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Old 03-26-2012, 06:24 PM   #251 (permalink)
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Quote:
Originally Posted by eco_generator View Post
Wow, those graphics answered a question that had been rattling around in my head. Over-simplified, a car is pushing a 'bubble of air' and dragging a 'vacuum'. I wondered if there was a point where the trailing car's 'bubble' would start to fill the 'vacuum' of the leading car. This seems to be the case at around 1 vehicle length +/- .... anything beyond that and the leading car keeps its full aerodynamic drag.

I might have it all wrong in my head, but this is still very interesting information to know!
If you look at the bottom image of the stock cars drafting,you see that the lead car is unaffected by the trailing car,but the trailing car is already picking up some 'draft.'
This is going to very from vehicle to vehicle.
When I get the railroad comparisons posted,you'll start to see some similarities in drag as a function of following distance.
The main theme is that the lowest drag for both the 'tug' and towed vehicle always occurs when bumper to bumper.By closing the gap between the two,the absolute lowest drag is achieved.Which is the premise for the gap-filled trailer.

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Old 03-26-2012, 11:11 PM   #252 (permalink)
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Originally Posted by aerohead View Post
In the images posted earlier for the railroad boat-tailed tailcar,all the boat-tailing was done in plan-view.
The length of the tail is 1.74X the body width for the full stinger.It's within 2.5% of the template,so it's going to just slide in between Mair's 22-degree,and Emmelmann's 23-degree max slope angle protocol for attached flow.The base drag is completely eliminated.Which would be the lion's share of road load for a light truck on the highway.)
length of the tail would then be 3.5 times the half a body width (two sides).

the arctan(3.5) is about 74, leaving the angle 16 degrees to the direction of travel.

My theory is there is theory, and what works in the real world. I think for the sides you have to run a shallower angle because direction of wind in relation to the car is a big deal, where when looking at the top of the car, you can run a steeper angle as teh top of the car doesn't care what angle the wind is.
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Old 03-26-2012, 11:14 PM   #253 (permalink)
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Quote:
Originally Posted by aerohead View Post
perhaps, we can only present the data to the motoring public and let them decide whether they'll use any of it for their particular situation.
Generations of aero researchers have already lived and died without ever witnessing large scale adoption of the technology.I'm pragmatic enough to expect nothing significant within whats left of my lifetime.
people are stupid.

Aero, you and Aerocivic have REALLY got my brain going.

People do not realize WHAT THIS MEANS. THIS IS A BIG DEAL, AND YES I AM SHOUTING!!!!!!!!!!!!!!!1

HOLY CRAP!!!!!!!!!!!!!!

THANK YOU!

I'm gonna go build one.
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Old 03-27-2012, 06:18 PM   #254 (permalink)
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tail

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Originally Posted by drmiller100 View Post
length of the tail would then be 3.5 times the half a body width (two sides).

the arctan(3.5) is about 74, leaving the angle 16 degrees to the direction of travel.

My theory is there is theory, and what works in the real world. I think for the sides you have to run a shallower angle because direction of wind in relation to the car is a big deal, where when looking at the top of the car, you can run a steeper angle as teh top of the car doesn't care what angle the wind is.
For Johansen's boat-tailed tail car he appears to have just factored the width of the car as it is more narrow than tall,and to economize on material,utilized this parameter for the contour.Rumpler did this with his cars as well as Jaray with his small cars.The fastest Bonneville streamliners,be they 4-wheel or 2-wheel,all use plan view boat-tailing as the dominant criteria.
The surface friction drag coefficient for the tail car is Cd 0.05,so when you subtract this out,it leaves Cd 0.05 for the tail itself.It would be Cd 0.045 if 'ideal',according to theory.
So to achieve Cd 0.045 it looks like the contour is plenty good with a max tangent angle of 22-23 degrees,which falls very close to Mair's 1969 research,and Emmelmann's from the 1970s.NASA's boat tail for the Ford Econoline of 1980 isn't remarkably different.
With respect to angles,or contour for top and sides,ideally,they would be verified in a wind tunnel to guarantee that no vorticity was being introduced.
If we limit tangent angles to respect the 22-23 degree 'rule' we ought to be safe.
Kenny Lion's BMW motorcycle streamliner which I crewed on in 1990 was measured at Texas Tech's wind tunnel at Cd 0.11.
Walter Korff's design of the Summer's Brothers Goldenrod is rated at around Cd 0.115.This car was designed for a top speed of 555 mph.It ran a 1-way pass at 425 mph in 3rd gear on 'slush,' on the low power HEMI engines.
So the plan-view streamlining is known to perform just fine
The non-ideal streamlined train gets over 50% better mpg at 100 mph than the 'standard' railroad train.
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Old 03-27-2012, 06:29 PM   #255 (permalink)
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people

Quote:
Originally Posted by drmiller100 View Post
people are stupid.

Aero, you and Aerocivic have REALLY got my brain going.

People do not realize WHAT THIS MEANS. THIS IS A BIG DEAL, AND YES I AM SHOUTING!!!!!!!!!!!!!!!1

HOLY CRAP!!!!!!!!!!!!!!

THANK YOU!

I'm gonna go build one.
Things are what they are, for whatever reasons may be.
Personally,I've been amazed whenever these 90+ year old technologies are placed into service,and the performance produced.
I've had a blast learning and actually verifying some of the claims made so long ago.
In college,as engineering students,we were told that the goal of good design was the most efficiency with the least amount of resources.Aerodynamic streamlining can help with this.

'Efficiency' isn't a very sexy topic but it is a way to get rich slow.
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Old 03-31-2012, 04:17 PM   #256 (permalink)
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8-new pictorial tables

*I'm posting 8-new pictorial drag tables up in #248 permalink which will keep the 10-newest images together.
*By comparing the 'anatomy' of the different configurations and inspecting their respective drag coefficients,you can discern the difference how certain features impact aerodynamic drag.
*Most of the numbers are straight out of the wind tunnel.I did interpolate or extrapolate a few values based on drag curves constructed from published data for configurations not 'broken out' in the original studies.
*If you have the original 'Figures' used in the developments,you'll understand where 'I was going.'
*The last 2- tables are specific to the trailer project and by inspecting all 10-tables and cross-referencing them,you should be able to understand the linear thought progression leading up to the invention of the trailer.
*Abbott and Von Doenhoff's wing work establishes the significance of a 'gap' and its impact as a function of size and position along the airfoil.These values can be 'seen' within the railroad,drafting,convoy,and trailer images.
*When NASA gap values are considered with respect to the 'Standard' locomotive railroad train,you'll see tight correlation to the 3-bus convoy drag of #2 & #3 bus.
* Base drag of tail cars on trains can be deduced from drag of leading car vs tail car and vary as a function of surface quality,surface friction and boundary layer effects.Base drag IS the Cd of the wake turbulence itself.
*I've mixed buses and rail cars together.The air doesn't care.It's all the same as far as fluid mechanics goes.
*On line #6 of the railroad components table you will see a self-powered passenger rail car of 'Ideal' form with theoretical drag minimum of Cd 0.08.This form is remarkably like the aero-modded JEEP Cherokee vs Prius in the Stunt-Busters episode thread.
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There's always more to add,but I need to take some time to deal with Caesar ,domestic engineering,and femme-bot.And Bonneville.
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These 10-tables should arm you with enough data to be dangerous.They required 96 man-hours to put together,or over the cost of 19 of Hucho's books.
If you don't have Hucho's book.Get one.
If you'll eat dry dog food for one week,you can save enough in groceries to cover the cost of the book.Once you have his book,you'll be able to save enough to eat caviar if you like,which will easily take the taste of dog food out of your mouth.Your health won't suffer.I know this for a fact.
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Old 03-31-2012, 05:23 PM   #257 (permalink)
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I found it interesting that the lead car would get some drag reduction out of drafting. Also look at how the following cars can be so far back and still see improvements. Note how the template seems to have little to no effect on drafting numbers. I'm guessing they have more to do with effects in the broader air stream. We have seen this in the CAD studies members have shared here- air velocity behind the car is much lower than the surrounding stream. I'm not sure how that all comes together in the end but it's something to think about.



Also strange how the trailer he draws does not fit the template. Perhaps there is implied plan taper and edge radii?



I have to admit I don't have the book (imagine asking your dad for money to buy an aerodynamics textbook) but will get one as soon as I have any significant income!
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Old 03-31-2012, 05:40 PM   #258 (permalink)
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draft

As the 2nd car approaches it's riding in the turbulence of the #1 car.It takes on all the base drag as it comes close and will have a negative pressure at its nose.This is why combustion air intakes are at the base of the windshield where there's a bit of static pressure and radiators must be sized to deal with the heat flux, as no ram air is present while in the draft until they slingshot.
The turbulence behind #1 car extends for hundreds of feet and you see #2 picking up a 'draft' at some distance behind.
#1 car itself does not see a benefit until #2 is quite close.
If #1 were a 'Template' car,#2 would see no 'draft' benefit,as there would be no wake to draft inside of.
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Old 03-31-2012, 05:47 PM   #259 (permalink)
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trailer

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Originally Posted by Sven7 View Post
I found it interesting that the lead car would get some drag reduction out of drafting. Also look at how the following cars can be so far back and still see improvements. Note how the template seems to have little to no effect on drafting numbers. I'm guessing they have more to do with effects in the broader air stream. We have seen this in the CAD studies members have shared here- air velocity behind the car is much lower than the surrounding stream. I'm not sure how that all comes together in the end but it's something to think about.



Also strange how the trailer he draws does not fit the template. Perhaps there is implied plan taper and edge radii?



I have to admit I don't have the book (imagine asking your dad for money to buy an aerodynamics textbook) but will get one as soon as I have any significant income!
I drew the trailers and there is no real 'Template' scale implied.I just wanted members to understand the deductive reasoning behind the concept,as we morph from drafting cars or buses/rail cars into a trailer,comparing drag values from the whole suite of tables.
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Old 03-31-2012, 08:31 PM   #260 (permalink)
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Okay, so can we say turbulence acts similarly to wake? Are they different words for the same thing?

I didn't look close enough to recognize PRK Nice job- I thought it was out of a textbook. It's nice to see that (if I'm reading this right) as long as the trailer form fits inside the "template", it will reduce drag, without factoring in RR and such.

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