Upstream shear-layer stabilisation via self-oscillating trailing edge flaplets

Abstract

This is the published version of the paper.This version of the publication may differ from the final published version. Permanent repository link AbstractThe flow around a symmetric aerofoil (NACA 0012) with an array of flexible flaplets attached to the trailing edge has been investigated at Reynolds numbers of 100,000-150,000 using high-speed time-resolved particle image velocimetry (HS TR-PIV) and motion tracking of the flaplets' tips. Particular attention has been made on the upstream effect on the boundary layer evolution along the suction side of the wing, at angles of attack of 0 • and 10• . For the plain aerofoil, without flaplets, the boundary layer on the second half of the aerofoil shows the formation of rollers as the shear-layer rolls-up in the fundamental instability mode (linear state). Proper orthogonal decomposition analysis shows that non-linear modes are also present, the most dominant being the pairing of successive rollers. When the flaplets are attached, it is shown that the flow-induced oscillations of the flaplets are able to create a lock-in effect that stabilises the linear state of the shear layer, whilst delaying or damping the growth of non-linear modes. It is hypothesised that the modified trailing edge is beneficial for reducing drag and can reduce aeroacoustic noise production in the lower frequency band, as indicated by an initial acoustic investigation. List of symbols

A Numerical Simulation on the Airfoil S833 Equipped with Flapping Trailing Edge Fringes

“…Furthermore, Talboys et al (2019) indicated that the tonal noise can be removed when the passive flaplets are affixed to the pressure side of the airfoil, whereas the tonal noise can be attenuated if the flaplets are placed on the suction side of the airfoil. Talboys et al (2018) also analyzed different motions of the trailing edge flaplet and found that the movement of the trailing edge is primarily due to small scale turbulent structures, and therefore, the flaplets are able to stabilize the shear layer on the suction side to promote the performance of the airfoil. Our experimental investigation showed that the real fringe feather edges installed on an airfoil enables the large-scale vortex to break down into small-scale turbulence (Yang et al, 2015), demonstrating that the vibrating long flexible fringe contributes an important vertical velocity in the flow field around the trailing edge.…”

A parametric study of the effect of self-oscillating trailing-edge flaplets on aerofoil self-noise

“…The Eigen frequency of the flaplets needs to be determined in order to see at what frequency the flaplets will oscillate in the flow. The set-up and image processing for this response test is the same as was used in [21]. The normalised response of the long, baseline and short flaplets is shown in Fig.…”

“…However the authors have two concepts of thought on the source of the reduction. The first is the hypothesis of the lock-in effect documented in [21] (flaplets act as pacemaker), which plays an important role in the stabilisation of the T-S waves. Because of the different Eigen frequencies of the flaplets, they are modifying the wake with different frequencies, hence the different acoustic frequency reductions.…”

“…The present acoustical study continues a series of original experiments of the authors on arrays of individual self-oscillating elastic elements attached to the trailing edge of an aerofoil to reduce self-noise. This type of trailing edge modification with arrays of elastic flaps has been proven as a novel effective way for passive noise cancellation [21,22,23,24,25,26]. This mainly refers to tonal noise, which is generated by the periodic shedding of vortices from the trailing edge of an aerofoil.…”

“…This mainly refers to tonal noise, which is generated by the periodic shedding of vortices from the trailing edge of an aerofoil. The noise reduction that can be achieved with the flexible flaplets was attributed to a lock-in process reported in [21], which causes the fundamental instabilities in the flow over the aerofoil -the periodic vortex shedding -to lock-in their frequencies with the natural frequency of the oscillating flaplets. Therefore, these attached oscillators effectively act as pacemaker, keeping the flow instabilities to remain in the state of linear growth.…”