Diffuser Design

[iamuwere]

I drive a small formula car in SCCA competition. Top speeds of mid 130's with good flat cornering. We are allowed a flat bottom (55" width through the sidepods) with no more than a 1" deviation in height across the floor pan. Area before the front axle and after the rear axle are free. The obvious area to work with is the rear diffuser design.

I not yet found any good rules for diffuser design. Currently there is about a 10deg rise over approx 24" length. One center strake (straight) with straigt side plates.

Any good rules on diffuser design or good references with practical application?

Thanks,
jim

 

[btrueblood]

"Horizontal "channelizers" in the diffusors would certainly work to improve the flow separation but only the last channel that sees the ground would now contribute to down force. "

Sreid,

Based on your argument, a wing in a wind tunnel generates no (externally measureable) lift. Literally thousands of technical papers have been written proving otherwise.

 

[btrueblood]

pvw,

Yeah, a liquid manometer could work, but I envisioned you careening around corners in a cart at 120+ mph, and wondered if you'd have time to look at the vibrating tube banks... Solid state is a lot easier, even when we're talking about bench testing.

"Don't forget the Rear tires that also create a narrowing and turbulence leading into the diffuser.
I remember an artical from a williams designer who admitted that the rotation of the driveshafts going through their diffuser actual created more downforce than a clean tunnel because it helped with reattachment. (Active arodynamic device?)"

Hmm, neat idea. Yes, the drive axles for the car, if exposed to the aft-going flow, would generate downwards force (Coanda effect). Also, if the axles were located near the throat of the diffuser, could help keep the flow attached to the upper surface (they would act the same as a blown lip).

"You are right about a long throat gives more downforce but you must use vortex generation or exhaust activation to keep it attached on the steeper diffuser or a sucking device such as a low mounted wing."

Yes. This is the age-old problem of diffusers (preventing seperated flow, maximizing the total pressure recovery), and doing a bad job of it (having massively seperated flow in the diffuser) results in a much higher drag coefficient (poorer wake fill behind the vehicle), and poorer flow rates/velocities in the underbody low-pressure zone, resulting in lower downforce. Having done a bit of research on v.g.'s recently, I wouldn't spend much time or money on them. They work okay a certain specific speeds/flows, but typically lose effectiveness at "off-design" conditions. Also, the job I was working on had a "ground plane" passing at supersonic speeds, such conditions tend to make the effect of "enhancements" made on the moving surface much less effective. Not sure if our results would apply at the much lower speeds of an automotive application. In any case, my comments here sound as if they are moot - you may be restricted by SCCA rules against putting a turning vane or similar horizontal aero device in the flow, so the big hammer of turning vanes may not be usable by you. If so, then explore the website link I gave you, as well as the NASA Langley server in general. Also, there are a lot of ASME, AIAA and SAE papers written over the last century or so that have direct bearing on diffuser design, and the design of swirl vanes and vortex generators to help prevent or delay flow detachment.

As regards blowing in a diffuser, can you direct engine exhaust out the diffuser lip? Also, can you devise a way to add heat rejection (from engine cooling water, e.g.) along the upper wall?