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Old 08-01-2009, 01:57 PM   #31 (permalink)
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Craig's brother

Quote:
Originally Posted by Rokeby View Post
This little clip rom Craig Vetters site is telling...

http://www.craigvetter.com/Movies/Streamlined_test.mov

blunt end forward, taper aft is the way to go for low drag.

I'm not sure the narrator, Craig's brother, has got the forces right. He
speaks of air pushing on the front of the car, when I think the real
issue is air pulling on the back as aerohead says.

I'm sure there are also unidentified interactions between the airflow
around the car and the tube due small clearances.

Although it is still technically "vaporware," Mitsubushi's all-electric iMiev
Sport Air concept car seems to be the best embodiment of these concepts:

Autoblog Green
In the context of a wind tunnel,the air would be impacting the body,in a positive pressure gradient up to the point of maximum cross-section,beyond which,the air would be receding away.With proper contouring,as Craig has already accomplished on his Rifle,etc.,the body could maintain attached flow all the way back,finally breaking away into a relatively turbulence-free wake.-------------- Without the proper contour,the flow would separate,and the base pressure behind the body would be that of the point of separation,acting to "pull the body back."---------- So it's the pressure differential at work here.Separated flow breeds high Delta-P,zero separation yields losses attributable only to friction,internal flow,and induced drag.------------------------------------ The Mitsubishi looks pretty good! I held a protractor up to the computer screen and measured around 22-degrees for the roofline exit profile,just at Mair's ragged-edge for attached flow.I wish they would provide an overhead plan view of the car so we could inspect that aspect of the cars architecture.I wish they'd do that for all cars!----------- Thanks for sharing that!

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Old 08-01-2009, 02:20 PM   #32 (permalink)
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[QUOTE=aerohead;119009]---- The template respects streamline flow with an economy of structure and literally guarantees attached flow,at least if you don't cheat the minimums.----------- YOU CAN make the body with higher fineness ratio,I just want everyone to understand that there will be a small penalty to extra skin friction.

I agree that it is good to avoid extra skin friction, but when only one study suggests that flow can stay attached to a shape at 2.5 : 1, and hundreds of tests at a more suitable Reynold's number disagree, I want to see some tuft tests. In the NASA tests for streamlining trucks, you can see a tailcone extension eliminating a Kammback, while increasing the angle a bit, and the tell-tales show separation there, at an angle that is still less than this "ideal."
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Old 08-01-2009, 02:27 PM   #33 (permalink)
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putting wings to rest

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Originally Posted by aerohead View Post
I've been thinking that we should do a separate thread exclusively for wing sections.I've been playing a game of catch up,reading a little deeper into my texts and see the wings with mixed feelings.Here are some random observations1) wings operate in free air which is virtually turbulence free.(2) automobiles operate in air which is all turbulence.(3) Virtually all the drag of a wing (unless it stalls) is skin friction.(4) Only about 7% of an automobiles drag is from skin friction,with 55% from profile drag.(5) Wings reach their lowest Cd(min) at aspect ratios of 4-5,and "practical" aspect ratios for wings are in the range of 3-9,and as high as 20 for sailplanes.(6) a wing used for a automobile body could have only a fractional aspect ratio,its tabular data unusable according to Abbott and von Doenhoff.(7)"Flight" Reynolds numbers of 6-million can be achieved at Re 2,000 due to turbulence in ground-effect,and at 20-mph.(8) What would otherwise be a "laminar" wing in free air,will transition to turbulent boundary layer at normal automotive ground clearance and low road velocity.(9) crosswinds would be the same as angle of attack change to a vertical wing section,Cl would quickly climb,as would drag( for example,a Clark-Y in a 17-degree relative wind would see a 180% drag increase),creating a pitching moment about the aerodynamic center.(10)0.0005-inches roughness or less is permissible for at the leading edge of a laminar wing,so dried bug juice and insect remains would be enough to scuttle the low drag of these high performance sections,not to mention free turbulence.----------------------------------------------------- Wings are great and I love what they do.I'm just having a time of it trying to wrap my brain around using their performance criteria in the context of an automotive body.Jaray's form does resemble a Clark-Y from the side,but with all the rounding off of edges on the side and nose,"pumpkin seed" seams a better fit for a descriptor.With the "mirror-image" my brain sees the body of a "pair-of-grins" falcon in full stoop at 250-mph.
This is an addendum to what I presented above in hopes of tempering enthusiasm for symmetrical wing sections,and providing a context in which to contemplate their use1) The fundamental assumption for all sub-supercritical -velocity wing theory models used to construct section data is that of 2-dimensional,chord-wise-only flow. (2) The fundamental assumption for all road vehicle aerodynamics is that of 3-dimensional flow,hence,the Navier-Stokes Equations of Spherical Coordinate System,employed in CFD modeling. (3) To presume the "applicability of section data" as presented in "THEORY OF WING SECTIONS" for 3-dimensional flow,in ground-effect and "outside turbulence",violates the fundamental premise under which all the data were conceived,and is stated so by the authors on page#28,paragraph#4,sentences-#2,#3,and #4 of their text.
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Old 08-01-2009, 03:23 PM   #34 (permalink)
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one study

[QUOTE=Bicycle Bob;119013]
Quote:
Originally Posted by aerohead View Post
---- The template respects streamline flow with an economy of structure and literally guarantees attached flow,at least if you don't cheat the minimums.----------- YOU CAN make the body with higher fineness ratio,I just want everyone to understand that there will be a small penalty to extra skin friction.

I agree that it is good to avoid extra skin friction, but when only one study suggests that flow can stay attached to a shape at 2.5 : 1, and hundreds of tests at a more suitable Reynold's number disagree, I want to see some tuft tests. In the NASA tests for streamlining trucks, you can see a tailcone extension eliminating a Kammback, while increasing the angle a bit, and the tell-tales show separation there, at an angle that is still less than this "ideal."
I have been unable to post any attachments since I posted the template.I've asked for help each computer session since and so far no help,so I'm deadlocked as far as that goes.---------------- I had additional documents scanned and attempted to attach with the other 5-documents but EcoModder message came up saying I was "maxed-out".------------------------------- The DVL research was conducted at Re 3,000,000.The work is included in Fig.5.13,page 69,Aerodynamic Drag,by Hoerner,1951.Why Wolf Hucho chose to include it in his table would be a matter of conjecture on my part.The form does exhibit one of the lowest Cds for a 3-dimensional form and is of lower drag than airship bodies of greater fineness ratio,of which it shares great resemblance.The airship Los Angeles,with fineness ratio of 7.23:1 only increases to Cd 0.045,when fins,gondola,and engine nacelles are taken into account),a 12.5% increase,however a 25% increase in ground-effect.----------------- The study does not suggest that this is the only form that will support attached flow.It does claim that a body of revolution with a fineness ratio this small CAN have full attached flow.So I chose it as a minimum,as with smaller fineness ratio,the exit architecture exceeds Mair's 22-degree maximum for a boat tail,suffering separation and attendant rise in profile drag.----------------------------- Since it is Hucho that says only a long tapering tail can prevent separation,and Jaray says that a half-body of a streamline body of revolution is the lowest 3-dimensional form for a vehicle body,my pea-brain seized on The DVL 2.5:1 as the logical minimum.Just a starting place.Obviously,this isn't a one-size-fits-all solution.The data suggests that it could be a no-brainer go-no-go,or template for which to contemplate potential fuel savings from aft body modifications,which Hucho suggests is our only path for truly significant drag reduction.He also states that in the context of cars which are designed specifically for low drag,the science is good and you can lean hard on it.------------------------------------------------ With respect to the NASA van,I think the exit angle off the boat tail is around 20-degrees,same as for my VW.If this is the same photo you're referring to,then I can tell you that it works and got me an extra 4-mpg.The important thing is the gentle curvature leading into the tail as you will see in Mair's boat tail research.Without that,the flow immediately separates,with little gain.------------------------------------------- I have a tuft-test photograph of the boat-tailed and streamlined T-100.There is very little perturbation of the airstream and all tufts are straight back at the terminus.The trailer will be a follow-on to the boat tail,providing,through the use of the gap-fillers,a continuous boat tail taper the a point about 10-feet aft of the tailgate.She's a little longer than needs to be,but that's where I'll be "living" on my way to and from Bonneville and Battle Mountain.The rig comes in at about 5.3:1 on the ground and 2.65:1 in "mirror" so she's a little over my minimum but I'll welcome the extra space inside.-------------------------- If you can post the NASA image I'd appreciate it,and be able to compare to what I have.Mines the truncated tail,only a couple of feet,maybe a meter in length.Thanks!
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Old 08-01-2009, 07:02 PM   #35 (permalink)
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Quote:
Originally Posted by aerohead View Post
...
I wish they would provide an overhead plan view of the car so we could
inspect that aspect of the cars architecture.I wish they'd do that for all cars!
aerohead,

here's the best I can come up with for an overhead shot of the iMiev Sport
Air concept car.





Not very good, but you can see that in essence it has a rectangular plan-
form at all levels with verry little side-to-side taper aft of the max cross
section.

I notice that the prototype has nowhere near the swoopy lines that the
artist's drawing did... Reality, that is salability and day-to-day survivability,
rears its ugly head.

Hope this helps.

Last edited by Rokeby; 08-01-2009 at 07:19 PM..
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Old 08-01-2009, 10:43 PM   #36 (permalink)
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Aerohead,

I PM'ed you but perhaps you didn't get it. I would be happy to post your images and files. PM me with your email address and I'll send you mine.

If I'm ever in the area, I'd be glad to show you how I work. In the meantime, I have thought of doing more detailed posts here if people were interested. I could do little videos, I suppose.

Thanks again for all the advice.
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Old 08-04-2009, 06:43 PM   #37 (permalink)
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Quote:
Originally Posted by Rokeby View Post
aerohead,

here's the best I can come up with for an overhead shot of the iMiev Sport
Air concept car.





Not very good, but you can see that in essence it has a rectangular plan-
form at all levels with verry little side-to-side taper aft of the max cross
section.

I notice that the prototype has nowhere near the swoopy lines that the
artist's drawing did... Reality, that is salability and day-to-day survivability,
rears its ugly head.

Hope this helps.
You can see some boat tailing of the roof structure although the body has less camber.Thanks for the link!
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Old 08-04-2009, 06:51 PM   #38 (permalink)
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PM

Quote:
Originally Posted by orange4boy View Post
Aerohead,

I PM'ed you but perhaps you didn't get it. I would be happy to post your images and files. PM me with your email address and I'll send you mine.

If I'm ever in the area, I'd be glad to show you how I work. In the meantime, I have thought of doing more detailed posts here if people were interested. I could do little videos, I suppose.

Thanks again for all the advice.
Thanks orange4boy,I left work early today to check PMs and had heard back from Darin and SVOboy.They think they have me fixed up with more capacity and this Saturday I'll try again with images.------------------------------ Your video is a great idea.I learn so much from observing and its a powerful teaching tool.At the build forum,such a "clinic" could get a lot of mileage from the members and lurkers worldwide.Nothing like class-A tin bashing!
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Old 08-08-2009, 04:10 PM   #39 (permalink)
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low fineness-ratio tuft study

Al helped me export a photo of the T-100 undergoing a tuft study to illustrate attached flow at low fineness ratio,something the template suggests as a "minimum" roofline curvature which still guarantees no separation.---------------------- The photo was taken by friends a couple years ago,on LOOP 288,Eastbound from FM 2164 at 60-mph.----------------------------- As you can see from the image,while there IS some lateral flow along the flanks,the flow is clean as the flow exits the body.Kamm's car had this "spanwise" lateral flow,which while not ideal,is nevertheless attached,and further aft,can deliver energy to the wake for pressure recovery.NOTE: the boattail becomes the basis for the articulated gap-fillers with the trailer project,providing for continuous flow to the trailer tail.
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Old 08-14-2009, 01:40 AM   #40 (permalink)
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Quote:
Originally Posted by Bicycle Bob View Post
I want to see some tuft tests. In the NASA tests for streamlining trucks, you can see a tailcone extension eliminating a Kammback, while increasing the angle a bit, and the tell-tales show separation there, at an angle that is still less than this "ideal."
What are the signs of separation in a tell tale. I was testing my kamm back and got flat tell-tales that scooted side to side a bit. Do they have to be straight and motionless or is a bit of side to side ok?

What is a good angle to start with for a kamm back. Profiles are nice but my printer ain't that big.

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