The experiments on two tandem circular cylinders were conducted in a horizontal soap film tunnel for the Reynolds number $\text{Re}=60$, 80, and 100 and the nondimensional center-to-center spacing $\Gamma$ ranging in $1\sim 12$. The flow patterns were recorded by a high-speed camera and the vortex shedding frequency was obtained by a spatiotemporal evolution method. The secondary vortex formation (SVF) mode characterized by the formation of a secondary vortex street in the wake of the downstream cylinder was found at large $\Gamma$. Moreover, some typical modes predicted by previous investigations, including the single bluff-body, shear layer reattachment, and synchronization of vortex shedding modes, were also revisited in our experiments. Further, numerical simulations were carried out using a space-time finite-element method and the results confirmed the existence of the SVF mode. The mechanism of SVF mode was analyzed in terms of the numerical results. The dependence of the Strouhal number Sr on $\Gamma$ was given and the flow characteristics relevant to the critical spacing values and the hysteretic mode transitions were investigated.

• Received 22 June 2009

DOI:https://doi.org/10.1103/PhysRevE.81.036305