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The effects of periodic wake disturbances on flat-plate turbulent boundary layers Holland, Robert M.

Abstract

Compressor and turbine sections of axial flow turbomachines are subject to periodically disturbed flows. The phenomenon is due to upstream blades creating unsteady periodic wakes that interact with the boundary layer and free stream flows about downstream blades. This study modelled these flow conditions, with circular cylinders periodically traversing in front of a turbulent flat plate boundary layer inside a low speed wind tunnel. These cylinder wake disturbances simulated the unsteady flow pattern in the compressor section of an axial flow turbomachine. Both time-averaged and ensemble-averaged data were interpreted in this study. The free stream wake disturbance was found to be similar to simple analytical predictions andthe boundary layer is believed to be responsible for a phase lag of the cylinder wake. The ensemble-averaged turbulence intensity increases dramatically with periodic unsteady flow, causing an increase in local coefficient of friction, boundary layer thickness, momentum thickness, and displacement thickness. Vortices are believed to be the predominant structure in the unsteady cylinder wakes. The vortex structures and turbulence effects tend to break down and dissipate as the flow proceeds downstream. Vortex theory can be used to explain and predict the velocity deficit and phase angle profiles seen in this study.

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