Forcing function and steady loading effects on unsteady aerodynamic gust response

This research was directed at understanding two issues facing the turbomachine aeroelastician: (1) defining the relevant aerodynamic forcing function to a blade row; and (2) predicting the resulting unsteady aerodynamic blade forces. Utilizing rotating rows of perforated plates, circular cylinders, and airfoil cascades, unsteady periodic flow fields were generated. The measured unsteady flow fields were compared to the linear-theory gust profile. Wakes generated by fundamentally different phenomena do not generate similar forcing functions and the fundamental gust modeling assumptions inherent in linear theory analyses are not valid. When the experimental forcing function exhibited the profile of a linear-theory gust, the resulting response on the downstream stator airfoils was in excellent agreement with the linear-theory models. The complexity of the forcing functions and the resulting pressure responses increases from perforated-plate, to circular-cylinder, to airfoil-cascade-generated unsteady flow fields. The airfoil-cascade forcing functions and pressure responses correlated poorly with both the linear-theory gust profile and response predictions. The effect of steady-unsteady flow field interaction was investigated by utilizing symmetric and cambered airfoils at various angles-of-attack. The time-averaged loading in the unsteady flow fields was found to be equal to the steady loading in steady flow fields for the lower angles-of-attack where stalling was not evident. The unsteadiness in the flow field was shown to delay the onset of stall, with large flow field unsteadiness delaying stall more than small flow field unsteadiness. Similar unsteady chordwise pressure response trends appeared at various angles-of-attack depending upon the level of steady loading, but the chordwise trends were independent of the airfoil profile or cascade solidity. A linearized, incompressible, thin-airfoil, perfect-fluid theory analysis was developed. This analysis models the effects of solidity, stagger angle, interblade phase angle and reduced frequency and is capable of predicting the unsteady aerodynamic response of cascades detuned by alternate blade spacing, chord lengths, leading edge locations, and elastic axis locations. Predictions from this model correlated well to response data from an alternate blade-spacing detuned cascade. (Abstract shortened with permission of author.