Abstract
The low-speed fans used for automotive engine cooling contribute to a significant part of the global noise emitted by the vehicle. A low-order sound-prediction methodology is developed considering the blade sweep-angle effect on the acoustic predictions of the turbulence-impingement and the trailing-edge noise-generating mechanisms. We modeled these through the application of a semianalytical method based on Amiet’s airfoil theory, appropriately adapted via a strip-theory approach accounting for rotation and modified to include the blades forward curvature. Sweep was already shown in the literature to reduce the noise emitted by isolated airfoils, but its effect on rotating machines was not yet well understood. In this study, we show that the effect of the sweep-angle is to globally reduce the emitted noise by the fan and to change the sound distribution of the sources along the blade span. Thus, the sweep-angle must be considered not only because it yields a better comparison with experimental results but also because wrong conclusions on the dominating noise-generating mechanisms can be drawn when this effect is not taken into account. The investigation is finally complemented by a sensitivity analysis focusing on some of the key parameters characterizing the acoustic prediction.
Highlights
Several industrial fields, such as the aeronautical, automotive, wind-turbines and the ventilation of buildings sectors are concerned with the acoustic radiation of rotational machinery [1,2,3,4]
An extension of Amiet’s theory to rotating blades, initially developed by Schlinker and Amiet [30] and subsequently by Rozenberg et al [15], is firstly proposed in order to take into account leading-edge and trailing-edge noise mechanisms
The overall radiated fan noise is calculated as the summation of the sound individually emitted by each strip to which the single-airfoil theory is applied, assuming that the circular motion can be approximated by an equivalent local translation
Summary
Several industrial fields, such as the aeronautical, automotive, wind-turbines and the ventilation of buildings sectors are concerned with the acoustic radiation of rotational machinery [1,2,3,4]. Following the semianalytical works carried out in [12,13,14,15] for the previous two broadband mechanisms, the aim of this study is to implement semianalytical methods, based on Amiet’s airfoil noise theory [16,17], able to take into account the effect of having swept leading- and trailing-edges These methodologies were already proven to accurately model the noise emitted by static swept airfoils, to the authors’ knowledge, no application to low-speed rotating blades has been carried out so far including sweep, in order to predict the overall broadband emitted sound. The pressure load on the fan suction side presents similar distribution over the blades, with higher values at their leading-edge tip, as Figure 3b shows For these reasons, only one blade has been used for the following acoustics results. 7 non--distributed fan blades, making it possible to deal with them separately: in (a), the relative velocity is illustrated on a plane normal to the fan rotating axis, showing higher velocities at the blade trailing-edge tips; in (b), the distribution of pressure indicates that the most loaded zones are the blade leading-edge tips
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