Abstract
Pressure fluctuations on the suction side of a NACA 0018 with trailing-edge add-ons are obtained from integration of time-resolved stereoscopic and tomographic particle image velocimetry data and compared to the ones computed from Lattice–Boltzmann simulations. The airfoil is retrofitted with solid and slitted serrated trailing edges and measured at 0° and at 12° angles of attack. At 0° angle of attack, the boundary-layer thickness and the intensity of the pressure fluctuations are found to decrease along the edge of the serration from its root to its tip. The spectra of the pressure fluctuations additionally show a change of decay in frequency along the serration edge. This last finding has important repercussions for noise-prediction models, which usually assume the turbulence and the slope of the pressure spectra to be “frozen” in the streamwise direction. Results from this study also indicate that the pressure-fluctuation modification along the serrations scales with the local boundary-layer parameters, which can be obtained from experimental and numerical data. In particular, the pressure spectra collapse into a single profile when the local boundary-layer thickness and skin-friction coefficient is employed, instead of the parameters of the incoming flow. The analysis is further extended to flow fields at positive angle of attack, where serrations are known to exhibit lower performance in noise reduction. At incidence angle, the scaling with the local parameters shows that the spatial distribution of boundary-layer thickness and pressure fluctuations is uniform along the serration. This evidence might indicate a positive correlation between the noise-reduction performance of serrations and the spatial change of pressure spectra (and local boundary-layer thickness) along their edge.Graphical abstract
Highlights
The intensity and the spanwise correlation of the surfacepressure fluctuations beneath a turbulent boundary layer at the trailing edge of an airfoil are the most relevant quantities for the estimation of turbulent boundary-layer trailing-edge noise [TBL–TEN (Brooks et al 1989)]
The integral parameters of the turbulent boundary-layer convecting past the trailing edge of the NACA 0018 airfoil are measured by Particle Image Velocimetry (PIV) and compared to numerical simulations
The study is motivated by a possible improvement of the simplifying assumption of frozen turbulence, which is not able to correctly represent the correct pressure-fluctuation distribution along serrated geometries
Summary
The intensity and the spanwise correlation of the surfacepressure fluctuations beneath a turbulent boundary layer at the trailing edge of an airfoil are the most relevant quantities for the estimation of turbulent boundary-layer trailing-edge noise [TBL–TEN (Brooks et al 1989)]. This broadband component of airfoil self-noise is dominant in wind turbines (Williams and Hall 1970). More complicated geometries have been designed by employing slits (Arce León et al 2015; Gruber 2012), brushes (Herr and Dobrzynski 2005), iron (Avallone et al 2017), and randomly shaped trailing edges (Chong et al 2013). These geometries have already proven to reduce noise with respect to a straight trailing edge, a huge difference is reported between their analytically predicted noise reduction (Howe 1991; Lyu et al 2016) and the measured one (Oerlemans et al 2009)
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