Cd and blunt front ends

[ttoyoda]

So as an extreme example, when you have a tailgater 'drafting' behind your ultra slick car, he is actually ruining you aerodynamics by following too closely ?
You are pulling him in your wake, and if it is a large SUV then he just ruined your aero big time.

I don't know about that. I would argue that if your tail-gator is close enough, and most any shape, he acts as an extended tail. The flow gets fully filled in around your tail, and the tail-gator gets the separation drag behind his tail (assuming his flow seperates). Don't forget that the tail-gator is providing power to keep his vehicle moving.
If you were to take a teardrop shape and cut it in half, (the front half is the semi-circle and the back half is the triangle section) but keep the two halves together, and measure the drag force on the front half and the rear half, each would contibute to the total drag.
I do notice that if a huge truck tailgates me I can feel the car speed up, but the following distance of the truck is too close for me to be comfortable with.

So if I drive a Hummer and had an Aptera tailgaiting me, then the .Cd on the Dummer would go down as well since the flow would reattach onto the Aptera ??

To some extent yes. It would be best for the hummer if the front end of the aptera was cut off flat, and the aptera needs to follow close.

I have read about this sort of thing - reattaching flow and such.

Read more. This is the key to the drag of most shapes we can drive on the road, at the speeds we can drive them. Once you get to really streamlined shapes like wing profiles, and aircraft speeds, the surface skin drag becomes more significant, because there is no (little) flow separation. As the speed of the aircraft approaches the speed of sound, all the rules change, so do not make your car look like a supersonic fighter. All the calculations and equations for airflow of low speed objects assume the air is not compressible. That is obviously not really true, and is what changes everthing near the speed of sound.

Lastly, bullet shapes are chosen for many reasons, not just drag. Many bullets are supersonic as well. Any bullet that makes a ripping noise as it flies thru the air is going sonic or supersonic.

[tasdrouille]

Re the SUV - Aptera convoy.

In this case It's just about pressure drag. You got high pressure up front and low pressure behind, the differential causes the drag. The hummer will fill the Aptera's low pressure wake an reduce the pressure differential. The Aptera "gives" some of it's low back pressure to the SUV reducing again its pressure differential.

Two vehicles traveling together have less drag than the sum of their individual drag. That is one of the basic "laws" of pressure drag.

[aerohead]

Originally Posted by .Cd

Speaking of falling rain drops and such, in real life, they just look like a blob and rarely take on a teardrop shape.

I've seen high-speed photos of raindrops and they're elliptical,totally unlike a teardrop,and they are constantly deforming,as they're not solid.

[aerohead]

Originally Posted by .Cd

I have read about this sort of thing - reattaching flow and such.

So as an extreme example, when you have a tailgater 'drafting' behind your ultra slick car, he is actually ruining you aerodynamics by following too closely ?
You are pulling him in your wake, and if it is a large SUV then he just ruined your aero big time.

( In this case, it you might end up getting better MPG by driving 85 MPH and not having a string of SUVs in you wake ... right ? )

So if I drive a Hummer and had an Aptera tailgaiting me, then the .Cd on the Dummer would go down as well since the flow would reattach onto the Aptera ??

In NASCAR,the race cars have basically a Cd0.30.When a challenger pulls up behind in a close draft,the Cd of the lead car drops to 0.21,and the Cd of the drafting car drops to 0.18.Hucho addresses this in his book.GM calls it "convoy" driving and is actually being considered in the U.S. to conserve fuel.It was part of a Discovery Channel show about "Future Cars" that aired a decade or so ago.

[aerohead]

Sorry I'm very late to the party.I think Frank Lee hit it on this one.Below transonic flow ( about 250-mph ) a hemispherical fore-body is a much as you would need for attached flow.And those clever wind-tunnel guys have been able to pear it down even to less.Even Hucho's work at VW was to soften the front of the Rabbit only enough to allow attached flow.And a Rabbit doesn't look super aero.but nonetheless,it has perfectly attached forebody flow.The Camaro looks like the nose is extended,the nose is dropped,and above the parachute it looks like there is a very steep rear spoiler.All these things would add up to drag reduction.Some of the photos show the hood scoop taped up,don't know about the grille.That would be a 12-percent drag reduction right there! I don't think Cd 0.20 is a reach.

[Cd]

Originally Posted by Duffman

Gen2 Camaros VEE in the front and there is a good rake in the grill too, like I said before you are being harsh on the stock design, it's not a van or a 1/2 ton.

They have a lot of potential, but stock they were a rolling parachute.

[Cd]

So Phil, can I bring my .Cd down to under point 20 without installing a boat tail ?
( I drive a '93 Civic hatchback )

I have read that the new Tahoe hybrid has a .35 drag coefficient.
Ignoring .CdA, this would mean that the SUV would have a better .Cd rating than a Ford Focus hatch at point 36, despite having a much larger wake.

How does wake size effect .Cd ?

Thanks ! ( And Phil ...are you ever going to make it down to the Texas Mile ? It's a lot closer than Bonneville ! )

[ttoyoda]

Quote:
Originally Posted by .Cd View Post
Speaking of falling rain drops and such, in real life, they just look like a blob and rarely take on a teardrop shape.
I've seen high-speed photos of raindrops and they're elliptical,totally unlike a teardrop,and they are constantly deforming,as they're not solid.

When people talk of the teardrop shape they do not mean it literally. A water drop in mid-air wants to be a sphere due to surface tension, and water drops are very small so the surface tension forces are dominant.
Here is a "teardrop" shape:
http://www.my-town.com/sailing/foils/naca12.gif

.Cd,
That white camaro in your pictures has decals stuck to the front to give the appearance of headlights and the turn signals. For all I know the grill could be a decal too. So the front is a lot smoother than it looks.

[Cd]

Yes they are all decals - even the grille - like NASCAR.

I wasn't talking about that part of the car though. What puzzled me was the flat 'bumper' with a very sudden change in curve around the sides of the car.
Also, what puzzles me is the wide open wheel wells that have protruding tires, the lack of a belly pan, and the parachute assembly hanging down in the breeze.
I know that the low ground clearance doesn't allow much wind under the car, but still I would think it would spill inward from the sides and hit the wheels, exaust and all the other junk under there.

They put 'moon' wheels on the car, but the tires themselves still protrude when seen along the sides of the car. I would have thought for sure that the Z28 style side spats would direct air away from them and keep the area less turbulent, but to my surprise, all they did was create more downforce. They did not help the .Cd at all.
Strange.

Regarding raindrops and such, some meteors that have entered the atmosphere have a perfect 'teardop' shape like what you are refering to. They solidify as they fall. Thanks for sharing your thoughts.

[ttoyoda]

.Cd:

In this picture:




What would you estimate the height of the front of the car to be? About one and one-half front rim diameters?

So with a 15 inch rim (just guessing, I have no idea) that would be
22.5 inches. The height of the front of that car is exactly the height from the floor to the center of my knee. If you can find a cardboard box, cut it so it is only 22.5 inches high. Now contemplate the box's Zenness.

Walk around the box and look how gawd-durned low that little farker is.
Take the box outside and set it next to cars. Cars dwarf the box.

Bottom line: The front of the camaro is so low, and the frontal area of the front of the car is thus so small, I don't think the shape matters very much to the total drag. They must have put a dry sump on the motor so they could get rid of the oil pan, to be able to get everthing that low. The carb lives in that snorkel. The underside near the engine looks like it plows up the ground.