Evaluation of Analytical Aeroacoustic Models for a Low-Speed Axial Ventilation System

2.5 Noise Prediction by Analytical or Numerical Models

Small ventilation systems with high rotational speed are used for the cooling of avionic racks and cabin air conditioning. These compact systems of axial or mixed flow type are mainly designed by optimizing their efficiency and weight. The acoustic levels emitted by such systems are of key importance and design rules based on a long-term experience of acoustic measurements are already accounted for. In the present work, analytical aeroacoustic models that can be informed with mean flow parameters from steady-state numerical simulations will be evaluated for a typical axial fan system.
A low-Mach number ducted axial fan (LP3) from Safran Ventilation Systems is investigated in the present study. A steady Reynolds-Averaged Numerical Simulation has been performed using ANSYS CFX. The computational domain includes a long constant duct section of an angular sector of the machine containing the rotating hub, three rotor blades and four stator vanes, and the downstream hub diffuser. In the present simulation, the blades are considered to be identical and uniformly distributed, the heterogeneity due to three thickened vanes are investigated in a dedicated study [1]. The tip Mach number is 0.3, and the hub-to-tip ratio is about 0.5 at the vane leading edge, limiting the number of cut-on modes in the duct section. In particular, the blade numbers have been selected to ensure that the first two blade passing frequencies are cut-off. Nevertheless these tones appear in the acoustic spectra measured both in Safran Ventilation Systems test-rig and in the experimental installation of this fan at Ecole Centrale de Lyon [2].
The tonal noise sources will be investigated analytically focusing on the rotor-wake interaction on the stator vanes using the wake deficit parameters extracted from the RANS simulation. The role of the evanescent acoustics mode generated in this system will be investigated. Indeed, the configuration accounts for both slowly varying duct sections, in the inter-stage and in the diffuser, and the sharp annular to cylindrical section changes upstream of the rotor and downstream of the diffuser. The evanescent modes could become cut-on modes in the cylindrical sections and responsible for the emergence of tones in the far-field acoustic spectra. Considering the large number of vanes in the small duct diameter, the influence of cascade effects will be investigated by comparing the acoustic levels obtained with an isolated airfoil response or a cascade response on this configuration. The broadband noise contributions from the rotor self-noise and the turbulent wake impingement will also be estimated using isolated airfoil response in infinite constant duct. The noise sources will be modeled from statistical turbulence parameters extracted in the RANS simulation.

[1] M. Pestana, M. Sanjosé, S. Moreau, M. Roger and M. Gruber, “Investigation on the noise of an axial low Mach-number stage with a heterogeneous stator.” In FAN 2018.
[2] M. Pestana, A. Pereira, E. Salze, J. Thisse, M. Sanjosé, E. Jondeau, P. Souchotte, M. Roger, S. Moreau, J. Regnard and M. Gruber, “Aeroacoustics of an axial ducted low mach-number stage: numerical and experimental investigation.” In 23rd AIAA/CEAS Aeroacoustics Conference, AIAA-2017-3215, 2017.
[3] M. Daroukh, “Effets de la distorsion sur le bruit tonal d’un turboréacteur moderne”, PhD. Institut National Polytechnique de Toulouse, France, 2017.