Abstract
In the gap region of tandem cylinders, within the reattachment regime, bi-stability is seen to be cellular. Direct numerical simulations at a Reynolds number of 500, and a gap ratio of 3.0, show that shedding of gap vortices occurs in spanwise cells, with lengths between 0.3 and 2.7 cylinder diameters. These unstable vortex cells tend to be asymmetric with respect to the gap centreline, so that vortices are repeatedly shed from just one gap shear layer for several periods. The unstable cells appear within the basic spanwise cell structure dictated by the three-dimensional instability, and their cell lengths do not exceed those of the basic cells. Unstable cells intermittently become spanwise coherent, and this leads to a significant increase in drag amplitude. The mode change in the gap is associated with low-frequency variation of the reattachment and separation points on the downstream cylinder, causing low-frequency modulation of the vortex formation length.