  The radio lobes of the radio galaxy Hercules A contain intriguing ring-like
structures concentric with the jet axis. To investigate the occurrence of such
features, we have used hydrodynamic simulations of jets with a range of Mach
numbers (from M=2 to 50) and densities (down to a ratio of 1E-4 relative to the
background) to generate ray-traced images simulating synchrotron emission from
the time-dependent shock structures. We compare these images with observations
of Hercules A, and consider the physical nature and temporal evolution of the
most plausible configurations. We find that the observed ring-like structures
are well explained as nearly annular shocks propagating in the backflow
surrounding the jet. We infer that the jet is oriented at between 30 and 70
degrees to the line of sight, consistent with radio depolarisation observations
of Gizani & Leahy. The observational lack of hot-spots at the extremities of
the radio lobes, and the possible presence of a buried hot-spot near the base
of the western lobe, are explained in terms of the intrinsic brightness
fluctuations and dynamics of the terminal shock of an ultra-light, low Mach
number jet that surges along its axis due to intermittent pinching and
obstruction by turbulent backflow in the cocoon. We conclude from the
appearance of both sides of the Hercules A, that both jets are on the
borderline of becoming fully turbulent.
