Rear Diffusers

[jimepting]

I have been avidly following the interesting thread on boattail extensions for the Insight 1 now active in another thread here. Not to highjack that thread, I'll just ask a question that has been puzzeling me.

Is there a role for a defuser of some sort for the underneath of boattail extensions? I know that there is lots of concern about how the air underneat flows, and it influences, in most minds, the maximum departure angle of the underside.

I'm a Formula 1 fan and I recall how the introduction of an innovative rear diffuser in 2009 by the Brawn team gave them a significant aerodynamic advantage. It is true of course that the speeds are dramatically different.

I also see some "funny little things" going on on the rear of the stock Insight 1and the Prius that cause me to wonder. The Insight 1 has an undercut section on the bottom of the rear bumper which I'm guessing is aero driven. The Prius uses strakes(right word?) behind the rear wheels to form a sort of channel for air exiting the rear underneath.

Just thought I'd ask. I'm still pretty much a neophyte at the aero stuff and want to get Hucho's book soon as I can get $100 tegether.

[MetroMPG]

Not as familiar with the Insight 1's underbody, but the features behind the the tires on the gen 2 & 3 Prius are classic "boat tail" tapered bodywork meant to minimize the cross sectional area of the trailing wake.

I tried out XXL variants of the same thing when I built & tested my car's boat tail:



You can't see it well in the angle of this photo, but that structure tapers from the full width of the rear tire (as on the gen 2 & 3 Prius) down to about 1 inch wide at its tip.

They may have a dual purpose and serve as a kind of guide/channel for flow exiting the middle of the car; I don't know for sure. Would need to find an aerodynamicist to ask.

[silverinsight2]

Source insightman.com



Source Autospeed .com

[aerohead]

Scientifically,I can only default to what Hucho shared in his work.However some of the members are having success with angles steeper than what is recommended and they're being careful about quantifying results ( science of the real world dyno ).I think the case-specific-basis is at work on this one.
Personally,I was losing some speed with an aggressive diffuser angle and had to tape it shut with cardboard.
Since top speed and drag go hand in hand I can only surmise that there is truth to the notion that they can be too steep.
To borrow a phrase from Darin,' tuft-testing is your friend.'