First Electric van boat tail

[JSH]

Originally Posted by JohnForde

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.

GM designed the rear trim for the aerodynamic requirements of the van in stock conditions - not to keep the boundary flow attached to a streamlined tail. Yes, there are other considerations besides straight aerodynamics.

It has been about 5 years since I was responsible for installing aerodynamic aids and talked routinely with our design engineers that do aerodynamics but spray control is another consideration. Keeping things like mirrors, cameras, and windows from accumulating grim and dirt.

You have a good plan - build out a prototype and test it. What looks good on paper doesn't always work , nor does what works in computer simulations or even wind tunnel testing. The road tells the true tale.

[aerohead]

Originally Posted by JohnForde

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.

1) In Hucho's 2nd-Edition, Aerodynamics of Road Vehicles, back in the chapter on 'Commercial' vehicles, he shows a Cd breakdown for a 'van' or 'bus'-like vehicle. 'Softening' of the trailing edges of the van with a radiused surface, of around 7% of the square-root of the frontal projected area 'did' develop an overall Cd reduction ( I don't have that with me so I can't say ).It would also affect yawing moments in crosswind gust, as Airstream uses in the very stable RV travel Trailers.
2) The 'cuff' would be subject to the same 'laws' as affect the 'aft-body' of a dimpled golf ball, except that the ZEVO's turbulent boundary-layer thickness would be 'enormous' compared to the golf ball, limiting momentum transport from the outlying inviscid free stream, to the shear layer of the TBL.
3) Qualitatively, on your 'cuff', the TBL would be unstable @ 22-degrees, with full flow separation by 25-degrees.
4) The 'cuff' isn't 'large' enough as a 'lead-in' region, to adequately direct the flow towards the 'base', as those used by Jaray, Lay, Breer, Reid, Fachsenfeld-Kamm, Korff, Mair, NASA, Daimler-Benz, etc..
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For ' rapid change in object shape or surface direction' ( Tharanga Jayathunga ) boat-tails:
on the Cd 0.295 'Windsor' body, 8-degrees, top and sides provided the lowest drag, Cd 0.2419 ( 18% ).
By 'rounding' the edges of the same tail, drag fell by a total 31%, @ Cd 0.2035.
This boat tail constituted a length equal to 20% of total body length of the vehicle.
PhD Jeff Howel et al., at Loughborough University, UK, have also investigated the 'Windsor' body. With a 'full-length boat tail they got down to Cd 0.133.
As long as the tail can support attached flow, the new drag coefficient will be simply a function of the new wake area, compared to the original.
If your 'target' remains a 20% drag reduction, you just need a tail that will produce a wake which is 20% smaller than what you've got.
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The ZEVO is 109.0" tall, and 106.0" wide, for a gross area of 80.2361-sq-ft.
The frontal area is in the neighborhood of 69.6024-sq-ft. You'd be looking for a wake of 55.68 sq-ft.
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In 1944, the Luftwaffe's bombs used a tail-cone of 13-degrees contraction slope, giving Cd 0.11, same as a Peregrine Falcon, or Emperor Penguin. I wouldn't exceed this angle, unless you build 'curve' into your final design.

[aerohead]

I ran an energy balance for the ZEVO, based on the reported range performance for:
63-mph, 8,800-lb test weight, approx. 69.6024-square-feet frontal area ( 6.4662 meters-sq. ), 178-kWh 'useable' ( 5.2811-gallons gasoline equivalency ), 100% SOC-to-Zero% SOC, 360-miles range, 68.1674-mpg-e, 494.444 Wh/mi., 31,149.999-Watts/hour, 41.7728- bhp-e, @ 95% BTE-e, 39.6841-hp Road Load, best-case Cfrr= 0.0055 lb/lb.
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Using the test weight, Cfrr, and road speed, I got a Rolling-Resistance horsepower requirement of 8.1312-hp.
Subtracting this from the road load leaves 31.5529-hp left over to satisfy the road aero power requirement.
Using standard air, @ rho= 0.002378 slugs, 92.4-feet/second air velocity, 550-lb-ft/sec ( horsepower-work conversion factor ),the stated frontal area, the only drag coefficient which can satisfy the energy balance is Cd 0.2658, for a CdA 18.501-sq-ft ( 1.7188 meters-sq ).
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Conditions/caveats:
* Steady 63-mph travel
* Calm
* Dry road
* Smooth concrete road
* Flat road
* Air-conditioning 'OFF', 'shutters' closed
* 100% belly pan
* Flush glazings
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GM's website for the Brightdrop ZEVO 600 provides very little pertinent data, necessary for a 'proper' analysis of the vehicle.
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A 20% reduction in wake area would yield Cd 0.21265, an 18.88% increase in range ( 428-miles ), @ 415.8 Wh/mi..

[JohnForde]

Thank you Aerohead! Your numbers are very accurate. The Zevo has been consuming 526 wH/mile, expending 33 kW to maintain 63 mph. (1.9 mile/kWh)

I did my first road test with the new WT yesterday, about 100 miles and saw no noticeable improvement over stock. After shaking off the depression I remembered I designed the structure to be adjustable and have a plan for how to fix what's wrong.

The cuff of 19" at 17 degrees is creating airflow separation. I am going to add a spacer to set my doors 2" closer to the edge on each side. I will then use a 6' x 4' coroplast to completely cover the gap and the door. I will tape the leading edge to the Zevo's body about 12" in front of the cuff. I should still be able to adjust the doors between 5 & 20 degrees.

When I get new tail lights down low on the step bumper I will be able to extend the doors 24" lower.

[freebeard]

Tuft testing?

[JohnForde]

Originally Posted by freebeard

Tuft testing?

Yes! I will tuff test after adding the rounded 6'x4' coroplast,

[ennored]

Originally Posted by aerohead

the only drag coefficient which can satisfy the energy balance is Cd 0.2658, for a CdA 18.501-sq-ft ( 1.7188 meters-sq ).

I've been told it's actually in the 0.37 neighborhood. Worse than the Rivian EDV which is supposedly about 0.05 better. Neither have been in a big enough tunnel to get a number that can be trusted though.

[aerohead]

Originally Posted by ennored

I've been told it's actually in the 0.37 neighborhood. Worse than the Rivian EDV which is supposedly about 0.05 better. Neither have been in a big enough tunnel to get a number that can be trusted though.

I finished the calcs for 'Winter', and 'Seasonally-Averaged' and I'll put them in their own permalinks.
I was unable to find an official:
* Cd
* Af
* Tire size ( width )
* Tire type
* Ground clearance
* I'm unsure what John's actual weight is during travel.
* Unaccounted unknowns for:
elevation
wind
precip.
air density
curve drag
underbody quality
actual rolling-resistance coefficient
'traffic'
regen

[aerohead]

I ran the numbers for the reported 'Winter' performance.
@ 275-miles range:
* 647.27-Wh/mi
* 40,778.18 Wh/hr
* 54.684-bhp-e
* 51.95-hp Road Load
* using same 8.1312-hp rolling-resistance power absorption
* 43.819-hp aero road load
* Plugging into the drag power equation, with Af 69.6024 sq-ft, 92.4 feet/second velocity, air density @ 0.002378 slugs, and solving for Cd.......
* Yields exactly Cd 1.0000 vs Cd 0.2658 'Summer'

[aerohead]

Using simple average between the 'summer' and 'winter' energy consumption yields a 'seasonally-averaged' 570.85-Wh/mi.
* @ 63-mph, this yields 35,964.07 Wh/hr.
* converting to brake-horsepower yields 48.228-bhp-e.
* using the industry-average BTE-e of 95%, yields a Road Load of 45.228-hp.
* subtracting out the estimated 8.1312-hp rolling-resistance power absorption yields 37.685-hp aero power absorption.
* plugging into the aero power equation with the unaltering standard air, velocity, and Af, spits out a 'seasonally-averaged Cd 0.3174.
NOTE: Cd 0.3174 compares within 98.9% agreement with A. Gilhouse's Cd 0.314 simple wind tunnel model results, of 1981, for 'softened'-nose, roof edge radii, and rear radii, on a 'bus'/ 'motor coach.'
Qualitatively, the Gilhouse 'stromform'-nosed model of same features as the above model, could produce a Cd in the neighborhood of 0.294. The nose of this model better represents that of the ZEVO 600.
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If we take the more conservative Cd 0.3174, the target drag reduction of 20% would occur with a 'wake' area of 55.68 sq-feet, @ Cd 0.2539.