This paper discusses the importance of the unsteady interaction in transonic turbomachinery stages. Although the flow in a turbomachine is inherently unsteady, most current calculations for routine design work exploit the steady state assumption. In fact, unsteady flow effects are often taken into account for mechanical integrity checks, such as blade flutter or forced response, or heat transfer issues associated with circumferential nonuniformities, whereas steady state calculations are usually selected for the aerodynamic design. In this work, some cases are discussed in which significant departures are found between steady and time-averaged results, and the basic fluid mechanisms responsible for them are examined. Finally, a current perspective of unsteady computational fluid dynamics (CFD) calculations for the aerodynamic design is given.

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