The benefits of different film cooling geometries are typically assessed in terms of their time-averaged performance. It is known that the mixing between the coolant film and the main turbine passage flow is an unsteady process. The current study investigates the forms of unsteadiness that occur in engine-representative film cooling flows and how this unsteadiness affects the mixing with the mainstream flow. Cylindrical and fan-shaped cooling holes across a range of hole blowing ratios have been studied experimentally using particle image velocimetry and high speed photography. Coherent unsteadiness is found in the shear layer between the jet and the mainstream for both cylindrical and fan-shaped cooling holes. Its occurrence and sense of rotation is found to be controlled by the velocity difference between the mainstream flow and the jet, which is largely determined by the blowing ratio.

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