Abstract
The flow past a NACA 0018 airfoil with sawtooth trailing edge serrations has been investigated using stereoscopic particle image velocimetry (PIV). The serration flap angle and airfoil incidence are varied in order to study the effect of secondary flow establishing between the suction and pressure sides of the serrations. The flow topology around the serrations is inferred from the analysis of time-averaged streamlines close to the airfoil surface and from the wall-normal flow velocity in between serrations. Additional PIV measurements with a plane in cross-flow highlight the formation of streamwise vortex pairs. The flow behavior is further characterized in terms of its turbulence statistics. Noise emissions are measured with an acoustic phased array in combination with beamforming. The serrations are found to be effective in reducing noise, and their application is studied for different degrees of airfoil incidence and serration flap angle.
Highlights
Broadband airfoil noise emissions that originate due to the interaction of the airfoil turbulent boundary layer with the sharp trailing edge (Brooks et al 1989) have been shown to be effectively reduced using trailing edge serrations
A NACA 0018 airfoil profile has been CNC-machined into an aluminum model with removable trailing edge inserts, a chord C = 20 cm and a span of 40 cm
Instead, following Howe’s approach, one expects an additional reduction of noise emission since the streamline deflection tends to reduce θ even further, with the flow becoming more parallel to the trailing edge
Summary
Broadband airfoil noise emissions that originate due to the interaction of the airfoil turbulent boundary layer with the sharp trailing edge (Brooks et al 1989) have been shown to be effectively reduced using trailing edge serrations Evidence of this has been observed in acoustic measurements. An analytical model for the prediction of the noise emitted from a sawtooth serrated trailing edge has been proposed by Howe (1991a, b). Experimental studies (Gruber et al 2013; Moreau and Doolan 2013) have reported less effective noise reduction for serrated trailing edges than that predicted with the above-mentioned model. In order to relate the noise reduction efficiency to flow field effects, the present work combines stereoscopic particle image velocimetry (PIV) and acoustic phased array measurements of sawtooth trailing edge serrations on a NACA 0018 airfoil. The noise emitted by the serrated trailing edge is monitored with a microphone array yielding the power spectral density of the acoustic pressure fluctuations in the relevant frequency range
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