In this CFD Article I show how the Diffuser of a Race Car effects the Downforce and Balance.
The majority of you landing on this page most likely already have an idea of the effect that Ride Height has on Downforce for an F1 or DSR / SR2 / LeMans Prototype Race Car. In general, the lower you can get the Underbody of the Race Car to the ground, the stronger the Ground Effect will be, and this will increase Downforce. It does however get a little more complicated than that. As you get closer to the ground, Ride Height Sensitivity increases. In addition, once the Race Car Underfloor gets too low the flow underneath can be blocked and not allow enough air through the diffuser. Once this happens, Downforce plummets.
The following article will talk about these implications in Formula One and other racing series such as DSR/SR2 and LeMans or other Formula Cars or Prototypes.
It is well known that the lower a Race Car rides to the ground, the more Downforce it generates. However what happens as the Underfloor is moved in a larger range, from half the normal Ride Height to several times higher? Approximately how close can the Underfloor of a Race Car get to the ground before Downforce begins to diminish? What does the Diffuser Angle do to these trends?
Lets first look at some existing results and see what we can decipher from them.
The first results we will look at are from George  ( Cooper referenced in the images was Wind Tunnel Data). It is clear that the Downforce increases pretty much linearly from the maximum Ride Height until brought down to a certain Underbody Ride Height. This point represents a point at or near the optimal Ride Height for generating maximum Downforce. Lowering the car beyond this optimal Ride Height results in a loss of Downforce for the Race Car. Also take note of the 2nd image showing a higher Diffuser Angle. The Wind Tunnel Data indicates worse performance, possibly due to flow separation caused by the high Diffuser Angle. However the CFD Solvers show improved performance. This is another indicator of something I've mentioned before, which is that any results I get with my own analysis at or near the regions producing stall shouldn't be taken at face value.
Images from George 
For the experiment covered in Milliken  a Symmetric Airfoil was used. This experiment used multiple Ride Heights and for each Ride Height the Angle of Attack of the body was changed. It can be seen that for a given Angle of Attack, as the Ride Height is lowered the Downforce is increased, up to a point.
The basic trends from my previous article should still apply, so at reasonable Ride Heights, such as around 37.5-50mm, the data should correlate with my results from my CFD Bluff Body SR2 Diffuser Angle Article which was run at 37.5mm Ride Height.
Based on the information above, I made my best guess as to what the CFD Data should resemble once all the different SR2 Diffuser Angles and Ride Heights were run. My hypothesis was that steeper Race Car Diffuser Angles would result in lost performance at higher Ride Heights, and shallow Diffuser Angles would lose performance closer to the ground. The basic sketch of this can be seen below.
My initial intention was to just make the runs at a single Diffuser Angle. However at a Diffuser Angle of 10 degrees for this Bluff Body the CFD Results did not match with the known trends. The simulations would fail to converge after a Ride Height of 35.0mm was reached. Though this could be taken as an indication the trend is being reinforced (since my solver seems to fail to converge around stall points) it was still not very confidence inspiring. I changed my meshing method to more accurately mesh the Underbody of the Race Car Bluff Body, and the trend showed up. It was at this point that I decided to make the runs over many Diffuser Angles in order to see if the meshing problem persisted for blocked flow at higher Diffuser Angles. If the meshing went smoothly without any trouble and the trends continued to show then I'd have a pretty good indication my initial hypothesis was correct.
It took about a week and a half of almost non stop CFD SR2 Bluff Body Simulations to get these Data. The case of the 10 degree Diffuser Angle was run over all Ride Heights in its range at 2.5mm intervals. In the interest of saving time, all other Bluff Body Diffuser Angles were only run at points of interest.
For each Diffuser Angle, a few key Ride Heights were run in order to observe the trend in the data. Once this was done, I made judgment calls as to what other Ride Heights to try for that given Diffuser Angle. The Freestream Velocity chosen was 40m/s, and standard atmosphere was used.
First lets look at the post processing and visuals before looking at the data.
Here is an image showing a Ride Height range from 42.5mm down to 25.0mm for the 5 degree Diffuser Angle. As the Underfloor of the F1 Bluff Body gets closer to the ground the vortices seem to increase in strength.
Note that for the last 2 runs for the 5 degree Diffuser Angle the mesh is different. This was due to RAM limitations and the meshing method used.
It was observed that for high Diffuser Angles the blocked flow was achieved, however for low Diffuser Angles it was not as easy to show the phenomenon. It was possible that due to RAM limitations the mesh sizes needed to properly model such low ride heights just weren't achievable with the computing platform used. It took a lot of tinkering with the mesh to get the 5 degree and 7.5 degree Diffuser Angled Bluff Bodies to Stall. Since the Meshing method used to generate these data were different from the other runs the exact values for the stalled runs should not be counted on.
The reason for this was likely that higher Diffuser Angles begin stalling at higher Ride Heights than their small Diffuser Angle counterparts. Due to this, the higher Diffuser Angle Bluff Bodies do not require extra special attention underneath the car, and my typical meshing method could be used. For the low Diffuser Angles, stall did not occur until very close to the ground at which point the mesh sizes required to accurately model this flow went beyond the capability of my laptop.
Below it can be seen that the Data very closely represent my hypothesis above. A notable exception can be seen however in the data for the 15 degree Diffuser Angle case. For some reason the 15 degree Diffuser Angle has a very obvious difference in the slope of the Lift as a function of Ride Height curve. This could be due to the fact that 15 degrees is near the stall point to begin with, or it could just be an error in the CFD. Without another form of verification all I can do is speculate.
It is clear that the 5 degree diffuser generates less Downforce than the other cases, which makes sense. It is also able to ride lower before reaching Maximum Downforce. For each incrementally higher Race Car Underbody Diffuser Angle it was clear that the minimum Ride Height before Diffuser Stall increased. The maximum Downforce generated was achieved with the 10 degree Bluff Body Diffuser at 35mm.
The effect of Race Car Diffuser Angle and Ride Height on Drag was not as clear as that for Lift. The general trend was that Drag increased as Ride Height decreased and as Diffuser Angle increased. The trend was expected, but such variation in the curves was not expected.
The trends for Lift to Drag ratio were as expected, with the exception of the lower slope for the higher Diffuser Angles. Additionally it is interesting to note that the best Lift to Drag ratio was achieved at 37.5mm, rather than at the point of maximum Downforce which was at 35mm.
The results matched pretty well with the hypothesis. Deviations from predictions included the slope of the lift curve for the 15 degree Diffuser Angle and convergence failure at and near the stall point for low Diffuser Angles. Once the convergence failed, a different meshing method was used and the trends showed through correctly.
As the Underbody of the SR2 Race Car got closer to the ground the Downforce was increased, until a point was reached at which the flow stalled and Downforce suddenly dropped. As Diffuser Angles increased, the Ride Height at which Stall occurred increased.
Drag tended to increase as Ride Height was decreased, and also as the Diffuser Angle increased.
Lift to Drag tended to be the greatest at or near the point of highest Downforce.
 George, A Computational Study of Idealized Bluff Bodies, Wheels, and Vortex Structures in Ground Effect
 Milliken, Race Car Vehicle Dynamics (He got the image from Stollery and Burns)
I write these articles to help. Please let me know if this was helpful through shares likes and comments so I know you guys find this interesting and a good area for me to focus on.