Wheels and wheel houses contribute up to 25% of the total aerodynamic drag of passenger cars and interact in a complex way with their surroundings. Rims and tires induce complex flow separation mechanisms in a highly unsteady regime and the proximity to the ground enhances these phenomena. To have a clearer understanding of the flow mechanisms that develop around wheels and inside wheel houses, the effect of tire deformation and vehicle ride height on the aerodynamics of passenger cars has been investigated with unsteady CFD simulations. Tire deformation is modelled with an empirical formulation that provides close-to-real deformed shapes, while vehicle ride height changes are made by applying vertical translations the vehicle body. Slick tire geometries and closed rims have been analysed and their rotation has been modelled with a tangential velocity component applied to their surface. The investigation has been conducted in three steps: different car heights and tire deformation levels have been investigated separately and then combined, classifying the results on the basis of the drag of the vehicle. Results show that even small tire deformation levels can significantly affect the aerodynamic drag, thus deformation should be included in simulations and treated with caution.

This research proposes a numerical methodology to assess the behavior of a car towing a generic caravan in crosswind conditions. Driving with crosswind can lead to dangerous situations. The vehicle can have stability issues and eventually crash. Different crosswind and vehicle velocities have been analysed using a car-caravan dynamic model with accurate aerodynamic loads. The forces generated by the action of the wind are calculated with CFD simulations and applied to the vehicle to perform dynamic simulations. Results show that also non-extreme crosswind condition could lead to dangerous situations.

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<div class="section abstract"><div class="htmlview paragraph">Wheel and wheelhouses contribute up to 20-30% of the aerodynamic drag of passenger cars. Simulating the flow field around wheels is challenging due to the complexity of the flow structures generated by tires and rims, wheel rotation, tire deformation and contact with the ground. High accuracy is usually obtained with transient simulations that treat rim rotation with the Sliding Mesh (SM) approach, which is also computationally expensive. Previous studies have confirmed that the application of a tangential velocity component to the rim surface is unphysical for open rims, while a Moving Reference Frame (MRF) is lacking accuracy and the averaged results depend on the initial spokes position. These methods do not consider the dynamic nature of the problem. This work proposes the use of the Actuator Line (AL) and Rotor Disk (RD) approaches as alternatives for simulating open rims with much lower computational cost. They are based on the introduction of volume forces to resemble the presence of the spokes and require a preliminary aerodynamic characterization of the body. The new methods are compared to SM, Rotating Wall Boundary Condition (RWBC) and MRF applied to an isolated rotating wheel case. A good agreement is observed for the most relevant flow features, with consistent reductions in computational cost. Specifically, AL keeps the transient features of the problem, while RD provides a much less expensive steady-state solution. As a result, both AL and RD are promising alternatives for the simulation of rotating wheels.</div></div>