Effects of surface roughness on the two-dimensional flow past circular cylinders II: fluctuating forces and pressures

Abstract

Wind tunnel experiments on circular cylinders (CCs) in a cross-flow, at relatively low Reynolds numbers, are reported in this paper and an attempt is made to answer the question of which types of surface roughness are most efficient in triggering a transition of the flow so as to simulate the mean and the fluctuating pressures occuring at ultracritical Reynolds numbers (when the Kármán vortices reappear). This experiment study was carried out using the wind tunnel of the UFRGS and the results of the second part of the tests are presented in this paper. In this part, root mean square and peak fluctuating values of force and pressures, power spectral density and sectional and spanwise cross-correlations were measured on models with three types of surface roughness: sand paper, wire mesh screen and ribs. All models were tested in smooth flow, but the model with ribs was also tested in turbulent flow. In all cases the flow regime was ultracritical. The main conclusions were as follows. The fluctuating values measured on the rough CC (at relatively low Re values) were larger than those observed on the smooth CC (at high Re values) in the ultracritical regime. The results of power spectra indicate that flow organization is quite good and apparently similar in both cases. The results of cross-correlations confirm that rough CCs present good organization of vortex shedding, comparable with that observed in the subcritical regime.