Manipulation of 3D blunt body turbulent wakes: drag reduction and wake equilibrium

Combination of passive and active flow control are used to manipulate the turbulent wake of a 3D blunt body in order to analyse the impact of flow asymmetries on aerodynamic drag. An Ahmed-like body with a square-back is mounted in the test section of a wind tunnel to produce a canonical turbulent wake at Re = 500000. By using passive perturbations around the model or by changing its ground clearance, the large-scale asymmetry and dynamics of the unforced recirculation region are modified almost at will. Depending on the initial unforced equilibrium, additional pulsed blowing along all or selected edges of the base produces a very different impact on the drag. On top of a global boat-tailing effect resulting in drag reduction (up to 10%), the reorganization of the recirculation region equilibrium plays a key role in the observed drag changes. In particular the symmetrisation of the wake appears to be one of the main mechanisms involved in drag reduction. This provides key ingredients to adapt forcing strategies for drag reduction in presence of various wake asymmetries and topologies typical of the flow around such body. The change between wake types being essentially caused by minor geometric and flow conditions changes, this is essential for industrial automotive applications.