• bluff-body
• CFD
• hairpin vortex
• near-wake
• numerical simulations

In the present work the main findings of a numerical research activity aimed at characterizing and reducing the base drag of bluff bodies are presented. The flow around an axisymmetric body with a sharp-edged base perpendicular to its axis is considered. Variational MultiScale Large Eddy Simulations carried out at Reynolds Re = 550000, based on the body length and the freestream velocity, corresponding to Re_d = 96000, based on the body diameter are analyzed. Direct Numerical Simulations are performed at a Reynolds numbers
roughly two orders of magnitude lower, i.e. Re_d = 1500 are also considered. VMS-LES and DNS simulations show that a decrease of the base suctions − and thus of the base drag − is directly proportional to an increase of the
length of the mean recirculation region behind the body. Indeed although the different set-ups and Reynolds numbers in the numerical simulations, in
all cases the data collapse on a single straight line when the base pressure is plotted against the length of the mean recirculation region behind the body. The lengthening of near-weak region, in turn, can be obtained by increasing
the boundary layer thickness before separation. Moreover, it is fundamental that the length of the mean recirculation region is connected with the location of the incipient instability of the detaching shear layers. It is shown that the
location of this instability can be moved downstream, and thus base drag can be reduced, by increasing the thickness of the separating boundary layer. The analysis of the instantaneous dynamic of the wake has been carried out for
all numerical simulations. The vortex indicator Λ2 is used. Hairpin vortical structures are found for on simulations. The DNS simulations at Re_d = 800
and Re_d = 1500 show a different wake dynamics. In particular, at the lower Re_d the flow is characterized by a planar symmetry and the plane of symmetry remains costant over time, while at the higher Re_d the flow is characterized by
vortical structures forming and evolving on planes rotating with time. These two different behaviors of the flow are consistent with the studies present in literature. The VMS-LES simulations show a wake dynamics similar to the are found at Re_d = 1500 in DNS, but the hairpin vortices form chose to the body base and shed at higher frequency.