The Response of Turbomachine Blades to Low Frequency Inlet Distortions

An analysis is presented of the unsteady lift on a cascade of airfoils moving through a circumferential inlet flow distortion. The flow model is based on blades of finite chord, but small pitch/chord ratio. Two separate methods are used to derive the unsteady lift on the blades, leading to the same analytical expressions in both cases. The analytical solution is compared with earlier investigations of the flow past isolated airfoils (Sears), with numerical results for airfoils in a cascade (Henderson/Daneshyar, Whitehead and Smith) and with actuator disk results. It is shown that the present simple model provides results that are consistent with the numerical calculations for small pitch/chord ratios. In addition, the present method provides a means for resolving a discrepancy between existing theories as to the behavior of the cascade lift coefficient at low reduced frequency.