Abstract
The application of open-porous materials is a possible method to effectively reduce the aerodynamic noise of an airfoil. However, the porous consistency may have a negative effect on the aerodynamic performance of the airfoil, since very often the lift is decreased while the drag increases. In a recent investigation, the generation of trailing edge noise of a set of airfoil models made from different porous materials was examined experimentally. The materials were characterized mainly by their airflow resistivity. Besides the material, the chordwise extent of the porous material was varied, which was done by covering the front part of the porous airfoil with a thin, impermeable adhesive foil. Acoustic measurements were performed in an open jet wind tunnel using microphone array technology, while the aerodynamic performance was measured simultaneously using a six-component balance. In general, both the airflow resistivity and the extent of the porous material have an influence on the trailing edge noise. However, if a suitable material is chosen, the results show that a noticeable reduction of trailing edge noise is possible even with only a small chordwise extent of the porous material.
Highlights
Airfoil trailing edge noise is a dominant source of noise in several applications, including wind turbine noise, fan noise and airframe noise
The basic trend that can be observed for the partially porous airfoils is that the lift coefficient increases with increasing airflow resistivity r and decreasing extent s of the porous materials
When examining these sound maps it has to be kept in mind that the noise source of interest is only the one positioned at the spanwise center of the trailing edge, between two stronger noise sources that are due to the interaction of the trailing edge with the wind tunnel shear layer
Summary
Airfoil trailing edge noise is a dominant source of noise in several applications, including wind turbine noise, fan noise and airframe noise. There are different methods to reduce this noise by modifying the blade, which includes the use of flow permeable materials. Several studies on this passive modification exist, including experimental studies on brush-like or slitted trailing edge extensions [1,2,3], on fully porous airfoils [4,5] and on airfoils with trailing edges modified by metal mesh sheets [6] or porous foams [7,8,9]. There are a variety of analytical [12] and numerical studies [13,14,15,16] on porous trailing edges for noise reduction available
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