Tonal Fan Noise Prediction and Validation on the ANCF Configuration

F2 -Tonal Noise Modelling

The NASA Active Noise Control Fan (ANCF) rig has been intensively studied at the Aeroacoustic Propulsion Laboratory facility at NASA providing both aerodynamic and acoustic measurements (D. Sutliff et al.). This low hub-to-tip ratio axial fan provides an excellent test bed for aeroacoustic code validation. Recently a 3D compressible unsteady simulation has been performed on this fan using a Lattice-Bolzmann Method particularly adapted to the low-speed Mach number of this configuration (A. Mann et al.). This simulation including the full geometry of the installation reproduce accurately the acoustic measurements made in the anechoic facility. It complements the experimental database providing a direct insight into the aerodynamic sources (mainly the rotor wakes impacting on the stator), the in-duct and far-field acoustic propagation.
The present work aims at validating an analytical noise prediction code OPTIBRUI on the well documented ANCF configuration. The software accounts for the complex description of the rotor and stator geometry including varying lean, sweep stagger and camber angles, non-uniformly spaced blades and several analytical models are implemented including free-field or in-duct acoustic propagation models and isolated (Goldstein, M. E., Amiet R. K.) or cascade blade responses (de Laborderie, J.) The analytical models require flow modeling as input and the results obtained with analytical, experimental and numerical flow descriptions will be compared. In particular, the effect of the skewed excitations will be investigated. Some recommendations on the flow extractions from the CFD will be made using the unsteady results from the above mentioned LBM simulation. An extrapolation model to compute the gust at the stator vane leading edge from an upstream extraction will be investigated.
In a second step, the analytical model for rotor/stator interaction will be extended for heterogeneous stator following the work of M. Roger. As a demonstration, a parametric study of the acoustic radiation based on typical stagger angle variations require for proper load balancing will be built upon the original homogeneous ANCF stator geometry.