Base-drag reduction of an axisymmetric bluff body through boundary-layer and near-wake modifications.

In the present work, the main findings of an experimental and numerical research activity aimed at characterizing and reducing the base drag of bluff bodies are presented. We consider the flow around an axisymmetric body, which can be viewed as a simplified model of a road vehicle. The results of experiments, LES and DNS simulations show that the base suctions - and thus the base drag - decrease in direct proportion to the increase of the length of the mean recirculation region behind the body which, in turn, can be obtained by increasing the boundary layer thickness before separation. Although the different set-ups and Reynolds numbers in the experiments and numerical simulations imply significant differences in the near wake dynamics, in all cases the length of the mean recirculation region present behind the body seems to be 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 results of the present analysis may be useful to devise further strategies for pressure drag reduction.