Engine bay undertray aerodynamics

Some time ago I started to do some CFD analysis to see if I could do any simple changes that improved the aerodynamic properties of the car, without altering its appearance too much. It turned out that I had too much faith (or more probably - too little knowledge) of what the CFD software could provide me, and after putting in a great deal of work I finally gave up.

It is said that from an aerodynamic point of view, one of the most important features of a car's body design is how it looks underneath, and even if my previous CFD adventures failed it pointed me in the same direction. To fit an engine bay undertray is not very complicated and doesn't affect appearance much, but before I started I wanted to confirm if it would improve things or just be a waste. This time I made a much simpler 3d-model of the car, and had much lower expectations of what the CFD software could help me with.

Standard under body

The results should be taken with lots and lots of salt!

Standard
Drag: 879N
Downforce: 427N

Undertray only
Drag: 793N
Downforce: 455N

Undertray and skirts
Drag: 809N
Downforce: 590N

Undertray, skirt and splitter:
Drag: 858N
Downforce: 604N

All calculations are in an air speed of 160 km/h. The standard design does generate a bit of downforce, because the car body is at a small angle compared to the direction of the air flow. The ground clearance is 15 cm, minus the engine oil sump. The design measurements are just guesses as I didn't take time to go out to the garage and measure. (it does look a bit short). No rounded edges. The body is solid on the upper side.

An underdray reduces drag but doesn't give much improvement in downforce. On the other hand, extending the undertray with a 5cm skirt improves downforce quite a lot. I also tried to add a small splitter in the front and see if that made a difference (see top banner image) but the small gain in downforce is not worth the effort.

One apparent issue with fitting an undertray is of course heat. I have a plan for lowering intake temperatures (more about that later), but if the heat from the exhaust primaries will be just too much and start to melt things I don't know.

Caterham has a ready made undertray on their website, but I've already bought a sheet of 1 mm aluminium and started to cut out the under tray. I'll post more when (and if) I finish it.