The significant impact of ribs and small-scale roughness on cylinder drag crisis

Abstract

The impact of flow-normal ribs and small-scale surface roughness on the drag and vortex shedding of a circular cylinder was investigated. Three rib heights, four relative rib spacings and three different forms of micro-roughness were combined to produce 28 unique surface coatings for the cylinder. The drag was measured in a wind tunnel for Reynolds numbers in the range 20,000<Re<160,000, representing nearly a decade change centred about the drag crisis. The drag measurements were complemented by hot-wire measurements in the wake to investigate the vortex shedding frequency. The results show significant average drag reduction, up to 23%, for most of the ribbed geometries compared to a smooth cylinder for Re<160,000. Increasing the rib height was found to reduce the critical Reynolds number and increase the minimum drag coefficient. Varying the rib spacing resulted in an ‘‘optimal” spacing, approximately five times the rib height, that caused the lowest critical Reynolds number. Increasing the micro-roughness resulted in a reduction in the critical Reynolds number and an increase in the minimum drag coefficient.