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Old 10-07-2020, 12:10 PM   #861 (permalink)
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numbers @ DARKO

Quote:
Originally Posted by JulianEdgar View Post
Based on quoted top speed compared to the standard vehicle, I work it out at Cd=0.33.

Addition: When Aerohead was sending me details on his vehicle for inclusion in my book it soon became obvious that a lot of what he was saying was of somewhat doubtful validity - like an absurdly low Cd measured in a tiny wind tunnel. I then asked for the top speeds in standard and aero-modified forms, did the calculations and got a Cd of 0.37. Dr Wolf then corrected my maths and got a Cd of 0.33, the one I have quoted above. I chose not to run any claimed Cd figure next to the truck.
Some things to ponder:
1) You're comfortable publishing Cds from A2 Wind Tunnel.
2) DARKO's design consultant was Gary Eaker, designer of the A2 Wind Tunnel.
3) You have expressed a threshold of 5% blockage ratio for automotive wind tunnels.
4) Alan Pope allows 10% blockage ratio for 1:1 - scale automotive tunnels. Hucho allows 10% blockage ratio for automotive wind tunnels.
5) Hucho allows 20% blockage ratio for curved-walled tunnels.
6) Hucho allows 30% blockage ratio for adaptive-wall wind tunnels.
7) Hucho allows in excess of 30% blockage ratio for adaptive- wall tunnels if only zero-yaw measurements are targeted.
8) Hucho allows even higher blockage ratios if 'streamlined' vehicles are tested at zero-yaw in an adaptive-wall tunnel.
Please explain your conclusion about 'tiny' tunnels.

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Old 10-07-2020, 04:13 PM   #862 (permalink)
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Quote:
Originally Posted by aerohead View Post
Some things to ponder:
1) You're comfortable publishing Cds from A2 Wind Tunnel.
I quoted in my Veloce book some tests carried out by a major magazine using an "unnamed aerodynamicist in an unnamed wind tunnel". From what you've said since it appears those tests were done in one of the tiny wind tunnels. Had I known that, I wouldn't have put the material in the book!

Quote:

2) DARKO's design consultant was Gary Eaker, designer of the A2 Wind Tunnel.
3) You have expressed a threshold of 5% blockage ratio for automotive wind tunnels.
4) Alan Pope allows 10% blockage ratio for 1:1 - scale automotive tunnels. Hucho allows 10% blockage ratio for automotive wind tunnels.
5) Hucho allows 20% blockage ratio for curved-walled tunnels.
6) Hucho allows 30% blockage ratio for adaptive-wall wind tunnels.
7) Hucho allows in excess of 30% blockage ratio for adaptive- wall tunnels if only zero-yaw measurements are targeted.
8) Hucho allows even higher blockage ratios if 'streamlined' vehicles are tested at zero-yaw in an adaptive-wall tunnel.
Please explain your conclusion about 'tiny' tunnels.
Easy - car manufactures and professional race teams don't build enormous wind tunnels with moving floors because they like spending money. If a tiny wind tunnel and a fixed floor would do, they'd be using them. They don't.
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Old 10-07-2020, 04:59 PM   #863 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
I quoted in my Veloce book some tests carried out by a major magazine using an "unnamed aerodynamicist in an unnamed wind tunnel". From what you've said since it appears those tests were done in one of the tiny wind tunnels. Had I known that, I wouldn't have put the material in the book!



Easy - car manufactures and professional race teams don't build enormous wind tunnels with moving floors because they like spending money. If a tiny wind tunnel and a fixed floor would do, they'd be using them. They don't.
1) It's given, that 'tiny' wind tunnels will generate perfectly acceptable data for streamlined cars, at zero-yaw, with the condition that a blockage correction factor ,from a large wind tunnel is provided.
2) The Toyota Prius provides the large tunnel blockage correction factor. We have Cds from Toyota, A2, and DARKO for that specific car.
3) Professional race teams wouldn't necessarily be germane to ecomodding.
4) For a couple of decades, the USEPA has required cross-wind-averaged Cds from automakers, from which they can set proper loads on the dynamometers used by Mobile Sources to establish compliance with federal emissions standards.
5) Market demand for higher profile vehicles such as SUVs and pickups has changed the degree by which vehicle wheels are 'integrated' into vehicles ( or not ), making wheel drag an issue, whereas before these vehicles existed was not really on he radar.
6) Obama-era CAFE standards drove aerodynamics, of which they've now been rolled back under the present administration.
7) The cost of the tunnel is passed onto the consumer, and is a complete tax write-off, so it really doesn't hurt the bottom line.
8) Are they 'better'? Only re-testing of earlier vehicles, with and without the rolling road is the only way to quantify that one. Some of MIRA's testing showed that in some circumstances, drag due to stationary vs rotating wheels are a 'wash.'
9) Gene Haas Racing offers a rolling road tunnel to all comers. $ 4,000 / hour. DARKO, @ $500 / hour was all I could absorb.
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Old 10-07-2020, 05:05 PM   #864 (permalink)
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Quote:
Originally Posted by aerohead View Post
1) It's given, that 'tiny' wind tunnels will generate perfectly acceptable data for streamlined cars, at zero-yaw, with the condition that a blockage correction factor ,from a large wind tunnel is provided.
Er, no. I can't think of even one profesional tech paper that uses a tiny wind tunnel for full size car tests. Even at zero yaw. But hey, if you say it often enough, someone might believe you!

Quote:
9) Gene Haas Racing offers a rolling road tunnel to all comers. $ 4,000 / hour. DARKO, @ $500 / hour was all I could absorb.
It would have been far better to have spent that money on a hotel in Arizona or Utah and done some decent testing on an empty road.
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Old 10-07-2020, 05:55 PM   #865 (permalink)
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professional tech paper

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Originally Posted by JulianEdgar View Post
Er, no. I can't think of even one profesional tech paper that uses a tiny wind tunnel for full size car tests. Even at zero yaw. But hey, if you say it often enough, someone might believe you!



It would have been far better to have spent that money on a hotel in Arizona or Utah and done some decent testing on an empty road.
How about a textbook published by a seasoned professional like Hucho?
Everything I said comes from his chapter on wind tunnels.
And remember, I gave you caveats :
* only 'Streamlined' cars tested
* Tested at zero-yaw
* Curved or Adaptive walls ( A2 has an adaptive ceiling )
* With a blockage-ratio calibration model tested in a large tunnel.
All conditions were satisfied according to Hucho.
* Hucho read all the tech papers in preparation for publishing his book. I know of no revelations which have occurred since 1986, that would overturn his basic premise.
* I've done my testing either at the Chrysler Proving Grounds, Bonneville International Speedway, or DARKO.
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Old 10-07-2020, 06:18 PM   #866 (permalink)
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Quote:
Originally Posted by aerohead View Post
How about a textbook published by a seasoned professional like Hucho?
Sorry, I should have said:

Over the last 33 years I can't think of even one profesional tech paper that uses a tiny wind tunnel for full size car tests. Even at zero yaw.

I keep forgetting we're working over completely different timeframes....
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Old 10-07-2020, 06:25 PM   #867 (permalink)
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33-years

Quote:
Originally Posted by JulianEdgar View Post
Sorry, I should have said:

Over the last 33 years I can't think of even one profesional tech paper that uses a tiny wind tunnel for full size car tests. Even at zero yaw.

I keep forgetting we're working over completely different timeframes....
With pickups at 36-square feet of frontal area, compared to 31.5 back in Hucho's 2nd-Ed days, and full-size SUVs just as large, I suspect that that if nothing else, this 'size' issue has driven tunnel dimensions. Along with greater ground clearance, and faster legal speed limits.
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Old 10-09-2020, 11:18 AM   #868 (permalink)
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tiny tunnel post script

It dawned on me that, in addition to passenger cars, the major automakers must also test full-size vans, delivery vehicles like the Sprinter, and all classes of commercial vehicles, up to Class-8, as Volvo, Isuzu, and Daimler-Benz does, as with their Freightliner Tractors, to mention a few; as well as Pentagon defense products from General Motors Canada.
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Old 10-09-2020, 08:27 PM   #869 (permalink)
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Quote:
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At DARKO ...
I had asked whether 0.25 cd was the measurement, and so I will take these two words, in context, as confirmation. 'Twas much fund crowd-funding your visit to that facility here. Thanks.
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Old 10-10-2020, 02:45 AM   #870 (permalink)
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Quote:
Originally Posted by aerohead View Post
It dawned on me that, in addition to passenger cars, the major automakers must also test full-size vans, delivery vehicles like the Sprinter, and all classes of commercial vehicles, up to Class-8, as Volvo, Isuzu, and Daimler-Benz does, as with their Freightliner Tractors, to mention a few; as well as Pentagon defense products from General Motors Canada.
A typical Aerohead argument, where he goes up some blind alley dementedly flogging a dead horse. (There: two metaphors in one sentence, and they actually work together.)

Professional race teams also invest in huge, moving floor wind tunnels - and they're not testing trucks in them...

It's been known forever that small wind tunnels have major problems in giving accurate data - not only because of the blockage factor but also because the length of the tunnel test section influences flow behind the car.

Quote:
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I had asked whether 0.25 cd was the measurement, and so I will take these two words, in context, as confirmation. 'Twas much fund crowd-funding your visit to that facility here. Thanks.
Well that might help explain the emotional investment that some people seem to have in Aerohead's tiny tunnel testing, but unfortunately the physics doesn't actually care about how much people gave to the cause.


Last edited by JulianEdgar; 10-10-2020 at 05:29 AM.. Reason: Typo
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