Vehicle wakes in side wind

Doctoral thesis, 2021

There is a global push to reduce the energy consumption of passenger vehicles with increasingly stringent targets and regulations. More than one-tenth of Europe's greenhouse gasses are due to passenger vehicles. The aerodynamic drag is a major contributor to energy consumption responsible for more than a quarter of the vehicle's energy usage. Thus, improving the aerodynamic drag will help us achieve our greenhouse gas emissions targets.

Vehicle aerodynamics is typically assessed and developed in idealised conditions using low turbulence wind tunnels and numerical methods. Several aspects influencing vehicle aerodynamics are often neglected such as traffic and wind conditions. This thesis explores the effects of steady wind, or yawed flow, on the wake of vehicles. The goal is to increase the knowledge of the full wake behaviour at yaw and how it is related to the aerodynamic drag. For this, an optimisation method is used throughout this work to generate robust, low-drag, reference geometries. The optimisation is done at different yaw angles, allowing asymmetric geometries at yaw. The cycle averaged drag, which takes into account the driving cycle as well as the wind distribution, is also considered to create symmetric geometries which are insensitive to yaw. The optimisation is focused on base cavities and trailing edge modifications to these cavities.

Generally, the low-drag configurations have a more balanced wake, with and without side wind, where the recirculating flow in the wake is aligned with the vehicle. The improved balance allows the wake to move more freely which often increases the large scale coherent motions of the wake. These unsteady motions are linked to increases in drag in the literature, however, in this work, improving the wake balance was found to be the more important indicator of the overall drag. At yaw, the coherent unsteady motions are reduced as a result of the wake being locked in a more stable, but higher drag, upwash or downwash dominated state.

The wake becomes increasingly downwash or upwash dominated at yaw by a large rotating structure in the wake. The yaw insensitive designs have a wake that is slightly biased towards the top or bottom of the base at zero yaw to counteract the movement of the wake at yaw. Optimising the geometry without considering yaw can reduce the performance over the entire operating range. This highlights the importance of considering several operating conditions during vehicle development.