StereoPIV analysis of the near wake of a square-back bluff body

The automotive market is, nowadays, one of the most competitive and innovative in the eld of transportation: optimization, e??ciency and emission reduction are probably three of the main key concepts. Generally, a vehicle can be treated as a blu body, even if years and years of aerodynamic development lead to a drastic improvement in drag and e??ciency. Nev- ertheless, city cars and small/medium cargo vans are tipically known for their high level of practicality, linked to the characteristic square-back shape. As a consequence, this kind of blu bodies tend to develop big and complex three dimensional wakes and, therefore, a very high level of drag. Therefore, the comprehension and in-depth knowledge of the structure of the wake and its characteristic dynamics, often complex, unsteady and unstable, is the basis of the development and optimization process of square-back vehicles. Having investigated and understood, therefore, the nature of the wake, it is also possible to evaluate possible methods of aerodynamic control, whether passive or active, in order to reduce the contribution of the wake to the overall drag of the vehicle. The present study is experimental: a blu body reproducing a medium sized cargo van will be used. In order to evaluate the forces acting on the body a balance will be used, while the characterization of the vehicle ow eld will be possible by means of an optical technique. That said, the main topics addressed in this study are the following: ˆ A complete characterization of the wake and its unsteady behaviour, in terms of shape, direction, energy, momentum, turbulence, vorticity, characteristic modes and so on, both in a quantitative and qualitative manner. Wind tunnel experimental testing is ideal, with the test section equipped with a stereoscopic PIV (particle image velocimetry) measurement system; ˆ A statistic study of all the near wake and its vortical structures, including cross/auto- correlation between the areas of greatest in uence of the wake; ˆ The time dependent evolution of the wake, studied by means of probability distribution functions related to vorticity magnitude and location; ˆ The identi cation of the most important dynamic modes involved and the casual relation- ship. The eventual presence of bistability and the general analysis of the modes developed in the wake can be assessed by means of the POD (proper orthogonal decomposition) method applied to the stereoPIV measurements. Once the natural behaviour of the wake is known and described, we will focus on the aero- dynamic active control of the wake through blowing jets. We will de ne the conditions of maximum e??ciency and maximum drag reduction as a function of the jets arrangement and 1 2 their intensity. Air jets are continuous and located on each edge of the rear face of the van: the blowing will be convergent, forcing the wake to stay in a smaller volume. It is, therefore, possible to repeat the same analysis as above in order to evaluate the level of drag for each case and estimate an eventual energy saving.