Noise generation and propagation within an aircraft air distribution system

Abstract

The flow in an air distribution system is heavily affected by the wide range of complex components used to control the air. The complexity of the system inevitably leads to increased turbulence and interaction between flow over different components. The work presented in this thesis aims to provide improved understanding of the way in which these interactions occur and the effect that they have on the noise produced in the system. For this purpose, a series of simulations have been performed on a clean duct and ducts with single and double orifices using a Reynolds averaged Navier-Stokes turbulence model.

These simulations allowed for an investigation of the velocity and turbulent kinetic energy fields within the duct and the effect of the separation distance of the orifice pairs to be quantified. Further work was conducted on a single orifice configuration, where the turbulence levels upstream of the orifice were controlled using a ring placed at varying separation distances from the orifice. The effect of the varying turbulence levels on the pressure fluctuations, turbulence and noise sources was investigated using large eddy simulation. The presence of additional upstream turbulence, produced by the ring, was seen to increase the turbulence levels and pressure fluctuations downstream of the orifice. The noise source was investigated for all separation distances between the ring and orifice and was found to be dominant in a region close to the upstream corner of the orifice on the upstream face. The propagation of both existing noise and the noise produced by the orifice were investigated in the plane wave range for a single orifice. In the first investigation, sound waves were introduced in the duct upstream and allowed to propagate through the orifice. Following this, idealised sound sources were placed on the surface of the orifice in a position corresponding to the previously identified dominant noise source. The upstream and downstream propagation of this source was then quantified and showed that the effect of the orifice causes a slight predominance of the propagation in the upstream direction, primarily at lower frequencies.

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