Particulars of the Cross- and Spanwise Near-wake Development of a
Short Semicircular-section Shell, through the Transition Re-range (60 ≤
Re ≤ 600)
Affiliations: University of Poitiers - Laboratoire dEtudes
Aérodynamiques - UMR CNRS 6609, 86960 Futuroscope cedex - France. e-mail
: [email protected]
Abstract: Within the fundamental context of quasi-2D bluff-body near-wake
investigation, we analyse the way the initial wake vortices (primary and
secondary vortices) form and develop with time behind a short (5.2:1)
cylindrical semicircular shell (hollow at the back), after an impulsive start
in translation. The study includes the transition regime and non-usual
cross-sectional shapes. The structure of the vortices is examined by means of
precise visualizations both in the mid-cross and mid-span planes. The
particulars induced by the hollow face are deduced from a comparison with the
cases of analogous solid cylinders and more particularly with the full
semicircular cylinder (Boisaubert et al., 1996; Coutanceau et al., 1998). The
crosswise analysis shows that, except some fundamental difference at the
separation initiation and some differences in details, a similar development of
the Bénard-Kàrmàn-vortices is observed, thus : - the
regime of the occurring structures changes beyond the first phase of
development (i.e. t* ≥ 6) when Reynolds number is passing through a
critical value Re_c (here 120 ≤ Re_c ≤ 140) - Kelvin-Helmholtz
instabilities are detected for Re = 600 in the rolled-up separated shear layers
originating from the sharp body-edges. On the contrary, the spanwise analysis
reveals that the hollow face plays an important role in the time-development of
the induced body-end three-dimensionalities (end-vortices and opposite spanwise
currents) and intrinsic three-dimensionalities respectively. Thus, as opposed
to solid bodies similarly limited in span, the emergence of clear patterns of
secondary Taylor-Görtler-like vortices in the near wake have been
experimentally proved for the first time when Re passes beyond the transition
value of Re_t ≈ 200. The consequence on the global structure of the flow
is deduced.
Keywords: three-dimensional formation, cylindrical shell, cross and spanwise visualization, primary and secondary instabilities