In the present study, the flow through the fan stage of a high bypass ratio turbofan at windmill is studied numerically. First, steady mixing plane simulations are validated against detailed experimental engine test-bed measurements, at several locations within the fan stage and close to the core/bypass flow splitter. Good agreement between the numerical and experimental results is obtained. A local flow analysis is proposed, evidencing several characteristics of the flow in windmilling: in the rotor, the size of the separation zone is found to increase from hub to tip, and in the stator, massive flow separation occurs at mid-span, which leads to the formation of two streamwise counter-rotating vortices. Then, the Nonlinear Harmonic (NLH) method is applied to a section (at 70 % of the relative span) of the fan stage. A modal analysis is performed, showing a specific behavior at windmill: the massively separated flows in the rotor and the stator entail strong rotor/stator interactions modes. Finally, the unsteady flow pattern is examined: the velocity defect of the rotor wake, 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. The implication of this qualitative flow behavior on the method to extract CFD results for comparisons with experiments is discussed.

This paper aims for the analysis of experimental and numerical results of windmilling flow topologies far from freewheeling condition. Two major cooling fans were investigated: a baseline design and an innovative one meant to reach good performance in both compressor and turbine modes. Experiments are conducted with global and local characterizations to determine energy recovery potential and local loss mechanisms. Also, tests were performed on a turbofan engine to confirm some trends observed on the cooling fans. The numerical study is carried out with mixing plane steady simulations, the results of which are in fair agreement with experimental data. The difference of local topology between freewheeling and highly loaded windmill demonstrates that classical deviation rules such as Carter's are not well-suited to highly loaded windmilling flows. Finally, under certain conditions, the minor influence of the stator on the rotor topology indicates that nonrotating elements can be considered as loss generators.

A detailed study of the air flow through the fan stage of a high-bypass, geared turbofan in windmilling conditions is proposed, to address the key performance issues of this severe case of off-design operation. Experiments are conducted in the turbofan test rig of ISAE, specifically suited to reproduce windmilling operation in an ambient ground setup. The engine is equipped with conventional measurements and radial profiles of flow quantities are measured using directional five-hole probes to characterize the flow across the fan stage and derive windmilling performance parameters. These results bring experimental evidence of the findings of the literature that both the fan rotor and stator operate under severe off-design angle-of-attack, leading to flow separation and stagnation pressure loss. The fan rotor operates in a mixed fashion: spanwise, the inner sections of the rotor blades add work to the flow while the outer sections extract work and generate a pressure loss. The overall work is negative, revealing the resistive loads on the fan, caused by the bearing friction and work exchange in the different components of the fan shaft. The parametric study shows that the fan rotational speed is proportional to the mass flow rate, but the fan rotor inlet and outlet relative flow angles, as well as the fan load profile, remain constant, for different values of mass flow rate. Estimations of engine bypass ratio have been done, yielding values higher than six times the design value. The comprehensive database that was built will allow the validation of 3D Reynolds-averaged Navier–Stokes (RANS) simulations to provide a better understanding of the internal losses in windmilling conditions.

Commercial transport fuel efficiency has improved dramatically since the early 1950s. In the coming decades the ubiquitous turbofan powered tube and wing aircraft configuration will be challenged by diminishing returns on investment with regards to fuel efficiency. From the engine perspective two routes to radically improved fuel efficiency are being explored; ultra-efficient low pressure systems and ultra-efficient core concepts. The first route is characterized by the development of geared and open rotor engine architectures but also configurations where potential synergies between engine and aircraft installations are exploited. For the second route, disruptive technologies such as intercooling, intercooling and recuperation, constant volume combustion as well as novel high temperature materials for ultra-high pressure ratio engines are being considered. This paper describes a recently launched European research effort to explore and develop synergistic combinations of radical technologies to TRL 2. The combinations are integrated into optimized engine concepts promising to deliver ultra-low emission engines. The paper discusses a structured technique to combine disruptive technologies and proposes a simple means to quantitatively screen engine concepts at an early stage of analysis. An evaluation platform for multidisciplinary optimization and scenario evaluation of radical engine concepts is outlined.

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.

This paper aims at evaluating wall-modeled LES capabilities to accurately predict turbulence properties in an aero-engine turbomachinery configuration. To do so, two LES numerical setups are compared, the first one using a 2nd-order space and time numerical scheme on a user-defined mesh, the second one using a 3rd order accurate in space and 4th order time numerical scheme on an automatically adapted mesh. Numerical results are compared to hot-wire anemometry measurements. Turbulence is evaluated through a triple velocity decomposition, enabling the evaluation of stochastic velocity fluctuations. A very fair agreement is evidenced regarding the numerically predicted turbulence spectra and integral turbulence time scale. Turbulence intensity values is however over estimated downstream of the stator, underlying the need for a second mesh adaptation.

This paper aims at validating LES capability if applied to an actual turbofan configuration at nominal regime, if compared to RANS and experimental measurements. For assessment, averaged radial profiles are compared in 3 axial planes-before the stage, between the rotor blade and stator vanes, and downstream of the stator. RANS and LES results are in very good agreement, but found to be shifted compared to the measurements and for some quantities. An analysis of the unsteady axial velocity is then proposed, investigating root-mean square of axial velocity. Tip-leakage, as well as two boundary layer transitions are evidenced in the rotor. An estimation of the integral turbulent timescale is finally proposed in the whole domain, using autocorrelation of the axial velocity. Suctionside horseshoe vortices are found to be very coherent, as well as the stator corner vortices. Regions of large timescale are moreover evidenced between rotor and stator wakes.

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