This paper aims at underlining the existence of some generic properties of windmilling flows, partially spread in the literature but never clearly stated. Two kinds of axial machines are investigated from compressor mode to highly loaded windmill: a conventional fan with poor turbine performance and an optimized fan able to reach high efficiencies in both compressor and windmilling operations. Both simulations and experiments are used to perform the analysis. Three particular behaviors were identified as typical of fans operating at windmill: the inverse stacking of the speed lines visible in (Π, Qm) turbine maps, the appearance of a slope change on the loading-to-flow coefficient diagram at windmill, and a threshold effect occurring at highly loaded windmill.

Achieving the objectives of the American and European long-term air transport in terms of emission reduction requires to shift to hybrid/electric aircraft by 2050. This implies the development of distributed propulsion systems using multiple low-diameter lightly-loaded shrouded rotors, operating at high rotational speed and moderate flight Mach number. The relevance of characteristic maps of such a propulsive system is discussed in this paper. It is based on an integrative approach, that takes advantage of the two different formalisms generally found when dealing with aeronautical propulsion : either propeller diagrams (external aerodynamics) or turbomachinery maps (internal aero-dynamics). It outlines the main parameters representing the shrouded rotors performance and the parameters that drive them in a self-similar way. This is done by dimensional analysis, taking into account the geometric and operating parameters of both the rotor and the shroud and their combined variables. The self-similarity is observed with potential flow and RANS computations, which credits the proposed formalism as a powerful tool for scaling the shrouded rotors.

The determination of the rotational speed and massflow of the fan of a turbofan at windmill is critical in the design of the engine-supporting structure and the sizing of the vertical stabilizer. Given the very high bypass ratio obtained at windmill, the flow in the fan stage and bypass duct is of prime interest. Classical CFD simulations have been shown to predict such flows accurately, but extensive parametric studies can be needed, stressing the need for reduced-cost modeling of the flow in the engine. A Body Force Modeling (BFM) approach for windmilling simulations is examined in the present contribution. The BFM approach replaces turbomachinery rows by source terms, reducing the computational cost (here by a factor 5). A shaft model is coupled to the BFM source terms, to drive the simulation to a power balance of the low-pressure shaft. The overall approach is thus self-contained and can predict both the massflow and the rotational speed in the windmilling regime. Comparisons with engine experimental results show the proposed model can predict the rotational speed within 7 %, and the massflow within 5 %. Local analysis and comparisons with experimental data and reference blade calculations show that the work exchange, in term of total temperature variation, is predicted within 0.5 K, and the overall total pressure ratio within 1 %. However, the losses in the stator are largely underestimated, which explains the discrepancy for the massflow predictions.