The transition of the boundary layer subjected to unsteady wake-passing in a linear cascade of ultra high lift profiles has been investigated at the Avio Aerodynamics Laboratory. The blade profiles are representative of the turbine nozzle mid section of a long range aeroengine. Measurements were performed at the cruise Reynolds number. A surface hot-film array was adopted to survey the boundary layer nature and the periodic variations related to the passing wakes. A phase-locked ensemble averaging technique was employed in order to separate the random fluctuations from the periodic ones. Results have been represented in space-time plots in order to provide an overall view of the time-dependent phenomena in terms of the quasi wall shear stress statistical moments, that are important parameters for the analysis of the boundary layer transition and separation. Passive control devices may be adopted to suppress boundary layer laminar separation at critical conditions (low Reynolds numbers, ultra high lift profiles). In the present experimental investigation a wavy step device has been mounted on the suction side of the blade. The effects of this boundary layer control device on the transition process and profile losses have been investigated at cruise Reynolds number, with and without incoming wakes.

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