The Challenge of Unsteady Separating Flows

Unsteady effects produce strong or unusual changes in the flow separation characteristics of many modern fluid dynamic devices. The peculiar physical phenomena are described that are involved in unsteady flows past cylinders, airfoils, wings, and rotating blades and in unsteady flows within turbomachines and diffusers. The recent fundamental research on these topics is reviewed briefly. The combination of unsteadiness and flow separation manifests itself in vortex shedding, buffet, dynamic stall, stall flutter, shock-wave boundary-layer interaction, compressor stall, and transitory stall in diffusers. These complicated fluid dynamic processes, which are extremely difficult to predict, often produce undesirable fluctuating forces, vibrations, and aeroelastic instabilities. Much additional research and engineering efforts are needed to develop the tools for predicting and controlling the effects of unsteady flow separation.