This paper documents the effects of surface roughness on boundary layer transition in separation-bubbles under low free-stream turbulence conditions (<1%). The experiments were performed on a flat surface, upon which a pressure distribution similar to those prevailing on the suction side of low-pressure turbine blades was imposed. The test matrix consists of four variations in the roughness conditions, including a reference test case with a smooth surface. The remaining roughness levels are typical of in-service turbine blades in gas turbine engines. The measurements were performed at flow Reynolds numbers of 350,000 and 470,000, based on the length of the test surface. The separation, transition inception, transition completion, and re-attachment locations, and the streamwise intermittency distributions in the transition region are documented for each of the test cases. Increasing surface roughness is shown to result in earlier transition inception, and consequently, a reduced size of the separation-bubble. However, the presence of surface roughness does not appear to have a significant effect on the rate of transition within the separation-bubble.

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