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Old 10-04-2023, 03:16 PM   #10 (permalink)
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Julianís video is a great example of testing: on a real car, in the real world, with attainable equipment (the pressure scanner is on the expensive side, but you can make measurements like this with very cheap equipment).

I know for a fact there were multiple back-and-forth discussions between the expert reviewers, as they argued over points and concepts and even word choices. Some of these even made it into the final version of the book, where two contrasting opinions are each given a sidebar so the reader can see the discussion and differing views (and Iím happy to have contributed one of those, contrasted with Rob Palinís view on the next page). Aerodynamics isnít a settled science. There is lots to investigate and research and learn still. The expert reviewers wouldnít have allowed their names to be attached to this book if they didnít stand behind everything in it.

That said, there is nothing untrue in any of Julian's points:
-Pressure drag accounts for 80-90% of the drag on a typical road car
-Pressures vary because of variations in local flow velocity (where flow is attached) and separation
-We donít have to assume or imagine pressures on a car because we can measure them, on real cars on the road
-Air behaves unintuitively and sometimes our mental models are incorrect
-To reduce pressure drag, we can reduce pressure on front-facing surfaces or increase pressure on rear-facing surfaces
-This also includes oblique surfaces where a pressure vector has a horizontal component because of the surfaceís inclination
-Rear spoilers, if shaped correctly, can not only reduce lift but also drag
-Tail extensions can reduce drag
-Reducing the base area exposed to the wake can reduce drag
-Low pressure on front-facing surfaces can contribute thrust and reduce drag
-YOU can measure pressure changes on the road and SEE if changes you make are reducing pressure drag

Thatís all I have time for, for now. Say my name three times and maybe Iíll appear again in future.

Originally Posted by aerohead View Post
It was difficult for me to watch.
* It's intended for non-engineers. If the expert reviewers and proof readers considered that only lay people would be reading these materials, perhaps a certain 'latitude' was given. I don't know.
* Kind of a dog's breakfast.
* Contextual comments are made without the context, and passed along as if they were universal truths.
* At least one comment could never withstand scientific rigor, as there's counterfactual evidence available in the public domain.
* The data is never captured as in official investigations.
* The data isn't presented as in official investigations.
* The data is never normalized to reflect 'standard' conditions.
* There's a risk that the data is being misinterpreted.
* Teachable moments fall through the cracks.
* Mechanisms associated with the observed phenomena never mentioned.
* I experience over-simplification and confusion with respect to rather sophisticated fluid mechanics principles, which might not best serve the audience. And it involves the basics Dr. Hucho insisted engineers learn, one-third of the reason he published his 2nd-Edition.
I'll do some more thinking on this between now and Thursday when I return. I've got some materials at home I'd like to revisit.
UIUC Aerospace Engineering
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