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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First, I want to preface what I'm going to say with an opening statement directed towards VMAN455, in hopes that he'll communicate it to Julian, whom I believe he remains in contact.
There has been 'strong' back-and-forth interplay in regards to the ASTs and 'expert' opinions.
When asked, I never received actual transcripts of conversations with the panel of experts responding to queries about the ASTs, so I've never known the actual language used in discussions, which might explain certain 'responses', including the late Dr. Barnard's.
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It appears that the language regarding the 'template' shared by R.H. Barnard is cribbed directly from SAE Paper# 2000-01-0491,'A New Aerodynamic Approach to Advanced Automobile Basic Shapes,' by Dr. Alberto Morelli, in the ABSTRACT, Introduction, page-2, and page-4.
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Professor Morelli provides artwork from Paul Jaray's patents, associated with this text.
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If, somehow, Dr. Barnard based his comments on 'these' images, rather than the ASTs, then I'd be in total agreement with his wording.
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Since neither Mr. Edgar, nor VMAN455 are mechanical engineers, aerodynamic engineers, aerodynamicists, nor aeronautical engineers ( yet in the case of VMAN ), and neither are in possession of THE THEORY OF WING SECTIONS, or Boundary-Layer Theory, then there's a possibility that some things got 'lost in translation,' and Dr. Barnard was responding to a different topic; and neither Mr. Edgar, nor VMAN would be the wiser. They wouldn't know what they don't know.
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I'll leave it to all interested individuals to compare the wording and make whatever conclusions they derive from the text.
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If Dr. Barnard 'did' base his criticisms of the ASTs on Jaray's depictions, then nothing he wrote would be germane to the AST's. They're a completely different animal. And all this gnashing of the teeth has been completely unnecessary.