Effect of Vortex Cell in the Diffuser

In the search for more downforce at the rear of my car for the KVRC challenge I came up with a design for a cavity in the diffuser, my idea behind this was to help extract air out from under the car, the cavity may be too much of an aggressive start to the diffuser and may cause issues.


This first design was not compliant with the rules for KVRC and it was also pointed out that this concept had been trialed on wing profiles and had been called a ‘Vortex Cell’.

I decided to see if there was any benefit to the use of a ‘Vortex Cell’ in the diffuser that would meet the rules of the KVRC challenge.

I took the opportunity while testing some updates to my car to run 2 tests with the only difference in the car being the ‘Vortex Cell’ in the diffuser.

Using OCCFD I ran 2 light tests and got the following results.

Standard Diffuser Diffuser with ‘Vortex Cell’
Cd.A 1.56 1.56
Cl.A -5.04 -4.67
COP (m) 1.67 1.61
Drag (N) 1882.05 1880.81
Downforce (N) -6065.96 -5624.47

From the results, we can see that with the ‘Vortex Cell’ drag has been reduced but this could be attributed to slight variation in the 2 tests as the difference is minimal.

The loss of downforce and the shift in the center of pressure forward indicate the ‘Vortex cell’ must be affecting the efficiency of the diffuser.

Using the Virtual Stopwatch supplied by Competition car engineering we can see that such a small change to the car and the loss of downforce can have a large impact on lap times at different circuits.

Standard Diffuser Diffuser with ‘Vortex Cell’
Nurburgring 385.48secs 387.56secs
Monaco 84.05secs 84.52secs
Silverstone 104.6secs 105.55secs
Sepang 108.65secs 109.23secs
Sao Paulo 82.2secs 82.63secs


Standard Diffuser Diffuser with ‘Vortex Cell’
nocell underside withcell underside
surfacelic no cell underside surfacelic with cell underside
surfacelic no cell surfacelic with cell

From the results it does not look like an additional investigation into a ‘Vortex Cell’ in the diffuser would be beneficial at this time.