This weekend I intend to fabricate some side sills (or skirts). Whatever you call them, I intend to have them run as long as I can between the front and back wheels, be around 100mm high, and fabricated from 4mm acrylic plastic.
It's worth mentioning that I have already panelled the underside of the car, so I'm trying to keep air underneath the car on the flat sections rather than moving laterally and creating drag on the wheels.
As I've been doing with my other mods, I've modelled it in SolidWorks FlowSimulation to see what the aerodynamic science says.
On the "Cut Profile" below, the modified sills are on the right. The blue air is where air velocity has remained highest (or lowest drag if you will). No major difference visible here but I've drawn a simple line in photoshop and there is less drag when the extended sills are fitted.
***All images are thumbnails which can be clicked***
In the image below the modified sills are on the bottom. When looking at velocity lines we want laminar flow and we can see that the sills have a positive effect again - there are more
dark blue lines flowing off the top of the car and mixing with the wake. This means again there is less drag under the car and the air is mixing better in the wake. You can also see more red and orange turbulent air under the unmodified vehicle.
Finally a view of what is happening underneath, and you can see the laminar flow over my underbody panels is being better contained and kept at high speeds (the dark blue lines again). The sills are the black extensions and to re-iterate they are just bits of plastic in a straight line - not shaped at all.
I hope this helps anyone who is looking to improve their underbody aero. I'm going to bed, because in the morning I'm getting out my saw and glue!