CFD - Bluff Body - Splitter Preliminary

CFD - Bluff Body - Splitter Length Downforce Distribution

Introduction

Though Downforce is a big player in Race Car Performance, there are other factors that will determine cornering ability and ultimate handling. One of this is the Aerodynamic Balance of the Race Car. You may generate road crushing levels of Downforce, but if all that Downforce is on the Rear Tyres it won't do your lap time a whole lot of good. For the Race Car to go around the track as quickly as possible a good Aerodynamic Balance is required.

Other words used to describe a Splitter are Front Spoilers or Air Dams [1]

Hypothesis

A very simple explanation for how Splitters work is that the high pressure that develops at the front of the Race Car is trapped by the Splitter, and this high pressure is applied to the top surface of the Splitter which pushes it down. In general and within a reasonable range, the longer the Splitter, the more Downforce.

Splitter [2]

Furthermore since the Downforce is being created due to a Difference in Pressure between the top and bottom of the Splitter, the Downforce is being created at the location of the Splitter which is ahead of the Front Wheel Center Line. Due to this, a Splitter will not only increase Downforce but should also help if a Race Car is experiencing Understeer.

I predict that as I increase the Splitter Length on this DSR/SR2 Race Car that Downforce will increase and the Force Distribution on the Tires will move forward.

Simulation

Unfortunately (as far as the website is concerned) I've been busy with projects for clients so I didn't have time to run as many simulations as I'd have liked. This run just had 3 variations. First of all, a new car was designed to fit within the new SR2 Regulations and to more accurately represent a Race Car instead of the highly primitive Bluff Body I was using before.

The first car was run without a Splitter, the next ran a 50mm Splitter, and the last ran a 100mm Splitter. I used the same mesh sizes as before, this car has on the order of 1.5million cells. Such a low cell number leaves much to be desired, but that's the way it will be until I am able to justify buying a new machine.

The SR2 Race Car Bluff Body was run at 40m/s, with a Diffuser Angle of 12.5 degrees, and a Ride Height of 37.5mm. There was a slight change in the raised nose between the car with no Splitter and the car with Splitters. This was due to a design that didn't make for good meshing and was updated. Due to my projects I just didn't have time to rerun the Race Car with no Splitter.

Analysis

With the exception of an unexpected Rear Downforce jump with the 0mm Splitter Race Car, the data from the CFD Simulations fit well with the expected aerodynamic trends.

Splitter Length Front Load Rear Load Front % Rear % Downforce
0 mm 328.20 N 796.82 N 29.17 % 70.83 % 1125.0 N
50 mm 330.61 N 791.46 N 29.46 % 70.54 % 1122.1 N
100 mm 334.43 N 792.23 N 29.68 % 70.32 % 1126.7 N

DSR SR2 CFD Splitter Length Race Car Aerodynamic Pressure Distribution Front Top

This animation shows the effects on the Pressure Field caused by the change in Splitter Length. Note the jump in Pressure Distribution due to the change from the old version of the model to the newer more meshable version. For a more conclusive result the car should have been run with the updated bodywork and no Splitter, but as I mentioned I am too busy with paying projects.

DSR SR2 CFD Splitter Length Race Car Aerodynamic Pressure Distribution Front Bottom

The flow under the car is changed somewhat by the increased Splitter Length. I hope to go in to more detail on this topic in the future, but for now here is a view of the Pressure underneath the DSR/SR2 styled Race Car Bluff Body.

DSR SR2 CFD Splitter Length Race Car Aerodynamic Pressure Distribution Plot Balance

Finally I show the Downforce Distribution based on the length of the car. The red area on the left of the plot is where the splitter is clearly visible. The overall Force in this area is downward. Notice the peak forces don't really seem to change, but the area of that plot increases and due to this area having a negative center (as far as Lift is concerned) and a movement to the left as the Splitter Length is increased. As the Splitter Length is increased, the Downforce increases, and the Force on the Front Wheels increases due to the Center of Pressure shifting to the front of the Race Car.

Conclusion

It is clear that the Splitter does help increase Downforce as well as help to shift some of the Force to the Front Tyres. However if the Center of Pressure is too far off, getting a good Balance just won't be possible just by changing the Diffuser Length alone. In the future to get the Balance better on my design I'll have to add things such as Dive Planes, and likely a Front Diffuser and other details.

References

[1] Race Car Vehicle Dynamics - Milliken

[2] formula1-dictionary.net/splitter.html

 

- Keith Young

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