Design Strategies to Mitigate Unsteady Forcing (Preprint)

Abstract : The ability to predict accurately the levels of unsteady forcing on turbine blades is critical to avoid high-cycle fatigue failures. Further, a demonstrated ability to make accurate predictions leads to the possibility of controlling levels of unsteadiness through aerodynamic design. Several ingredients were essential to the success achieved in this study. First, judicious post-processing of CFD solutions was required to ensure that proper periodicity was achieved, and this was contingent upon an understanding of basic concepts in digital signal processing that are essential to the accurate calculation of unsteady forces on airfoils. Second, time-resolved predictions were subjected to a thorough and rigorous validation study for the physics observed in the turbine of interest in a relevant environment. Third, a clear understanding of the necessary steps to obtain the most accurate solution possible given the fidelity of the predictive system employed was required, and this followed naturally from knowledge gained in the validation study. Finally, it was pertinent to ensure that design changes to reduce forcing did not result in new sources of high unsteady loading. The ability to predict accurately the levels of unsteady forcing on turbine blades is critical to avoid high-cycle fatigue failures. Further, a demonstrated ability to make accurate predictions leads to the possibility of controlling levels of unsteadiness through aerodynamic design. This lecture presents a successful example of forcing-function prediction and control during the design cycle of a modern gas-turbine engine. 3D time-resolved computational fluid dynamics was used within the design cycle to predict accurately the levels of unsteady forcing on a single-stage high-pressure turbine blade.