A book to buy and read

[aerohead]

Originally Posted by JulianEdgar

I actually don't think people get it.

I used to wait after Aerohead posted something that was completely wrong - wait for someone to say "No, that's not right", but what people would do was to instead thank him for being misled!

A classic example: Aerohead saying that on the current Porsche Cayenne, the airflow separates at the top of the windscreen and is separated right across the roof. Absolute garbage - which if accepted, is going to lead people astray in almost any aerodynamic car modification that they might try to subsequently think-through on a squareback car.

And there are so many of his posts like that...

When I first came here, I couldn't believe some of the weird things people believed about car aero. And what really gobsmacked me was that they were said with an air of absolute certainty. I knew they couldn't have come from any formal references (textbooks or SAE papers) so I started to look at where they did come from. In nearly every case, they came from what Aerohead was spouting.

I initially tried correcting him (eg look at the Tesla paper on wheel design and don't keep saying that full wheel covers are always best for low drag), but instead of gratefully updating his knowledge, he just poured shXX on Tesla! I couldn't believe it! Then I realised Aerohead isn't really interested in getting correct information out to other posters; he's interested in repeating - ad nauseum - his old misunderstandings.

But as I often say - don't believe me! Go and read the Tesla paper for yourself. Go and put some tufts on a Cayenne. Read some good aero books - like the one this thread was originally about.

As Vman455 has found, as soon as you start looking at the references Aerohead quotes, reading some aero texts and doing some testing, you will quickly find out for yourself that Aerohead writes a great deal of misleading rubbish.

* That's a rather remarkable observation. I have no recollection of ever saying such a thing.
I do recall making an association to lift with an attribution to the mutilation of the roofline, where the raked-roof truncation occurs. As the separation line is artificially moved closer to the suction peak by the stylist, the pressure is lower than if the roofline simply continued all the way to the rear of the body ( more like a RAV4 ), and since this lower static pressure is occurring 'over' the rear of the body, it's inducing lift. The separation creates turbulence which is incapable of transmitting momentum downwards, and with no reattachment, the wake gets dosed with lower pressure, which lowers base pressure, raising pressure drag, exactly what Hucho said we should avoid.
* I have the Tesla road test data from Germany. There's some context which is not addressed in their presentation. As is also true for the WS12 Aerodynamic Performance paper by Cranfield University et al..
* And again, if ventilated wheels are categorically, absolutely, with no reservation, superior to MOON, convex discs, why do automakers like Ford, General Motors, and Volkswagen choose them for land speed record attempts if their 'ventilated' wheels could clearly mean the difference in a world record? I believe that this is a fair question.
* If caveats and conditions exist, they need to be clearly addressed in any preamble to research findings.

[aerohead]

Originally Posted by JulianEdgar

The context of the book and its author are also interesting.

Dick Barnard has a strong background in aviation aerodynamics, where as he says, it's much easier to use equations to make predictions. When we were working on my book, he was always very cautious about advancing theories about what was actually happening in car aero. He used to say to me words to the effect:

"Tell them what is happening, not why it is happening. They (our readers) don't need to know why it is happening, and often it will be too complex for them to understand anyway."

I often remember those words when I read Aerohead's theories - the ones that usually don't match actual, measured, reality.

From what Dick - and also Dr Wolf of Porsche - have said to me, it's very easy in car aero to underestimate the complexity of what is actually going on. It's like ignition timing in car engines - you can theroise all you like but the interactions are so complex that timing charts are still constructed by testing.

1) Your mother chews castor beans on the left side of her mouth and dies.
2) Your father chews castor beans on the right side of his mouth and dies.
3) Your sister chews castor beans on both sides of her mouth and dies.
4) Your brother chews castor beans only with his incisors and dies.
5) Your other brother 'masticates' castor beans in a mortar and pestle, swallows the mash and dies.
6) Ad infinitum.
--------------------------------------------------------------------------------------
At some point, a theory about not ingesting poison takes on an air of practicality.

[aerohead]

Originally Posted by JulianEdgar

(1) When I first started commercially writing (about 37 years ago!) I assumed that the bigger the words I could put in, the better. After all, weren't they great examples of my writing abilities?

(2) When I became a school teacher, I realised that understanding was key - and to use the required words to achieve that.

(3) When I started teaching professional writing, I realised that the clearer I could make the concepts, the better.

(4) When I started writing books, I realised that the simpler the words in which I could express the concepts, the better.

My book chapters - and, I hope, the writing I do here - come in at about Flesch Kincaid 9-10. That is, 9-10 years of schooling.

I haven't run a test on Aerohead's verbiage but I'd guess FK = 13-16. It might just be that Aerohead is a crap writer, but my gut feeling is that he deliberately writes in obscure language.

There are conventions used in fluid mechanics and aerodynamics, which have unique specificity. They have a particular meaning. Like Verjungsgungsverhaltnis. It means only one thing.
Mechanical engineering requires the use of very specific, multidisciplinary language.
There's less confusion when certain words are understood by all to mean a certain particular thing.
My exercise with Hucho's subject index for example, was predicated upon my own difficulty in navigating his book, as certain, very important topics failed to make it into the back of the book, where an interested reader might have gone initially, targeting specific information to no avail. Since you didn't do it, it was left to someone else.
Go ahead and do the verbiage test. That 'll be fun.

[JulianEdgar]

Originally Posted by aerohead

* That's a rather remarkable observation. I have no recollection of ever saying such a thing.
I do recall making an association to lift with an attribution to the mutilation of the roofline, where the raked-roof truncation occurs. As the separation line is artificially moved closer to the suction peak by the stylist, the pressure is lower than if the roofline simply continued all the way to the rear of the body ( more like a RAV4 ), and since this lower static pressure is occurring 'over' the rear of the body, it's inducing lift. The separation creates turbulence which is incapable of transmitting momentum downwards, and with no reattachment, the wake gets dosed with lower pressure, which lowers base pressure, raising pressure drag, exactly what Hucho said we should avoid.

What Aerohead said on 12/5/2020:

Originally Posted by aerohead

* I believe that the lift issues that you believe to be the major reason for rear lift would actually be attributed to separated flow over a horizontal portion of the aft-body, ahead of the transom. Squarebacks are incapable of generating rear lift.

*The Cayenne and I-Pace both will create this separation with, zero chance of re-attachment, as they violate the limits of the Mair/Buchheim departure slope angles, necessary to allow for re-attachment.The plan-view section of the roof exposed to the turbulence is at the lowest static pressure, compared to the suction peak at the windshield header,and this low, acting over the 'lever arm 'spanning the distance to the tail creates the 'moment' which lifts the tail. Simply extending the roof to the back of the vehicle cancels the moment, leaving a higher base pressure, and lower pressure drag.

You can be confident that there is no separation at all on the roof of the Cayenne - this is just two versions of Aerohead's weird theories of lift on squarebacks. (Or, if you want to overlook the inconsistencies - no lift on squarebacks.)

Or is Aerohead actually trying to say that lift is caused by the wake pressure acting on the angled rear glass? (It's so hard to understand when he writes in such an obscure style - what is a transom? I didn't know the Cayenne was a boat.)

Either way, the theories are wrong, as I explained in detail in the original post.

[JulianEdgar]

Originally Posted by aerohead

Go ahead and do the verbiage test. That 'll be fun.

Sure, it takes only moments.



It's the sort of writing style I use as the 'bad example' when I am training technical people in writing skills! (FK of 18 is very poor indeed.)

[aerohead]

Originally Posted by JulianEdgar

What Aerohead said on 12/5/2020:



You can be confident that there is no separation at all on the roof of the Cayenne - this is just two versions of Aerohead's weird theories of lift on squarebacks. (Or, if you want to overlook the inconsistencies - no lift on squarebacks.)

Or is Aerohead actually trying to say that lift is caused by the wake pressure acting on the angled rear glass? (It's so hard to understand when he writes in such an obscure style - what is a transom? I didn't know the Cayenne was a boat.)

Either way, the theories are wrong, as I explained in detail in the original post.

1) the separation occurs right where the raked truncation is.
2) there are some wind tunnel images of smoke flow testing of other SUVs in which you can clearly see the boundary layer leave the roof, exposing a void underneath, which is of low pressure. Turbulence takes on the pressure of the separation line which created it.
3) since it's impossible for the flow to reattach, the entire wake is subjected to this low pressure over the backlight. If you draw the force vector at the backlight, you'll see the vertical component of lift acting on that incline, acting over the rear overhang.
4) this low pressure decreases the the base pressure, which in turn, raises the pressure drag.
5) since Hucho told us that the whole point of streamlining is to reduce, or eliminate separation and its attendant pressure drag, Porsche designers have succeeded in purposely raising the Cayenne's drag. It's not as bad as the Macan ( Cd 0.37 ), but it's pretty nasty.
6) 'transom,' we can use it. We're talking fluids.
7) Both flavors of Mercedes-Benz GLC are Cd 0.31 by comparison. The RAV4 is Cd 0.30.
8) See if you can find lift data for the Nissan Cube ,or Kia Soul. They would be a good examples of contemporary squarebacks.
9) as to 'theories', get ahold of Wolf Hucho, he studied under Hermann Schlichting, author of BOUNDARY LAYER THEORY, every aerodynamicist must study Schlichting's work.

[aerohead]

Originally Posted by JulianEdgar

Sure, it takes only moments.



It's the sort of writing style I use as the 'bad example' when I am training technical people in writing skills! (FK of 18 is very poor indeed.)

I'm just talkin' like a PhD. No apologies.

[Fat Charlie]

Originally Posted by JulianEdgar

What Aerohead said on 12/5/2020:

On this side of the equator we put the month first. Our toilets also go counterclockwise, the way God intended.

[JulianEdgar]

Originally Posted by Fat Charlie

On this side of the equator we put the month first.

No, in the UK they put the day first, so it's not 'this side of the equator'. As with many writing conventions, Australia takes after the UK.

[freebeard]

Verjungsgungsverhaltnis? Vas ist das?

The only date format that sorts correctly is ISO: 2020-09-07.