ON NEAR-WALL TURBULENCE-GENERATING EVENTS IN A TURBULENT BOUNDARY-LAYER ON A RIBLET SURFACE

The boundary layer over a drag reducing riblet surface is investigated using hot-wire anemometry and flow visualisation. The concept of a 'r iblet sublayer' is introduced, and a definition is proposed in terms o f a region of reduced turbulence energy production formed near the wal l by the addition of riblets. The hot wire records are examined using a modified form of quadrant analysis, and results obtained over plain and riblet surfaces are compared. Close to the wall, the addition of r iblets produces a marked reduction in the occurrence of 'ejection' (2n d quadrant) events. A corresponding increase in the incidence of 'swee p' (4th quadrant) events is accompanied by the development of a strong tendency toward a preferred event duration, and a preferred interval between events. These changes diminish rapidly with distance from the surface, becoming almost undetectable beyond y+ = 40. They are discuss ed in the light of flow visualisation results, and interpreted in term s of mechanisms associated with the interaction between the riblets an d the inner boundary layer flow structures. A conceptual model of the flow mechanisms in the riblet sublayer is proposed.