  Wakes of upswept afterbodies are often characterized by a counter-rotating
streamwise vortex pair. The unsteady dynamics of these vortices are examined
with a spatio-temporally resolved Large-Eddy Simulation dataset on a
representative configuration consisting of a cylinder with an upswept basal
surface. Emphasis is placed on understanding the meandering motion of the
vortices in the pair, including vortex core displacement, spectral content,
stability mechanisms and overall rank-behavior. The first two energy-ranked
modes obtained through Proper Orthogonal Decomposition(POD) of the
time-resolved vorticity field reveals a pair of vortex dipoles aligned
relatively perpendicularly to each other. The dynamics is successfully mapped
to a matched Batchelor vortex pair whose spatial and temporal stability
analyses indicate similar dipole structures associated with an |m|=1 elliptic
mode pair. This short-wave elliptic instability dominates the meandering
motion, with strain due to axial velocity playing a key role in breakdown. The
low frequency of the unstable mode (Strouhal number StD =0.3 based on cylinder
diameter) is consistent with spectral analysis of meandering in the LES. The
wake is examined for its rank behavior; the number of modes required to
reproduce the flow to given degree of accuracy diminishes rapidly outside of
the immediate vicinity of the base. Beyond two diameters downstream, only two
leading POD modes are required to reconstruct the dominant meandering motion
and spatial structure in the LES data with < 15% performance loss, while ten
modes nearly completely recover the flow field. This low-rank behavior may hold
promise in constructing a reduced-order model for control purposes.
