Comparative early development of wake vortices behind a short semicircular-section cylinder in two opposite arrangements

As a first step in a more general study of the influence of the body shape upon the initial time-development of wake vortices, we consider the case of a 5.20 aspect-ratio semicircular-section cylinder, fitted with two endplates, and with the rounded side and the flat side in turn facing the oncoming current. The flow structure is analysed by means of a detailed qualitative and quantitative analysis of numerous flow visualization pictures, for Reynolds numbers Re ranging between 60 and 600. Beyond the first phase of development, necessary for the vortex-shedding process to take place (t* greater than or equal to 6), a change in the flow evolution with Re is found for both body configurations, at a critical Reynolds number Re, whose final value is, within about 5%, 190 and 140 for the rounded and flat fore body respectively. However, this change varies with the body shape. Thus, above Re = 200, it is shown that the reversal of the body compared to the free stream (flat-forebody configuration) implies a clear difference in the wake development with a quasi-symmetrical shifting of the vortex cores to the rear of the recirculating zone and a complete annihilation of the process of vortex shedding, at least during the limited period of time corresponding to the present observation. The consequences of the modifications of the wake behaviour are quantitatively evaluated by considering the time- and Re-evolution of the wake geometrical parameters and of the axial velocity distribution; they are related to the body geometry.