A study of the effect of serration shape and flexibility on trailing edge noise

Abstract

In this study, we investigated the performance of flexible trailing edge serrations of various shapes for airfoil self-noise reduction through anechoic wind tunnel experiments. A flat plate model was tested at zero angle of attack. The chord-based Reynolds number was between 1.6 × 105 and 4 × 105. The boundary layers were fully tripped near the leading edge. Add-on type trailing edge serrations were cut from flexible polyethylene terephthalate sheets. It is observed that compared with rigid serrations, flexible serrations can achieve an additional broadband noise reduction up to 2 dB–3 dB at high frequencies, and the effect also depends on the geometry of the serrations. Complementary deformation measurement and aerodynamic force measurement show that flexible serrations can align better with the flow and are expected to reduce the crossflow intensity near the serration roots, which has been related to the extraneous high-frequency noise generated by serrations in previous studies. An inviscid model is proposed to predict the wake structure and the loadings for serrations of various shapes. Although the model over-predicts the crossflow speed due to the omission of the viscous effect, the relative intensity corresponding to different serration geometry is consistent with experimental observations. Last, we show that the recent analytical noise prediction model [B. Lyu and L. J. Ayton, “Rapid noise prediction models for serrated leading and trailing edges,” J. Sound Vib. 469, 115136 (2020)] for a serrated trailing edge still significantly overpredicts the noise reduction capacity by serrations and does not reveal the role of serration shape properly. This indicates the necessity to include the non-frozen turbulent properties near serrations in the future prediction models.

Benchmarking of the NACA 633-018 Trailing-Edge Noise in a Broad Reynolds Number Range as Part of the IEA Task 39

“…There is a fair scaling with 𝑆𝑡 𝛿 * 𝑆𝑆 , which supports previous results that showed that the noise reduction maximum depends on 𝑈 [35,36]. Two regions of noise reduction are identified for both serrations in the LRM results (AWB and TUD-A) The second noise reduction peak is similar to the results of P. Zhou et al [37], which were also measured at similar 𝑅𝑒 𝑐 number as the LRM. The HRM measurements (DTU) show two noise reduction regions in the sawtooth case, but only one in the iron case.…”

“…At this point we cannot rule out that the noise reduction mechanisms behave differently at high Reynolds numbers. [37] found that this second peak was highly dependant on the serration flexibility and the flow alignment. The serration deformation was dependant not only on the stiffness and flow speed but also on the aerodynamic loading, which is directly related to the serration geometry.…”

Analytical Green's function for the acoustic scattering by a flat plate with a serrated edge

“…2018; Zhou et al. 2020). Second, the Green's function would provide a more intuitive understanding of the effects of serrations by showing the scattering characteristics of simple sound sources, thereby lending insights into the physical mechanism of noise reduction by using serrations, and more importantly informing on new techniques of suppressing TE noise.…”

“…Moreover, the Green's function itself is fundamentally important in many important aspects concerning TE noise. First, this would open the possibility of examining the consequences of many assumptions that have been open to heated debate, such as the validity of frozen turbulence that has been called into question in a number of recent studies (Ragni et al 2018;Zhou et al 2020). Second, the Green's function would provide a more intuitive understanding of the effects of serrations by showing the scattering characteristics of simple sound sources, thereby lending insights into the physical mechanism of noise reduction by using serrations, and more importantly informing on new techniques of suppressing TE noise.…”

Aerofoil self-noise radiations subjected to serration flap angles

“…An improved understanding of the underlying physics would almost certainly provide a new avenue to elevate the serration technology to the next level. For example, the flexible serrated trailing edge studied by Zhou et al (2020) is found to achieve additional 2 − 3 dB broadband noise reduction at high-frequency. Setting a spanwise periodicity for the trailing edge serrations can also achieve further level of broadband noise reduction (Woodhead et al 2017).…”