This paper provides an experimental investigation into the use of leading edge (LE) serrations as a means of reducing the broadband noise generated due to the interaction between the aerofoil’s LE and impinging turbulence. Experiments are performed on a flat plate in an open jet wind tunnel. Grids are used to generate isotropic homogeneous turbulence. The leading edge serrations are in the form of sinusoidal profiles of wavelengths, λ, and amplitudes, 2h. The frequency and amplitude characteristics are studied in detail in order to understand the effect of LE serrations on noise reduction characteristics and are compared with straight edge baseline flat plates. Noise reductions are found to be insignificant at low frequencies but significant in the mid frequency range (500 Hz–8 kHz) for all the cases studied. The flat plate results are also compared to the noise reductions obtained on a serrated NACA-65 aerofoil with the same serration profile. Noise reductions are found to be significantly higher for the flat plates with a maximum noise reduction of around 9 dB compared with about 7 dB for the aerofoil. In general, it is observed that the sound power reduction level (ΔPWL) is sensitive to the amplitude, 2h of the LE serrations but less sensitive to the serration wavelength, λ. Thus, this paper sufficiently demonstrates that the LE amplitude acts as a key parameter for enhancing the noise reduction levels in flat plates and aerofoils.

/pdf/airfoil-noise-reductions-through-leading-edge-serrations-4z45kpzxge.pdf

Airfoil noise reductions through leading edge serrations

On the mechanisms of serrated airfoil trailing edge noise reduction

This paper presents the results of a detailed experimental investigation into the effectiveness of sinusoidal leading edge serrations on aerofoils for the reduction of the noise generated by the interaction with turbulent flow. A detailed parametric study is performed to investigate the sensitivity of the noise reductions to the serration amplitude and wavelength. The study is primarily performed on flat plates in an idealized turbulent flow, which we demonstrate captures the same behaviour as when identical serrations are introduced onto three-dimensional aerofoils. The influence on the noise reduction of the turbulence integral length scale is also studied. An optimum serration wavelength is identified whereby maximum noise reductions are obtained, corresponding to when the transverse integral length scale is approximately one-fourth the serration wavelength. This paper proves that, at the optimum serration wavelength, adjacent valley sources are excited incoherently. One of the most important findings of this paper is that, at the optimum serration wavelength, the sound power radiation from the serrated aerofoil varies inversely proportional to the Strouhal number , where , and are frequency, serration amplitude and flow speed, respectively. A simple model is proposed to explain this behaviour. Noise reductions are observed to generally increase with increasing frequency until the frequency at which aerofoil self-noise dominates the interaction noise. Leading edge serrations are also shown to reduce aerofoil self-noise. The mechanism for this phenomenon is explored through particle image velocimetry measurements. Finally, the lift and drag of the serrated aerofoil are obtained through direct measurement and compared against the straight edge baseline aerofoil. It is shown that aerodynamic performance is not substantially degraded by the introduction of the leading edge serrations on the aerofoil.

/pdf/performance-and-mechanism-of-sinusoidal-leading-edge-155s5tomuj.pdf

Performance and mechanism of sinusoidal leading edge serrations for the reduction of turbulence–aerofoil interaction noise

Experimental Investigation of Airfoil Self Noise and Turbulent Wake Reduction by the use of Trailing Edge Serrations

https://bura.brunel.ac.uk/bitstream/2438/9870/1/Fulltext.pdf

An experimental study of airfoil instability tonal noise with trailing edge serrations

https://bura.brunel.ac.uk/bitstream/2438/10041/1/Fulltext.pdf

Design and performance of an open jet wind tunnel for aero-acoustic measurement

This paper represents the results of an experimental program with the aim of controlling the flow in a highly unstable 90 deg curved diffuser. The diffuser, which is an integral part of an open jet wind tunnel at the University of Southampton, has the unique configuration of extreme shortness and high area ratio. In this study, several passive flow control devices such as vortex generators, woven wire mesh screens, honeycomb, and guide vanes were employed to control the three-dimensional diffusing flow in a scaled-down model. Although less successful for vortex generators, the other devices were found to improve significantly the uniformity of the flow distribution inside the curved diffuser and hence the exit flow. This study suggests that a cumulative pressure drop coefficient of at least 4.5 at the diffuser exit with at least three guide vanes is required to achieve adequate flow uniformity at the diffuser exit. These flow conditioning treatments were applied to the full-scale diffuser with exit dimensions of 1.3×1.3 m2. Flow with comparable uniformity to the scale-model diffuser is obtained. This study provides valuable guidelines on the design of curved/straight diffusers with nonseparated flow and minimal pressure distortion at the exit.

A Parametric Study of Passive Flow Control for a Short, High Area Ratio 90deg Curved Diffuser

This technical report presents the design of an open-jet, blow-down wind tunnel that was newly commissioned in the anechoic chamber at the ISVR, University of Southampton, UK. This wind tunnel is intended for the measurement of airfoil trailing edge self-noise but can be extended to other aeroacoustic applications. With the primary objectives of achieving acoustically quiet and low turbulence air jet up to 120m/s through a 150mm x 450mm nozzle, several novel noise and flow control techniques were implemented in the design. Both the acoustical and aerodynamic performances of the open jet wind tunnel were calibrated in detail after its fabrication. It is found that the background noise of the facility is adequately low for a wide range of exit jet velocity. The potential core of the free jet is characterised by a low turbulence level of about 0.1%. A benchmark test by submerging a NACA0012 airfoil with a tripped boundary layer at zero angle of attack into the potential core of the free jet was carried out. It was confirmed that the radiating aerofoil trailing edge self-noise has levels significantly above the rig noise over a wide range of frequencies. The low noise and low turbulence characteristics of this open jet wind tunnel are comparable to the best facilities in the world, and is believed to be the first of its kind in the UK.

P.O.A.L. Davies

Papers

Design and performance of an open jet wind tunnel for aero-acoustic measurement

A Parametric Study of Passive Flow Control for a Short, High Area Ratio 90deg Curved Diffuser

Design and characterisation of a quiet, low turbulence open jet blow down wind tunnel in ISVR