Standard roof shape vs Kammback with higher frontal area?

[Big time]

Which one offers the lowest overall drag:

A. A standard shaped roof where the roof is mostly parallel to the ground

Or B. a Kamm placed over a standard roof?
The roof continues upward inclination beyond the standard shape roof height to slowly taper down.

Roof camber data seems to show that while Cd decreases with higher roof camber, overall CdA goes up as the increase in frontal area won't compensate for the lower Cd.
In This Chapter the Modifications That Were Carried Out To

But there's a catch as this data is for a symmetric shaped roof camber where both upward and downward slopes are equal.
What happens when the upward slope (front) is steeper than the downward slope (rear)? Of course the max roof camber point goes forward.

[arcosine]

I've read somewhere the resigned Saturn in 1996 had a taller, cambered roof for more head room and has lower drag than the flat roof of the prior years, even thought the frontal area is more. Looks like the optimal cd is .06 height to camber ratio. That would be about 3 inches for a 4 foot long roof. It all depends on what car you have. The transition from windshield to roof should be as smooth as possible.

[ChazInMT]

Without a picture or illustration this is impossible to judge. It would be like describing a gymnastics move to us and asking what kind of score we’d give it. Case by case basis thing and without visuals, it’s all just Blah Blah Bladitty blah. The link you posted is a POS that cannot be used without a membership and I don’t feel like spending $9 to try and figure out if there is anything there.

[arcosine]

MY are we having a bad day? I read the link just fine.

[kach22i]

Originally Posted by Big time

Roof camber data seems to show that while Cd decreases with higher roof camber, overall CdA goes up as the increase in frontal area won't compensate for the lower Cd.

You might want to do a search in this forum, look for the tread when a similar topic came up regarding semi-truck tractor trailers.

The prevailing argument was that the lowest possible frontal area (flat roof) would be preferred over a larger and taller trailer with aerodynamic roof hump (tear-drop).

However, I did find many links from the UK which argued otherwise. They were in all fairness from companies making aerodynamically shaped truck trailers. Most were smaller in overall volume and load capacity than the ones we are more familiar with here in the USA, which accounted for even more striking differences.

What you see here in this thread is an agreement that the devil is in the details, and a carpet coverall conclusion best not be made.

There are just too many other factors which could easily change any general assumption. For instances as mentioned the transitional arc over the top of the windshield and so forth which may affect actual air flow.

EDIT: Page 8 of the link you provided touches on your question when it mentions the suction of a flat roof due to loss of boundary layer adhesion.

In This Chapter the Modifications That Were Carried Out To

In this context or example I would have to agree with the author of that book. The bubble roof or roof arc is better in the case illustrated, and for the reasons they state.

[Cd]

Yes, there is a thread around here on just this very thing. ( I can't remember what it was titled )
There is a European maker that has actually reduced drag by adding frontal area to the top of the trailer.
With a flat top trailer, you would have to add length or modify the rear of the trailer to taper the wake downwards.
With an added 'hump' the air follows over the hump and continues downward at a more tapered angle , but without any additional length - but at the expense of added drag due to added frontal area .
Does it ever balance out ? Not sure, but the experts seem to 'hiss' at any mention of frontal area being used as a means to reduce drag

[Cd]

With the trailer mentioned above, the ' hump' doesn't add that much frontal area because the trailer has so much length.
With a car or SUV or whatever, the length is very short and the hump would add a larger percentage to the frontal area versus the length to work right.

That's my speculation .

It would be super easy to test. Just place a large sheet of thin plywood on the roof of the vehicle and test it at different profile heights. ( strapped down of course - DUH)

[arcosine]

A long cylinder has a cd of .82 and a streamlined body .04. If the long cylinder is placed inside a streamlined body, the drag will be lower even thought the frontal area is greater. An extreme example.

[Cd]

Originally Posted by arcosine

A long cylinder has a cd of .82 and a streamlined body .04. If the long cylinder is placed inside a streamlined body, the drag will be lower even thought the frontal area is greater. An extreme example.

Thanks for that info. Arcosine I suspected that to be the case, but does this scale up ?
If you had a box shape as large as a jumbo jet and added a teardrop top on it you would have to add so much frontal area that it would cancel out any benefits eh ?

Dumb question yes, but I'm .....well ....dumb.

[Cd]

http://ecomodder.com/forum/showthrea...-suv-8918.html

Here is the original post.
I agree that it would be better to graft a kammback onto the tail end to lower the wake, but if the vehicle length must remain the same then this idea certainly looks interesting .