Teardrop aerodynamics... in reverse!

[MetroMPG]

Originally Posted by hypermiler01

But I am sure that everyone here, including yourself, is already well aware that the required angle of taper is not the same for all speeds, but varies in direct proportion to speed, and that what I said about faster speeds needing a longer, more gradual angle to prevent detachment is the truth.

At the speeds automobiles travel, Cd is effectively constant.

Otherwise we'd be provided qualifying data (test speeds) by automakers & testers to put their figures in context, and without that context it would be impossible to compare different vehicles' drag coefficients to one another.

The drag coefficient can be considered constant for objects moving at high Re, e.g. a car at highway speed - source

When the flow is turbulent the Reynolds number is large, and the drag coefficient CD is approximately constant. - source

Originally Posted by aerohead

drag coefficients are stable up to transonic flow

I'll be the first to admit I'm far from being an expert in the subject of aero, but that seems like a pretty fundamental concept.

[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.

[hypermiler01]

In general, Cd is not an absolute constant, for a given shape body. It varies with the speed of airflow (or more generally with Reynolds number). A smooth sphere, for example, has a Cd that varies from about 0.47 for laminar (slow) flow to 0.1 for separated (faster) flow.

Drag coefficient - Wikipedia, the free encyclopedia

[MetroMPG]

Sure, that wikipedia quote is correct - for small, smooth spheres where the comparison is between a low Reynolds number regime (laminar) against turbulent flow.

But you're skipping over the important point that at high Reynolds numbers, for large objects like cars, the change in speed does not significantly affect Cd. Which is what was said in the three sources quoted.

Here's a fourth:

cD ... remains nearly constant for large Re#'s in the laminar region and approaches a constant value after the transitional region. As a car is very wide compared to something like a tennis ball, the Reynolds number is high - very high in fact (on order of 10^6 @ 55mph and 10^5 at ~3mph). - source

And a fifth:

In general, the dependence of [production cars'] drag coefficients on Reynolds number is very small and sudden changes do not occur. This demonstrates that the predominant part of the drag of these vehicles is pressure drag. (Source: Aerodynamics of Road Vehicles, Hucho Ed., 1998)

[hypermiler01]

cD 0.07 Nuna, World Solar Challenge winner 2001-2007

[TestDrive]

Two more views of the nose of the Nuna 4.

[NeilBlanchard]

Hi,

The wheel shrouds and cockpit canopy are blunt, and really, so is the leading edge of the "solar wing" body. It is very slim and the "angles" of the (curved) surfaces are gentle, so the radius does not have to be very big -- but it is there, and it is not a sharp edge.

But, the leading angles are pretty close to the trailing angles, so -- it that why this has the lower Cd? Also, this "car" is about as un-streetable as you can get, and it doesn't enclose very much volume. So, vehicles like the Aptera are compromising (!!!) in order to have two people in comfort, have ease of entry, and practical cargo space, etc. -- and not have an 8-10 foot long nose sticking out in front...

[aerohead]

Chrysler's Carl Breer knew that cars would go better backwards because his team took a car body off it's chassis and re-installed it reversed, to find that it went faster and got better mpg.Before the 1934 introduction of his Chrysler Airflow,he hired a professional race car driver for a publicity stunt,by driving a Chrysler DeSoto across the U.S. "backwards",to demonstrate the aerodynamic point of the upcoming Airflow.---------------------------------------------- America didn't get it.They still don't.

[aerohead]

Originally Posted by hummingbird

I think the ideal shape is neither the forward teardrop nor the reverse one. The objective here is to avoid a detached flow- the forward teardrop pushes a sizable wall of air, the reverse one allows turbulence to form at the end face, increasing drag.

The Ideal shape would be a combined teardrop with the front end formed by a reverse teardrop tail and the back end formed by a normal teardrop tail. The intermediate section can be tubular shape that then reaps benefit of a parting already taken place - something like the bullet trains of Shinkansen Japan or TGV France.

The "ideal" teardrop has the lowest drag for subsonic flow.If you lengthen the nose forward, the drag goes up do to increased skin friction.Anything you do to alter the "ideal" teardrop increases it's drag.That's why they refer to it as "ideal".

[TestDrive]

Originally Posted by aerohead

Anything you do to alter the "ideal" teardrop increases it's drag.That's why they refer to it as "ideal".

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