for in noise prediction models by assuming that the turbulence distortion upstream of an airfoil can be approximated by the distortion upstream of a cylinder with a diameter equal to the airfoil LE nose diameter. However, the validity of this assumption was not addressed in the literature so far. Therefore, this paper discusses the turbulence distortion in the stagnation line of different airfoils and cylinders to verify this assumption and to investigate the airfoil geometric parameters relevant to turbulence distortion. Experiments were performed in the Aeroacoustic Wind Tunnel of the University of Twente. The inflow turbulence was generated by a circular rod and a grid. It was evaluated upstream of a NACA 0008, NACA 0012, NACA 0018, and two cylinders with diameters corresponding to the maximum thickness and the LE nose diameter of the NACA 0012. The results show that the turbulence distortion in the stagnation line of the airfoil can be approximated well by the turbulence distortion upstream of a cylinder. This indicates that the rapid distortion theory for a cylinder can be a good approximation of the turbulence distortion upstream of an airfoil. Consequently, this theory can be combined with the Amiet LE noise prediction model to account for the turbulence distortion effects in the LE-radiated noise. However, the cylinder diameter must be representative of the airfoil average thickness in the LE region. Furthermore, the results show that the airfoil geometric parameters that influence the turbulence distortion are the average thickness of the airfoil in the LE region and the location of maximum thickness. These parameters affect the turbulence characteristics close to the LE differently. The airfoil geometry downstream of the maximum thickness location has a negligible effect on the turbulence distortion.