"Influencing" the air flow rather than "shaping" it.
 

Sorry for a confusing title but my simple mind couldn't figure out a better wording.

I guess what I am wondering about is, is it (to an extent) as beneficial to direct airflow unattached as it is attached?

The example I am seeing is this. We know an aerpcap on a pick up it the best for aero. Airflow stays attached and is "shaped" for lack of a better term. But the cab spoiler/half tonneau is rather close in benefit, but seems unattached so that flow is maybe more "influenced".

If this is the case, then could the same principle apply to say the underbody of the same pickup. Rather than a full belly pan (attached or "shaped") but rather a fin arrangement, directing or "influencing" the flow outward as opposed to directly under.

We know that an air dam is the way to go, so this isn't a real world application. Just a mental exercise I guess so I can try and learn something.

Thanks for any insight.

I don't think they're the same situation. The aerocap is in free air while the underbody is a plenum with four open sides and a moving roof. There will be interference drag and the wake of the four wheels.

The only 'fin arrangement' I can think of are the canted side skirts under some semi trailers. And the strakes in this illustration:

https://ecomodder.com/forum/member-f...dynamics-c.jpg

"As beneficial" may be a stretch.

I was thinking something similar to the under skirts on a semi trailer but much smaller and originating at the center under the front bumper going rearward to the rear tires. Like a ships prow, but just the edges. I'm no artist by any means, but I drew this here at lunch. It probably only confuses this more though.

Not so confusing at all.

https://ecomodder.com/forum/member-f...gt-one-tm1.jpg
ToyotaGT One TM1

Current practice puts it above a full bellypan.

Do you think two big strakes would outperform more, smaller ones?

I really had no idea what it would do or if it would work at all. The general thought I had was if some of the flow was moved from under the bed, it might lessen the negative effects. And if it had any benefits, it might be cheaper and easier, at least for some people, than a full belly pan.

Again, more of a mental exercise, but also a way to get to open a discussion, either positive or negative, on something other folks may have thought of.

I failed to get an angle that shows this off, but this Vette had skegs or difusser fences that flaired from inside the wheel to flush with it's outer face.
https://ecomodder.com/forum/member-f...bonnevette.jpg
One could do similar behind the front wheels and then fill in with two or three more pair. It would need A-B testing.

Something similar to that but originating at the center of the front bumper area and ending about where the D in Goodyear is at the rear wheels or slightly ahead of that. Maybe not so close to the ground though. Just something to guide the flow that direction like the cab spoiler guides the flow in the half tonneau arrangement.

shaped/influenced
 

1) The 'wing/ half-tonneau' values you quote are only for a circa-1988, half-ton, Ford F-150, long-bed, of Cd 0.535, and 31.6-sq-ft frontal area.
2) Any later evolution of the F-150, or other legacy maker 1/2-ton would be an unknown quantity.
3) The 2012 F-150 received Brett Herndon's patented 'Aerolid' and it registered a delta Cd 0.067.
4) We have no wing / half-tonneau data for that truck.
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5) As to the underbody, no one has officially tested a specific truck, back-to-back, comparing the difference between an airdam, and full belly pan with diffuser.
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6) Successful operation of a 'fin' is predicated upon 'clean' air. Clean air does not exist downstream of an airdam. Clean air would only exist along a proper belly pan.
7) There would be some clean air 'below' the elevation of the airdam, although fins would add frontal area, drag, and interference drag, not appreciably improving anything, as they cannot undo the damage from the turbulence 'above.'

skirts
 

If rocker panel extensions, the depth of the airdam, are added behind a wraparound airdam, all the way to the rear, a pool of dead air can be captured within its perimeter.
Underbody flow will travel over this upside-down pool of stagnant air, almost as if it were a solid panel. Short of an actual belly pan, it's about the best you could hope for.
You would lose the benefit of a diffuser ( quite significant! ), as they cannot function in the absence of a belly pan. ( A Cd 0.025 penalty on the 1983 Audi 100-III )
The 1983 Pontiac Firebird Trans Am is an example of one car which benefitted from this technology.

under the truck
 

The flow below the truck does not possess the kinetic energy present in the 'upper body' flow, which creates the vortex captured underneath the 'wing' and tailgates upper trailing edge; which is telegraphing the low pressure of the vortex core, under the open tonneau, to the inner face of the tailgate. That provides the drag reduction!
Pickup trucks don't have a rear 'valence' which might act like a mini-tailgate. Sometimes they don't even have a rear bumper.
You'd be expecting a pressure phenomena which is structurally impossible for it to exist.