Meandering dynamics of streamwise vortex pairs in afterbody wakes.

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.