Quote:
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.
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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.