First Electric van boat tail

[Piotrsko]

Yup that's how I thought it worked. Did think part of turbulent was not attached, hence the added drag. thought with vg the bubles are smaller hence less drag than you get with natural unattached flow.

[aerohead]

Originally Posted by Piotrsko

Turbulent is attached? If that's true, gonna have to do some serious rethink.

1) The flow separation off the steeply-inclined upswept tail is the source of the fuel-robbing drag.( upside down Porsche 911 RS Carrera ).
2) The low core-pressure of the vortices' center of circulation , induced by the VGs, provides a more 'negative pressure gradient, and 'vacuums' the separated TBL back towards the fuselage boundary.
3) With flow attachment reaching further rearwards on the tail boom, there's additional flow deceleration before the flow finally does separate.
4) With additional flow deceleration, we have some additional pressure recovery.
5) That pressure recovery increases base pressure.
6) The increased base pressure reduces the pressure drag.
7) The lower pressure drag allows a lower overall drag, impossible with the larger initial turbulent wake.

[aerohead]

Originally Posted by freebeard

.

TBL is our 'friend.'
* Some momentum from the 'ideal flow' just 'outside' the TBL is transferred by viscous-shearing all the way through the TBL, to the body's surface ( boundary ), something a laminar boundary-layer cannot do.
* All the air immediately adjacent to the boundary is 'dead.' It's completely at 'zero' velocity.
* Since the 'minimum pressure' on the body is 'forwards ' of the aft-body, all the increasing-pressure, aft-body flow 'wants' to be 'forwards' of the aft-body.
* The vertical momentum vector component colliding with the 'dead' air adjacent to the body, is the only thing 'holding' it in its place ( as if a slightly elevated machine gun were 'strafing' an area ).
* If the body surface is angled in excess of just a few degrees more than that of the 'local' streamline's orientation, there won't be enough vertical force vector transmitted into the boundary ( the atmospheric 'bullets' simply 'ricochet' off the top of the dead air ), and the boundary air will begin to 'migrate' forwards, seeking the low pressure upstream.
* This is the 'genesis' of flow separation.

[JohnForde]

https://photos.app.goo.gl/Z1gMN9wTwVyc5UFY7

[aerohead]

* Your fabrication looks 'good to go' for testing. I like to think that the tape will be good for 63-mph.
* I looked at P. W. Bearman's Windsor body testing from 1984. At a 15-degree rear slope, vortex-drag had increased by 300% compared to zero-slope.It would continue to grow to 900% by 32.5-degrees slope, where it finally 'burst', creating an all-squareback wake, as if it had no slope.
* On the Ahmed body, drag minimum occurred at a slope of 9-degrees.
* Clarkson University reported 10-degrees for their 'sweet spot '.
* And the Lawrence Livermore Lab. 11-degrees closely-matched NASA's Ford Econoline-based 'Shoebox's curved boat tail's lead-in' contour, even though it was 'curved.
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The 'empirical testing results' of actual aft-body contours appears to corroborate the historical strictures of Professor Hermann Schlicting's 'Boundary-Layer Theory '. Dr. Wolf-Heinrich Hucho studied under Professor Schlicting, and he emphasized that all of these 'historical' trends, observed for well over a century now, should be used to inform the direction of our own research.
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I think the takeaway for us is that:
* Just as has been said by all the 'pros', the surface of body must always 'live' within a few degrees of the local streamline.
* If the body cross-section varies to 'quickly', we'll introduce a super-deceleration of the boundary-layer.
* This super-deceleration will introduce the 'adverse pressure gradient' responsible for flow reversion, eddy-rollup, separation, and turbulence.
* Once separation/turbulence has occurred, any hope of additional pressure recovery is lost forever.
* Base pressure will be 'low'.
* Pressure drag will be 'high'.
* Total drag will be 'high'.
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For simple, prismatic boat tails, with no curvature, the pressure regimes which remain within the 'Goldilocks' zone of attached flow, appear to reside inside the range of 10-degrees-to-11-degrees.
What John Forde's R&D may provide us for the very first time in aerodynamic history is, how far can we 'stretch' that zone by introducing vortex generators into the calculus.

[JohnForde]

VGs installed
Doors full height after new low tail lights
Top view with angles

[JohnForde]

Seeking advice.
Today I thought for sure I would see an appreciable decrease in drag. My sample was only 15 miles but any improvement was negligible. I have a 4 sided box 66" long at 16 degrees. The entry cuff is curved HDPE at about 11 degrees. The top is 16 degrees. There are vortex generators on top & both sides.
The Zevo is 84" W. The WT doors 'aperture' @ 50"W.

I cannot do a tuft test because I do not have a partner/chase vehicle.

Included are 6 photos and a video. Thoughts?

https://photos.google.com/photo/AF1Q...YZmJw5oCr5sSsJ

[CigaR007]

Top seems to be too steep at 16 degrees. I would try 11 degrees on all 3 sides and flat at the bottom, based on the referenced study posted in this thread. Pressure gradient must be all over the place. Very difficult to assess without any visual cues (ie. Tufts testing)

Is the structure "air-tight" at the attachment location and along the sufaces ?

[freebeard]

I cannot do a tuft test because I do not have a partner/chase vehicle

ecomodder.com/forum/showthread.php/measuring-aerodynamic-pressures-40081.html

Julian Edgar favored the Dwyer Magnahelic shown at Pressure Testing Toolkit, there's a link there to a DIY howto. You can run tubing the length of the vehicle.

[JohnForde]

Originally Posted by CigaR007

Top seems to be too steep at 16 degrees. I would try 11 degrees on all 3 sides and flat at the bottom, based on the referenced study posted in this thread. Pressure gradient must be all over the place. Very difficult to assess without any visual cues (ie. Tufts testing)

Is the structure "air-tight" at the attachment location and along the sufaces ?

Pretty close to air tight. The hole high in the center is actually plexiglass to allow the clearance lights to be seen from the rear. There is about a half inch gap there. I may tape it.