Drag Bucket Paradox, sometimes less aero = less drag

[freebeard]

I have read many threads about the front curvature, but they are usually dismissed under the banner that the big gains are in the rear, but this information seems to suggest this is not quite the whole truth.

I know, right? But with no intervening posts, nothing was gained by quoting the whole thing.

I'd even go so far a to say that the facetting isn't a problem. The dihedral angles are less than 10° and the air sees a shallower angle everywhere except the bands running around the front-rear 'equator'. I suspect at some undetermined frequency the effect would be like golf-ball dimples, reattaching flow.

I went to all the trouble because this relates to a post I promised months ago about my Clark Cortez motor home.

[Tesla]

Didn't even think about the quote thing, reflex action, point taken.

I stumbled across a little info on dimpled bike wheels, posted a thread on it, they said they were doing hexagonal dimples and it seemed to be the actual angles which were responsible for the boundary layer tripping.

I would be interested to know a bit more on what effects were coming together to indicate that an object in the 2.5:1 to 4:1 had the best aerodynamic profile regarding ground vehicles?

Like for instance what is the drag of a flat square plate, then using same face extend it to a cube and then rectangle to the appropriate ratio's and beyond, what is actually happening there. Think there was an image of drag for different shapes somewhere, will have a look around.

What's with the Clark Cortez motorhome?
Had a google, looks like the Dryden Van in it's first incarnation as the "shoebox".

[freebeard]

I see your dimpled wheel thread never achieved escape velocity.

Have you seen these? (PRK is aerohead)





As for the Clark, from an early post on this site:

As I declared in my Introductions post, I've got too much on my plate. Four potential modding projects and one has to go to finance another. I'm not going to get into resale value or sunk costs.
...

So, I solicit your comments. Just keep in mind there's still 3 projects to go. This one looks like a lot of work; but as I've hinted, the progression on the motorhome is Class-A brick==>rat-rod flatbed truck==>3-wheeled rat-rod flatbed truck==>The Template.

It looks all shiny in that picture, but it's actually a 9000 pound rust bucket with an 800lb payload. I'm trying to find an X-ray view so I can explain the plan.

[Tesla]

Yeah, think everyone is over the dimples, just put it up as I thought they had some interesting comments on why they sometimes don't seem to work.
If you just want to push a box through air, then the detail isn't that important, but if you want to streamline that box, then minor details are crucial.
That seems obvious to me with the number of discrepancies in drag reduction from different mods by different individuals, so for my part I am trying to understand the detail. Better to waste my time on the computer in comfort than waste it along with fuel doing A-B-A testing on a poorly designed modification.

Anyway, regarding the Cortez, didn't mean to be disrespectful, I do get "foot in mouth" occassionally.
Don't know what your greater plans are, but it probably comes down to considering what your options and goals are, purchase price is irrelevant as that is history and is wrapped up in emotion which doesn't make for good financial decisions.
If you want an RV, then restoration may be a viable option, if you need a utility vehicle, then conversion may be viable, but this should be costed fully, most of the time it will go well over costed budget and may be more than just buying a truck, etc. etc.

As for the shapes, yes that was the list I was refering to.
Pity they don't have the half sphere backwards to compare it, but the cylinder forms further down indicate some of this effect,
Flat circular plate = 1.17
Cylinder ratio <1 = 1.15
Cylinder ratio >2 = 0.82

So same frontal and rear profile and a reduction of 30% roughly in the Cd just because of the extra length. The only thing that I can put that down to would be the developement of a thicker boundary layer and hence a subsequent reduction in wake size.
Or maybe the extra length allows for reattachment of air blown out by the bow wave, resulting in the creation of turbulent lobes that are contained either side of the forebody and virtually simulating a teardrop shape?

Most vehicles are way short, so if you improve frontal flow, you just tighten the boundary layer, you don't have the length to get the benefit of reduced skin friction, but you suffer the penalty of a stronger wake.
With a longer vehicle you do have the potential to gain a benefit from reduced skin friction, but the question is whether the thinner boundary layer will result in the same negative impact with the wake.
Irrespective, it seems to me that they may need slightly different approaches to gain the maximum benefits.

I found the post below which also had a little bit of info in the diagrams posted.
http://ecomodder.com/forum/showthrea...pes-11183.html

Now, overall there is no denying that most of the information indicates that there is a far greater benefit in tapering the rear, and in most cases the front effect is minimal, but in some cases I think the front has a much greater bearing on the overall outcome particularly with longer vehicles like vans.

[freebeard]

Anyway, regarding the Cortez, didn't mean to be disrespectful, I do get "foot in mouth" occassionally.

I have no idea what that's all about. So, OK.

Don't know what your greater plans are, but it probably comes down to considering what your options and goals are...

When that whole working for a living thing ran out of steam, I found myself with 4 vehicles, each with a potential project associated. The post I linked explains the first design, seen in my profile picture. The second post is here. And the third is here.

As for the chart, the cube get a 25% improvement just turning it 45°!

Thanks for the link to that post. I hadn't seen the illustrations before. I'll read the rest of it tomorrow.

[aerohead]

Originally Posted by Tesla

I went back and had a look around for some ellipsoid drag data, mainly found info on airships, the one thing was that all these shapes seem to centre on the 2.5 - 4:1 Fineness ratio.

So what's going on there?
If you can take the template or reverse it or use an ellipsoid with the same Fineness ratio and basically get the same drag.

We have a sphere, which I think I read had a Cd of 1, if we put a cone on the front or rear it comes close to the template drag or if we split it and insert a cylinder between the two halves, we again get a drag figure approaching the template.

This has to be related to the boundary layer and it's developement over a distance, the front and rear angles don't change but drag does.

So this suggests to me that in some cases the frontal design may have a significant bearing on the overall drag.

I know that being in the impact/pressure zone there is almost guaranteed attachment with almost any frontal design, but seems to me there is something a bit more subtle there than just plain attachment.

I spent over 20-minutes addressing all your questions and was rewarded by losing it all when I hit Submit Reply.I've worked on it since yesterday when I saw this post.I'm not going to do it again.
Here's the deal.Forget all about the front of the vehicle.
Buy Hucho's book.It will answer all your questions.

[aerohead]

Originally Posted by freebeard

There's a Delete button too. Just sayin'.

And thanks for that video. I liked the real-world examples of things we would get simulations of today.



I get this in general, but:

• What's a 'reverse template'? Why can't you see out of it?
• What's an 'ellipsoid'. Is that like a Dymaxion shape?

*Fachsenfeld wind tunnel tested a L/D=6 streamlined body of revolution,both 'forwards' and in 'reverse.'
*The drag was 22% higher when reversed,owing to the wake,although if wheels were added,as a 'car',the drag would only be 14.66% higher(Cd 0.136 vs 0.12) than the 'Template' (estimating)
The hitch would be that the windscreen inclination would be so extreme that you'd have no forward vision,only a reflection of the inside of he cabin.
*The ellipsoid family are a series of bodies of revolution which are based on ellipses of varying fineness ratios.They are 'double-ended,like a lozenge,and would also pose a serious challenge to outward visibility.
From Hoerner and Hucho's drag measurements,the ellipsoid of minimum drag has an L/D= 2.5.As a road vehicle it would be L/H= 5 with Cd 0.11 in ground proximity,without wheels,according to R.Barth's research.Dymaxion Car is asymmetrical longitudinally,more like an airship than an ellipsoid.

[Tesla]

Originally Posted by aerohead

I spent over 20-minutes addressing all your questions and was rewarded by losing it all when I hit Submit Reply.I've worked on it since yesterday when I saw this post.I'm not going to do it again.
Here's the deal.Forget all about the front of the vehicle.
Buy Hucho's book.It will answer all your questions.

Bummer when that happens, on big posts I usually copy them onto a word doc then submit reply, if all good then delete the doc, otherwise I can copy it back and try again.

Always appreciate the effort and detail you put into your posts and you must get tired of repeating yourself about focussing attention on the rear.

Apart from the "but why" condition I have, there are multiple reasons why I am trying to nut out all the other aero avenues.

My vehicle is a wagon and a big one at that, so parking it anywhere is already difficult as the world goes to smaller cars and parking places to match, therefore a major permanent attachment to the rear is out of the question.
I am always looking at potential work I can do on the rear, so although I do ask questions about front, sides, under & over, the rear is always there in the forefront of my thinking.

Another issue I am struggling with at the rear, I can do maybe 18" off the roof and tapering down to about 6" on the sides at bumper level, although I think there would be a good benefit in it in both tapering, good seperation and a nice shallow cavity to keep vortexes of the rear face, but the method of fixing is an issue.

I have barn doors which are flush at roof level, but set in at the sides, so do I attach to the doors or the body or have a split system which needs significant engineering though to hold firm and mesh neatly when opening and closing the doors.

Anyway just outlining that so you know am not ignoring what you say about rear end tapering.

[freebeard]

Originally Posted by aerohead

I spent over 20-minutes addressing all your questions and was rewarded by losing it all when I hit Submit Reply.

Doncha hate that? If it's the vBulletin forum software timing out your log-in; here's what you must do:

Always put Go Advanced/Submit Reply at the end of a line that goes "Select All, Copy, think 'the first time I trust the forum software and skip that it will bite me', Go Advanced/Submit Reply."

When it does happen again, logging back in in the dialog down the page is better than the log-in box in upper right. Sometimes it will get you back with the text intact.

Sometimes, if vBulletin loses it's footing big time; and can't even open a viable Message window, you need to paste the Clipboard somewhere so you can Copy the URL of the page and just Open a fresh instance to regain control.

Don't ask me how I know all this.

HTH

[radioranger]

I'm guessing all these box trucks would benefit from towing super lightweight trailers for highway trips. and maybe altered roof lines like an add on roof bump