Stall flutter is an aeroelastic phenomenon occurring near the stall line for fans at part speed. In certain operating conditions, acoustic waves generated by blade vibrations can propagate in the intake and the reflection at the opening can lead to acoustic driven flutter, which can reduce flutter margin significantly. Therefore, evaluating the acoustic feedback of the intake on fan flutter has a great importance during the design process.

This paper aims to compare three numerical methods with different levels of fidelity to characterize this acoustic feedback. To do so, one can consider the reflection rate and the phase lag caused by propagation and reflection at the fan leading edge position. These quantities are used to assess the stabilizing or destabilizing effect of intake on fan flutter. The first radial order modes with an azimuthal order between one and three are studied. The geometry considered here is a realistic bellmouth intake.

The first method is based on an analytical model from the literature. This model is a fast low fidelity method which considers slowly variable sections of the duct and aerodynamic field variations in the axial direction only. Inputs are the geometry, farfield conditions, the massflow and an imposed modal acoustic source modelling the fan.

The second method is an FEM implemented in the acoustic solver Actran. The fan is modeled like in the analytical method. Non uniformity of the flow in the radial direction, significant around the spinner and the intake lip, is included in the simulation.

The third method is to run unsteady 3D CFD computations with the URANS solver elsA. The full-annulus computational domain includes the fan and the whole intake with a significant portion of the upstream domain. The blades vibrate on a given eigenmode and the mesh is deformed at each iteration. A wave splitting method is used to analyze the reflection at the intake opening depending on the mode and the operating point. These computations aim to bridge the gap between local aerodynamic phenomena around the vibrating blade and modal acoustic propagation through the intake.

The upstream and downstream propagating waves predicted by the three methods are compared. It appears that the analytical method overestimates the pressure amplitude ratio and predicts the phase lag with a low precision. The main problematic element seems to be the curved shape of the intake lip. Nevertheless, predictions are good enough to locate zones over the compressor map corresponding to a potentially destabilizing phase lag. On the other hand, the small discrepancies between Actran and elsA results validate the usage of an acoustic solver to locate the risky operating points. However, only the URANS simulations takes into account the fan response to the acoustic reflection on the intake opening, leading to the prediction of flutter.


Prediction of the Acoustic Reflection in a Realistic Aeroengine Intake With Three Numerical Methods to Analyze Fan Flutter

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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