Broadband Noise Reduction Of Linear Cascades With Trailing Edge Serrations

Abstract

The noise reduction potential of trailing edge serrations in a linear cascade of seven loaded airfoils is investigated experimentally for Re= 5.5 × 10 5 and M= 0.23. Turbulent boundary layer trailing edge noise is considerably reduced in the low-to mid-frequency range similarly to previous isolated airfoil investigations. A high frequency noise increase is also found for St= fδ/U > 1.2. The cascade effect is neither found to enhance nor to limit the noise reduction of serrations, except in the very low frequency range where the noise reduction potential found for isolated airfoils is not recovered. The loading of the airfoil is not affected by the serrations but the drag is increased by 14%. High speed particle image velocimetry is carried out in the near wake of a serration tooth and compared to the straight edge. The suction side turbulent boundary layer is nearly suppressed from the airfoil surface which might partly explain the observed noise reduction. Conversely the pressure side boundary layer turbulence crosses the troughs of the serrations. A third turbulent region generated at the very trailing edge is identified as a consequence of the Kutta condition. The elongated geometry of the sawtooth serrations makes it vanish faster with the downstream distance than in the straight edge case. Nomenclature b [m] Half-chord (= c/2) s [m] Blade pitch λ [m] Serration wavelength h [m] Amplitude of the serrations ω [rad.s − 1] Acoustic puslation, St Strouhal number δ [m] Boundary layer thickness, m δ * [m] Boundary layer displacement thickness, m θ * [m] Boundary layer momentum thickness, m α c [deg] Geometrical angle of attack β c [deg] Air inlet angle U [m.s −1 ] Mean flow velocity ρ [kg.m −3 ] Fluid density C p Pressure Coefficient C D Drag Coefficient σ = c/s Cascade solidity Subscripts 1 Upstream quantities * PhD Student2 Downstream quantities p Pressure side quantities s Suction side quantities Superscripts a Mean quantities a ′ Fluctuating quantities

Flow topology and acoustic emissions of trailing edge serrations at incidence

“…The straight trailing edge shows higher u ′ RMS and v ′ RMS on the suction side which continues downstream while another region of higher RMS is seen to originate from the trailing edge. This behavior was previously observed in the PIV measurements conducted in Finez et al (2011). The serrated trailing edge with ϕ = 6 • shows this effect as well, and this region of increased fluctuations is seen to originate from the root of the serrations.…”

“…While wind tunnel experiments have revealed that a reduction in noise is often measured when using serrations, a slight increase in noise at relatively higher frequencies has also been reported (Moreau et al 2011;Gruber 2012). It has been argued in Dassen et al (1996) and Oerlemans et al (2009) that the noise increase at higher frequencies might be due to the misalignment of the serrations with the flow in the wake of the airfoil. Gruber et al (2011) has ascribed this phenomenon to an increased turbulence activity between the serration teeth.…”

“…performed both in wind tunnel experiments (Gruber et al 2013;Moreau and Doolan 2013;Dassen et al 1996) and on full-scale wind turbines (Schepers et al 2007;Oerlemans et al 2009). For the latter, airfoil self-noise reduction is relevant in relation to the observance of noise limits established by local regulations.…”

“…For the latter, airfoil self-noise reduction is relevant in relation to the observance of noise limits established by local regulations. Furthermore, experimental studies related to the flow around serrations and surface pressure characterization have also been previously performed (Chong and Vathylakis 2015;Finez et al 2011;Arce et al 2015). …”

An Experimental Investigation on the near-field turbulence for an airfoil with trailing-edge serrations by using 3-D hot-wire

“…Among the fan-OGV sources, Trailing Edge(TE) noise significantly contributes engine broadband noise 2 . TE noise source has first been characterized experimentally by Brooks & al.…”