Aerodynamic Streamlining Template: Part-C
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Aerodynamic Streamlining Template: Part-A
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The above is the intro the the templates which were posted separately by aerohead. Thanks for the charts and template. Good timing for me because I'm going to start my kamm-back soon. I have several neophyte questions that have not been answered so far (that I could find). 1. Does the optimal fineness ratio change with speed? I am hoping that if I make mine lower, that is to make the slope steeper, but I stay below say 55mph, will the flow stay attached? My aim is to reduce the size of the tail, and I could live with the slower speeds to get the wake and tail as small as possible. I don't think the Egg will ever go "super-critical". Sounds like you would need a flux capacitor for that. 2. Does the kamm-back or boattail have to be closed off to get the best result? How much difference is there between a closed off and an open ended tail? |
Keep in mind, the right chart that shows the .25 t/c being optimal is at a reynolds number of 400,000, much much lower than a car traveling at 55mph.
Orangboy, for your vehicle, you want to experiment with shapes at RE of around 11,200,000, considering your vehicle is 15 feet long stock, and i'd expect atleast 5 feet to be added for a boat tail RE = 9360 * SPEED(in mph)*DIMENSION(in feet) Using a program called DesignFoil, i have found that the best shape for a vehicle traveling 60mph has the location of minumum pressure located 70% the length of the cord(from front to back). The shape is NACA 67-025. http://i422.photobucket.com/albums/p...Naca67-025.png However, this shape has a center of pressure that is out in front of the shape 8% of the chord length. What this means is its very susceptible to cross winds, and will get blown around. When you move the max width forward a bit, the cd increase slightly, but the center of pressure moves backward drastically, meaning its better in crosswind. For example, going from NACA 67-025 to NACA 66-025 (70% to 60%), the COP is moved back to 30% of the length (IE, its now inside of the shape) |
I just thought of some other questions I have not seen answered elsewhere.
Regarding the point of stagnation and the underbelly. Does the air under the car have a lower or higher pressure? I think it's a low pressure area but it's not obvious to me. Is it better to have a sharply defined, upward sloping air dam parallel to the lowest points on the car or a bumper curved under the car from the stagnation point (like in the template?) the trailing edge of which terminates parallel to the lowest point under the car? Thanks. |
From everything i have read, it seems that the optimal design is to have no airflow underneath the car. The flip side is that this is not really practical in a road car because there needs to be some sort of ground clearance for speed bumps, parking lot entrances (some are not level with road), etc. So the best answer i can give is that the least amount of airflow under the car is ideal.
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I would like to hear what aerohead thinks about what i have talked about so far.
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I think you are right regarding the least airflow under a body is best but at some point assuming the same ground clearance are you not increasing frontal area if you block off all under flow. It could be this is an advantage anyhow. If the larger frontal area is offset by reduced drag. I have not heard this spelled out yet. Thoughts?
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Well, there has been some discussion about totally covering the front to eliminate all the air going under the car, even if the airdam is lower than the underbody. Its a common practice at the salt flats and in nascar.
http://www.carolinaautomasters.com/I...l/PICT0079.JPG http://www.saltflats.com/Graphics/JP...%203%20033.jpg |
B*tchin' Camaro!
So what you need is: 1) a stiff rubber lip at the front for aero 2) a stiff upper lip to deal with the guff from the unwashed masses. 3) a nascar decal. Got it. |
Thanks for the source postings. The bible I've been going by is "Theory of Wing Sections" by Abbott and Von Doenhoff, which takes up the history from just after that first chart. Being concerned with wings, it makes sense for them to not go beyond 21% thick, as they want to leave something in hand for some angle of attack. That had persuaded me to go to 25% on a strut myself.
The NACA tests progressed to "laminar flow shapes" of which the 67-series is the most extreme. These are almost lab curiosities, first achieved in production on the P-51, using bondo over the flush rivets. No such shape can tolerate a blemish, let alone a wheel opening, without tripping the boundary layer into turbulence in a wake spreading 15 deg to each side. The "standard roughness" in these tests, which double the drag, consists of .011" grit, covering 5-10% of the area, just within 8% of the length from the leading edge. (like small bug strikes) Here are a few selected data points from my book: The lowest cd shown was for a shape that didn't make a stable wing, so there may be much room for exploration. That wing is the 0010-35, in the old system, 10% thick. At Re from 3 to 9 million, the cd is .003 near-polished, and .009 "rough." The 2424 shape is similar to the 2412 It gets a cd of .008 smooth and .013 rough, at 24% thickness, while the 12% thick shape gets .0065 smooth and .010 rough. Moving on to the later laminar shapes, which rely on perfect conditions to maintain a laminar boundary layer back as far as the second term in the identifier, usually the widest part of the shape. A 63-006 shape is a 6-series, (laminar) with 30% laminar flow, no camber for lift, and 6% thick. - ie: 100" long, 6" thick. The 67-025 is 70% laminar, no camber, 25% thick. The 66 and 67s are slipprier than the 63 and 64s, but who can wait for 70% of the way back for the first seam, wheel opening, or other feature? Shape: cd around 6 mil Re. cd with standard roughness 63-006 .0043 .0087 63-021 .0055 .011 64-009 .0043 .0087 64-015 .0047 .0097 64-021 .0054 .018 66-006 .0032 .0085 66-021 .0041 .013 These all show a clear preference for the slender shape, despite the extra surface drag. The text helps illustrate the trends with increasing speed, and gives examples that improve, such as the 65-418, with less separation as you go faster. This is not uncommon, but the devil is in the details, and these shapes are only a rough indication about designing practical cars. Re: underflow - it is good if done right, but blocking it off is a cheap fix that gives downforce, which is good for road racing, not cruising. Underflow is not really hard to visualize; it is like a duct, but with the boundary layer on the flat side moving and helping stabilize things. After F1 banned the vacuum cleaner cars, they did quite well with underbody venturi systems. |
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I've been using a RE of 6million for all of my tests. It seems perfect for a 10foot chord at 65mph. I have read that the 6 series NACA foils are susceptible to imperfections on the body as well. The reason i am so interested in the shape, while not practical for a 4 wheeled car, is because it seems quite ideal to use for a bicycle and/or motorcycle. Funny thing i say that because there are quite a few high speed HPV's that use a shape very close to the 65,66, and 67. Example 1: Sam Whittingham's Varna Diablo: the worlds fastest HPV @ 82mph http://www.varnahandcycles.com/gallery/g12.jpg Example 2: The Cheetah bike http://www-eng.lbl.gov/~osborn/cheetah/pop_sci.jpg This team used the 6 series foil from top to bottom when shaping their fairing, as seen here: http://www-eng.lbl.gov/~osborn/cheetah/wirefairing.jpg They went 65mph with this design. |
Yes, the NACA 6-series has been very popular with HPV builders, although between the seams, ripples, and vibration, they probably seldom get up to even 20% laminar flow, which makes the shape just a reasonable guess. The Varna is completely done by artist's methods. George sends persistent theorists to argue with me and leave him be, and won't even run a tuft test.
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http://ecomodder.com/forum/member-or...y-supplied.jpg
http://ecomodder.com/forum/member-or...5-top-view.jpg I love how these look. The charm of the hand drawn templates is something I miss with most everything done with computers now. I think this is a good visualization tool. I have been trying to find something like this for a while. Now I just have to decide where to cut it off. I may just build a full length detachable cone section aft of the kamm. Why should Basjoos have all the fun? |
Somehow, it looks almost identical to the Clark-Y, a very popular wing of the period, so you'd probably want to adjust it nose down a degree or so to kill the lift.
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The golden egg is blushing at your inadvertent compliment.:o She weighs a sprightly 3800lbs. I'm fairly certain she won't be lifting off at highway speeds.
Or perhaps you're right and I'm going to be the proud owner of the first ever STOL minivan.:thumbup: Isn't lift unavoidable when streamlining? The shape is a wing after all. Or are you referring to keeping the underbelly pointing down so as not to create pressure under the car? I suppose added to the lift, that would start to really add up. The Previa is already nose down because I have removed all the rear seats so if that's the idea, I'm good. |
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speed/open-closed
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Phil, your post has reminded me to add "enclose rear of Kammback" to my to-do list. Thanks.
It's easy to do with a piece of cut plexiglass that still preserves my rear view. |
thinks
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Watch that ground plane!
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wings on the ground
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 observations:(1) 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.
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"The air going under the car takes a straighter path to the rear than the air going around or over the car and according to Bernoulli's theorem,it's pressure would be higher than those other two pathways.-------------------------"
If the path for the air from the front stagnation point to the low pressure zone at the rear is very smooth, the pressure difference, along with the restriction from the ground, can move the air as quickly as the air going around. This is one of the major techniques to produce downforce for racing - the venturi bottom. |
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Since these shapes are so susceptible to imperfections (bug guts, cracks in shape, roughness in shape, etc), what then would be the optimal shape for a road vehicle? Is it worth even attempting to use a low drag foil shape, like the NACA 6-series, on a road car at that rate? Are you suggesting that a simple shape following the template you have posted would have less drag and more laminar flow than a car designed around a NACA foil, considering every day road conditions? Would it be safe to say that a shape which has the least average drag from -10 degrees to 10 degrees would be the optimal to select for a road vehicle? |
More darned questions....
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By the way here is the link to my newly finished roof and sides extensions, soon to be kamm back. I'd be thrilled if you would give it a look. It was mentioned to me to concentrate on the rear end. It's a start. http://ecomodder.com/forum/showthrea...evia-9437.html Thanks guys! :thumbup: |
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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 |
as quickly
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shapes
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darned questions
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Craig's brother
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A lonely messenger
[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." |
putting wings to rest
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one study
[QUOTE=Bicycle Bob;119013]
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here's the best I can come up with for an overhead shot of the iMiev Sport Air concept car. http://www.youtube.com/watch?v=6PcvEQVR1mY&NR=1 http://www.youtube.com/watch?v=LlIoP...eature=related 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. :p Hope this helps. |
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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low fineness-ratio tuft study
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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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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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