Abstract

Tip leakage flow is one of the major sources of noise in axial fans. The complex flow in the gap between the rotating fan blade and stationary casing contributes to noise emission due to several features, e.g., flow separation on the pressure side edge, contraction and acceleration of the flow in the gap and interaction of the tip leakage vortex with the blade edges. The design of the blade tip using winglets helps to reduce noise emission in the tip region. 

In order to investigate this effect on stationary airfoils, experiments were conducted in the aero-acoustic wind tunnel of the DLR in Braunschweig. Among other parameters the influence of blade tip design and tip clearance were investigated on a NACA 5510 airfoil. For some of these configurations, numerical simulations were done using PowerFLOW. The acoustic performance was evaluated directly from the compressible pressure in the far field as well as from Ffowcs Williams and Hawkings propagation emitted from the airfoil. 

All investigated winglet designs reduce sound emission at low frequencies below 1000 Hz, which is in agreement with the measurements. With the application of a large radius on the pressure side edge, local flow separation is prevented, leading to low-frequency noise reduction. High-frequency self-noise from the trailing edge is underestimated in simulation. This is attributed to the very small trailing edge thickness of 0.8 mm. Variation of trailing edge resolution was investigated. In general, the noise reduction effect of winglets for this stationary airfoil is less compared to winglets in real fan applications.

Presentation slides