Effects of surface roughness on the two-dimensional flow past circular cylinders I: mean 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 which types of surface roughness are more efficient in triggering a transition of the flow so as to simulate the mean and fluctuating pressures occuring at ultracritical Reynolds' numbers (when the Kármán vortices reappear). This experimental study, with models placed in smooth nearly two-dimensional flow, was carried out using the wind tunnel of the Universidade Federal do Rio Grande do Sul (UFRGS). The results and conclusions of the first part of the tests, in which the mean values of the force and pressure coefficients were measured in the range 50 000 ⩽Re⩽400 000, are presented in this paper. The models had an aspect ratio l/ d=6.1 and the tests were made with three types of surface roughness (sand paper, wire mesh screen and ribs), each with four different roughness heights (0.0018 ⩽ κ/ d ⩽ 0.0123). All the roughness elements were applied to the whole surface of the CC. All roughness types proved to be quite efficient in triggering regime transitions and in allowing the establishment of the ultracritical regime. In this regime, however, the greater the relative roughness, the greater were the differences observed between the parameters (such as drag or pressure coefficients, separation angle, etc.) measured on the rough CC (at low Re values) and those observed on the smooth CC (at high Re values). The smallest relative differences were observed on the model with ribs.