When developing aerodynamics in CFD, we need to test our designs in the flow conditions we might expect in reality.  The further our test deviates from real life, the less likely it will be that our performance gains will be realised.

Over the years that we’ve been doing motorsport CFD, perhaps the most extreme example of getting the test conditions right has been in the world of rallying.  If you’ve ever watched footage of a rally car power-sliding around a corner, you’ll know what we mean.  Cornering (curved) flow, steer, high slip and roll angles!

EXTREME! Curved flow, high slip, roll and opposite steer lock.

EXTREME! Curved flow, high slip, roll and opposite steer lock.

Creating the Power-slide


Thankfully, bramble’s powerful kinematics make running at the extreme attitudes of a rally car no more difficult than testing in a straight-line.

We’ve created a CFD model of a rally car to demonstrate how easy it is.  This car was downloaded from GrabCAD and was created by Benali.  Please checkout both their sites!

image of rally car run through cfd simulation

Once the model was added into bramble, we simply set the test conditions in the map. Specifying a corner radius of 50m, plus some yaw and roll.  We then launched the case and bramble took over, updating wheel and suspension positions, the domain and all associated boundary conditions.

Easy Performance Wins


The initial CFD model is lacking downforce and so we’ve decide to make a few modifications to improve it.  To start with, we added a simple modification to increase front loading: a skirt on the front bumper.  We tested 50mm and 100mm height skirts.

Convergence Criteria

Rally Car CAD model with 100mm Skirt added

Adding the skirts has a large positive impact, increasing front downforce, reducing drag and even slightly increasing side-force acting towards the corner centre.

Increasing the skirt from 50mm to 100mm was beneficial and therefore it might make sense to continue to increase the height.  In reality though, the skirts receive significant punishment from the the road and cannot be too large.  100mm might already be too big, so we’ll leave it there!

Effect of Adding Skirts on Full Car Forces. Adding skirts increased downforce.

Effect of Adding Skirts on Full Car Forces

Analysing the Data


We can look at the lift accumulation on the baseline and modified rally cars to understand where the downforce gain comes from.  This shows us an increase in downforce (negative lift) being generated over the front portion of the vehicle (i.e behind the skirt).

Then by looking at the centreline slice of total pressure (energy in the flow) we can see a region of low energy, low pressure flow being the skirts.  This will lower pressures under the floor, sucking the car to the ground.

The wake from the skirt will also be shrouding the wheels, reducing the stagnation pressures on their front faces and so reducing drag.

Next steps


Now that we’ve added some front downforce the next thing we will look at is the rear wing.  The design of the rear wing on a rally car is interesting as it needs to not only generate downforce but also side force to help stabilise the car as it slides.

But that’s a story for another time…

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