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Old 08-15-17, 09:09 AM  
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Default Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Originally Posted by xirallicGDB-F View Post
So what degree are you setting the diffuser at? I assume 7/10? Also is there an advantage to be had by sweeping the diffuser gradually to the desired angle vs. a straight panel? I haven't come across such info in my book, at least not yet.
Kind of a slow day at work so I've been doing some internetting. Mostly on F1technical about diffuser shapes.

Mostly all I wanted to know was how the leading edge shape of the diffuser effects the downforce/drag but that is very hard to find information on (so far).

There are three main "types" of diffuser shapes being discussed, flat rake, concave curved rake, and convex curved rake. There is also some mention of a hybrid concave/convex diffuser but that’s probably not real relevant here. My current diffuser design has a convex entry and flat rake exit.

Concave (which I didn't know was a thing) has generally lower drag and lower downforce but in a given set of regulations can be the best option. From what I've read though if you have freedom in your design (as I do) then concave is not likely to be the best choice. Interestingly it seems as if the concave design actually relies on airflow separation to create a bubble of air in the diffuser to energize the airflow.

Convex seems to be a better performer in general, by producing more downforce but also more drag. I’m sure the increase in drag (as compared to concave) is associated with the continuously attached airflow.

Most of the "comparison" information I have read so far only compares concave to convex and not to the flat rake that a lot of "cars" seem to be using (possibly due to simplicity or to motorsport regulations). In fact motorsport regulations are really confusing the whole subject because people are doing the best with what they are allowed to do and that can look very different from one car to another.

The flat rake diffuser with a sharp transition a the throat of the diffuser has a higher expansion coefficient and moves the point of lowest pressure forward closer to the throat than the convex diffuser.

The convex diffuser has a smoother/gentler shape for airflow and has a lower expansion coefficient as a result. I believe the “effective length” of the diffuser is slightly reduced as well, which could be a determining factor for a motorsports team working within design regulations.

One thing that does seem clear though, is that the gentler shape of the convex diffuser is likely to be less prone to airflow separation. To me this indicates the possibility for raising the angle of attack a degree or two, without negative results as compared to a similar flat rake diffuser. But this still includes a lot of variables such as how the airflow behind the car is acting and how the angle of attack corresponds to the wake. This also does not speak to the tradeoff of diffuser length versus angle of attack for the flat rake versus convex diffuser either.

Another topic discussed is the ability to use exhaust gasses to try and seal the airflow inside the diffuser from the turbulent airflow off of the rear wheels. This, while being a useful design element will not be practical for anything less than open wheel cars or bonkers time attack cars.

There was also some discussion about having a gap between the diffuser and the rear bumper for airflow from the engine bay/transmission tunnel to be relieved at the rear, and that you should NOT seal that up. The discussion catalyst was actually someone who had taken the diffuser and sealed it up to the bottom of the rear bumper and wound up having heat retention issues because there was no way to relive that heat. The gap over the diffuser solves this, although heat retention with a full on racecar/endurance racer is still potentially an issue and may necessitate trans/diff coolers.

I have also seen several flat bottoms that include a NACA intake for the rear diff as well, but they are usually at least a foot or two upstream of the diffuser throat. I’m starting to think that they may not be detrimental after all, but I’m still not ready to try it myself.

Another common design element I am seeing is the vertical “gurney flap” on top of the rear edge of the diffuser. My understanding is that this helps to further energize the diffuser and the small(er) amount of air above it and reduce the size low pressure zone behind the car

On a related but not super applicable note, there are a lot of venturi shaped flow meters for pipes/ducts. The only thing I came up with about straight vs curved edges is that the straight edges produce sharp pressure changes and that by using curves instead the flow meter is more accurate. To me that says the curved edges produce a more stable airflow profile. But that says nothing about the actual overall pressure change or how that relates to creating downforce


So after all that reading, my takeaway is that without testing or working within a given set of regulations there are no fixed “answers” regarding diffuser design that applies to all situations, and that airflow separation (except in the concave design) is always a bad thing. With that in mind, I feel good about the entrance of my diffuser having a curve to it for a smooth airflow transition. I will keep looking around to see if I can find anything else.
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