Conclusions and Future Work

1. At midspan, the differences in the stator exit ft>w boundary conditions between stage and stator only experiments are more significant for the vane passing frequency than in the Low Engine Order frequency. Closer to the endwalls, the differences become remarkable.

2. None of the modeling approaches match the measured low engine order exit flow. Only the global behavior is represented by the computations.

The single passage models of minimum and maximum spacing overes­timate the exit ft>w variation at 20% and 50% of blade height. At 90% of blade height, the shift of significance from 1st harmonic to the spa­tial average indicate the different physical effect causing the low engine order.

The linearized method overestimates the variation of stator exit fbw. The correct prediction of viscous effects as the wake deficit can not be expected, and a reasonable applicability is only granted when viscous effects are negligible for the rotor blade excitation. Future investigations will clarify this.

Both multi passage viscous methods give similar results even though the codes and the modeling approaches have been different. This gives confidence in the results of this type. Nevertheless, the calculated 5th EO amplitudes are significantly smaller over the whole span than those of the experimental data. The reason for this discrepancy is not yet clear – Since different scaling approaches, different meshes, and differ­ent solvers were used and still very similar results were achieved, these parameters do not seem to have a large infhence on the results. Possi­ble reasons could be connected to the large differences observed in the predicted and measured secondary fbws.

The largest differences between the models and the experiments have been detected at 90% span. Future modeling refinements will investigate if and how these flow variations can be computed, and what influence they have on the low engine order excitation of the rotor.

From the present analysis it is concluded that the physical effect causing the low engine order variation is probably the higher flow acceleration in a smaller pitch passage. At 90% span, the tip secondary ft>w seems to be affected by the pitch variation causing large changes in the stator exit ft>w field. Future analyses of the ft>w field will clarify this.

The computed harmonics will be used for unsteady rotor computations to estimate the unsteady rotor blade pressure, which will be compared to measured data as soon as available. The importance of different physical effects on the low engine order excitation will be estimated.

Acknowledgments

This work was initiated and supported by the European Community under the "Competitive and Sustainable Growth” Programme (1998 – 2002) in the Project "Aeroelastic Design of Turbine Blades II" (ADTurBII), contract num­ber G4RD-CT – 2000-00189.

The opportunity to use the codes SliQ provided by Rolls-Royce plc and Vol – Sol provided by Volvo Aero Corporation, Sweden is gratefully acknowledged. We also want to thank all collaborators who contributed to the present work, especially Jeff Green and Robert Elliott from Rolls-Royce plc for their project management and support with ideas and Christian Lenherr from ETH Zurich for the multi passage VolSol computations during his student project.

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