Quote:
Originally Posted by Vman455
I hope he doesn't mind my excerpting his book here, but this quote, from Chapter 4 "The Aerodynamic Design of Family Cars" in Road Vehicle Aerodynamic Design: An Introduction, is salient to the discussion surrounding the 'template' on this site in recent months:
A few comments:
We know from the literature on car aerodynamics and direct measurement that the range of variation of shapes that can support attached flow (that is, that are streamlined) is large; I posted a quote from a 2010 paper by Hucho on another thread here yesterday regarding the leeway available to designers to change fastback backlight angle without increasing drag, for example, and tuft test images posted here by a handful of people show attached flow over a range of shapes.
This excerpt considers the impact of practical factors, such as occupant room, and physical parameters, such as ground clearance, on the 'ideal' form. Even if we were building cars from scratch--which most of us aren't--slavishly following a 'template' will not by default obtain the best results.
Now, consider that most of us are modifying already-existing production cars, cars that are not half-bodies of revolution, cars that may have air dams or limited underbody paneling or extensive underbody paneling from the factory, cars with narrow tires or wide tires, cars with high ground clearance or low ground clearance, cars with varying rear-body shapes that translate to varying flow fields and pressure profiles, cars with all sorts of differently-optimized (or not-optimized!) aerodynamic and styling details.
I used to think, when I didn't know any better, that extending along a 'template' was the solution--based on what I read on this site. I now realize that this an incredibly simplistic view, and one that does not take into account the facts that 1) there is no single optimum 'template', and 2) the variation in shape and thus flow over the cars we modify is in no way consistent, so a "one size fits all" approach simply cannot produce the best results possible.
For example, a few weeks ago I measured surface panel pressures and found that the flow speed down the center of the roof of a Prius is faster than at the outside edges, and that over the rear window the opposite is true. The 'template' assumes uniform pressure and thus uniform flow speed over its circular section. Given that this is not the case on a Prius, why would a person assume that extending it with a half-circular tail at a specific angle is automatically the best solution to decrease drag? Maybe it isn't. Perhaps the optimum solution doesn't involve fitting a tail at all, if doing so would increase difficulty of parking or not being able to fit in a standard garage, for instance, but looking at other areas of the car to decrease drag. Perhaps a tail that does not have a half-circular cross section would reduce drag more than one that does. How on earth would one know without testing? Without measuring, I would never have known that the flow speed over the rear body is not constant side-to-side in the first place!
'Templates' are like security blankets: someone guarantees results with a minimum of thought or effort. But after trying it, testing is infinitely more rewarding, not very expensive, and not that difficult; the hardest part is really just finding the time to do it, and that's not a huge hurdle. More importantly, testing will reveal what the air is actually doing over your car--no guessing or predicting necessary. Go try it!
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1) sounds like Barnard is describing the Morelli CNR 'banana' shape, developed in 1976, from a theoretical, 2-D, numerical model of WW-II era, which is constrained for zero lift and pitching moments.
2) Goro Tamai covers this technology at length in his book, "The Leading Edge.' MIT utilized the Morelli form with their solar race team.
3) at 'normal' ground clearance, the Morrelli form has 79% higher drag than the 'template.'
4) at 'ideal' ( drag minimum ) ground clearance, the C.G. is so high that the NHTSA might deem it unacceptable as a candidate for commercial passenger car manufacture.
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5) as to the velocity profile over the 'template', you are correct.
6) no extrapolation to the Toyota Prius is implied.
7) the original premise of the 'template' was as a guide for roofline elongation modification, a prerequisite to low drag.
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8) your 2010 data from Hucho may be conditional, contextual, as well as any tuft-test imagery.
9) Hucho provided quite a lot of design latitude for vehicles up to Cd 0.15.
10) below Cd 0.15, a designer is much more constrained.
11) in order to approach Cd 0.09, to Cd 0.07, a designer would be required to keep with the half-body of revolution, with no rear slope angle any steeper than 22-22.5-degrees.
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12) the body cross-section may morph longitudinally, from a half-semicircle, to more rectangular section, and back again, as long as the pressure profile is maintained. This was already demonstrated by Jaray as of 1922, Kamm, General Motors, Funderburk, etc.. Commercial/ Industrial ASHRAE, HVAC ductwork technology takes advantage of this capability, and I have nearly 6-years of firsthand experience working around it with the company's Pitot-tubes, MAGNEHELICs, anemometers, flow-hoods, U-Tube manometers, and inclined manometers
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13) 'Parking' was addressed by Fachsenfeld in the 1930s. I reproduced the technology and tested in the latter 1990s. That should be in my photos album here at the forum.
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14) As to 'testing', the 'template' is already pre-tested. It's simply off-the-shelf-technology, for 98-years now. That's the whole point to the exercise. It's a known quantity. It's not an unproven hypothesis. It can be taught as 'theory' in fluid mechanics because it's been corroborated, time and time again, worldwide
15) whatever you're modifying, the ground reflection, mirror-image criteria remains. Whether 'square' or 'round' the aft-body contour must not deviate from from Hucho's ( Julienne's, Lanchester's, Eiffel's, Jaray's, Lay's, Heald's, Aberdeen Proving Ground's, Electric Boat Division of General Dynamics Corp., Prandtl's, Reid's, Horner's, Kamm's ) l/h = 5:1 streamline half-body if you ever expect to achieve really low drag. If you have a problem with it, take it up with NASA. I'm just the messenger.
16) And as to Barnard, why the supposition that he's the sharpest knife in the drawer?