The wheel wake is a major part of the flow field of a Formula 1 car and of particular interest right now with the new regulations for 2019. The exposed wheels contribute a lot of the overall drag to an F1 car as well as producing lift. Therefore, understanding the aerodynamics of the wheels is important.
Graph showing forces acting on the wheel, final values are the output of the CFD simulation
The lift (around 100N for this simulation) is produced by the wheel is the stagnation point just in front of the tyre contact patch. This creates a region of high pressure, resulting in the lift. Also, the airflow over the top of the tyre accelerates, producing lower pressure which acts on the upper surface. The flow around a wheel is dominated by a large wake and major 6 vortices, 3 vortex pairs. These contribute to the large drag associated with the wheel as well as lift. The wake is a result of flow separation, as the air flows over the bluff body of the wheel. The wake is characterised by a region of flow containing lots of eddies and recirculation. The reduced dynamic and total pressure in the wake combined with the high pressure in front of the wheel results in a large pressure drag (aprox. 150N at 100mph).
Wake profile immediately after the wheel
Wake profile 1m behind the wheel
Legend for the above wake profiles, white being the lowest, pink the highest
High pressure on the front surface of the wheel
When compared to the flow over a non-rotating wheel the separation point shifts forwards, this results in a lower lift as the flow velocity over the top of the wheel reduces due to the detachment, hence a reduced pressure coefficient. The vortices previously mentioned only form on the rotating wheels.
An image of the wheel wake where some of the major vortices are visible
The front wing is the main tool for reducing the impact of the front wheels on the aerodynamic performance of the car. Firstly, the wing tip vortices, of which there are two main ones, shed off the top and bottom of the end plate, interact with the wheel. However, the placement of these is vital, for example if positioned wrong they can delay separation of flow over the wheel increasing the lift. When positioned correctly the vortices cause acceleration of the flow around the front corners of the wheel, reducing the pressure difference between the front and back faces. There is also the often mentioned outwash, which directs airflow away from the wheel instead of flowing around it. The aerodynamics of the wheel are highly sensetive to changes in the height of the wing from the ground and the overlap between the front wing and the wheel. This means that the aerodynamics of the front wing will be different for 2019 than in 2018, due to the new regulations.
It is not only the front wheels that produce a wake, the rear wheels have in a similar way. This causes a problem for the diffuser. Despite have a region of low pressure behind them, the turbulence in this region acts as a blockage (the reason for this will be covered in a following article), reducing the lateral expansion and extraction of the diffuser. As a result diffuser performnace is impacted by the wheel wake.