Nonlinear Harmonic Simulations of the Unsteady Aerodynamics of a Fan Stage Section at Windmill

In the present study, the unsteady flow through the fan stage of a high bypass ratio turbofan at windmill is studied numerically. The Nonlinear Harmonic (NLH) method is applied to a section (at 70 % of the relative span) of the fan stage. First, steady mixing plane simulations at windmill are used to perform a grid convergence study based on the prediction of the massively separated flow occurring on the lower side of both the rotor and stator due to highly negative angles of attack. The unsteadiness of the flow is then examined for the isolated rotor and stator, showing that, for this 2D case, negligible natural unsteady flow effects arise. This supports the use of the NLH method to account only for deterministic unsteady rotor/stator interactions. NLH simulations are then performed, and the influence of the number of harmonics is assessed, based on the analysis of wakes. Contrasting the results with the nominal operating point simulations shows that less harmonics are needed for the windmilling case: this is due to the much larger wake behind the rotor associated to massive separation at windmill, which is more conveniently represented by Fourier series than the sharp narrow wake of the nominal point. Finally, the unsteady flow pattern is examined: the velocity defect of the rotor wakes, which periodically increases the flow angle on the stator, is shown to trigger a periodic movement of the reattachment point at the trailing edge of the stator, associated with vortex shedding from the lower side of the vane.