Development of Separation Phenomena on a Passenger Car
The general shape of a vehicle influences its aerodynamic performance through separation phenomena and flow structure development. Since these flow characteristics have a direct influence on the energy efficiency, safety and comfort, it is essential to study their formation and evolution into the freestream. The energy efficiency is determined by the aerodynamic drag, while the safety and comfort aspects are dependent on the noise generation, vehicle soiling, handling and stability.
The objective of this work is to achieve a more detailed physical understanding of the development of flow structures by analysing their surface properties and their evolution into the freestream. The concept of limiting streamlines is used to investigate and characterize the near wall flow, and surface properties such as the surface pressure, the wall shear stress and the vorticity are analysed and correlated with the flow patterns. The detachment of the flow from the surface and its development into the freestream are investigated using 2D streamlines and flow properties such as vorticity.
This study is based on numerical simulations of a detailed full scale passenger car of the notchback type. Results are compared to experimental flow visualisations and pressure measurements performed on a full scale vehicle. Special focus is put on the flow around the antenna, the flow over the rear window, the flow downstream of the front wheel and on the base wake flow.
Based on this analysis, it is found that the surface patten can be used to identify evolving flow phenomena. Analysing the limiting streamline pattern and 2D planes, together with the vorticity distribution, makes it possible to predict and study occurring flow phenomena. Flow structures developing in the main flow direction were the most dominant and are the least suppressed. It is shown that the only mechanisms, of flow detaching from the surface, must be either through singular points or along separation lines. The study of particular areas around the vehicle shows different flow phenomena and explains the formation of flow structures. Familiar phenomena such as the A-pillar vortex and the trailing vortices behind the vehicle are discussed. For instance, it is shown that, in the near wake an up-wash zone is created (crucial for contamination) and in the far wake, the two trailing vortices create a down-wash; both phenomena emanate from the vehicle base.