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.
Highlights
The flow around circular cylinders and the resulting forces have been studied extensively over the last century
If a cylinder has ribs that are an order of magnitude larger than the micro-roughness superimposed on top of the ribs, unified parameterisations of the roughness, e.g., S10z from the ISO 25178 standard used by Hsu et al (2019), are strongly biased towards the ribs
The present study investigates the drag and vortex shedding of a 417 mm long circular cylinder with a diameter of d 1⁄4 75 mm
Summary
The flow around circular cylinders and the resulting forces have been studied extensively over the last century. After the roughness is applied with one of these methods, the surface with unknown drag is scanned to find the relative roughness ks= d, and compared to experiments of an artificially roughened surface with the same ks=d, to find the equivalent drag This is equivalent to using the Moody diagram for pipe flows (Cengel and Cimbala, 2014), and has been used with some success for circular cylinders as well (Achenbach, 1971; Hsu et al, 2019). If a cylinder has ribs that are an order of magnitude larger than the micro-roughness superimposed on top of the ribs, unified parameterisations of the roughness, e.g., S10z from the ISO 25178 standard used by Hsu et al (2019), are strongly biased towards the ribs This could result in numerous surfaces with effectively the same ks=d but very different drag. Correlations are drawn between the surface topologies and the drag of the body
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