Three dimensional structure of the unsteady wake of an axisymmetric body

The near-wake of an axisymmetric body has been investigated using base pressure tappings and 4 large scale Tomographic Particle Image Velocimetry (TPIV) at a Reynolds number of ReD = 5 3.2 × 105 , based upon model diameter. Insights into the near-wake dynamics are provided by the 6 application of Proper Orthogonal Decomposition (POD) to the pressure and the TPIV datasets. 7 The first two POD modes show that the axisymmetric topology seen in the time averaged field is 8 the result of the combination of different reflectional symmetry preserving states, each one featuring 9 a hairpin vortex surrounded by an annular structure developing in proximity to the wake closure. 10 The ‘head’ and the ‘tails’ of each hairpin vortex appear to be dynamically linked, as also proven by 11 the existence a second pair of modes, visible only in the TPIV dataset, featuring a twisted two-lobe 12 structure. The analysis of the temporal evolution of the radial position of the centre of pressure over 13 the model base reveals the existence of two different low-drag scenarios, characterised respectively 14 by the restoration of the axial symmetry or the selection of a single plane of reflectional symmetry. 15 The first state is reported to become the only admissible low-drag configuration when the short-time 16 wake dynamics are removed from the unsteady pressure signal.

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