Modification of subcritical cylinder flow with an upstream rod

Abstract

In the current research, a small circular rod is placed upstream of the main cylinder. This is used to change the incoming flow and, thus, impact the vortex shedding process of the wake as well as the resultant aerodynamic forces. The experiments are carried out in a wind tunnel with a subcritical Reynolds number of Re = 32 000. The sectional pressure distribution around the main cylindrical model is obtained to calculate unsteady aerodynamic forces. The experimental results show that the gap ratio markedly affects the aerodynamic forces exerting on the main cylinder. The optimal gap ratios for drag reduction are found to be G/D = 1.5 and 2.0, where the average drag and unsteady lift forces exerting on the main cylinder are suppressed. In addition to pressure measurements, we also apply the high-speed particle image velocimetry system to obtain flow characteristics of the cylinder wake as well as the gap flow. Two different modes of gap flow are found, i.e., the cavity mode and the wake impingement mode, and the transition value between them is around G/D = 1.5. Moreover, at the appropriate gap ratio, the wake vortex shedding pattern of the main cylindrical model changes from antisymmetric to symmetric.