The interactions between the turbulent structures beyond the fan rotor blades and the Outlet Guide Vanes (OGV) are mainly responsible for aero-engines broadband noise emission at approach conditions. More particularly, sound power levels in the bypass duct are mainly generated by the turbulent pressure fluctuation over the stator vanes, whereas acoustic levels in the intake can also be attributed to the rotor self-noise. 1 The present study is only devoted to the interaction mechanism. Modern turbofans are characterized by composite fan blades with low count and large chords, and the struts are integrated to the OGV. Although these technological effects should be ideally taken into account in the aeroacoustic predictions, classical fast-running industrial tools are still based on 3-D Reynolds Averaged Navier-Stokes (RANS) computations, restricted to a single blade channel, and assuming that turbulent wake characteristics can be assessed by current twoequation turbulence models. These informations are generally used to feed analytical formulations based on flat-plate2 or cascade aeroacoustic response models3 and generalized to annular duct propagation, 4–7 and thanks to empirical spectrum model assuming isotropic homogeneous turbulence (IHT). On the other hand, unsteady computations of a full rotor-stator stage are practically out of reach and are more devoted to quite simplified configurations. Indeed, rotor-stator applications based on Large Eddy Simulations (LES) or Detached Eddy Simulations (DES) are generally restricted to quasi 3-D simulations over a limited spanwise extent and by enforcing periodicity conditions over a single blade-vane channel. 8–10 Starting from the knowledge of the turbulent pressure over the vane surface, acoustic radiation can be theoretically obtained by means of a FWH-based formulation. However, the non-physical periodicity conditions of inflow turbulence as well as the radial strip restriction appear to be too restrictive for accurate predictions. Recently, the lattice Boltzmann Method (LBM) has been firstly applied to turbomachinery noise predictions with an impressive direct acoustic simulation11, 12 including the full rotor-stator stage and the wind tunnel walls. In the present paper, an alternative method is suggested to provide a realistic description of the turbulent wake of a modern configuration from Snecma, based on the computation of an isolated blade row (one blade channel) with a full 3-D DES. Although the sound radiation is still described by a simple response model using an Amiet-based code developed by Onera, the focus is set on the analysis of representative turbulent structures impacting an idealized OGV. Simulations are performed using the Onera solver elsA and using a Zonal Detached Eddy Simulation (ZDES) approach initially developed by Deck. 13 Just as DES method, a continuous transition between RANS and LES is checked, but a more flexible choice adapted to the nature of the local flow is permitted with the ZDES approach. The objectives here are twice:(i) firstly, to check the consistency of isotropic homogeneous turbulence (IHT) assumption used in the current RANS-based predictions tools by comparing with present LES-based turbulence spectra (without any assumption);(ii) secondly, to evaluate the accuracy benefit of a ZDES-Amiet coupling methodology, since the CPU cost is considerable compared with a usual RANS approach.

The simulation results are compared with available experiment issued from a Snecma aero-engine test rig (RACE rig, at Saclay in France) providing hot-film radial scanning in the rotor-stator interstage plane and wall-mounted microphone …