Streamlining. More important, the front or the back?

[betasniper]

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.

[aerohead]

Originally Posted by sendler

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.

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.

[sendler]

Originally Posted by aerohead

The chart and its values are only for a fuselage of the fineness ratio shown.

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.

[aerohead]

Originally Posted by sendler

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.
.
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.
.
Foil sim shows the x,y code for the given shapes but it can't be edited.

*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.

[mikeyjd]

Originally Posted by aerohead

*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.
--------------------------------------------------------------------------
*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.
--------------------------------------------------------------------------
*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.
-------------------------------------------------------------------------
*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.
-------------------------------------------------------------------------
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.

[Frank Lee]

He IS an aerodynamics book, on legs.

[freebeard]

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

Figure 2 is a ‘drag accumulation’ plot which shows hows the drag builds up along the length of the vehicle. It’s is interesting to note that the total drag of the car up to the front wheels is almost zero for the straight ahead flow and negative for the 15 deg yaw case. In other words, a significant amount of thrust is being generated around the front of the car which offsets the stagnation pressure acting on the nose.

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

*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.

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

[aerohead]

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!

[sendler]

Originally Posted by aerohead

I wish their Paper had gone into greater detail to break out pressure drag and skin friction effects.

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.

[aerohead]

Originally Posted by sendler

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.
.
.
http://ecomodder.com/forum/showthrea...rag-25378.html
.
.
Add whatever skin is needed to get the most ideal shape you can conveniently use and you will have the lowest 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.