Tribute to Syd Mead or F-117 aerodynamics?

[Vman455]

Originally Posted by Hersbird

The problem is his scale on the Ram and the Ford is silly small, it's like he modeled a toy car and is running that without changing the dimensions. Doesn't a smaller, shorter car make it more difficult for air to reattach? By making the Ram and Ford toy sized wouldn't that give them disproportionately high Cd numbers in the comparison?[/url]

Yes. If he really didn't adjust the size (it looks like the dimensions are 1/10 what they should be), that will throw off the Reynolds number, which is proportional to length, and you won't get an accurate modeling of the airflow over the real truck unless you increase fluid speed to compensate.

[freebeard]

Did someone say smaller truck?

https://www.youtube.com/watch?v=l_Z_xNx9frA

He wants two passengers and a center console. I think three passengers and the Bollinger doghouse door would be cooler.

[redpoint5]

Originally Posted by Vman455

Drag coefficient does vary with the inverse square of speed*, but because of that inverse square relationship it's so small as to effectively be constant over the speeds a passenger car sees.

So that guy with the CFD program doesn't know what he's talking about at all then?

Last night Musk tweeted:
With extreme effort, Cybertruck might hit a 0.30 drag coefficient, which would be insane for a truck. Requires tweaking many small details.

"Extreme effort" is code for not gonna happen because effort is greater cost and/or reduction in aesthetic quality. So he basically said it's gonna be above .3

[Hersbird]

Originally Posted by Vman455

Yes. If he really didn't adjust the size (it looks like the dimensions are 1/10 what they should be), that will throw off the Reynolds number, which is proportional to length, and you won't get an accurate modeling of the airflow over the real truck unless you increase fluid speed to compensate.

I believe you can get close with a model but you also have to scale the speed down the same. It sill will give a .05-.1 higher Cd even then. Better would simply use the real height and width as it's not a real wind tunnel but a computer simulation.

If it's already over .3 then adding wipers and mirrors is not going to get any better. Only if by "extreme" they mean ditch the fender flairs, keep the tires inside the body and maybe even flatten the roof. The roof may not change the CD much but dropping 3-4 square feet off the frontal aera will be even better.

[Hersbird]

Originally Posted by redpoint5

So that guy with the CFD program doesn't know what he's talking about at all then?



"Extreme effort" is code for not gonna happen because effort is greater cost and/or reduction in aesthetic quality. So he basically said it's gonna be above .3

Now inside EV's is linking that garbage Tesla Ram Ford comparison.

https://insideevs.com/news/385397/te...rd-raptor/amp/

[Vman455]

Originally Posted by Hersbird

I believe you can get close with a model but you also have to scale the speed down the same. It sill will give a .05-.1 higher Cd even then. Better would simply use the real height and width as it's not a real wind tunnel but a computer simulation.

Scale *up* the speed. Reynolds number is proportional to test length and speed, so if length decreases speed must increase to achieve the same Re.

[Vman455]

Originally Posted by redpoint5

"Extreme effort" is code for not gonna happen because effort is greater cost and/or reduction in aesthetic quality. So he basically said it's gonna be above .3

After the Model 3 was unveiled he tweeted that they were aiming for 0.21, and that car ended up at 0.23. So I read this the same as you--it isn't going to hit 0.30. (I'm also remembering Mercedes 2010 announcement that within 5 years all their cars would be at Cd 0.20. 9 years later, we're still waiting).

[aerohead]

Originally Posted by ldjessee00

Not sure I saw someone compare the Cybertruck to the Citroen Karin from the 80s...

Citro?n Karin

The forebody is quite similar,and online,there are images of forebody wind tunnel tuft-testing of cars similar in architecture,perhaps by Bertone (sorry,didn't flag the maker).
The penalty for the hard edges of the forebody/A-pillars is only Cd 0.008 (2.5% on a Cd 0.35 vehicle).
Good call on the Citroen!

[aerohead]

Originally Posted by redpoint5

That guy claims the truck has something like a .41 Cd at 60 MPH, and somehow it drops to .3 at 88 MPH. How does Cd change with speed? I thought it was a constant value.

Up to the debut of the Cybertruck,there's been nothing in the aerodynamic literature to suggest that between 20-mph,and 250-mph,the drag coefficient would ever change.Once beyond the supercritical Reynolds number the Cd would remain constant,until transonic flow,and attendent compressibility effects began to drive the drag higher with localized shockwaves.
Also,in Justin W. Martin's CFD presentation, he's saying that the Cybertruck's Cd is below nearly any 1/2-ton pickup,with the 2019 RAM 1500 @ Cd 0.357 as the current industry benchmark for lowest drag.
We really need to see the Tesla at 8-inches ground clearance,with its body inclination optimized for highway cruising,as Ford did with their Probe-IV and Probe-V concepts.It's entirely possible that the Tesla's belly can become a full-length diffuser, front airdam can deploying downwards,and same for an optimising rear diffuser.Tires partially shielded.

[aerohead]

Originally Posted by Hersbird

His .40 seems reasonable but he posted a 2nd video with some comparisons. The problem is his scale on the Ram and the Ford is silly small, it's like he modeled a toy car and is running that without changing the dimensions. Doesn't a smaller, shorter car make it more difficult for air to reattach? By making the Ram and Ford toy sized wouldn't that give them disproportionately high Cd numbers in the comparison? It's just more bias. We know what the Ram can get and it's .36 Cd.

https://youtu.be/7741YdnAcR4

The scale won't be an issue as long as he selects a velocity which will ensure supercritical Reynolds number,and fully-turbulent boundary layer;something aerodynamicists refer to as varisimilitude.