Cd 0.38

[aerohead]

Just to illustrate some of the variability of shapes with identical Cd

[Cd]

I just don't understand.
How can the Smart car have the same Cd, yet have such a massive wake ?
The wake of the Z-28 is tiny in comparison.

Are the vortices just more controlled in that design, or what ?

[freebeard]

Cd (a dimensionless number) is what's being asserted. But I'm wondering for an equal A (area) does the height/width ratio have any impact? The Template would be .5, my car closer to 1. The Smart could be over 1.

This is based on the sides having twice the taper of the top.

[aerohead]

Originally Posted by Cd

I just don't understand.
How can the Smart car have the same Cd, yet have such a massive wake ?
The wake of the Z-28 is tiny in comparison.

Are the vortices just more controlled in that design, or what ?

*The car is highly detail optimized,with minimal turbulence before the tail truncation.
*It has extremely good sectional density,displacing the air nicely with it's leading edges for complete flow attachment,and gradual reduction in cross-section allowing separation-free flow to the truncation,with perfect pressure recovery.
*It's fineness ratio is adequate for Cd 0.38.
*While it DOES have a relatively large wake,the base pressure is higher due to the careful contours and pressure recovery upstream.Just like a 2015 Ford Econoline which shares the same Cd.

*It's 'Stromform' nose is of the lowest drag.

*It's the only car with a proper K-form truncation.
*It has zero-induced drag penalty.
*There is essentially zero vorticity.
*The base pressure of the Z-28's wake will be the same as at it's early separation point,way forward on the back slope.Very low pressure! Quite a penalty!

[aerohead]

Originally Posted by freebeard

Cd (a dimensionless number) is what's being asserted. But I'm wondering for an equal A (area) does the height/width ratio have any impact? The Template would be .5, my car closer to 1. The Smart could be over 1.

This is based on the sides having twice the taper of the top.

Kamm and Fachsenfeld tested this at the FKFS and the short answer is no,it didn't make a difference.
They varied the width of their airfoil-shaped model from around 230mm,to 1,000mm,holding the height constant,and recorded the same Cd, on a frontal area basis.
Any car of any scale will have an identical Cd as long as Reynolds number is accounted for.They call is dynamic similarity,or verisimilitude.
The tunnel velocity must be such that a turbulent boundary layer is guaranteed.

[kach22i]

Originally Posted by aerohead

*It's the only car with a proper K-form truncation.

I had to look that one up.

http://ecomodder.com/forum/showthrea...orm-28260.html

This thread will cover the 'K-form' which is commonly referred to as the "Kamm-back",which is a misnomer due to the fact that it was invented by Baron Reinhard von Koenig-Fachsenfeld.

I had the same reaction as "Cd", still amazed.

[aerohead]

Originally Posted by kach22i

I had to look that one up.

http://ecomodder.com/forum/showthrea...orm-28260.html


I had the same reaction as "Cd", still amazed.

Aero's a funny thing.So far,the best for cognitive dissonance is this comparison,both cars Cd 0.311

[skyking]

thanks Phil. This all gives me hope that my Kamm efforts are the way to go.

[Cd]

Originally Posted by aerohead

Aero's a funny thing.So far,the best for cognitive dissonance is this comparison,both cars Cd 0.311

But ...but ...it has a pointed nose.

[UFO]

Originally Posted by skyking

thanks Phil. This all gives me hope that my Kamm efforts are the way to go.

I agree. I still think a flat spoiler would look cooler, or at least strange in a striking way, but I bet you get better results with your Kamm ideas.