Roof Spoiler

[aerohead]

Originally Posted by Joggernot

The person was very proud of the new after-market add on. Not an OEM piece. Yes, it was on a Hyundai Genesis, but the year wasn't given.

That's critical information. Contemporary KIA/ Hyundai products are already at the lower end of the production car drag spectrum, and I couldn't imagine why Luc Donckerwolke would have put that device on a car.
I'd bet a cup of coffee and a donut that the roof spoiler has degraded the aerodynamic efficiency and stability.

[aerohead]

Originally Posted by JulianEdgar

I just saw this, and immediately thought: that must be from about the worst current textbook on car aero. And I was right - 'Theory and Applications of Aerodynamics for Ground Vehicles'.

That para is absolute garbage. To show the 'quality' of this book, note how reference 9, cited in the para, isn't even in the chapter references...

A spoiler, common in sports cars, is a negative lift device.

Yes.

It reduces the lift by slowing down the flow over its upper surface.

What surface? Not the spoiler, as this suggests.

A negative lift wing is the most common type of spoiler.

No, a wing is not a spoiler - how basic a mistake can be made?

When lifting devices are used, it is important to place them in the proper location at the rear of the vehicle, or they may turn out to negate the very effect for which they have been incorporated [9].

Ref 9 not included in chapter references (there are none for the book as a whole). It's highly unlikely that any spoiler will create lift on any modern car (where most lift comes from attached flow), unless something really weird is done so that the spoiler deflects air massively downwards.

For example, a spoiler on the vehicle rear roof only adds to the lift.

I don't think this is the case - in fact I think this is balderdash on any modern car with attached flow over the rear window. But OK, now where is the evidence for this? None is presented.

The book is full of mistakes - staggering that it was published by the SAE. If you want to learn about spoilers/wings/etc, Katz, Hucho or Scheutz are the gold standard - especially when compared to this book!

Without specifics, I can imagine a common situation where the comment would be spot on.
If the spoiler is there to provide a surface of reattachment, then it IS addressing a situation in which the low pressure over the backlight/boot region is caused by the early 'fast' flow separation.
By providing for flow reattachment, a locked-vortex is established, over which the fast inviscid flow CAN decelerate, and by the time it reaches the top of the spoiler, is at a higher static pressure, plus the spoiler acting as a dam, sequestering the low pressure immediately over the boot, away from the base of the car, where it cannot effect the wake.
In this case, both drag and lift are reduced do to the 'slower' air.
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As to a 'wing' not being a 'spoiler', it may be a issue of semantics.
Anything that spoils lift is technically a spoiler, regardless of the actual device or technology. For example, a venturi is a 'spoiler.'
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Your comment about 'most lift comes from attached flow,' ought to include caveats, as there exists counterfactual evidence to your claim.
Streamlined bodies have completely-attached flow, yet generate zero lift.
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Your book contains mistakes about 'wrapped' flow and lift. ' He who sins not, cast the first stone.'

[JulianEdgar]

Originally Posted by aerohead

Without specifics, I can imagine a common situation where the comment would be spot on.
If the spoiler is there to provide a surface of reattachment, then it IS addressing a situation in which the low pressure over the backlight/boot region is caused by the early 'fast' flow separation.
By providing for flow reattachment, a locked-vortex is established, over which the fast inviscid flow CAN decelerate, and by the time it reaches the top of the spoiler, is at a higher static pressure, plus the spoiler acting as a dam, sequestering the low pressure immediately over the boot, away from the base of the car, where it cannot effect the wake.
In this case, both drag and lift are reduced do to the 'slower' air.

Yes, as I already said, it's possible to dream up a situation where a roof spoiler creates lift. (You seem to have missed the part about it being a roof spoiler.) But as I also said, with any modern car with attached flow on the rear window (and the quoted book shows a sedan) then it's extremely unlikely. And it's certainly not common.

So a good example of Aerohead writing material that is not relevant as he seems to be talking about a boot spoiler, not the roof spoiler that's under discussion.

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As to a 'wing' not being a 'spoiler', it may be a issue of semantics.
Anything that spoils lift is technically a spoiler, regardless of the actual device or technology. For example, a venturi is a 'spoiler.'
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If Aerohead is arguing it, black is white and white is black. A wing is not a spoiler, and a spoiler is not a wing. If Aerohead believes they are different only semantically, that's fine - another example of a misleading post from him.

Your comment about 'most lift comes from attached flow,' ought to include caveats, as there exists counterfactual evidence to your claim.

Again, a typical misleading Aerohead statement - he just loves misquoting what others write. I didn't just say: 'most lift comes from attached flow', I said:

It's highly unlikely that any spoiler will create lift on any modern car (where most lift comes from attached flow), unless something really weird is done so that the spoiler deflects air massively downwards.

Note my specific reference to modern cars: and yes, in modern cars, where there is attached flow over the upper surfaces, most lift does in fact come from attached flow.

Streamlined bodies have completely-attached flow, yet generate zero lift.

An aircraft wing is a streamlined body with attached flow - yet it generates no lift? Dear me - yet another wrong statement from Aerohead. (I think we've been here before, and if I remember correctly, Aerohead then had his own definition of what a streamlined body comprises.)

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Your book contains mistakes about 'wrapped' flow and lift. ' He who sins not, cast the first stone.'

I don't think so. I've now had chapter by chapter feedback on the book from:

• the head of Jaguar Land Rover aerodynamics
• an ex-Tesla aerodynamicist
• the head of Porsche aerodynamics
• a professor of aerospace engineering and author of two books on car aero
• a Formula 1 aerodynamicist

...not to mention of course feedback from the tech consultant when I wrote the book, who happens to be a world-renowned aerodynamicist.

None of them suggest any mistakes about 'wrapped flow' and 'lift'.

And you know what, I think I'd trust their opinions over Aerohead's ideas....

[aerohead]

Originally Posted by JulianEdgar

Yes, as I already said, it's possible to dream up a situation where a roof spoiler creates lift. (You seem to have missed the part about it being a roof spoiler.) But as I also said, with any modern car with attached flow on the rear window (and the quoted book shows a sedan) then it's extremely unlikely. And it's certainly not common.

So a good example of Aerohead writing material that is not relevant as he seems to be talking about a boot spoiler, not the roof spoiler that's under discussion.



If Aerohead is arguing it, black is white and white is black. A wing is not a spoiler, and a spoiler is not a wing. If Aerohead believes they are different only semantically, that's fine - another example of a misleading post from him.



Again, a typical misleading Aerohead statement - he just loves misquoting what others write. I didn't just say: 'most lift comes from attached flow', I said:

It's highly unlikely that any spoiler will create lift on any modern car (where most lift comes from attached flow), unless something really weird is done so that the spoiler deflects air massively downwards.

Note my specific reference to modern cars: and yes, in modern cars, where there is attached flow over the upper surfaces, most lift does in fact come from attached flow.



An aircraft wing is a streamlined body with attached flow - yet it generates no lift? Dear me - yet another wrong statement from Aerohead. (I think we've been here before, and if I remember correctly, Aerohead then had his own definition of what a streamlined body comprises.)



I don't think so. I've now had chapter by chapter feedback on the book from:

• the head of Jaguar Land Rover aerodynamics
• an ex-Tesla aerodynamicist
• the head of Porsche aerodynamics
• a professor of aerospace engineering and author of two books on car aero
• a Formula 1 aerodynamicist

...not to mention of course feedback from the tech consultant when I wrote the book, who happens to be a world-renowned aerodynamicist.

None of them suggest any mistakes about 'wrapped flow' and 'lift'.

And you know what, I think I'd trust their opinions over Aerohead's ideas....

* I was addressing the ability of a decklid spoiler to slow the air down and increase pressure.
* Anything that spoils lift is a 'spoiler'. Wings included.
* I'm in disagreement with your broad-brush assertion that, with modern cars, that 'most lift comes from attached flow.'
*It's my opinion that, caveats/ conditions need to be spelled out.
* If you have an industry-wide statistical analysis which demonstrates that for the entire vehicle population, that causality of lift is directly associated with a statistically significant proportion of vehicles, only then could one make such an argument.
* And just for the benefit of the reader, allow that there are exceptions to your general claim.
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* If you're only testing vehicles possessing contour-compromised roofline profiles, which violate the ' ground rules of fluid mechanics' as Hucho refers to them, all your data will suggest that presumed attached flow is responsible for lift. An inescapable intellectual cul -de -sac.
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* Wing sections are not streamline bodies, in the strict sense of the term. Wings operate in two-dimensional flow. As mentioned elsewhere, every wing profile has an angle-of-attack at which zero-lift is achieved. In the back of their book, Abbott and Von Doenhoff provided tables for all extant wing profiles, and the tables provide dedicated columns just for the zero-lift data.
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* 'Streamline bodies' denote 'streamline bodies of revolution', and for automotive application, ' half-bodies of revolution.' This is technical language specific to road vehicle aerodynamics.
* The 'aerodynamic streamlining template' is based upon a half-body, derived from a streamline body of Cd 0.04, the drag minimum known, for a body of which the aft-body contraction contour does not exceed 22-degrees as measured off a horizontal projection.
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As I was not a witness to your communications with your team of aerodynamicists, I've no idea about the specific language chosen in your exchanges which would lead to your conclusions.
Your choice of 'wrapped airflow' is a very unfortunate choice of wording, it is not a 'technical' term used in the profession, and extremely problematic with respect the reader experience.
Not everyone excels at technical writing.

[JulianEdgar]

Originally Posted by aerohead

* I was addressing the ability of a decklid spoiler to slow the air down and increase pressure.
* Anything that spoils lift is a 'spoiler'. Wings included.
* I'm in disagreement with your broad-brush assertion that, with modern cars, that 'most lift comes from attached flow.'
*It's my opinion that, caveats/ conditions need to be spelled out.
* If you have an industry-wide statistical analysis which demonstrates that for the entire vehicle population, that causality of lift is directly associated with a statistically significant proportion of vehicles, only then could one make such an argument.
* And just for the benefit of the reader, allow that there are exceptions to your general claim.

The discussion was about roof spoilers, so as usual, Aerohead's post (which as about boot spoilers) just sows confusion.

A wing is not a spoiler, and a spoiler is not a wing, in any technical automotive use of the words. I am glad Aerohead reiterates his misconception so that can be no confusion in the minds of people reading this that his mistake was just a typo.

I am quite happy to stand by my point that most lift on modern cars comes from attached flow. Just look at any CFD image or wool tuft / pressure testing of any modern car shape. There are plenty around to look at!

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* If you're only testing vehicles possessing contour-compromised roofline profiles, which violate the ' ground rules of fluid mechanics' as Hucho refers to them, all your data will suggest that presumed attached flow is responsible for lift. An inescapable intellectual cul -de -sac.
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So far we have discovered that, according to Aerohead, wool tuft testing cannot be trusted, smoke testing doesn't show what it is supposed to - and now, pressure testing is invalid as well. No doubt subsequently we will get to the invalidity of measuring overall lift. Most people would find it pretty hard to maintain a theory when all the quantitative evidence is against it, but not Aerohead.

* Wing sections are not streamline bodies, in the strict sense of the term. Wings operate in two-dimensional flow. As mentioned elsewhere, every wing profile has an angle-of-attack at which zero-lift is achieved. In the back of their book, Abbott and Von Doenhoff provided tables for all extant wing profiles, and the tables provide dedicated columns just for the zero-lift data.

I am glad Aerohead reiterates his misconception that a wing is not streamlined; then there can be no confusion in the minds of people reading this that his mistake was just a typo. As I have previously said, Aerohead has his own definition of 'streamlined' - one that doesn't match any normal technical automotive use.

-------------------------------------------------------------------------------------
* 'Streamline bodies' denote 'streamline bodies of revolution', and for automotive application, ' half-bodies of revolution.' This is technical language specific to road vehicle aerodynamics.
* The 'aerodynamic streamlining template' is based upon a half-body, derived from a streamline body of Cd 0.04, the drag minimum known, for a body of which the aft-body contraction contour does not exceed 22-degrees as measured off a horizontal projection.
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So Aerohead has said before. But, with respect, so what? It's his theoretical hobbyhorse, but it is one that is basically ignored (1 -2 pages max in a whole book, if that) by all the current major authoritative texts on automotive aerodynamics. Why do they ignore it? Because it's of such little significance.

Not everyone excels at technical writing.

Aerohead is certainly right about that.

[aerohead]

Originally Posted by JulianEdgar

The discussion was about roof spoilers, so as usual, Aerohead's post (which as about boot spoilers) just sows confusion.

A wing is not a spoiler, and a spoiler is not a wing, in any technical automotive use of the words. I am glad Aerohead reiterates his misconception so that can be no confusion in the minds of people reading this that his mistake was just a typo.

I am quite happy to stand by my point that most lift on modern cars comes from attached flow. Just look at any CFD image or wool tuft / pressure testing of any modern car shape. There are plenty around to look at!



So far we have discovered that, according to Aerohead, wool tuft testing cannot be trusted, smoke testing doesn't show what it is supposed to - and now, pressure testing is invalid as well. No doubt subsequently we will get to the invalidity of measuring overall lift. Most people would find it pretty hard to maintain a theory when all the quantitative evidence is against it, but not Aerohead.



I am glad Aerohead reiterates his misconception that a wing is not streamlined; then there can be no confusion in the minds of people reading this that his mistake was just a typo. As I have previously said, Aerohead has his own definition of 'streamlined' - one that doesn't match any normal technical automotive use.



So Aerohead has said before. But, with respect, so what? It's his theoretical hobbyhorse, but it is one that is basically ignored (1 -2 pages max in a whole book, if that) by all the current major authoritative texts on automotive aerodynamics. Why do they ignore it? Because it's of such little significance.



Aerohead is certainly right about that.

I'll repeat from what I posted many months ago. Julian is a 'writer.' I'm uncertain that he's a 'reader.'

1)'[L]ow drag can only be achieved when the separation at the rear is eliminated.' Hucho, 2nd-Ed. page 16 ( template)
2)' [T]he optimum shape in terms of drag is a half-body, which forms a complete body of revolution together with its mirror image- produced through reflection from the roadway.' ( template) Hucho, page 15.
3) ' [T]he value of Cd 0.15 can be realized with more than one single body shape.' Hucho pg 201
4) ' Lower drag can only be achieved by extending the length of the vehicle's body.' Hucho pg 201
5) ' The drag coefficient for...passenger cars may be plotted against vehicle length.[I]f the evaluation is limited to vehicles that were developed for the lowest possible drag coefficient ( template ),( the correlation discerned between greater lengths and lower drag ) this expected trend in in fact confirmed.' Hucho pg 202
6)' A closer approach to the value of the basic body without wheels ( Cd 0.07 - Cd 0.09 ) is only achievable through further integration of the wheels into the body.' ( template ) Hucho, page 201
7) ' The drag and lift of a body depend strongly upon the angle of attack.'
Hucho, pg 202, Re Stollery & Burns, Ref. 4.82 ( bodies of revolution / template)
8) ' It is very unfortunate that numerous ( lift ) investigations on basic bodies are inconsistent.' Hucho, pg 205 ( bodies of revolution )
9) ' A more systematic investigation is needed to generate the basic knowledge on the aerodynamics of bluff bodies close to the ground.' Hucho, pg 206 ( bodies of revolution / template )