# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 5 Fringe-Type Trailing Edge Extensions
Several studies revealed that the noise reduction characteristics of different brush or slit configurations were mostly related to the geometry parameters of the trailing edge extensions (spacing, length, and diameter), rather than on the flow conditions (flow speed, angle of attack, etc.). [PERSON] and [PERSON] [143; 147] conducted a comparison of the respective noise reduction potential for different brush lengths and fiber diameters on a flat plate model. They summarized that the highest low frequency (1.6-2.5 kHz) noise reduction was achieved for a medium length brush (30 mm) with the thickest fiber diameter of 0.5 mm, while more promising mid to high-frequency noise reduction was provided by medium diameter brushes (0.4 mm) with longer length (30 mm to 60 mm). A further investigation on the NACA 0012-like airfoil [144; 148] revealed that: (1) A flexible brush was generally more favorable than a stiff brush (also observed in [149; 150]), but not essential since the latter could also achieve a noise reduction. (2) The mean spacing of the adjacent fiber materials was the crucial design parameter for either a brush-type trailing edge or a solid slit trailing edge: small fiber spacing (\(<\)1 mm) provided a significantly increased noise reduction capability (also noticed in [128]) (see Figure 15). (3) Longer brush-type or slit configurations (exceeded a minimum fiber length of about 15 mm) led to better noise reduction since they became more flexible and provided a larger flow-permeable region to achieve the desired hydrodynamic absorption. (4) When the fibers were not oriented along the flow direction, excess broadband noise increase of about 2 dB was observed. (5) The achievable noise reduction was irrespective of the flow velocity, in the examined range of Reynolds numbers. (6) The brush-type trailing edges nearly did not affect the noise spectra by changes in angle-of-attack, and thus the overall noise reduction appeared to diminish with a growing angle-of-attack since the noise emission of the solid reference NACA 0012-like airfoil significantly decreased in the mid-frequency range. [PERSON] et al. [146] found that when the open-area ratios of the slit trailing edge were the same, noise reduction increased with decreasing slit separation or increasing number of slits. Figure 14: Experimental setups in the AWB test section with example of trailing edge brush or slit devices, reprinted from [148] with permission from [PERSON]. Similar to trailing edge serrations, broadband excess noise contributions at high frequencies (\(>\)10 kHz) were also analytically predicted or experimentally observed for the brush-type [143; 147] or slit [122; 146] devices. [PERSON] and [PERSON] [143; 147] speculated that this effect may be due to the decrease of the spanwise correlation scale, which is introduced by the transformation of spanwise vorticity into streamwise vorticity. On the other hand, [PERSON] et al. [145] suspected that excessive high-frequency noise components might be linked to the micro-jets through fibers. Up to now, the effect of fringe-type devices on the aerodynamic performance has not been experimentally tested. [PERSON] and [PERSON] [151; 152] numerically studied the effect of two different stiff brush-type devices (cylindrical or rectangular) at the realistic A340-type slat trailing edge on the flow field and force coefficients. The results showed that cross vortex caused by the design-related steps at the root of the cylindrical brush hairs impaired the flow around the slat trailing edge and thus adversely affected the aerodynamic properties, i.e., reduced the lift coefficient while increased the drag coefficient, especially when the length and diameter of the brush hairs were increased. On the other hand, brushes with a rectangular cross-section (i.e., slit trailing edges) had only a minor influence on the slat wake and momentum loss, and thus the loss of lift coefficient was insignificant compared to the reference slat trailing edge configuration.