Reverse wake?

[aerohead]

Originally Posted by Phase

Is there a distance listed on how far those vehicles are from each other in that picture above? I’m curious to how far behind a big pick up truck has to be behind my Ioniq for me to see any type of benefit

That data is in the original drag table at the source.
Chrysler Corp. also published the same thing for NASCAR drafting.
All we have data for is either buses, notchbacks, or semi-trailers.

[freebeard]

Some guy named PRK in 2012:

[Phase]

What’s the explanation for the front car benefitting? Filling in the wake behind it? Increasing air pressure behind leading car?

[Piwoslaw]

Originally Posted by Phase

Is there a distance listed on how far those vehicles are from each other in that picture above? I’m curious to how far behind a big pick up truck has to be behind my Ioniq for me to see any type of benefit

That big pickup truck probably wants to go faster than you, so just let it push you

[freebeard]

Originally Posted by Phase

What’s the explanation for the front car benefitting?

That's an interesting question isn't it.

I picture the wake as tiny, invisible clutching fingers, trying to drag the car backward, reletively speaking. The pressure difference can creep forward behind some certain body features, presumably the tail end of the wake (small narrow and close to the ground) can influence the parts further forward.

Push the bow wave of a second vehicle into it and counterintuitive things might hanppen.

[Piwoslaw]

Originally Posted by freebeard

Some guy named PRK in 2012:

What surprises me is that buses #1 & #2 (bumper to bumper) retain their Cd regardless of whether bus #3 is tailgating them or not (1st and last example).
I would think that #3 would somehow impact #2's Cd, even if to lesser effect than #2 did for #1.

Maybe we need to look at #1 + #2 as 1 vehicle, check its Cd, and see how #3 effects that?

Originally Posted by freebeard

I picture the wake as tiny, invisible clutching fingers, trying to drag the car backward, reletively speaking. The pressure difference can creep forward behind some certain body features, presumably the tail end of the wake (small narrow and close to the ground) can influence the parts further forward.

Push the bow wave of a second vehicle into it and counterintuitive things might hanppen.

I saw those "clutching fingers" also as proportional to the surface area of the wake's cross section, more or less. Not sure how wakes with same area but different perimeters compare? Going OT, sorry.

How the wake "pulls" the vehicle back is already difficult for many people to imagine, it takes a good imagination. But how the presence and distance of a tailgating vehicle effects the wake isthe next level of abstraction for me.
It seems to imply that those "clutching fingers" depend not only on the area of wake, but also on its length? So it is a 3 dimensional problem, not 2D. Plus speed adds yet another dimension... Wild.

[aerohead]

Originally Posted by Phase

What’s the explanation for the front car benefitting? Filling in the wake behind it? Increasing air pressure behind leading car?

The wake itself has a drag of Cd 0.0945.
When the second car rides the rear bumper, the lead car essentially loses it's wake. ( Cd 0.315 - 0.0945 = Cd 0.2205 )
In the two-car NASCAR draft, the two vehicles become an aerodynamic singularity, sharing one frontal area. The lead car no longer has a 'base.'
The trailing car has no dynamic pressure acting on it, except at it's windshield.
Wetted area of the trailing car is 'invalid', as it's immersed within the thickening turbulent boundary layer which behaves differently from the laminar flow of the lead car.
Base pressure is transferred completely to the trailing car.

[Phase]

How close does the rear car need to be? I’m still not seeing a distance

[aerohead]

The highest benefit is @ zero-gap, then degrades from there as shown in the pictorial graph.
This graph was intended for the 'fully-boat-tailed, gap-filled trailer' invention, as well as the bus pictorial graph.
You can see where the sedan hits Cd0.315 @ infinity, it's 'naked' Cd.
At that distance between the two identical cars there's no effect.
The buses react differently depending on position, showing a change at distances of 20-meters, 62.5-meters, and 50-meters, depending.
The NASCAR racers are Dodge Chargers, circa 1968. You could do a search for their exterior dimensions. That's what I did.

[aerohead]

If link works
http://https://www.researchgate.net/...fig2_235908987