Wake dynamics of canonical symmetric bluff bodies

Bluff body flows are ubiquitous in everyday life and present inherent challenges, particularly in the separated recirculation zone. Increased understanding of wake dynamics can help to increase efficiency, minimise instabilities and facilitate model development for numerical simulations and flow control. One prominent feature which has gained significant interest recently are large scale, long time motions linked with symmetry breaking. The present work explores two canonical bodies with a particular focus on modal decomposition and non-stationary spectral methods to shed light on these behaviours.

Following previous tomographic PIV measurements of a fully axisymmetric body, eddy resolving numerical simulations were performed to provide additional detail of the multi-stable wake. Initially, a coarse grid Hybrid RANS-LES approach was applied though shear layer instabilities were weak which delayed separation and ultimately suppressed the meandering motions. By contrast, a wall modelled LES approach showed exploration of all azimuthal positions via a low frequency shift mode and confirmed the hairpin structure found in experiment. Fourier decomposition showed a dominance of the anti-symmetric, m=1 mode, which when coupled with a Conditional Space-Time POD revealed a strong radial flow inhomogeneity at wake closure. It was also shown that the azimuthal switching demonstrates a strong dependence on Fourier modes m=0 and 2.To explore the bi-stable wake in a new perspective, the fully-axisymmetric body was modified to form the half-axisymmetric body. The wake was characterised experimentally using PIV and base pressure measurements. Time-frequency representations enabled turbulent scales to be separated and reconstructed which showed switches across the symmetry plane are strongly linked to the reversal of an oblique wave at the vortex shedding frequency (St_VS). Tomographic measurements further confirmed these findings with the inclination of spanwise arch-vortices inverting following a switch event. A novel hybrid wavelet-POD was also introduced to extract spatio-temporally coherent modes which oscillate at the instantaneous frequency and possess temporally varying coefficients. Results reveal a low-rank structure in the range St → 0 and St_VS.