Pickup truck aerodynamics

[aerohead]

Where the more-steeply falling roofline of the 'pup' is, technically, that steeper contour is going to create a flow deceleration and pressure increase, exceeding the 'threshold' for which the boundary-layer can remain attached.
It's the 'adverse pressure gradient' taught about in boundary-layer theory, always responsible for flow separation, which in turn is the reason for pressure drag, the main component of aerodynamic drag. To be avoided at all cost.
Lofting that contour back up to where the bedcover contour would continue, would reaccelerate the flow, restore a favorable pressure gradient, reattach the flow, and give you back, the otherwise lost pressure recovery, which will cut drag and rear lift, plus give you a little more useful storage volume.

[aerohead]

Originally Posted by j-c-c

Where the above 5 aero mods in any particular order?

Not really. Mr. Clark just presented them as a suite of five mods, that, taken together, would net that particular drag reduction, on that particular pickup.

[BlackGold6.4]

Originally Posted by aerohead

Where the more-steeply falling roofline of the 'pup' is, technically, that steeper contour is going to create a flow deceleration and pressure increase, exceeding the 'threshold' for which the boundary-layer can remain attached.
It's the 'adverse pressure gradient' taught about in boundary-layer theory, always responsible for flow separation, which in turn is the reason for pressure drag, the main component of aerodynamic drag. To be avoided at all cost.
Lofting that contour back up to where the bedcover contour would continue, would reaccelerate the flow, restore a favorable pressure gradient, reattach the flow, and give you back, the otherwise lost pressure recovery, which will cut drag and rear lift, plus give you a little more useful storage volume.

So during research the steeper back on the cap is what works but for my build it will depend on function and fitting gear required, for example; where the mattress fits above the pull-out gear drawers or how ever it's stored will determine where this sloop at the back sloops to. This will also determine how the pup on the back fits too. I won't have a whale tail out back but as what will fit for gear required while maintaining departure angle.

[aerohead]

I understand the design specification for the more gentle down slope on the 'cap.'
What stood out to me was, the more aggressive curvature on the roof of the swing-away 'pup' storage unit.
If there's a specific reason to do that, then it will be what it will be. If you can 'relax' the curvature, you'll get some additional drag reduction.
22-degrees maximum downslope on any 'roofline' is a safe bet.

[BlackGold6.4]

Originally Posted by aerohead

I understand the design specification for the more gentle down slope on the 'cap.'
What stood out to me was, the more aggressive curvature on the roof of the swing-away 'pup' storage unit.
If there's a specific reason to do that, then it will be what it will be. If you can 'relax' the curvature, you'll get some additional drag reduction.
22-degrees maximum downslope on any 'roofline' is a safe bet.

Yes I see what you mean, it can definitely be slopped less even matching the slope of the cap. Also the bottom can be slopped up a slight bit too I think. This will also be designed around the gear required to store in it, the big tire being the main thing. What I need to do is show a view from top to establish the gradual turn in from the sides. I know from the models online the longer and more gradual it extends out the back the better but I'll have to sacrifice some of that.

[freebeard]

An opportunity for the Fashenfeld tearing edge.

[BlackGold6.4]

Originally Posted by freebeard

An opportunity for the Fashenfeld tearing edge.

What is Fashenfeld tearing edge?, I assuming a tear drop edge to conform with the edge from another object. Like the edge of a truck box shape next to the body.

[freebeard]

This thread has aerohead's attention, I'll let him explain.

[aerohead]

Originally Posted by BlackGold6.4

What is Fashenfeld tearing edge?, I assuming a tear drop edge to conform with the edge from another object. Like the edge of a truck box shape next to the body.

Since you asked, I get to punish you with some foundational context
* Germany lost WW-I in part because they ran out of oil and rubber ( as they would also in WW-II ).
* In the runup to WW-II, Germany found out that a guy named Freiherr ( Baron ) Reinhard von Koenig-Fachsenfeld held patents on aerodynamics which might help Germany stretch strategic materials like oil.
* In exchange for the use of his patents, Germany created the FKFS, next door to Daimler-Benz ( Mercedes-Benz ) in Stuttgart,, giving Fachsenfeld free run of the place, and all its resources.
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* Koenig-Fachsenfeld is actually 'Kamm', in the context of the 'Kammback', as he held the patent for the 'K-form' roofline, something Dr. Kamm lectured about, but was not given official credit for it's birth.
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The 'tearing edge' was developed during 'Koenig's' R&D of the 'Kammback.'
* A roofline with a downslope contour/silhouette too steep to protect the boundary-layer was 'lofted' by the tearing edge, which:
1) pinched the air's pathway to the rear of the car.
2) this pinching re-constricted the streamlines.
3) since narrowly-spaced streamlines create 'faster-moving' air, the tearing edge is simply re-accelerating the flow,
4) which lowers the pressure,
5) from an adverse ( higher )pressure gradient ( responsible for flow separation ),
6) to a 'favorable ( lower ) pressure gradient' necessary for attached flow ( Dr. Thomas Wolf of Porsche refers to it as 'increasing the negative pressure gradient' ).
7) When the tearing edge reaches out to the 'reversal point,'
8) the nearest streamline re-attaches to the body, ' mending the flow' ( Goro Tamai ),
9) increasing pressure recovery [ which is the sole reason for 'aerodynamics' ( Hucho )].
10) When the air finally reaches the new 'trailing edge' of the tearing edge, and breaks away,
11) it's flowing at the slowest velocity ( Bernoulli Theorem ),
12) at the highest pressure ( Bernoulli Theorem ),
13) creating the highest base pressure in the wake ( direct measurement ),
14) which is the entire goal of 'aerodynamics.' ( Hucho ).
15) If the 'whole' tail of the car is present, there'll be 'zero' separation,
16) zero 'pressure drag',
17) only surface ( skin ) friction,
18) for lowest 'Total' drag.
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You could consider this a micro-course in boundary-layer theory, and all you would really need to know as an ecomodder. Or spend $75,000 to get you through your junior year in mechanical engineering, to learn the same thing.

[BlackGold6.4]

Ok cool, this would be the gradual slope I was thinking about that I can't do to the pup because it would make to long. So another thing I saw was small "bumps" placed a few inches apart along the leading edge to kinda force the air to follow the contour down. Have any of you seen something like this. Those bumps sorta looked like the clearance lights on a HD truck but closer together.