Bramble gives you the ability to model cars in cornering conditions where the wheels are steered, the body is rolled and, importantly, where the wind curves around the car rather than crossing it diagonally.
In this article we explain why engineers opt to use bramble’s corner modelling methods rather than a traditional yawed wind approach.
Aerodynamicists will look to maximise the amount of efficient downforce a car can produce when cornering as this will increase traction and hence the maximum speed the corner can be taken at.
A CFD model of a car in cornering conditions should have steered wheels and be rolled to relative to the ground mimicking the ‘on track’ attitudes. But the question is, how should we model the motion of the wind relative to the car?
As a car corners, the wind will tend to come onto the vehicle at a sideways angle (i.e. slightly from the driver’s left or right). This is known as the ‘yaw angle’. This size and direction of the yaw angle will depend on the cornering speed, the direction of the turn and the radius of the corner itself.
In both wind tunnels and traditional CFD methods, yaw is modelled by rotating the whole vehicle relative to the wind. As a result, the air will pass in a straight diagonal line across the vehicle.
But this isn’t what happens in reality. As the car takes a curved path around the corner, so the wind’s path will curve over the car. In the curved flow of a true corner, the front of the car will experience an opposite yaw direction to the rear of the car, unlike the traditional yaw approach where both the front and rear of the car will experience the same yaw direction.
In order to more accurately simulate what is happening in reality, shouldn’t we be curving our flow fields?
Whilst it isn’t possible to generate a curved flow field in a conventional wind tunnel, it can be done in CFD. Unfortunately, CFD testing has tended to copy wind tunnel conditions and so traditional yaw remains the predominant method of modelling cornering conditions.
We’ve run Ryan Vella’s RV22 car over a range of corner radii and also at their equivalent straight yaw angle (i.e. the angle the curved flow makes to the centreline of the car at the front wheel axle line).
These radii were 500m, 120m, 75m and 50m.
The graph to the right compares the measured rear downforce on the vehicle for the true corner and traditional yaw conditions. As can be seen, two different trends are predicted.
The gifs below show the contours of total pressure on slices through the RV22’s flow field in a 50m corner and its equivalent yaw angle of 2-degrees. These images show regions of high and low energy flows and are used to identify where key flow structures are.
From these images it is possible to see how the location of the key flow features differ with the different modelling techniques. Thus, it is apparent that an aerodynamic device optimised to a traditional yaw flow field may not work as well in reality if it experiences flow more like the curved flow field.