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Old 08-31-2017, 04:16 AM   #671 (permalink)
s_t
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Quote:
Originally Posted by freebeard View Post
I wouldn't worry about it too much. The Template presumes taper in plan to equal the taper in elevation, and a half-circular cross section. The racer's aphorism is "It isn't how you punch the hole in the air, it's how you close the hole in the air." IOW the rear is more important than the front.

That said the bluff forebody of the Template insures the flow is uniform enough that it can support the taper without shedding vortexes. Any disruption such as the windshield header you mention can spoil airflow 'downstream'.
Thanks. Been looking at more modelling and theory to try and understand it, and seems I was wrong about the 'high pressure' just above the windscreen. It's high velocity; which is low pressure I understand. But the transition between roof and windscreen would have to be rounded enough to prevent turbulence? Which i gather is what shedding vortexes are? Any guidelines on acceptable radii in this area?


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Why not go with the curved header and then a Tropfenwagen style rear with a flat top and tapered sides? It would have more interior space.
To avoid taking this thread off-topic I'll reply to this on my other thread (35515).

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Old 08-31-2017, 12:39 PM   #672 (permalink)
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The guideline for rectangular bodies is 4% of the gross width. Think of U-Haul trailers.

My basis for comparison is always the VW Beetle. The original Beetle had a windshield header that rolled 90 to meet the glass. Over the years the windshield was increased in height, later models had the header meet the glass at more like 45. But the overall contour didn't change.
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Old 09-02-2017, 02:18 PM   #673 (permalink)
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rounded enough

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Originally Posted by s_t View Post
Thanks. Been looking at more modelling and theory to try and understand it, and seems I was wrong about the 'high pressure' just above the windscreen. It's high velocity; which is low pressure I understand. But the transition between roof and windscreen would have to be rounded enough to prevent turbulence? Which i gather is what shedding vortexes are? Any guidelines on acceptable radii in this area?




To avoid taking this thread off-topic I'll reply to this on my other thread (35515).
The big dogs will tell you that there is no 'magic' radius for forebody edges.
4% of the square-root of the frontal area of the body itself is still a good guide.
In the forebody,up to the location of maximum body cross-section,there is a positive pressure gradient as the vehicle attacks the air in front of it.The air is essentially held against the body by this impingement.
A look at a 1975 VW Rabbit/Golf will show you a working minimum for radii.There is fully-attached flow everywhere on the VW's forebody.
It's the aft-body where the challenge comes,as all this area is in a unfavorable pressure regime,with no reason for the flow to remain attached,unless the body cross-section varies as it would in a streamline body.Hence the Template.
Any vehicle manufactured after 1982 would have respected Hucho's shape optimization techniques published in 1978.
Since forebody drag constitutes less than 6% of the overall drag,it's better to spend your energy and money on the aft-body.
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Old 09-02-2017, 07:03 PM   #674 (permalink)
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https://en.wikipedia.org/wiki/Audi_80

Maybe aerohead knows the improvement between the Audi 80 and the B1 Passat. DuckDuckGo gets confused by the Audi CD turbodiesel.
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Old 09-30-2017, 04:54 PM   #675 (permalink)
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Finally,confirmation on the shape

'finally got to test the 'Template' at DARKO,September 15,2017, 9-years after the latest 'Template'-C thread was published.
As a full-tail she came in at Cd 0.1209.
At Kamm's recommended 50% frontal area truncation,she measured Cd 0.1487.
With a 2% race-inspired cooling system she'd grow to Cd 0.1233.
As a mirror-less 'camera' car she'd remain at Cd 0.1233
A single,legal,SCCA-type race mirror would bump it to Cd 0.1283.
The 1/3-scale model has arbitrarily-wide fairings,to accommodate strap fastening to a ground-board.I suspect that the drag could be trimmed when at 1:1 scale.
IVECO has an 18-wheel, tractor, 'Glider', with a zero-porosity cooling system,
Such technology would allow for a zero-drag cooling system.
Also,with such little horsepower required to propel the 'Template' at highway velocity,there'd be little heat flux to deal with in the first place.
The model required 26-horsepower to overcome air resistance at an indicated 200-mph according to measurements.
NOTE:
During smoke flow imaging we could not exceed 30-mph air velocity in the test section,placing the body in a sub-critical Reynolds number flow regime.Streamline filaments diverged as expected,with smoke traces widening.A wake survey conducted with smoke revealed strong surface shear all around the transom,indicating fully-attached aft-body flow even at low Reynolds number.
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Old 09-30-2017, 05:23 PM   #676 (permalink)
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What is the approximate percentage of length discarded for the Kamm?
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Old 10-07-2017, 12:07 PM   #677 (permalink)
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percentage of length

Numbskull Knox never bothered to record it! I'll have to get back to you on that.Apologise for the delay.
Lookin' at the drawing,I'd estimate that the truncation is @ around 45% aft-body,measured from the roof apex,or,zero-point.

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Last edited by aerohead; 10-07-2017 at 12:10 PM.. Reason: add data
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