Want low drag? Don't follow a template.

[aerohead]

Originally Posted by JulianEdgar

Huh? I am using the template nominated in the tools section of the site, the one I have seen used numerous times when purporting to do each of the following (absurd) uses:

- Show where there is separated and attached flow on existing cars
- Guide the shape of rear extensions
- Show how rear spoilers on sedans should be positioned and shaped
- Allow the assessment of the ‘aerodynamic purity’ of cars)

If you want to pick a different template - fine, but it's an equally absurd approach.

I don't know how often it has to be stated, but 'low drag shapes' (let alone any sort of template) are given basically zero coverage in any current textbook on car aero. And it's not because they've forgotten to include it.

So you can either decide that this site has come up with an extraordinary breakthrough in car aerodynamics.... or this site is on the wrong track. I am afraid in any car engineering debates, I always back the professionals over amateurs - and professionals certainly don't use any type of aerodynamic template.

1) The 1st-gen Honda Insight and 2nd-gen Prius on are both considered 'Kamm' cars.
2) 'Kamm' was actually Baron Reinhard von Koenig-Fachsenfeld, owner of the German patent for 'K'-form vehicles. While Kamm had lectured on this form of vehicle, Fachsenfeld had already won races with , and sold the technology within the German automotive industry.
3) A great deal of research at the FKFS had to deal with dimensional analysis between Walter E. Lay's ' pseudo-Jaray', multi-element, built-up scale models, and Fachsenfeld's 'tweaking' into bread sliced, vertical truncations.
4) The models could predict where separation would occur. The models could predict the drag coefficient for any given body length, from zero-to-100% body length ( creating a streamlined half-body of revolution).
5) For small Verjungungsverhaltnis, the K-form was always lower in drag than the Jaray form, until a certain, critical ratio of Vejungungsverhaltnis was reached, after which, neither J or K form had any advantage with respect to drag.
6) For the 'short' vehicles, the K-form always offered more usable interior cabin space. Making it a favorite.
7) Lay reported as low as Cd 0.12 for his long-tail ( lang-heck ) model.
8) If my German translation worked out, then Fachsenfeld and Kamm were able to reproduce the same value with their model, which softened the upper tail surface into an arc, rather than a simple straight-sided cone.
9) BamZipPow's 1-wheel trailer roughly follows Lay's original tail architecture.
Pulling the trailer, Bam realizes mpgs unheard of with the 'naked' T-100, even in light of the obvious weight penalty.
10) Hucho wrote that half-bodies could eliminate separation. And produce Cds approaching that of the half-body without wheels, if attention was paid to wheel integration. Something Goro Tamai knows a lot about.
11) This isn't some abstract, theoretical exercise we're talking about. It's simply off-the-shelf-technology, empirically quantified, for 98-years now.

[aerohead]

Originally Posted by aardvarcus

Julian,

I truly have not noticed the prevalent use of that template here.

In a hemisphere, the top profile matches the side profile, somewhat by definition. Creating that would obviously be different than creating a profile only used for the top or the side. Hence why I do not understand your use of it. In the sense of separately formed top and side profiles, yes the vehicles I am copying use separately formed top and side profiles just like mine, not the same curve. In the sense of matching exactly, no not all of them in all dimensions.

I do not understand the constant relation of aerodynamic principals to engine air to fuel ratios. Air to fuel ratios could be related to my toaster. I set my toaster on 4, and it burns the bread. I don't know if your toaster reacts the same way. But I don't know what that has to do with AFR. And I don't know what AFR has to do with Aerodynamics in this context. I also don't think my diesel in my current project has strong opinions about AFR, so I will leave the air to fuel ratio to those who prefer spark ignition over compression.

Is modifying to mimic imparting different physics from designing from scratch?

I did test alternatives, some of which are documented on this site. I built the best one, but obviously as I did not test an infinite number I leave open the possibility of something better. I am not aware of anyone who has designed an add on aero device for a 2nd gen Tacoma that imparts more MPG gain than the final one I built, but I stand ready at any time to be bested by a better design.

I ran some overlays of the vehicles in your anti-template video, using the commonly used here AST-II, and found the results interesting. Five out of six were a perfect match, either in overall slope or the spoiler rising to hit the profile! The Sagitta was the one that did not match.

I find it interesting that 5 of the 6 vehicles used to disprove the template concept actually all follow a "template", one that is posted and commonly used here.

I believe that Hucho himself, wrote about Paul Jaray morphing templates ( viewed in front or rear elevation ) from, semicircular, to quasi-rectangular, and back, within the total length of a particular vehicle in order to address interior packaging issues. I used to see this almost on a daily basis with commercial and industrial ductwork.
One must only maintain each station cross-sectional area along the path, as if it 'were' a semicircle. Area rule. Sectional density.
--------------------------------------------------------------------------------------
The Volkhart V2 Sagitta is a 'Lange' car. Like the 1938 Porsche Type 64, 60K10 Berlin-Rome racer. Main body boat-tailing begins right at the front wheels, and the greenhouse is hoped to perform as a thick,2-D, symmetrical airfoil, shunting air 'around', rather than 'over' the car.

[aerohead]

Originally Posted by AeroMcAeroFace

Well I have been around here for a bit longer than that, not as a member though, and I don't remember anyone saying "Don't use the template in the tools section, it is wrong" Why is it in the tools section if it is the wrong tool?

Imagine a builder turning up at your house, wearing a tool belt with a hammer in it, but then every time he needs a hammer he goes back to the van to get a different hammer, uses it and puts it back in the van. You would question why he doesn't put the hammer that he uses in the tool belt.

It seems like whatever shape that a car maker uses there is a different template that fits.
Which is the one that is supposed to predict separation?

And if there is one that can predict separation (along the centreline), surely you need a correction factor for boundary layer thickening and transition to turbulent flow? Upstream effects are important and not usually considered.

I have seen so many people using the template in the tools section for their "assessment" of how aerodynamic a car is, rarely is there someone "correcting" them and using a different template to "assess" the car.

1) boundary layer thickness has nothing to do with passenger car drag.
2) above 20-mph, a passenger car is already at supercritical Reynolds number, with 100% turbulent boundary layer. And that's a 'GOOD' thing.

[aerohead]

Originally Posted by kach22i

Aerodynamics: The best value of all current Porsche models
https://newsroom.porsche.com/en/prod...ics-18554.html



I wonder if falling short of the aero-template provides additional opportunity to make the rear adjustable spoiler more effective for additional down-force.

VIDEO (different model)

I'll take a stab at this, as I've spent a week thinking about the patented Ford bi-wing spoiler on the Sierra / Scorpio.
Although Ford clearly knew that a streamlined roof would produce lower drag and lift, their specification for interior cargo space and reward vision precluded it use.
They ended up with separation-induced drag and lift.
Just adding the rear spoiler did not address a very low pressure, locked vortex, circulating over the first 1/3rd of the spoiler. Bad for drag and lift.
The wing, forward of, and above the spoiler, helped scavenge out the vortex, raise pressure over the boot, reduce lift, and drag, with a 'larger' however higher pressure wake, for total overall drag and lift reduction.
The bi-wing cut the Merkur XR4Ti from Cd 0.34, to Cd 0.32.
In 1988, Ford abandoned the bi-wing, raising the drag back up to Cd 0,34. They blamed lack of customer approval of the bi-wing aesthetic for it's demise.
It could be that the committee for the Taycan also had standards for rear vision which limited the rear contour. Just a guess.
If you buy one of Volkswagen Group's Lamborghinis, there's less interest in rearward vision. Same for Bugatti.

[aerohead]

Originally Posted by JulianEdgar

In an email to me last week, former head of Porsche aerodynamics, Dr Thomas Wolf, puts it this way:

"According to the Coanda-Effect the air follows the curved (upper) surface. According to Newton’s first law, there must be a force which pulls the air downwards. This force is exerted to the air by the car body and pulls the air downwards. According to Newton’s third law (action = reaction) there must be a force of the same magnitude in the opposite direction acting on the body."



There he goes again - now Dr Thomas Wolf of Porsche is wrong... but of course Aerohead is right!

On my count that's the sixth professional, highly esteemed car aerodynamicist that Aerohead has said is wrong. And people here pay attention to him? That's just amazing.

1) Best I can ascertain, the only Coanda technology to make it into the contemporary marketplace is Dyson's Coanda Air Curler hair-care product.
Coanda effect requires an auxiliary power source to power a high static pressure air-handler for the nozzle jet.
2) I know of zero production automobiles which employ this technology. Correct me if I'm mistaken.
3) The vehicle advancing through the atmosphere is all that is required to explain forebody flow.
4) Aft-body flow is dependent on the diverging exo-duct contour not being so severe that it generates too great a pressure rise. If it does, the streamlines diverge beyond the threshold with which momentum transfer through the TBL is so weak, it comes to rest, then reverses direction, moving towards the wind shield, rolling up into separated flow, eddies, then full-blown turbulence.
5) Turbulence cannot conduct momentum or pressure from the outlying streamlines, and pressure from the separation line aft, is at the pressure of the streamline outside the separation line. The source of low pressure for lift.

[JulianEdgar]

Originally Posted by aerohead

1) Best I can ascertain, the only Coanda technology to make it into the contemporary marketplace is Dyson's Coanda Air Curler hair-care product.
Coanda effect requires an auxiliary power source to power a high static pressure air-handler for the nozzle jet.
2) I know of zero production automobiles which employ this technology. Correct me if I'm mistaken.

Your definition and understanding are both wrong. Refer to Page 307 Aerodynamics of Road Vehicles (5th edition) and SAE paper 2001-01-1267.

This is what I mean: did you even pause for a millisecond to consider you might be wrong and Dr Wolf might be right? Your arrogance is just breathtaking.

[aerohead]

Originally Posted by JulianEdgar

Your definition and understanding are both wrong. Refer to Page 307 Aerodynamics of Road Vehicles (5th edition) and SAE paper 2001-01-1267.

This is what I mean: did you even pause for a millisecond to consider you might be wrong and Dr Wolf might be right? Your arrogance is just breathtaking.

Don't have the book. Won't be buying anything until I have the BEV and Spirit project is concluded.
I am familiar with Coanda, and his research, and his airplane based upon his theory, which never made it off the ground.
Does something published in 2001 expunge the aerodynamic historical record?
Does Dr. Thomas Wolf believe that he can erase history?
How about a couple of sentences which might shed light on this claim of his.
I just looked at an abstract of the paper. A waste of time. Rear spoilers, C-Pillars, vorticity, ................ not germane to a streamlined form.
What else do you have for me to waste my time with?

[JulianEdgar]

Originally Posted by aerohead

Don't have the book. Won't be buying anything until I have the BEV and Spirit project is concluded.
I am familiar with Coanda, and his research, and his airplane based upon his theory, which never made it off the ground.
Does something published in 2001 expunge the aerodynamic historical record?
Does Dr. Thomas Wolf believe that he can erase history?
How about a couple of sentences which might shed light on this claim of his.
I just looked at an abstract of the paper. A waste of time. Rear spoilers, C-Pillars, vorticity, ................ not germane to a streamlined form.
What else do you have for me to waste my time with?

Yep, that's Aerohead in a nutshell. Completely wrong - but in his own mind, always right.

To make it crystal clear: here's a man who thinks he is more expert at aerodynamics than the head of Porsche aerodynamics.

Just breathtaking.

[aerohead]

Originally Posted by JulianEdgar

Yep, that's Aerohead in a nutshell. Completely wrong - but in his own mind, always right.

To make it crystal clear: here's a man who thinks he is more expert at aerodynamics than the head of Porsche aerodynamics.

Just breathtaking.

Like I said, I learned enough from the abstract of the SAE Paper to know that the contents are not germane to our aero-modding.
And perhaps Dr. Wolf would like to walk over to Volkswagen, Lamborghini, Bugatti, SEAT, Audi, and the rest, and read them the riot act for their aerodynamic failings.
PS why don't we invite Dr. Wolf to the party, where he can wow us with his uber mentality.

[freebeard]

Originally Posted by JulianEdgar

Your definition and understanding are both wrong.

If true, then you should be able to address 3), 4) and 5).

Especially 4)....