“…Changes in the blade tip flow-field will have acoustic consequences. The change in end-plate geometry may result in an increase in the acoustic energy associated with the flow features induced by the endplate or alternatively the sound absorption may increase as a consequence of acoustic waves traversing regions of turbulent flow [56]. When considering the impact of blade tip end-plates, the characteristic time scales associated with turbulent flow are significantly shorter than those associated with acoustic waves.…”
Section: Flow Survey At the Blade Tipmentioning
confidence: 95%
“…The acoustic characteristics of industrial fans has been studied by Guedel [56] who concluded that turbulent shear layers result in sound absorption as a consequence of the rotation and stretching of vortical flow features passing though the turbulent shear layer. Before a comparative analysis of the four end-plate designs is possible, it is therefore necessary to establish if the flow-field in the blade tip region does, or does not absorb acoustic energy.…”
“…Changes in the blade tip flow-field will have acoustic consequences. The change in end-plate geometry may result in an increase in the acoustic energy associated with the flow features induced by the endplate or alternatively the sound absorption may increase as a consequence of acoustic waves traversing regions of turbulent flow [56]. When considering the impact of blade tip end-plates, the characteristic time scales associated with turbulent flow are significantly shorter than those associated with acoustic waves.…”
Section: Flow Survey At the Blade Tipmentioning
confidence: 95%
“…The acoustic characteristics of industrial fans has been studied by Guedel [56] who concluded that turbulent shear layers result in sound absorption as a consequence of the rotation and stretching of vortical flow features passing though the turbulent shear layer. Before a comparative analysis of the four end-plate designs is possible, it is therefore necessary to establish if the flow-field in the blade tip region does, or does not absorb acoustic energy.…”
“…Due to the limited experimental campaign, no transfer functions have been measured to identify the acoustic characteristics of the test section nor any assessment of spectral broadening of CROR tones; one of the effects caused by the open jet wind tunnel shear layer has been performed, see examples in Campos 16,17 and Guédel. 18 Figure 1.Pininfarina wind tunnel facility with full model installed.Figure 2.Out-of-flow microphone used for background measurements.…”
Section: Experimental Facilities and Instrumentationmentioning
confidence: 99%
“…Due to the limited experimental campaign, no transfer functions have been measured to identify the acoustic characteristics of the test section nor any assessment of spectral broadening of CROR tones; one of the effects caused by the open jet wind tunnel shear layer has been performed, see examples in Campos 16,17 and Gue´del. 18 Propulsion system design Eurotech designed and developed the CROR engines for this project from specifications provided by Alenia Aermacchi. The design of the CROR used in this investigation features two planes of 12 blades.…”
Noise from contra-rotating open rotors is a major obstacle to the adoption of this fuel efficient technology as a viable aircraft propulsion system. A better understanding of both contra-rotating open rotor noise generation and reduction has been achieved due to ongoing extensive research. One of the most recent research activities is the WENEMOR (wind tunnel tests for the evaluation of the installation effects of Noise EMissions of an open rotor advanced regional aircraft) project, which has been developed in response to the requirements described in the Clean SkyIntegrated Technology Demonstrators under the heading of Green Regional Aircraft. The project investigates the airframe installation effects of a 1=7th scale model of a regional aircraft equipped with two contra-rotating open rotors of the same rotor diameter, the same rotational speed and equal blade number. For this case, the blade passing frequency of rotor-alone tones and the frequency of relevant interaction tones cannot be distinguished due to the equal blade count of the two rotors. This study presents the tone directivity plots up to 4 Â blade passing frequency of the isolated WENEMOR single pylon contra-rotating open rotor engine in both pusher and tractor configurations at various angles of incidence and flow velocities. A linear array of 13 microphones is deployed for the far field sound measurements. The tone directivity trends show the efficient on-axis acoustic radiation at all blade passing frequency tones with the contra-rotating open rotor tone at 2 Â blade passing frequency dominating in the vast majority of the tests. The main objective is to compare the acoustic emission of pusher and tractor configurations tested under the same flow velocities and angles of incidence. The results suggest that the pusher configuration of the isolated contra-rotating open rotor tends to be slightly louder than the tractor at 2 Â blade passing frequency. However, it is shown that the acoustic performance of the isolated contra-rotating open rotor is complicated and sensitive to any change in the flow velocity and the angle of incidence. The increasing flow velocity and the increasing angle of incidence show limited consistency in proportional trends in the directivity plots of sound pressure levels. It is anticipated that the findings will be different for a more realistic case of installedon-model contra-rotating open rotor.
“…Applications of this model to the data from Candel et al (1975) showed good predictions of the overall trends followed by the sidebands. Other models developed for similar configurations include the work of Guédel (1985), based on Lighthill's analogy with a low-frequency assumption, and the work of Powles et al (2011) and McAlpine et al (2013) using high-frequency asymptotics and a weak scattering assumption.…”
The scattering of acoustic waves by a moving vortex is studied in two dimensions to bring further insight into the physical mechanisms responsible for the spectral broadening caused by a region of turbulence. When propagating through turbulence, a monochromatic sound wave will be scattered over a range of frequencies, resulting in typical spectra with broadband sidelobes on either side of the tone. This spectral broadening, also called ‘haystacking’, is of importance for noise radiation from jet exhausts and for acoustic measurements in open-jet wind tunnels. A semianalytical model is formulated for a plane wave scattered by a vortex, including the influence of the convection of the vortex. This allows us to perform a detailed parametric study of the properties and evolution of the scattered field. A time-domain numerical model for the linearised Euler equations is also used to consider more general sound fields, such as that radiated by a point source in a uniform flow. The spectral broadening stems from the combination of the spatial scattering of sound due to the refraction of waves propagating through the vortex, and two Doppler shifts induced by the motion of the vortex relative to the source and of the observer relative to the vortex. The fact that the spectrum exhibits sidebands is directly explained by the directivity of the scattered field which is composed of several beams radiating from the vortex. The evolution of the acoustic spectra with the parameters considered in this paper is compared with the trends observed in previous experimental work on acoustic scattering by a jet shear layer.
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