Most aerodynamic cargo box?

[aerohead]

Originally Posted by Tango Charlie

A while back, I was considering constructing a roof top carrier for my Vibe. On paper, it had many of Frank's parameters;

"...try to find out just how small the vertical height could be while still being functional, and spread the width out to the drip rails (or door gaps) on each side,...the front would be the least influential of all the surfaces of the box...but have it extend the curve and slope of the windshield... Let the back of the box follow the template down to a point."
and it looked like this;




But then I woke up and built this:

The rooftop box would have produced attached vortices,with 16.6% higher drag than a more organic transition.
Here are some images from Kamm's research:

[aerohead]

Originally Posted by Frank Lee

That doesn't change the radius equation does it?

One of Hucho's drag tables for leading edge radii showed that the drag hit a minimum and then essentially 'flat-lined' when the radius was equal to 4% of the body width.
For trailers,they used a percentage of the square-root of the frontal area.
But going further into Hucho's book,back in the commercial vehicle section,they show the 'bulbous bow' to have the lowest drag for crosswind,which is usually what we drive in.
So I would lean in the direction of a nose addition to a reversed cargo carrier.
Wal Mart sells a plastic wheelbarrow,who's 'body',reversed and inverted would make the beginnings of a pretty nice nose.About $50 (US).

[aerohead]

Originally Posted by ChazInMT

Ya know, for someone who's been around a while you still aren't getting it. Believe what you want about the front end needing to be not flat, fact is, the air creates its own aero shape in the front. In the back of a shape, the air gets really stupid and needs much help. Intuition tells us the air gets "spilled out", fact is, it does not. Granted, an aero front end does help some, but not as much as the back. I'd say it's a 20/80 split at best, 20% front and 80% back as far as how much you can improve the aerodynamics of something by improving its shape.

We aren't saying the carriers would be optimal by reversing them, only better, because that nice aero taper on the "Front" would bring the air back together much more efficiently, if reversed, than the big wide flat back it has now.

I made a simple drawing to illustrate what I mean.

Hucho's got a beautiful wind tunnel image regarding this sort of thing.I don't have it in my image file yet.
From memory,there is complete separation with zero reattachment until the body length reaches about 1.6X the body width,then there is complete reattachment.But we do pay for the separation,to the tune of Cd 0.85-.88.
With subtle rounding of the leading edges,the drag falls to Cd 0.45.
The Grumman OV-1 Mohawk Side-Looking-Airborne-Radar (SLAR) pod nose would be a good compromise
Here's a link
http://www.161recceflt.org.au/airfie...B%20Mohawk.JPG

[aerohead]

Originally Posted by PaleMelanesian

http://me.dyu.edu.tw/lecture/Vehicle_%20Design(2).pdf - Page 27

Don Sherman IS presuming attached flow in the forebody.So in all fairness we need to understand the context.
It IS Hucho,in his 1st English translation who tells us that at the time of printing (mid-1980s) that virtually all production cars have good enough noses so as to achieve attached flow over the windshield header and around A-pillars.
Some of the rooftop carriers have rather sharp trailing edges,so we'd want to be mindful when reversing them.

[aerohead]

Originally Posted by Varn

RE Streamlined brick and roof carrier.

I have seen the diagram before. It shows a bricks with radius curves on both ends not true bricks. I also recall a quote from Hucho (I think) saying that a 4" radius (or more) was required to keep flow attached. I might assume that the 4" figure would vary depending on the size and speed.

I have found that the main body of air is detached from the roof about a foot back from the windshield on my MK 2 vw Jetta, all front radii on it are tighter than 4". a gradual slope on the front of a carrier might be better than blunt front in my example. Probably a gradual slope on both ends would be best.

Here's one from Hucho:

[aerohead]

Originally Posted by phreaddy

I want to get a roof box for my 2009 VW Jetta Sportwagon. I understand the longer-is better argument, but I think that with wind coming off the windshield, you'd be better off with a shorter, stubbier (and/or shorter, wider) box placed as far back as possible -- that way more wind coming off the windshield would go over the box instead of getting shoved into the underside of it. And, yeah, I'd install it blunt-side forwards. Thoughts?

I'd like to see some peer-reviewed wind tunnel studies.Some of the flow interactions are so complex that it's hard to predict what will actually happen.
I suspect that some have been tested,but it is so far,proprietary info.

[Hersbird]

So my dad bought a car top carrier yesterday and I helped him put it on his 2006 Escalade. It was a Sears version of the Thule Evolution for $250. I tried to talk him in to a hitch carrier but he needs the hatch to open easily and often while travelling with dogs and the swing away ones cost much more. I then tried to get him to mount it backward but he wouldn't buy into it. It was a pain to install and thankfully it slid into the garage after disengaging the door opener and getting another 6'" of door lift as it was pouring rain. I bet he never takes it off as hard as it was reaching the wingnuts on the driver side. I will ask him later if he takes a hit in economy but he just bought the rig a month ago so he may not notice.

[Fat Charlie]

We've got friends who have an ancient Volvo wagon with a roof box. In the last seven or so years, it's never come off. I kept my mouth shut after my first "that can't be good for mileage" got a "well, it's an aero shape."

I saw it last week without the box.