Use of a small low-noise wind tunnel for determining the aeroacoustic noise produced by components on a vehicle

This paper describes an experimental method used to determine the aeroacoustic noise produced by a 'roof rack' placed on the roof of a vehicle. Testing was done on a vehicle roof positioned at the outlet jet of a small low-noise wind tunnel. A 'simulated vehicle cabin' was constructed beneath the vehicle roof that had similar absorption characteristics to an actual vehicle cabin. Sound pressure level measurements were made within the simulated vehicle cabin. The sound pressure level measurements were consistent with measurements made within the cabin of an actual vehicle in a large anechoic wind tunnel. The method could also be used to determine the in-cabin aeroacoustic noise produced by other vehicle accessories

Far-field sound radiation due to an installed open rotor

Future single rotation propeller and contra-rotating advanced open rotor concepts promise a significant fuel efficiency advantage over current generation turbofan engines. The development of rotors which produce a minimum level of noise is a critical technical issue which needs to be resolved in order for these concepts to become viable aircraft propulsors. Noise and emissions are subject to stringent legislative requirements, thus accurate models are required in order to predict the noise radiated from aircraft engines. In this article, the development of a theoretical model to predict noise levels of an installed open rotor is reported. First a canonical problem is examined: how to predict the pressure field produced by a rotating ring of point sources adjacent to a rigid cylinder. Analytic expressions for the far-field pressure from a rotating ring of single-frequency monopole and dipole point sources, located near an infinitely long rigid cylinder, immersed in a constant axial mean flow, are explicitly formulated. Illustrative results show how the far-field pressure is affected by varying the source rotational direction, source location and source radius. Next the solution of the canonical problem is utilized to formulate a more advanced model to predict the noise due to an installed open rotor. In this model, the rotor noise sources are represented by a distribution of rotating sources. The adjacent aircraft fuselage is modeled by the rigid cylinder, and the effect of the fuselage boundary layer and other steady distortions are neglected. Also neglected is the scattering from other surfaces such as the pylon, wing and centerbody. This distributed source model can be used to calculate the effect of scattering of open rotor noise by an adjacent cylindrical fuselage. The model can be used to calculate both rotor-alone tones and tones produced by periodic unsteady loading on the rotor blades. Practical examples are provided which show how the effect of blade rotational direction and propeller location relative to the fuselage affect the sound produced by the interaction of a pylon wake with a rotor in a pusher configuratio

Measurement of Sound in Airflow

The suitability of a number of different microphone configurations for making sound measurements in airflow was assessed. When a microphone is immersed in airflow, turbulence within the airflow interacts with the diaphragm causing the microphone to measure a noise level, which is due to the turbulence/diaphragm interaction and is not due to an acoustic wave. This turbulence-induced 'pseudo-noise' is equivalent to background noise and can interfere with sound level measurements if the pseudo-noise level is similar to the level of sound being measured. Instances where pseudo-noise may be a problem include measurements made out-doors where the microphone is subjected to atmospheric wind or measurements made in wind tunnels or HVAC ducts. In this paper a number of different microphones and microphone treatments were investigated for their suitability for minimizing pseudo-noise

Design of Automobile Components for the Minimization of Aeroacoustic Noise

In recent years, automotive manufacturers have invested significantly in measures to minimize the noise level within an automobile cabin. Today, aeroacoustic noise produced by airflow over car accessories such as vehicle side-mirrors, windscreen wipers and roof carrier systems make a significant contribution to the sound level within the automobile cabin. Consequently, the design of these components to minimize aeroacoustic noise has become important. This paper is concerned with minimizing the aeroacoustic noise of a roof carrier system

Open rotor centrebody scattering

This paper investigates the effect of acoustic scattering from the centrebody of an advanced open rotor engine. The physical mechanisms governing the scattering process are investigated and formulae for predicting noise levels are presented. It is found that centrebody scattering has a negligible effect on rotor-alone tones produced by a subsonic rotor, however, the scattering effect can be significant for rotor-alone tones produced by a supersonic rotor and certain rotor–rotor interaction tones. The paper concludes with an analysis which shows that the centrebody scattered field may be significantly reduced by applying an acoustic liner to the centrebody surface