Streamlining. More important, the front or the back?
 

I'm hoping someone with Solidworks flow simulation can run a simple experiment for us. Create a rough tear drop. Say 2 feet max diameter and 10 feet long. Round in the front and straight sides tapering back will work if it makes it easier to create the shape without the complex curves. Compare the performance of that with the same shape that has been cut off flat on the front at 80% of the radius. And to a shape with the round front but a straight cylinder all the way back to a flat back. And finally, to the original best shape that is truncated at 60% of the length with a flat back.

I think it's been pretty well proven that, all other things equal, there are more gains at the rear. Suppose another bit can't hurt.

Why? To what point?

It sounds like you want to compare a bullet, an 'ice cream cone' and a truncated teardrop, all at 5 to 1 fineness ratio. And a hollow-point ice cream cone.

That fineness ratio is long for a full body (2.5-4:1) but short for a blister on a larger body(10:1).

You could generate low resolution results in Solidworks. Those results *may* translate to similar comparisons in the real world, but what do you do with that information?

There are published results for similar forms.

Why not? So I know. Links?

performance
 

The 2' by 10' ice cream cone will have a free-flight Cd of around 0.086.
In ground effect it will be Cd 0.172.With wheels,maybe Cd 0.222,depending on how far they project outside the body.
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I'd like a drawing about the 80% nose to help think about that.
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The body with convex hemi nose and cylindrical section is Cd 0.201.
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For the truncated ice cream cone I'll have to get home and check my references.

http://ecomodder.com/forum/member-fr...ntitled-3b.jpg
http://ecomodder.com/forum/member-fr...ture4626-3.jpg
http://ecomodder.com/forum/member-fr...1264438870.jpg
"Knowledge Is Good" — Emil Faber

Not trying to be confrontational. Sometimes the subtext changes the answer. I don't think aerohead will mind if I point you to his albums. Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - aerohead's Album: Book illustrations
http://ecomodder.com/forum/member-ae...sugar-cube.jpg

Cross-posted. :( About the 80% nose—the front can be flat with a radius IIRC around 4% of the gross width.

60% truncation
 

Thanks freebeard!
From the fuselage chart it looks like the 60% truncation will cause a 28+% drag increase,so we'd be looking at Cd 0.284 with that tail chop.
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As to the nose truncation with leading edge radii,according to Hucho,that minimum would get you your flow attachment,and further rounding would net zero additional drag reduction.
************* The caveat is with 'crosswinds'. In a yawed flow condition (which is the statistical norm for N.America) the full,bulbous nose would have a distinct drag advantage.(think Oscar Mayer Weinermobile)

Thanks for the replies. My question stems from discussions of whether streamlining the front or the back of a motorcycle will make the most improvement if you were only to do one or the other. And, is it worth the trouble to run the tail at full length or will a Kamm truncation to 60% of the ideal length be only a marginal loss. The hemisphere in front of a cone was just my way of over-simplifying the shape so it would be still useful for analysis but also very easy to draw.
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The fuselage example above is much too long to for our discussion of Kamm truncation since most of the length along the sides is nearly straight. By the time you get to 60% of the length, you have made a long round nosed bullet with almost no angle remaining and haven't satisfied the other rule of thumb for Kamm which is to end with a width that is no more than half of the max width of the airfoil. The angle of my tail I have started to build is about 10.5* per side, 18 inches at the front, truncated to 8 inches at the back by 28 inches long.
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That it only takes 4% of the width for the radius of a round over on the front of my tail to harbor attachment is good news as that is only .75 inches which should be very easy to exceed with a pool noodle cut in half.

This drawing would be a good example of the tail I am building and what effects a truncation will have Though I am having trouble correlating the ratio that is displayed into our concept.
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http://ecomodder.com/forum/member-ae...ng-effect.html
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Can't say for sure. Yes it is our common knowledge that the big gains are at the rear however you are talking about a single track vehicle which- largely due to it's proportions- has less ground effect interference. In addition, I read posts from a very knowledgeable guy on here once that said he found good results with a front fairing and negligible results with a tail on bicycles (I would not have expected that).

A bicycle is mostly open, so it will benefit a lot from the front fairing, but I think a typical motorcycle will be more similar to a car i.e. a tail piece will be important. But the front wheel and forks are open, so you should look at Vetter and Allert Jacobs: http://ecomodder.com/forum/showthrea...diy-11970.html

My tests with tail sections on bicycles have not shown any improvement so far. I believe this is due to the fact the waste baskets are not as wide as the rider. A full fairing based on a vintage sailplane design is the next step. I posted the drawing URL in the #28 post of the Motorcycle Aerobody Research thread. It is similar to the Vetter nose with a rising tail. The nice part is that it is drawn with simple curves as in ellipses. If the tail is truncated at the sixth bulkhead the overall length is under 10 feet.

There, see? Instead of a nebulous target we have a clear one—maybe the worst possible case. Length challenged, with the biggest problems in the middle, not the front or rear. Fully enclosed, semi-enclosed, sit-on or sit-in? Whoa. Beware weather-vane effects from the enclosed front wheel.

And the question is a set-up. What you do at the front influences the rear more than vice versa, but why would you do only one or the other?

Still, thanks for asking. :) Do you know what college Emil Faber was past President of?

Ya see, motor sikles and bikes have this decidedly un-aerodynamic meat sack inevitably positioned between the front & back of them. I once read somewhere that a fairing that started the taper yielded good results.

Like dis.

http://i39.tinypic.com/vrauyt.jpg

Your labeling reminds me of my Faber College days. :)

Can't remember if 'badder' was better or worse than 'gooder'.

Adding bodywork to the rear is a necessity in order to create luggage volume. Truncating the tail creates a flat panel to mount the license plate on and saves 3-4 feet of length in the garage. How much drag is increased by the Kamm?
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I'm acually surprised that I can't find an online simulator that would let you play with different airfoils widths and lengths and allow you to add truncation to the tail.

There was a thread about this on side mirrors, although I suspect you're talking about something else. I remember they had also discussed a stepped partial boat tail looking like conical window louvers.

Interesting thread, and it's good to hear the reasoning behind it now. I would imagine a motorcycle is so complex a shape that it would be more effective to simply carve some rough bodywork out of foam and test it yourself than to play with primitive CFD forms.

:)

Yes, someone designed a full fairing on the front of a motorcycle that had the shape returning in before it ended, this showed a fairly (?) dramatic drag reduction to the motorcycle....I just keep waiting to see it utilized by someone, but have yet to find where someone has made use of the idea.

It was a full sized motorcycle, with a Goodest fairing design, I do recall the rearview mirror.

I know, Right...2 things I've picked up over the last couple of years here:

1st, Flow simulation is about 50 times more complex than what you think it is, the math & such required to predict what a bunch of air molecules are going to do, and what effect this will have as an object moves through them, is astounding. The computing power required to run a proper test is enormous, see this video for what it takes to image swirls. They had 4,096 CPUs working together to make this.

The Secret Life of Vortices: Evolution and Destruction of Vortical Flows

What's this mean to you? Finding someone online willing to let you play with their 4,096 CPU's to simulate your various designs is not likely. Not even mentioning inputting the data to the simulation, in order to properly test what you are wanting to do, would require knowledge of the system way beyond what the layperson could understand without years of education on it.

2nd, Because it is so complicated, the slightest error in math will yield results that are not accurate in the real world. This is why any major automaker spends many 10's of millions of dollars on wind tunnels to test models for aerodynamic properties.

While flow simulation may get you close, there is no substitute for real air moving over a real object and measuring the forces to see how changes in design affect the overall drag, lift, downforce etc. on a vehicle.

I used it on snowmobile hoods and pans starting in '93.

FoilSim works fine but you can't change the shapes.
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FoilSim III Student Version 1.4d beta
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Someone here was previously modeling a semi with solid works/ flow but hasn't chimed in yet. This sim I am asking for would be a piece of cake in the same vein. Not a big deal.

Vetter is way beyond this for four years now. But no one has ever had a chance to get in a wind tunnel yet to test whether it is really necessary to use the full tail or if a Kamm will get most of the benefit.
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https://sphotos-b.xx.fbcdn.net/hphot...36250449_n.jpg
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https://fbcdn-sphotos-g-a.akamaihd.n...06126256_n.jpg
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Danger, Will Robinson! That's a 2D sim of free air movement, not 3D in turbulent ground conditions.

CFD and wind tunnels are both resource intensive.

The vetter example is good. The next step is to have the front slide forward for ingress/egress/putting your foot down. The example used to be on the Ecomodder home page.

Start here.
http://ecomodder.com/forum/member-fr...ture4626-3.jpg



Or, the one you posted in #9.

Something like foil Sim would be more than good enough to give us an idea if we could manipulate the shapes to suit our questions in general.


I already commented on the fuselage chart. It is way too long in form factor. By the time you take away 40% of the length, the sides are already straight. It is no longer a Kamm. It is a round nosed bullet.




That is the correct shape but doesn't make sense. Where are they measuring "l"?

Looks like we have some choices. First is to try a search of free CFD software online. Second is to contact an area college that has a Mechanical or Chemical Engineering department that would have a student run some simulations on a mainframe. Third is to make a quarter scale model and put in front of a large window fan with thread for tuft testing.

form factor
 

The chart and its values are only for a fuselage of the fineness ratio shown.I selected it because I didn't do the research.It was tested at Germany's NASA,the DVL,where Kamm spent some of his time.
The 2.5:1 fineness ratio fuselage has the lowest drag.Cd 0.04
In ground effect its drag rises to Cd 0.08
When 'gross' wheels are added,it goes to Cd 0.12.
If you truncate it at 60% you should see the same 28% drag increase,or Cd 0.15.(drag of the body is figured separately with drag of wheels held constant).

Kamm's rule of thumb
 

Kamm's 50% frontal area wake body truncation was a concession to 'practicality' and had nothing to do with ideal streamlining.He knew they were still leaving 50 % or more of the drag on all the K-cars.

Kamm
 

*The full tail IS a Kamm tail.It just happens to be the whole thing.
*The fuselage chart is from the wind tunnel and shows exactly what happens when you chop portions of the tail away.
*Arado Flugswerke has published results of truncated fuselage wind tunnel studies.
*Most known aeronautical companies studied fuselage and wing truncation though the 1920s up and through WW-II.
*Lay did this wind tunnel research with truncated car models in 1933.
*Breer actually did the very first 'Kamm' tail in 1934 on a DeSoto Airflow test mule.
*K-Fachsenfeld wind tunnel tested the K-tail in 1935 and received a patent for it.
*K-Fachsenfeld was brought into FKFS to do research under Kamm.They repeated Lay's truncation research and refined the K-tail
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From the results of what's already been published we may be able to make fairly high confidence performance predictions without additional studies.

1/4-scale
 

You'll need 80-mph out of that fan to get the tufts right since they're dependent upon a turbulent boundary layer.

drawing
 

This is W.A.Mair's boat tail.
CAUTION!!!!!!!!!!!!!!!!
The values he presents here are with skin friction removed.If the friction drag is re-introduced the curve takes on a very different appearance.
I've posted an image of Mair's entire wind tunnel model elsewhere.

How about a water tunnel? 1/10th (.42m) scale would only require a 13 mph flow velocity to reach a Reynolds number of 2.5*10^6.

piece of cake
 

Ground proximity 3-D Wake flow cannot be predicted with CFD.
A model of the vehicle must first be built and tested to ascertain the wakes achitecture.
Then the wake is panel-ized along with the vehicle body surface vectors as an aerodynamic singularity and input-ed into the program.
If you've gone to the trouble to build the model you'll get 'real' results with that in a wind tunnel.And watch your Reynolds number.

Correct. And it is way too long and skinny to help us estimate the difference between the two Vetter tails pictured. The first 20% truncation of the fuselage basically chops off all of the boat tail already.
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Ground effect: Any minimal ground effect that the two bikes have will be quite similar and can be tossed out as a common factor. We are only concerned with the comparative values. Nothing absolute. The 22* ice cream cone is closer to the actual bikes although it is too short. An 11* ice cream cone, or better yet, 4 feet tall hemi front rounded wedge at 11* on each side would give quite a good means of comparison as we chop off length in the front or the back.
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Foil sim shows the x,y code for the given shapes but it can't be edited.

estimate
 

*If you can extend the lines of the Vetter fairing out to a point and then compare that to where they made the truncation you should be able to get a handle on the percentage of chop.
*The % drag difference per 10% of truncation difference won't really vary with fineness ratio.
*If you know the Cd of the profile your starting with,you should be able to predict the Cd at any given truncation.
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*One thing about the straight taper.It has been shown to have twice the drag compared to the more complex teardrop taper.Eiffel measured this.DVL measured it.Junkers measured it.
*If you run a straight,conical taper as long as the 'Template' as Lay,K-Fachsenfeld,and Kamm did,you can get to very low drag.
*If you go from the hemi nose,directly into the angle,without the Mair contour transition,you'll lose the game right there.Sorry,but it's hard science.
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*The thing about 'angles' is that they are measured at a tangent to the body,and will occur only as a constant after the curved Mair transition zone.
Without it,you'll have separation and a good chance that it will never re-attach.
*You can only cheat so much.
*You want to pull the body in as freebeard does with his redwood bender-board.I've been using 1" Schedule-40 PVC pipe as a 'french-curve' held over bulkhead station points which define major body vectors.The redwood or pipe will perform the calculus for you,plotting a smooth curve profile for fabrication.
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*It's better to have a truncated body of proper contour than a full-tailed body of pseudo-streamline form.This was the entire premise of Kamm's research and university lectures.
*If you can go in the 11-degree territory it will return great dividends compared to the 22-degree which would only yield around Cd 0.23 even with a full tail.
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I don't think you'll learn anything useful from the CFD.

On most of your post I feel like I'm reading an Aerodynamics book.

He IS an aerodynamics book, on legs. :thumbup:

I've dropped this into two threads already, oh well:1993 Dymaxion Concept Car Aerodynamic Analysis

http://www.totalsimulation.co.uk/wp/...cumulation.png

http://www.totalsimulation.co.uk/wp/...on-results.png

This Yawing coefficient must be what causes the Dymaxion to yaw into the wind instead of being pushed sideways.


I'm going to sort my redwood (up to 16' lengths) into clear and knotted. :thumbup:

CFD Analysis
 

I wish their Paper had gone into greater detail to break out pressure drag and skin friction effects.
I'm not sure how to interpret the numbers.
The Dymaxion Car was previously estimated at Cd 0.25.And probably from top speed with the Ford V-8 flathead,compared to the Ford Model-A,with same engine and Cd 0.838 if I remember correctly.
The Model-A topped out around 80-mph.Something good has to be happening for Bucky's car to attain 120+.
If I were of means,I'd talk Harrah's Auto Museum in Reno,Nevada into parting with their Dymaxion Car long enough to trailer it east to Ogden,Utah for wind tunnel measurements in the new tunnel there.
It's a nice tribute to Fuller for them to go to the trouble to do the CFD.
Thanks for sharing!:thumbup:

I hear a lot of talk about skin drag but it seems it is completely inconsequential compared to turbulent wake drag when talking about shapes moving through air. The NCFMF video shows that a round rod with the same max thickness as the ideal airfoil, and only a small fraction of the skin area (1/20th?), has 9 TIMES the drag.
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http://ecomodder.com/forum/showthrea...rag-25378.html
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Add whatever skin is needed to get the most ideal shape you can conveniently use and you will have the lowest drag.

skin drag
 

Yes,as ecomodders it's an intellectual culdesac.There's nothing we can do about it.
With respect to the CFD analysis though,it would be helpful if they told us what drag they were measuring or representing.Their data is presented in a format that's unconventional and begs expansion.Especially in light of the fact that they're reporting essentially half the drag previously reported by other researchers.