This paper documents an investigation into unsteady flow in a three-dimensional oscillating turbine cascade with emphasis on the influence of tip clearance. Systematic experimental measurements were performed on a low-speed turbine cascade rig. The cascade consists of seven prismatic turbine blades, with the middle blade being driven to oscillate in a three-dimensional bending/flapping mode. Blades were instrumented with pressure tappings at six span-wise sections to facilitate three dimensional steady and unsteady pressure measurements on the blade surface. The steady pressure measurements are complemented by CFD simulations. Both are in a good agreement and indicate a marked local pressure suction peak at 70–90% chord on the suction surface resulting from the tip-clearance vortex. The measured unsteady pressure shows that this tip-clearance induced suction peak has a significant destabilising influence on the aerodynamic damping at a large tip-clearance (5% chord). Whilst at a small tip clearance (1.25–2.5% chord), the tip-clearance actually has a stabilising effect. The behaviour is in line with a quasi-steady analysis.

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