Characterisation of the unsteady wake of a square-back road vehicle

Square-back shapes are popular in the automotive market for their high level of practicality. These geometries, however, are usually characterised by high aerodynamic drag and their wake flow dynamics present many aspects, such as the coexistence of long- and short-time unsteady modes, whose full comprehension is still far from being achieved. The present work aims to provide some contributions to this field. An extensive experimental campaign consisting of balance, pressure tapping, particle image velocimetry and single point velocity measurements has been carried out in order to characterise the dynamic behaviour of the wake developing downstream of a simplified square-back geometry. Tests have been performed considering the Windsor body, at a Reynolds number (based on the model height) of ReH = 7.7 × 10^5. New insights on how the long-time instability develops are provided. The instability is shown to stem from the mutual interactions between the four shear layers bounding the wake rather than being the result of the state of perturbation of a single shear layer. Changes in the level of interaction between two or more shear layers are also reported to affect the short-time unsteady modes. A drag reduction is reported every time the symmetry of the wake is restored, as a consequence of the increased amount of reverse flow impinging on the base of the model. This seems to be true regardless of the configuration considered (with or without wheels) and the type of optimisation strategy adopted, although it does not necessarily imply the complete suppression of the long-time instability. In fact, a certain level of mobility in the flow reversal seems to be inevitable every time the symmetry of the wake is restored. Several elements that can alter this behaviour are also identified. A change in the curvature of at least one of the four shear layers is shown to increase the frequency of the switches between bi-stable states, until eventually the long-time instability disappears replaced by low frequency flapping or swinging motions. Such changes can be triggered by applying perturbations on either a global scale or a more local scale. Overall, the results presented in this work help to bridge the gap between simplified geometries and more realistic automotive shapes, as far as the characterisation of the time averaged and main unsteady features of the wake is concerned, and provide insights that may allow in the future the design of more effective flow control systems for drag reduction.