Two new publications out now

FNG_Logo_5-01-01

An investigation of airfoil dual acoustic feedback mechanisams at low-to-moderate Reynolds number

Elias Arcondoulis, Con J. Doolan, Anthony C. Zander, Laura A. Brooks, Yu Liu

Abstract

An experimental investigation was performed in an anechoic wind tunnel involving acoustic measurements, hot-wire anemometry and surface flow visualisation techniques to investigate airfoil tonal noise generating mechanisms. Tests were conducted using a NACA 0012 airfoil at corrected angles of attack of 0° and 1.58° and Reynolds numbers of 50,000 to 150,000. A dual acoustic feedback model is presented, where feedback processes act independently on the airfoil pressure and suction surfaces between the point of boundary layer separation and the trailing edge. It is proposed that the tones generated on both airfoil surfaces, with the same or similar frequencies on each surface, interfere constructively. The primary tone possesses near exact frequencies on both surfaces, whereas the secondary tones have larger differences in frequencies between both surfaces, thus explaining their relative magnitudes based on acoustic superposition. This model provides a better comparison with the experimentally obtained tonal frequencies than the existing feedback models. Despite this agreement, the feedback model cannot perfectly predict the acoustic tones as the tones are not perfectly equispaced. An empirical feedback length is calculated by reverse engineering an acoustic feedback length scale by using the recorded primary tone as an input that also minimises the secondary tone prediction errors. This empirical length closely matches the dual acoustic feedback model presented in this paper.

Figures

https://doi.org/10.1016/j.jsv.2019.114887


Control of Aeolian tones from a circular cyliner using forced oscillation

Ruixian Ma, Zhansheng Liu, Guanghui Zhang, Con J. Doolan, Danielle J. Moreau

Abstract

Effects of forced transverse oscillation on the generation of sound from a circular cylinder immersed in uniform flow at a Reynolds number 150 and a Mach number 0.2 is investigated by direct numerical simulation. The cylinder is prescribed to oscillate sinusoidally with a constant oscillating amplitude ratio α of 0.2 of the cylinder diameter and oscillation frequency ratios = 0.2 to 1.4 of the inherent vortex shedding frequency. The impact of the oscillating frequency on the sound pressure is in accordance with that of the forces acted on the cylinder, by which three regimes are identified. In the first regime (≤ 0.7), the sound levels are slightly affected by the oscillation. In the second regime, at 0.8 ≤ F < 1.0 when the vortex shedding is entrained, an apparent reduction of sound pressure is achieved, together with a change in acoustic directivity; at 1.0 ≤ F ≤ 1.05, the sound generation is enhanced. In the third regime (F ≥ 1.1), the sound level further increases with increasing F, and the sound field recovered has a lift dipole character. The variation of the sound generation and propagation nature can be demonstrated by the interaction of the lift dipole and drag dipole.

Figures

https://doi.org/10.1016/j.ast.2019.105370