Three dimensional structure of the unsteady wake of an axisymmetric body

Abstract

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

Salient three-dimensional features of the turbulent wake of a simplified square-back vehicle

“…This is confirmed here, as an approximately 12 % drop is observed in the energy level of the first POD mode when wheels are added to the model (figure 9b). The weakening of the bi-stable lateral symmetry-breaking mode, following the loss of symmetry in the vertical direction, agrees also with the findings of Barros et al (2017), Bonnavion & Cadot (2018) and Pavia et al (2019b).…”

“…Two of the wings were mounted vertically at the start of the contraction whilst the third wing was mounted ≈ 30 mm from the floor at the start of the working section, to seed the model underbody (figure 1d). With the seeding system in place, a limited increase in the free-stream turbulence intensity is expected, although this has been shown to be relatively limited by Pavia et al (2019b). The seeding arrangement allowed a stream-tube with a square cross-section of ≈ 400 mm × 400 mm (in the test section) to be seeded.…”

“…A similar level of uncertainty is also observed at the corners of the volumetric field of view and close to the model base (for x * 0.25), as a consequence of the worsening of the signal-to-noise ratio. These regions of poor signal-to-noise ratio have been excluded from the analysis of the results, following the approach applied in Pavia et al (2019b).…”

“…In the present case, the 'snapshots method' proposed by Sirovich (1987) has been applied, due to its higher computational efficiency when the temporal domain is smaller than the spatial domain. Examples of application of POD to TPIV data can be found in Ceglia et al (2014) and Pavia et al (2019b). More information concerning the application of such a technique to fluid dynamic investigations is provided in Taira et al (2017).…”

Proper orthogonal decomposition analysis of coherent motions in a turbulent annular jet

“…On the other hand, in the laminar regime, Del Taglia et al [13] found that the occurrence of this flow asymmetry is influenced by the jet blockage ratio and the Reynolds number, which can be integrated as a state parameter. Similar kinds of asymmetric flow patterns were also observed in other flow configurations, such as in turbulent wakes past blunt bodies [17][18] .…”