Scaling properties of the aerodynamic noise generated by low-speed fans

Leakage Noise and Related Flow Pattern in a Low-Speed Axial Fan with Rotating Shroud

“…Compared to the previous works, see Canepa et al. [9][10][11][12], in the present study, a larger number of operating conditions have been investigated, and a different rotor has been tested, see Figure 1a. It also has nine blades of different geometry but it is an aluminum prototype while the previously studied rotor is a plastic one employed in a real cooling unit.…”

“…The good collapse of the characteristic curves of Figure 1b suggests that Re effects are independent of Ω and kinematic similarity is respected at the rotor inlet; together with the low Ma values, this indicates that aerodynamic similarity is also respected. Hence, constancy of Ψ during a speed ramp implies that ϕ is also constant and that the aeroacoustic similarity should be eventually respected, e.g., see Neise and Barsikow [15] or Canepa et al [10]. However, noise generation may be strongly affected even by flow details which do not affect the ϕ − Ψ characteristic curve.…”

“…α is an exponent close to one to be tuned case by case; it increases with the flow coherence and hence depends on both the operating point and the noise generating mechanism, that is on the considered part of the spectrum which is identified by St. (In fact, Equation (2) applies to SPL spectra computed with proportional bands, while 10 log 10 Ω 3+α has to be employed if ∆ f = const, e.g. see Canepa et al [10]).…”

“…Their interaction with the rotor blades results in noise in the low-to medium-frequency range, with the appearance of narrowband humps at subharmonic frequencies and a general increase of the broadband noise. The leakage flow noise often constitutes the major contribution to the overall radiation and its main features are known, but very few works related to it are available in the literature, e.g., see Fukano et al [2], Fukano and Jang [3], Piellard et al [4], Magne et al [5], Moreau and Sanjosé [6], Zenger et al [7], Na et al [8], Canepa et al [9][10][11][12], Zhu et al [13] and Sanjosé and Moreau [14]. In order to find effective solutions for noise reduction, a deeper insight in the leakage flow features and in the effect of relevant parameters is necessary.…”

“…Beyond an obvious dependence of such patterns on both gap geometry and operating conditions, a further, unexpected dependence on both rotational speed and pressure rise was also observed, due to the related rotor deformation. This raised a number of questions on the features of the phenomenon and also on the validity of the acoustic similarity, see Neise and Barsikow [15] or Canepa et al [10], which allows scaling the results of acoustic measurements related to different rotational speeds. Hence, it has been decided to perform a more systematic investigation on an aluminum rotor, which may be considered stiff.…”