Near wake coherent structures of a turbulent axisymmetric bluff body wake

The coherent structures of a turbulent axisymmetric bluff body wake are investigated based on synchronised near-wake velocity and base pressure measurements. The proper orthogonal decomposition (POD) analysis confirms the persistence of the laminar spatio-temporal symmetry breaking instabilities at high Reynolds numbers (here $R{e}_D=1.88\times 10^5$). The laminar rotational symmetry breaking mode randomly meanders in the azimuthal direction, and the unsteady laminar unstable eigenmodes manifest as asymmetric unsteady vortex shedding. Additionally, a coherent streamwise wake pulsation is identified (bubble pumping). Based on the symmetry-breaking property of the turbulent wake, the vector field $q$ is decomposed into two antisymmetric components $q^{+}$ and $q^{-}$ to perform conditional POD (CPOD) with a comparison of ${q{}^{\prime }}^{+}$ and $q^{+\prime }$, extracting antisymmetric modes in the stable asymmetric wake states. The asymmetry of the wake due to rotational symmetry break, quantified using the centre of pressure (CoP), is correlated to the base pressure using conditional averaging. The most probable symmetry-breaking wake state corresponds to a low-pressure (high drag) region, and two high-pressure (low drag) regions at the limit of axisymmetric (CoP $\to 0$) and highly asymmetric (CoP $\to \infty$) wake states are identified. The high-pressure wake state in the highly asymmetric wake is caused by the backflow, which results in a high-pressure region near the base edge. Conditional averaging based on the base pressure shows that the transition between high- and low-pressure conditions is coupled with wake asymmetry.