Teardrop aerodynamics... in reverse!

[Blue Bomber Man]

Phil, do you think in a week or so I could get some critique from you regarding airflow on the design of my vehicle? I'm just finishing the subframe and will be begin to design the exterior body. I'm hoping to have the wheels fully enclosed within say 2-3 inches of the ground and have a good taper towards the rear across both axis (width and height) but I would love to get some insight from someone that knows much more then I on the subject.

Thanks!
Ben's Custom EV Blog

[aerohead]

Originally Posted by winkosmosis

Here is something interesting.. The optimal shapes for holding pressure in a submarine are cylinders and hemispheres. But that is not the best for drag, so submarines used to be designed with an external hull that wasn't sealed, and it would be pointy on both ends.

submarine: Definition from Answers.com

General Dynamics Electric Boat Division has returned to John Holland's 1900 Albacore Tuna form for the Sea Wolf Attack Sub due to it's inherent low-drag and low acoustic signature.It's not as fast as the Soviet Union's Akula-Class sub which also uses the prolate ellipsoid nose and full boat-tail,but has lower fineness ratio and less skin friction.

[Cd]

How's this for blunt versus pointy fronts .....

http://www.google.com/url?sa=t&sourc...lR8xh-4BpNqK1Q

[aerohead]

Originally Posted by hypermiler01

Reynolds number


When the speed doubles, the Reynolds number also doubles and that is what causes the flow to detach. That is why fast cars need a more sloped shape than slow cars to have attached flow. And why the VW Beetle may have perfectly attached flow at, just for example, 20 mph, but does terribly on the highway.

That comparison picture I made shows the two different shapes with the exact same condition settings.

The major difference at the front is that the extreme wedge shape has almost NO STAGNATION. 100% of the air is scooped over the top and sides, leaving flow underneath completely unmolested. Well, since that is 2D, it is all scooped over the top. The effect is the same for 3D, just integrate to get the effect of a bunch of thin slices stacked together.

At very low velocity,low Reynolds number,and laminar boundary layer the flow would separate SOONER on the body of the VW Beetle,as the first pressure minimum would occur well ahead of the windshield area.
As velocity increased,Reynolds number would exceed the critical value necessary for the transition to turbulent boundary layer,and the separation point would be moved rearward to the area adjacent to the top of the backlight.
The drag coefficient would be constant for 'normal' driving velocities.
With 'wedge'-shapes,most of the body is in a positive pressure regime and the air is continuously accelerating into an area of decreasing pressure,a situation which is ideal for maintaining a laminar boundary layer.
The downside is the larger wake.
Impressive as is Ford's Probe-IV,Cd 0.154 'wedge',it is eclipsed by aft-body teardrop taper and Cd 0.137 of their Probe-V and its smaller wake.
For sub-sonic flow, drag is governed by flow separation,hence the boat-tail which eliminates separation.
For supersonic flow,drag is governed by shockwaves which are governed by the shape of the nose,something we need not be concerned with.
Below transonic flow ( which begins at about 250-mph with automobiles ) a semi-hemispherical nose offers optimized penetration and high-quality onset flow to the aft-body.

[Cd]

Below transonic flow ( which begins at about 250-mph with automobiles ) a semi-hemispherical nose offers optimized penetration and high-quality onset flow to the aft-body.[/QUOTE]

[Piwoslaw]

Originally Posted by aerohead

Below transonic flow ( which begins at about 250-mph with automobiles ) a semi-hemispherical nose offers optimized penetration and high-quality onset flow to the aft-body.

Like the nose of this Hummer?

[Cd]

Originally Posted by Piwoslaw

Like the nose of this Hummer?

Bugly !
Sort of like gold plating a terd.
They certainly could have decreased the Cd had they also rounded the edges on the windshield.
What were they thinking ?
( Is that a VW ' Thing ' convertible in the background ? I wonder what the Cd was on those. )

[Cd]

The Bugatti Veyron is the fastest street legal car at over 265 mph for the Veyron super sport, yet has a very rounded front end much like that Hummer.

[aerohead]

Originally Posted by TestDrive

Does it necessarily follow that putting a sharper nose on a streamlined half-body will increase Cd?

The argument is that it will not lower it,the additional surface drag associated with the added structure necessary to extend the nose forward to a point,by default,increases the profile drag,so there's no point doing it.
Once to 250 mph ( with cars ),there would be an advantage to the ogival nose.
If you'll look at any wind tunnel photo of a car 'in smoke' you'll notice that the streamlines are already deforming well ahead of the actual body of the car as they undergo what Ludwig Prandtl called a line of discontinuity,avoiding sharp turns,as if the nose was already 'pointed.'

[aerohead]

Originally Posted by Piwoslaw

Like the nose of this Hummer?

Piwoslaw,the nose would help measurably if not hurt,as Cd has mentioned,by the horrible A-pillars and windshield header,something Paul MaCready had a solution for.