This paper presents measurements of separation-bubble transition over a range of surfaces with randomly distributed roughness elements. The tested roughness patterns represent the typical range of roughness conditions encountered on in-service turbine blades. Through these measurements, the effects of size and spacing of the roughness elements, and the tendency of the roughness pattern toward protrusions or depressions (skewness), on the inception location and rate of transition are evaluated. Increased roughness height, increased spacing of the roughness elements, and a tendency of the roughness pattern toward depressions (negative skewness) are observed to promote earlier transition inception. The observed effects of roughness spacing and skewness are found to be small in comparison to that of the roughness height. Variation in the dominant mode of instability in the separated shear layer is achieved through adjustment of the streamwise pressure distribution. The results provide examples for the extent of interaction between viscous and inviscid stability mechanisms.

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