The present paper summarizes the main features regarding turbulent drag reduction via small narrow microgrooved surfaces, commonly referred to as riblets. The ability of such geometries to reduce turbulent skin friction has been well ascertained by numerous studies for these last two decades; their effectiveness has been tested under a variety of flow conditions. Besides the resulting tremendous amount of data, the way such grooves alter the turbulent friction is not as much documented since measurements inside these micro-geometry grooves are very difficult. The aim of this paper is to outline the main important features with the emphasis being placed on the latest results for various external flow conditions and to attempt to draw conclusions which will shed light on the understanding of the mechanisms of drag reduction by these passive manipulators.
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