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On the aerodynamic and aeroacoustic effect of passive trailing edge flaplets

Talboys, E. (2021). On the aerodynamic and aeroacoustic effect of passive trailing edge flaplets. (Unpublished Doctoral thesis, City, University of London)

Abstract

A passive, bio-inspired solution for improving aerodynamic and aeroacoustic performance has been investigated for this thesis. The work was a series of experimental campaigns carried out on a NACA 0012 aerofoil, with passive trailing edge flaplets. These flaplets are extended from the trailing edge, and are essentially cantilever elements that can freely oscillate in the flow. The thesis is split into three publications, where the first publication is an aerodynamic study and the latter two are aeroacoustic studies.

The first publication was a time-resolved particle image velocimetry (TR-PIV), focused on the rear part of the aerofoil. In this publication it was seen that when the flaplets were attached to the suction side of the aerofoil, non-linear instabilities in the shear-layer were damped. This is thought to be due to an upstream lock-in mechanism caused by the freely oscillating flaplets. These non-linear instabilities cause the shear-layer vortices to merge, therefore by damping these instabilities, the shear-layer rollers have a lower probability of merging. Hence, a thinner boundary layer and a predicted reduction in drag.

The first of the aeroacoustic studies was to investigate; firstly the effect of the flaplets on aerofoil self-noise and secondly whether there was any difference on the placement of them on the suction or the pressure side of the aerofoil. In both cases it was seen that there was a reduction in low frequency noise and an increase in high frequency noise. A reduction in tonal noise was also seen in both cases. However, it was almost completely eradicated for the case where the flaplets were attached to the suction side. This is thought to be due to the unintentional tripping of the boundary layer, caused by a small step created as the flaplets were attached to the aerofoil. As such there was no separation bubble on the suction side of the aerofoil, a necessary feature for aerofoil tonal noise.

The second aeroacoustic study was a comprehensive study, looking at changing different geometrical parameters of the flaplets and seeing what a change in length, width and inter-spacing has on the far field acoustic spectrum. What could be clearly seen was that there was a Strouhal number band of noise reduction, the same ‘low frequency’ noise reduction seen in the previous study, could be shifted by altering the length of the flaplets. The magnitude of this reduction band could also be increased with increased width and decreased with a smaller width flaplets. Another effect that was observed, was that as the flaplet inter-spacing was increased, the acoustic spectra trended back towards the original untreated trailing edge.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering Doctoral Theses
School of Mathematics, Computer Science & Engineering > Engineering > Mechanical Engineering & Aeronautics
[img] Text - Accepted Version
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