The Fastback bloc

[aerohead]

Originally Posted by JulianEdgar

I am not sure that is the best approach. In many cases, it appears more to be the shape of the C-pillars. I don't know of any comparative testing that has been published, but just by observation it's my guess (and only a guess) that C pillar designs that prevent airflow wrapping around the pillar onto the rear window reduce the strength of the vortices on fastback shapes. In other words, the more the airflow can be kept travelling longitudinally, the better.

If you look at the XL1, you can see this may well occur:



My tuft testing of the Tesla Model S shows this occurring:



..and the rear view shows the shape of the pillars well.



The Taycan is very similar:



Contrast that with my tufting of the XE Jaguar, for example, where you can clearly see airflow wrapping around the smoothly curved pillars:



...and look at how low the pressures are on the upper part of the rear pillars
(hotter colour = lower pressure).



(And if you look at the tuft and CFD images of the Jaguar, you can see - of course - that lift is created by the attached airflow... and not some weird theory that separation is responsible for lift on these shapes.)

The tufts indicate that the airflow over the roof is moving faster, and is at a lower local pressure,as per Bernoulli's Theorem.
The higher pressure/ slower air along the sides is attracted to the low pressure above it.
On a 'streamlined' body, the sides of the vehicle are boat-tailed for approximately 66% of body length, and their velocity and pressure profiles mimic that over the roof very closely. There's no 'attractive' low pressure over the roof to induce transverse flow which would otherwise roll-up into vortices.
If you watch the 'Streamlined Toyota Pickup in the Wind Tunnel' video, I spent quite a bit of the session imaging the rear of the boat-tail with smoke. You'll notice completely-attached flow all around the perimeter of the tail, indicating completely attached flow, free of vorticity. You'll also see the thickening of smoke filaments as the air decelerates and gains pressure, on its way to the separation line on the tail.
I'll bet you a donut and coffee that the XL1 is vorticity-free.

[aerohead]

Originally Posted by JulianEdgar

Porsche Taycan isn't far off that. Fastback shape, 0.22 Cd in Range mode, 0.02 CLr in Performance mode (ie rear spoiler up), 162 mph top speed.

Taycan's pretty close to the template, and the spoiler would be handy at those speeds. Hucho says that simply lofting the rear end of the Porsche up to the template contour would be even better for drag and lift, but he'd also say that the stylist would have the ultimate say in the matter. And probably, thumbs down.

[aerohead]

Originally Posted by JulianEdgar

Incidentally, note the shape of the Tesla Model S versus The Template.



See all that separated flow down the rear window?



Me neither.

You've failed to gain anything from our discussion about vortex-induced downwash, and how it gives the illusion of '
attached 'flow.
Put the Tesla Roadster-II under the template and see what they've done for a 250-mph car.

[aerohead]

Originally Posted by JulianEdgar

OMG, here's another.





How can the Jaguar possibly have attached flow on the rear window (albeit with a small centre/lower separation bubble.)

I mean the spoiler doesn't even 'reach up' to the right level! How on earth can the Jaguar generate positive pressures on the rear boot when it all should be in separated flow?

Anyone might think - gasp - that The Template is a complete crock. No, that would never do....

Why is it Cd 0.26 and not Cd 0.22?
Why is Spirit Cd 0.217 as a BEV? And lower if measured at A2. Even lower if measured at Toyota?
It's the positive pressures causing all the trouble on the XE. The TBL cannot tolerate that.

[aerohead]

Originally Posted by JulianEdgar

And it's not a fastback, but to show how utterly BS 'The Template' is, here's a sedan.



So of course airflow would separate at the end of the roof and remain separated all across the boot lid, no? And to gain flow re-attachment, you'd need a spoiler that 'reached up' to the template line, no?

Except, of course, that theory is simply rubbish.



Note the clearly attached flow down most of the window and on the trailing edge of the boot lid...

Exactly what Peter Brock did with the Shelby Daytona Coupe. And what Chrysler did with the Dodge Viper.

[aerohead]

Originally Posted by JulianEdgar

So, if you do almost no real-world testing, and rely on theory and shapes largely derived from the 1930s, it's possible to develop a very odd idea of what is actually going on.

Then, if your first premise is utterly wrong, you are potentially in a position where you build upon that faulty foundation and come up with a second premise, that is equally wrong. And so on...

First faulty premise: if the shape curves downwards faster than The Template, flow separation will occur.

Second faulty premise: if the shape curves downwards faster than The Template, flow separation will occur and therefore low pressures - and so lift - are due to separation.

Then, if you are completely seduced by your weird theory, you can extrapolate it into absurdities - a third faulty premise.

Third faulty premise: rear spoilers reduce lift by 'reaching up' to the streamline that would have occurred had 'The Template shape premise' (ie Premise 1) been true.

(To be fair, the third faulty premise is faulty only on cars with attached flow. Where separation occurred at the end of (say) the roof on a sedan of the 1960s, this was a valid idea [just not with The Template as the demarcator] and certainly not with cars of the last 30-odd years.)

The we add to the mix:

• a believer in these faulty premises who sometimes writes in an incomprehensible (but impressive-sounding) pseudo-technical English
  
  
• a largely uncritical audience that doesn't know much at all about the subject (sorry, guys)
  
  
• something I have noticed that dominates discussion groups with largely US contributors, and that is a really strong peer pressure to not call out rubbish being spouted by others (it's apparently seen as being rude)

.... and so we end up having completely invalid notions being accepted, regurgitated and defended.

One of my undergraduate majors was sociology. I sometimes think I should do a doctoral dissertation on hobby-technical BS spread by discussion groups...

Take it up with Hucho. I haven't shared anything that he didn't publish. You're a 'writer' okay. You just haven't mastered reading for comprehension. You should be correcting me scientifically, out of the same book the we share.
Since you don't or won't. what am I to think?
You can't present a logical, linear, coherent argument to defend your belief system while overturning Hucho's ground rules for fluid mechanics.
I suppose you also argue female anatomy with God.
You're a funny fellow. Indeed.

[JulianEdgar]

Originally Posted by kach22i

Again, flawed as any 3D template is and using only the latter half in 2D is, it's better than nothing at all for the novice experimenter.

The starting point is not flawed if it gets one to a working end point.

I have commented before about what I describe as an engineer's affinity for efficiency. This affinity for chasing an idealized solution often ignores less efficient designs which may lead to narrow thinking.

The attraction of the Morelli design for me and others is that it dares to color outside of these standard coloring book lines.

It is outside the box that we learn more about the box.

No, I think you have it quite reversed. Using a template of any type makes us think in 'the box' that the templates creates. Far better to test a variety of approaches and see what works, rather than have the template already directing us in a certain direction.

Concrete example: when I made the roof extension to my Skoda Roomster, I tested many different angles until I found that which gave the best results. The final design therefore took into account everything happening on the car - boundary layer thickness, wake pattern, etc. I didn't have any preconceptions as to what would work - which I would have had, if I'd thought a template (any template) would be a guide.

Thank you Julian for describing the differing boundary layer thickness dependant on the body forefront.

I have struggled to understand this before and your comments click into place many lose ends that went underexposed and undetermined in this forum in the past.

That's OK. Templates (and Aerohead's endless discussion of ideal shapes) I see as relevant only if we were all developing cars from scratch. If we are working with an existing car shape (ie modifying it) I think templates - any templates - are worse than useless.

[aerohead]

Originally Posted by kach22i

Great link, guess I have not seen that one before.

Has anyone superimposed the Mair's tail with the Aerodynamic Streamlining Template Part-C?

Yes, Photobucket seems to have removed the cork from it's butt.

Hurray!

It makes a fine template. So does Buchheim's 'Flow' body. I have about a dozen,but the 'template' satisfies every condition I've run across since 1974.

[JulianEdgar]

Originally Posted by aerohead

The plan-view of the two vehicle rear ends does nothing to inform us about the rest of the geometry of the two shapes. Conditions are everything.
The Polestar-I notchback is Cd 0.375.
Take a look at the Polestar-3 Precept. There's a reason for the change. Something Elon Musk knew all along.
Hucho gave us a half-dozen shapes with Cd 0.15. There's not a notchback in there. Nor any vehicles below Cd 0.15.
If there's a spoiler on a car, it means that there was a mistake with the original design and they've put a BANDAID on it.
Yes, a Euro-spec Mercedes-Benz CLA 180 Blue Efficiency, notchback and one of the BMWs is Cd 0.22. I've gone lower in my pickup truck.
Really low drag comes only from a fastback, and not just any garden variety. Hucho is crystal clear about it.

This is Aerohead's normal mix of irrelevancies, errors and half-truths.

If Aerohead believes the rear active spoiler on the Porsche Taycan is a bandaid put on a mistake, he appears to be suggesting he knows more about car aero than Porsche.

Mind-boggling.

[aerohead]

Originally Posted by JulianEdgar

I don't know how you get to that - Aerohead has repeatedly stated that his theories are correct in the face of contrary test results.

I emphasise: repeatedly.

How about we investigate those? Perhaps we'll discover 'conditions,' 'limits', 'caveats.'