Particulars of the cross- and spanwise near-wake development of a short Semicircular-section Shell, through the TransitionRe-range (60 ≤Re ≤ 600)

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 Benard-Karman-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-Gortler-like vortices in the near wake have been experimentally proved for the first time when Re passes beyond the transition value of Re_t a 200. The consequence on the global structure of the flow is deduced.