Originally Posted by aerohead
I laid in bed 'til after 2:00 AM thinking about it:
1) The black rubber 'corner' transition on each side is 'loose' aerodynamically. They need to be 'conformal' to a 'round-to-square' transition, and 'airtight', plus the region behind them, all the way to the trailing edge.
2) At the bottom, the 'floor' is positioned too 'high' aerodynamically, and it doesn't extend as far as the trailing edge of the tail.
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3) At the top, the longitudinally-attached vortices which would be present on a semi-trailer's 'all-square' edge boat tail, would not 'present.'
4) The pressure recovery would be contaminated by the leakage, as you've mentioned.
5) At the bottom, the belly pan of the ZEVO does not project into the new length, so it's laminar flow has become 'all-turbulence'. On the semi-trailer it doesn't matter, as, even though it is 'skirted' along the sides, there is no 'belly' ( NASA is the only semi-trailer project which ever had one ), and in between the skirts there'd only be turbulence. On a 'car' this might make for a delta-Cd 0.015 drag increase. Delta-Cd 0.035 increase if there were a working rear diffuser.
6) Between the top and the bottom, there could be enough added drag in play, as to 'occlude' the benefit of the tail.
------------------------------------------------------------------------------------- It all has to do with the Bernoulli Theorem:
* ZEVO's forebody has 'displaced' 'calm' air, which was 'all - static - pressure', and accelerated it to dynamic flow, of mandatory 'lower-static -pressure', the primary SOURCE of aerodynamic drag of road vehicles.
* The tail is attempting to give the flow an 'off-ramp' from this accelerated condition, 'de-displacing' the streamline filaments along a diminishing cross-section, towards the position they resided in before ZEVO came along.
* If the tail has 'holes' in it, the low pressure ' forwards', where the flow is faster, telegraphs through the 'breach', diluting what would have otherwise been higher pressure from the separation line at the 'rear' of the tail, moving at its lower velocity.
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With project 'Spindletop,' the shape of the diffuser section of the CRX's tail made over a 1.5-mph difference in top speed at Bonneville, almost a 5% difference in Cd.
Jeff Howell et. al., at Loughborough University, is getting a Cd 0.295 Windsor body, down to Cd 0.133 with a full 3D boat tail, of 'rounded' edges, and lacking a diffuser of any kind.
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