The effect of surface roughness on the flow regime around circular cylinders

Experimental research has been carried out to study the influence of surface roughness on transitions between the subcritical, critical supercritical and postcritical regimes around isolated circular cylinders in cross-flow. The experiments were performed in an open-jet wind tunnel, which was modified to obtain satisfactorily bidimensional conditions, with Reynolds numbers ranging from 2.6 × 10^4 to 2.8 × 10^5. The cylinders were covered with different types of standard commercial emery cloth, and the estimated relative roughness varied from 1.0 × 10^-3 to 12 × 10^-3. The signal from a hot-wire probe placed near the cylinder was analysed in order to ascertain the presence of vortex shedding and to obtain the Strouhal number. The mean pressure distribution around most of the cylinders were also measured, and then the drag coefficients were obtained by integration. The boundaries between the various flow regimes as a function of the degree of roughness and of the Reynolds number were tentatively established, assuming the critical regime to be characterised by the absence of regular vortex shedding, and the postcritical one by Reynolds-number independence. The results confirmed the striking influence of roughness on the flow regime; in particular, the critical regime can be impressively reduced, and it may even disappear in the case of highly roughened cylinders. The boundaries between the various regimes, as well as the supercritical and postcritical drag coefficients and Strouhal numbers, seem to be a function of both the size and the type of surface roughness. This is confirmation, therefore, that it is not possible to characterise the flow regime only by means of a Reynolds number based on the size of the roughness. Finally, it was demonstrated that strong supercritical and postcritical vortex shedding can take place from highly, but uniformly, roughened circular cylinders