On the Prediction of Unsteady Forces on Gas-Turbine Blades: Part 2 — Viscous-Wake-Interaction and Axial-Gap Effects

This paper is a contribution to the study of the generation of unsteady forces on turbine blades due to viscous wake interaction and potential-flow interaction from upstream blade rows. A computer program is used to compute the unsteady forces on a rotor. Typical results for isolated viscous-wake interaction (no potential-flow interaction) are shown. These results indicate that the first spatial harmonic of the unsteady force may decrease for higher stator-to-rotor-pitch ratios. This trend is explained by considering the mechanisms by which the unsteady forces are generated. The mechanism by which the viscous wakes affect the flow field to generate these unsteady forces is shown to vary with the stator-to-rotor-pitch ratio and with the outlet flow angle of the stator. It is also shown that by varying the axial gap between rotor and stator one can attempt to minimize the magnitude of the unsteady part of the forces generated by the combined effects of viscous-wake interaction and potential-flow interaction.