Rear Passive Control of Accelerating Bluff Bodies

We investigate the transient dynamics of the wake induced by a constantly accelerated D-shaped bluff body, starting from rest and reaching a permanent regime. We explore the effect of acceleration and rear geometry: a blunt base, a straight cavity and an optimized, curved cavity, obtained by adjoint optimization. TR-PIV was performed in a towing tank to characterize the near wake development in the early stages. Firstly, the wake is symmetrically developed with a pair of primary eddies attracted toward the body base. Eventually, the interaction between the upper and lower shear layers provokes the flow destabilization and the symmetry breaking, giving rise to a transitional vortex shedding regime. This process is sped-up when the curved cavity is used. In particular, the optimized geometry has been shown to limit the growth of the primary eddies, decreasing the recirculation region and providing with a more regularized transient vortex shedding. Finally, numerical simulations have been performed to evaluate this passive control in terms of drag evolution. Thus, the curved cavity produces a smaller averaged drag, together with less energetic fluctuations, regardless of acceleration.