  Direct numerical simulations at Re = 200 have been conducted of the flow past
rows of tandem cylinders. Local stability analysis shows that the wakes of two
tandem cylinders are characterised by the formation of a region where the mean
flow is locally absolutely unstable immediately behind the second cylinder,
followed by a long region that is convectively unstable. The location where the
flow changes from absolutely to convectively unstable provides a prediction of
where the placement of a third body could trigger a global change, which is
confirmed with simulations of the flow past three cylinders, and the flow past
two cylinders followed by a short flat plate. A third body placed downstream of
the absolute/convective instability transition location is effectively cloaked,
its presence having virtually no impact on the flow both upstream and
downstream. However, when a body is placed upstream of this location it
triggers a global change in the flow, its presence being broadcast throughout
the flow domain. However, a third cylinder placed well upstream of this
location does not trigger the global change. Sensitivity analysis of the mean
flow is conducted, and it is shown that the third cylinder does not simply act
as a small perturbation that can excite sensitive regions, but when it is very
close to the second cylinder it induces a mean flow correction that eliminates
the sensitive regions which may explain why it does not trigger the global
change.
