First Electric van boat tail

[Piotrsko]

There is a wiring harness somewhere along the frame or in an accessible spot up front. Find it and tap into that. The exception to this would be a land rover that has the harness in hollow body channels along the roof. They are still accessible, just got to find where they enter/exit that channel

[aerohead]

Originally Posted by freebeard

'Effective Back-Light Angle' ( we talked about here years ago ) [typically for a 'notchback' car ].
Take a straight edge, and lay it across the aft-body of a car's blueprint, resting from the backlight's header area, to the tearing edge of the car's rear deck.
Measuring from the horizon, the angle defined by the straight line constitutes the EBLA.
The term is attributed to Adrian Gaylard, aerodynamicist for Land Rover/Jaguar Cars, 2017.

[aerohead]

Originally Posted by JohnForde

Thanks JSH!
I do plan on keeping the boundary flow smooth. The width at the hinges is ~72"W. The hinges are 8 " behind the cuff which is 78"W. I will tape coroplast to the van's body to maintain this edge across the gap.
The hinges need to be 8" further aft to allow sight line to the tail lights.
The van's black plastic cuff has a 3" "rise" over 10" "run" for a slope of 16.7 degrees.
The red line in the photo is an even steeper angle to meet the orange line. The orange line is 3" below the cuff but ideal because it is in plane with the vans back up camera.
Photo in my previous post shows a lot more wood in the field of vision than is necessary.

--------------------------------------------------------------------------------------
It won't be an aspect of keeping the boundary-layer 'smooth', as it would be keeping it 'attached.'
* We know that drag is primarily as function of base pressure.
* We know that base pressure is a function of pressure recovery.
* We know that pressure recovery is a function of flow deceleration.
* We know that flow deceleration is a function of flow attachment.
* AND THE MOST IMPORTANT THING WE KNOW ABOUT AERODYNAMICS IS THAT FLOW ATTACHMENT IS A FUNCTION OF THE time-rate-of-change-in-deceleration-of-flow ( TROCIDOF ) along the aft-body contour **********!
-------------------------------------------------------------------------------------
Anything other than a 'streamlined' contour in the aft-body will trigger a 'super-deceleration' which is responsible for an 'adverse pressure gradient' which is the mother of all flow separation.
-------------------------------------------------------------------------------------
The air, immediately adjacent to the body surface is 'at rest'. It has 'zero velocity'.
--------------------------------------------------------------------------------------
The 'lowest' pressure on the body is at the windshield header, at the 'front' of the vehicle.
------------------------------------------------------------------------------------
The only reason flow can remain flowing 'rearwards', is that a horizontal vector of the kinetic energy from beyond the boundary-layer is being transported all the way 'through' the boundary-layer, literally 'pinning' the 'dead-air' against the body's surface.
--------------------------------------------------------------------------------------
If the contour of the aft-body 'converges' too rapidly in cross-section, the kinetic flow vector will simply ricochet off the top of the dead air.
--------------------------------------------------------------------------------------
Under the Bernoulli Theorem, should this happen, the flow would be required to 'decelerate', but it can't, as, it's 'always' been at 'zero' velocity.
-------------------------------------------------------------------------------------
The consequence is that, without being held by the flow vector, the flow begins to move 'forwards' ( high-pressure -to- low-pressure ) towards the lowest-pressure windshield header ( reversion ), rolling up into eddies, and finally full-blown 'turbulence' as it attempts to get there, detaching from the body as the origin the 'wake', as there's no longer anything there to prevent it.
-------------------------------------------------------------------------------------
All possibility of converting kinetic energy of accelerated flow back to static pressure at the 'base' of the body is lost forever.
-----------------------------------------------------------------------------------
As a compromise to build simplicity, you can intentionally design 'separation' into the tail, using 'straight', rather than 'curvilinear' surfaces, as long as the 'tearing edges' downstream occur where a 'streamlined' tail's surface would reside in the more complex construction. Box-cavities and Trailer Tail depend on this technology of captured-vortex, phantom surfaces. They can't perform as well as the more ideal curved surfaces, but they require only a fraction of the man-hours necessary to create the more favorable shape. You just need to be able to stomach the difference in performance.

[freebeard]

Box-cavities and Trailer Tail depend on this technology of captured-vortex, phantom surfaces.

A senior moment and the search engine is no help. Who was it that had a FIAT in the wind tunnel with a perforated base plate? It had fans in the rear wheels that pressurized the tires' wake.

[JohnForde]

Originally Posted by Piotrsko

There is a wiring harness somewhere along the frame or in an accessible spot up front. Find it and tap into that. The exception to this would be a land rover that has the harness in hollow body channels along the roof. They are still accessible, just got to find where they enter/exit that channel

Thanks for the nudge. A friend sent me a schematic pdf of where to look. Found it!

[freebeard]

I have a pair of tail lights from the Nissan Pulsar [NX? EXA?] that I like a lot. I haven't found a use for them yet, though.


3.bp.blogspot.com/-WtyJcgSxKgI/VdAqw8R2KpI/AAAAAAAAoPA/koTC39_ilDg/s1600/3.jpg

[JSH]

Originally Posted by JohnForde

I agree tail lights solve a great deal. To get at the wiring requires removing the cuff. I am working on getting info how to do that. I will have a pro do it and it will be expensive.

KEY QUESTION: Am I safe in assuming the stock "cuff" of 19 deg for 11" is designed to hold smooth flow? (diagram 1)

Extending the exact 19 deg angle for 10" more over lexan seems reasonable to me.

I can then experiment with door angle. I would love it to adhere at the same 18-19 deg. At the maximum legal distance of 48" behind the step bumper that would leave an 'aperture' of about 40", cutting aero drag in half from the 84" stock width.

If I cannot get flow to adhere one option would be to add tail lights, put the hinge an inch or two further to the side and build a rounded shallower cuff. That is a big project I am hoping to not have to do. (diagram 2)

A constant 19 degree angle is unlikely to keep the flow attached. GM did not design that rear trim for optimal aerodynamics.

You won't know until you test but if you are using straight panels I would shoot for 15 degrees. I would do some research on Class 8 trailer tails and see what angles they are using. No reason to reinvent the wheel.

As Aerohead said - it is a trade-off between build complexity, practically, and aerodynamic performance.

[freebeard]

KEY QUESTION: Am I safe in assuming the stock "cuff" of 19 deg for 11" is designed to hold smooth flow? (diagram 1)

I'd say no.

Especially when you consider crosswind conditions. Do the math, but a 55MPH wind at 90 degrees would be 45 degrees. A 10MPH wind at what sail boaters call a 'close-hauled' condition would involve trigonometry. Separation on the leeward side.

Windward and leeward
In geography and seamanship, windward and leeward are directions relative to the wind. Windward is upwind from the point of reference, i.e., towards the direction from which the wind is coming; leeward is downwind from the point of reference, i.e., along the direction towards which the wind is going. The side of a ship that is towards the leeward is its "lee side". Wikipedia

[JohnForde]

Originally Posted by JSH

A constant 19 degree angle is unlikely to keep the flow attached. GM did not design that rear trim for optimal aerodynamics.

You won't know until you test but if you are using straight panels I would shoot for 15 degrees. I would do some research on Class 8 trailer tails and see what angles they are using. No reason to reinvent the wheel.

As Aerohead said - it is a trade-off between build complexity, practically, and aerodynamic performance.

I actually think GM designed the cuff primarily for aerodynamics. But I am wondering if it is designed to allow flow separation but have some other benefit.

3" of rise over a 10" run is about 17 degrees.

Weather should be good here starting Tuesday so I may have a working, adjustable prototype late in the week and do some testing: both tuft and efficiency.

[aerohead]

Originally Posted by freebeard

A senior moment and the search engine is no help. Who was it that had a FIAT in the wind tunnel with a perforated base plate? It had fans in the rear wheels that pressurized the tires' wake.

That's gonna be Professor Alberto Morelli, of the Turin Polytechnic Institute's FIAT Tipo 'ring-vortex' research at Pininfarina.
It wouldn't apply to the ZEVO on account of the complete absence of cross-sectional vertical, and rooftop camber, allowing the 3-D relieving effect to operate, a fundamental prerequisite to the ring-vortex' viability.