Observation of the flow around rough-walled two-dimensional circular cylinders using infrared thermography

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The effect of surface roughness on the flow around a two-dimensional circular cylinder has been studied in a smooth and turbulent flow by means of a newlydeveloped flow-visualization technique using infrared thermography. The surface roughesses used in this study were provided by wrapping sand paper or by attaching ribs paralell to the cylinder axis. The locations of the transition from laminar to turbulent flow and of the separation of the laminar and/or turbulent flows were clearly detected by the flow visualization. The flow separation line was found to be approximately paralell to the cylinder axis regardless of the Reynolds number, from subcritical to transcritical range, within the limits of ks/D from 90×10-5 to 580×10-5, with k, and D being the equivalent sand grain roughness and the cylinder diameter, respectively. On the other hand, the flow transition was strongly affected by the degree and the local distribution of the roughness, in particular in the supercritical range; consequently, the transition line exhibited a wavy curve, which indicates a lack of the two-dimensionality of the flow. In the smooth flow, a steady asymmetric flow was observed in a Reynolds-number range for cylinders with ks/D less than about 300×10-5. The Reynolds-number range of the asymmetric flow became narrower with increasing ks/D.
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