On the noise reduction mechanisms of porous aerofoil leading edges

Abstract

This paper is predominantly an experimental study into the reduction of turbulence -aerofoil interaction noise by the introduction of aerofoil porosity. In this paper we study three scenarios applied to flat plates: (a) when the flat plate is fully porous, (b) when the flat plate is partially porous from the leading edge and (c) when porosity is introduced downstream of the leading edge.This paper shows that the noise reduction spectra collapse when plotted against non-dimensional frequency f l/U , where l is the length of porous section and U is the flow velocity. Narrow band measurements on a partially porous aerofoil have shown that its noise reduction spectra is characterised by a number of narrow peaks. This paper proposes two main mechanisms for explaining this behaviour. The noise reduction mechanisms are validated against noise reductions measured on a realistic aerofoil at relatively low angles of attack. One of the key findings of this paper is that, by using only a single row of holes downstream of the aerofoil leading edge one can obtain significant levels of noise reduction.

Reducing aerofoil–turbulence interaction noise through chordwise-varying porosity

“…Recently, Chaitanya et al. (2020) proposed source-cutoff and edge-to-edge interference as the two main noise-reduction mechanisms for fully and partially porous flat plates, when in the porous section the porosity is uniformly distributed. A possible alternative explanation for the source cutoff is due to the presence of evanescent surface waves of the form by the impedance-type boundary condition of the form , as previously observed by Rienstra & Hirschberg (2004) (where ).…”

“…This back scattering arises because a non-zero jump in pressure across the plate at the leading edge must be smoothly reduced to zero upstream of the plate. If the leading edge is porous, α L > 0, the back scattering is much weaker than if α L = 0, since there is communication between the upper and lower surfaces of the plate and the pressure jump across the plate at the (porous) leading edge is dampened (Chaitanya et al 2020). This gives rise to two possible reasons for noise reduction at low frequencies as we vary γ ; increased average plate porosity and back scattering effects result in edge-to-edge interference.…”

“…The use of porosity as an adaptation to traditional rigid impermeable aerofoils is a commonplace area of interest for minimising aerofoil–turbulence interaction noise (Geyer, Sarradj & Giesler 2012; Roger, Schram & De Santana 2013; Ayton 2016; Chaitanya et al. 2020). Both leading-edge noise, generated by upstream turbulence impinging on the aerofoil, and trailing-edge noise (also known as self-noise), generated by boundary layer turbulence scattering off the trailing edge can be reduced by replacing an impermeable aerofoil with a fully porous aerofoil (Geyer, Sarradj & Fritzsche 2010), or partially porous aerofoil (Geyer & Sarradj 2019).…”

Permeable Leading Edges for Airfoil and Fan Noise Reduction in Disturbed Inflow

“…With the aim of reducing the turbulence interaction noise, different leading-edge modifications have been developed in the past that affect the aerodynamic and acoustic properties of the airfoil. For isolated airfoils or flat plates, these modifications basically consist of leading-edge serrations [4][5][6][7][8][9][10], comblike structures [11], and the application of flow-permeable or porous materials [12][13][14][15][16][17][18][19]. Noise reduction techniques such as serrations or porous materials have also been used in so-called rod-airfoil configurations, which are used to study rotor-stator interaction noise [20][21][22][23].…”

Experimental and Analytical Investigation of the Distortion of Turbulence Interacting with a Porous Airfoil

“…Attenuation of the tonal components of the emitted far-field noise was measured in this case. Paruchuri et al [24] studied the turbulence-interaction noise reduction obtained by flat plates characterized by different extents of porous inserts. Notably, they observed that the use of a single row of holes situated downstream of the leading edge allows for a significant low-frequency attenuation without increasing the noise radiations at higher frequencies.…”