Vortex formation in the near wake of shallow flow past a vertical cylinder can be substantially delayed by base bleed through a very narrow slot. The structure of the wake associated with this delay changes dramatically with the dimensionless momentum coefficient of the slot bleed. At very low values, where substantial vortex delay is attainable, the bleed flow is barely detectable. For progressively larger values, various forms of jets issue from the slot, and they undergo ordered, large-amplitude undulations, not necessarily synchronized with the formation of the large-scale vortices. When the cylinder is subjected to appropriate rotational perturbations, in the presence of small-magnitude base bleed, it is possible to transform the delayed vortex formation to a form characteristic of the naturally occurring vortices and, furthermore, to induce a large change of the phase, or timing, of the initially formed vortex, relative to the cylinder motion.

These features of the near-wake structure are assessed via a technique of high-image-density particle image velocimetry, which provides whole-field patterns of vorticity, Reynolds stress, amplitude distributions of spectral peaks, and streamline topology at and above the bed, for both the delayed and recovered states of the wake. Among the findings is that even small bleed can substantially alter the patterns of streamline topology and Reynolds stress at the bed, which has important consequences for the bed loading.

These alterations of the near-wake structure occur in conjunction with modifications of the shallow approach flow, which is incident upon the upstream face of the cylinder. The topology at the bed, which is altered in accord with attenuation of the well-defined vorticity concentration of the horseshoe (standoff) vortex, shows distinctive patterns involving new arrangements of critical points.