Prediction of fan acoustic blockage on fan/outlet guide vane broadband interaction noise using frequency domain linearised Navier–Stokes solvers

Abstract

Fan/Outlet-Guide-Vane (OGV) interaction is a major broadband noise source in high bypass ratio turbofan engines. This paper assesses the acoustic shielding created by the fan rotor on the broadband noise generated by the interaction between the fan and the OGV. With this objective in mind, an efficient methodology to account for broadband noise transmission through a fan rotor using two-dimensional linearised frequency domain Navier–Stokes solvers is presented. The radial variations of the flow are retained using a strip-wise approach, which is key to estimate the fan blockage accurately. The methodology has been applied to the AneCom Aerotest (ACAT1) fan rig that is representative of a modern fan, at the three noise certification points and compared with acoustic measurements, namely, frequency spectra and azimuthal mode decompositions. Overall, good agreement on the impact of the rotor shielding on the upstream noise spectra has been achieved. Rotor shielding produces a decrease in the upstream emitted noise of about 2.5, 7 and 9 dB at Approach, Cutback and Sideline conditions, respectively. Thus, its modelling is of paramount importance for an accurate prediction of the engine’s noise footprint. The predicted sound pressure azimuthal decompositions show an excellent qualitative agreement with the experimental data, demonstrating the suitability of the methodology to predict this phenomenon. It is concluded that fan blockage cannot be predicted with conventional semi-analytical techniques based on flat plate approximations. The supersonic pockets and shock-waves present in the fan play a principal role in the upstream acoustic emission at high-speed operating points, which cannot be retained in flat plate-like methodologies. The simulations show that at Sideline, the broadband noise emitted by the engine core can become dominant, which was not anticipated. This conclusion is indirectly supported by measurements.