Experimental study of flow through a cluster of three equally spaced cylinders

Flow through a cluster consisting of three equally spaced circular cylinders was studied experimentally. The spacing ratio between the cylinders was P/D=1.35 and the operating Reynolds number was 2.1×103. Experiments were performed using flow visualization, particle image velocimetry, and laser Doppler velocimetry for a range of cluster orientation angles (0°⩽α⩽60°). For all cluster orientations, the results show large scale wake vortex shedding about five cylinder diameters downstream of the cluster. The large scale shedding frequency, when scaled by the projected height of the cluster, is equal to that of a single cylinder with equivalent diameter. For all cluster orientations, except α=60°, the wake is asymmetric. Jets forming between the cylinders lead to the formation of two distinct wake regions: narrow and wide wakes behind the cluster. For α=0°, a bi-stable wake regime is present in which the jet flow can be directed towards either of the two downstream cylinders, with no intermittent switching between the two wake configurations. For α=60°, the wake is symmetric about the mid-plane of the cluster, with narrow wakes behind the two upstream cylinders and a wide wake behind the downstream cylinder. For all orientations, small scale vortices form in the narrow wake and combine to produce large scale structures downstream. The results provide quantitative insight into the development of coherent structures in the near wake and their role in the formation of dominant vortices shed downstream of the cluster.