Help me understand aero: rear corner radii, yes or no?
 

This is one question I've never been able to answer to my satisfaction:

Regarding transitions from the side of the rear quarter panels of a vehicle to the rear-facing areas (ie the plane that the licence plate is on), what's better for lower Cd: transitions constructed with generous radii, or transitions that are sharp?

The reason I'm confused is because images I have seen conflict with each other and what I have read. What I see on a large number of ultra-sleek concept cars are side/rear transitions which look literally "chopped" - sharp 90 degree corners where the rear quarter panels meet the rear-facing bits.

EG: VW 1-liter car is a prime example; the PNGV cars; Ford Probe V.

Yet I have read (Barnhard, Road Vehicle Aerodynamic Design) that side-to-rear transitions should be generously rounded for reduced drag. With the caveat that generously rounded side-rear transitions are also less stable in cross-winds than sharp rear angles.

EG: several low drag production cars, notably the Insight, and the EV1, had generously rounded side-rear corners - the bumper covers below the tail lights anyway. Which makes me wonder if, because these are production vehicles, some concession to user-friendliness was made over aerodynamics (ie: round bumpers seem less likely to be damaged in minor scrapes than bumpers which have a sharp 90 degree corner).

Does anyone have any credible information or reasoning to help answer this?

I find the apparent "contradictions" on this design element between these otherwise very slick cars a bit confusing.

Yea...the rounded corners on the sides and sharp cutoff on the top seems to be the usual COMPROMISE used vs the ideal boat tail?

Yet supposedly...the CRV on this page...sharp top...rounded corners...

https://www.airtab.com/Images/gallery/CRV/sDSC05730.jpg

...claims 7-10% mpg gain with airtabs on sides and top?

I think someone could come up with the probable mpg for this CRV with an ideal boat tail and compare that with what would be possible with vgs....vs how it comes from the factory?

Been seeing factors relating mpg with CD values.

That's also my understanding, and you may be right that the problem is semantics, ambiguity, or my misunderstanding of Barnard's writing. The quote(s) in question (Road Vehicle Aerodynamic Design, an Introduction, 2nd ed., MechAero Publishing 2001):

This one doesn't specifically make it clear whether by rear-end rounding he's referring to the side-rear corner, or to plan-view tapering.

This one seems less ambiguous, however it is in a section about commercial vehicles (transports/vans), where the roof line is generally straight from front to back.

Maybe the distinction is that there's some aerodynamic benefit to radiusing the side/rear corner where there is otherwise no taper in the shape in plan/profile view.

But my gut tells me that if you've already achieved an proper tapered/boat tailed shape, the side/rear corner should be sharp, a la Kamm back.

These are all good points that could lead to a compromise vs. proper aerodynamics.

I've realized aero is a much more complicated subject since I decided to make an effort to go beyond "lay" or "pop" aerodynamics (which I like to describe as a general understanding of the fundamentals which I can mis-apply in specific situations :D ).

Good, reply clencher - thanks.

Still don't know the answer for sure though. Perhaps time to track down an expert and ask directly.

I should add these:

In the section Vehicles in cross winds:

and also...

Note the author is referring to separation from the vertical plane (side of the car), not the horizontal (roof or trunk)

Tape some tell tails on the back of the various vehicles you can find and see what is generated back there. You apparently want to avoid tapers that are too aggressive as it created more drag than a chopped off end so if you can't slope it back gradually then chop it off. Still haven't figured out if the roof foil on an xB really helps or just looks good. I do know that it is not cheep!

Just to be clear, I'm not talking about boat-tailing here, but the design of the corner where the side of the vehicle becomes the rear.

Just the last 4 or 5 inches of the length of the car.

Tuft testing is a good idea, but it's time-consuming. I'm trying to find the answer from someone who's already done the work (or knows about it).

I don't think that the side has that much effect on a car vs my "van" xB sides because the major air flow is over the roof and dropping over the trunk / rear hatch and the rear wheel wells totally screw up the air around the bumper region in back. I know my rear quarter is all open underneath and I haven't a clue what the air is doing back there other than the dirt that builds up on the outside lower half of the rear of the wheel opening. Once you put the rear wheel skirts on then things change however but volume wise the top air is much more a factor.

The top is usually clean...but the sides have mirrors...windows...wheel wells?

Might be that the air is so disturbed that round works OK?

As far as crosswind stability...they mention improvements for the CRV with airtabs?

Every once in a while I come back to this question.

Regarding this earlier thought:

... I realized that if you look at actual boats, the corner transition from the side/bottom to the transom is almost universally a sharp corner (with the exception of relatively uncommon double-ended hulls, which I believe are done more for safety in following seas than hydrodynamics).

That would seem to lend support to the idea that if you have a the ideal plan & profile taper (VW 1L car), the "transom" transition should be sharp. If it's not ideal (nearly all production cars), the corner should be generously radiused.

With a sharp transition, you're going to form an air pocket behind the car. With a rounded or completely tapered rear, you're not going to have the air pocket.

With a rounded or completely tapered rear, the air moving past the car is going to "stick" to the body (the boundary layer, anyhow). With the sharp transition, you get boundary layer separation from the body. Air slides better past air (the pocket behind the sharp transition) than it does to any "stationary" surface (the rounded/tapered rear).

This is not to say that having the rear taper in before the sharp break is a bad idea...it isn't. This makes the size of the air pocket you're dragging smaller, reducing drag. Something sticking out on the body that could 'trip' the boundary layer before the rear could also reduce the size of the air pocket, thus reducing drag.

Like everything else, you must balance surface area of the vehicle with the size of your separation zone.