On critical transition of flow over a circular cylinder roughened by textile materials

Experiments were made in a low-speed wind tunnel to study the effect of textile roughness on the critical transition phenomenon of flows over a 2D circular cylinder and a finite circular cylinder whose aspect ratio is 4, respectively. First, a comparison of the results of the two cylinder models without textile material fitted shows that the critical transition phenomenon takes place at higher Reynolds numbers for flow over the finite circular cylinder. Second, concerning the results obtained with the three kinds of textile materials fitted on the two cylinders, it is consistently shown that the critical transition Reynolds numbers for flows over the finite circular cylinder are lower than those for flows over the 2D circular cylinder. This finding is opposite to that of the smooth models. Moreover, discussion on the instantaneous flow characteristics in the critical transition range can be made with the plots showing the distributions of the instantaneous lift and drag forces measured.