Effects of surface roughness on the wake-induced instabilities of two circular cylinders

Wind tunnel tests are conducted to investigate the wake-induced vibrations (WIVs) of two circular cylinders with a centre-to-centre spacing of 4 times the diameter and attack angles varying from 0 degrees to 20 degrees for Reynolds numbers between 18,000 and 168,800. Two types of circular cylinders with different surface roughnesses are adopted, and the effects of the surface roughness and Reynolds number on the WIVs are examined. The results show that increasing the surface roughness of a cylinder changes the dynamic characteristic of the WIVs significantly. Two divergent types of wake-induced instability phenomena of distinct dynamic characteristics, namely oscillation trajectories, maximum amplitudes, and evolution of the vibrations, are observed. Changing the roughness of the upstream cylinder only has a smaller effect on wake-induced instabilities than changing the roughness of the downstream cylinder only. For a downstream cylinder with a rough surface, the cylinder only oscillates in a limited velocity range and exhibits "velocityrestrained vibrations" rather than divergent vibrations. Furthermore, it is found that the Reynolds number has strong effects on the wake-induced instabilities of the downstream cylinder, and the possibility that a cylinder experience instabilities is much lower at higher Reynolds numbers (Re>80,000) than at lower Reynolds numbers (Re<80,000). (C) 2019 Elsevier Ltd. All rights reserved.