A book to buy and read
 

Some threads here recently have, at least by implication, suggested that understanding car aero requires a high IQ - ie it is a hard topic.

I think that has been encouraged by some posters here who - either through intent or otherwise - choose to write about car aero in really obscure language. Others quote complex tech papers. I don't mind the latter, but I fear for many it makes the subject appear even harder.

I honestly don't think car aero is hard at all. As with any area of car technology, it's of course possible to delve into the subject at very difficult levels, but it's also quite possible to know enough to gain really good modification results - and to recognise when BS is being posted.

And there is a car aero book that is quite accessible, factually excellent and well written. It's also very cheap in earlier editions.

It's Road Vehicle Aerodynamic Design by RH Barnard.

Abe Books has copies from about US$6 plus postage.

The book doesn't over-simplify but at the same time isn't complex for the sake of being complex - a good middle line is walked.

This is an example of the writing:

https://i.postimg.cc/nVjSzvW8/IMG-0846.jpg

I can immediately see that this addresses no less than three misconceptions that have occurred in discussion on this group in the last few weeks!

I am not sure why the book isn't often quoted here.

I've been asked: what are the three misconceptions this passage addresses that have occurred in discussion in this group in the last few weeks?

1) Lift is defined in terms of frontal area, not plan area. Aerohead recently calculated the lift coefficient for a car using plan area.

Lift coefficient is defined in a similar way to drag coefficient: Lift = Dynamic pressure × Frontal area × C. Note that for convenience, the reference area is still the projected frontal area...

2) Lift decreases when flow separates. Aerohead argues that lift increases when flow separates. Barnard is talking about aerofoils but you can see from the context of the para he is also (correctly) applying that to car shapes.

...once the flow starts to separate, the lift begins to fall off....

3) The stalling (separation) angle is not easily able to be predicted for car shapes. Aerohead argues that his template shows this precisely.

... for a car shape, the complex geometry and high degree of three-dimensionality mean that the stalling angle is not readily predictable...

4) Lower lift is beneficial in normal cars. Aerohead (and others here) argue that it is of no consequence. Note the use of the word 'good'.

...recent good designs CL is nearly zero...

So there are four (not three) misconceptions briefly addressed in just the one passage!

Incidentally, if you can't understand lift/downforce, it's very unlikely that you will understand thrust/drag, and vice versa. They're all part of the one picture.

Is there anything of value in this?

https://www.youtube.com/watch?v=9A-uUG0WR0w

I think I skimmed through that video a while ago.

In general it's not worth getting excited about laminar and turbulent flow. Very little airflow on a normal car is technically laminar, and so looking at things in terms of laminar and turbulent flows tends to lead people astray ie create confusion.

I think it's much better to concentrate on separated and attached flows.

I found interest in the Moody diagram and breakdown of CFD (DNS/LES/RANS)

Something fishy about this grid though.

Viscid/inviscid isn't black and white, it's a gradient.

I don't think that is right. No air is inviscid. None. Inviscid fluid is imaginary - it doesn't exist.

Inviscid airflow is used only to greatly simplify the model of airflow that can be used. That is, it's a 'model trick'.

Wikipedia puts it well (I can give more formal references if anyone wishes. Anderson's books on aerodynamics are good in this regard):

Inviscid flow is the flow of an inviscid fluid, in which the viscosity of the fluid is equal to zero. When viscous forces are neglected, such as the case of inviscid flow, the Navier-Stokes equation can be simplified to a form known as the Euler equation. Using the Euler equation, many fluid dynamics problems involving low viscosity are easily solved, however, the assumed negligible viscosity is no longer valid in the region of fluid near a solid boundary.

In other words, if we're interested in pressures acting on the surface of the body (ie a solid boundary) - and with car aero, that is all that we are interested in - then using inviscid flow models is of no / very little use to us.

^^ Makes sense. Unobtainable asymptote.

lift
 

who cares?

plan area
 

If you'll stop associating 'wings' with automobiles, I'll stop using aeronautical conventions. The reason I posted what I did was extremely germane to the concept of a wing section as a car body, with respect to aspect ratio. And as always, your complete lack of perspicacity leaves you muttering incantions as you wander intellectually aimlessly in the dark like the automaton you resemble in your posts.

L vs T
 

A streamlined car can be devoid of turbulence, as it could be separation-free. That's Hucho's entire thesis for aerodynamic streamlining.
In that context, it's the most important topic of automobile aerodynamics.

context of 'inviscid'
 

The use of the word is contextual and dangerous if used loosely.
1) inviscid can describe an imaginary, frictionless fluid, incapable of supporting shear, or rotation, used in numerical models to establish streamline positions.
2) then the Bernoulli theorem is employed to establish the pressure distribution over the 'model' ,minus viscous effects.
3) then viscosity effects are incorporated to flesh out the total.
--------------------------------------------------------------------------------------
4) inviscid is also used to describe actual, ' real' fluid, outside the boundary layer, which telegraphs pressure through the boundary layer to the lowest strata of air, adjacent to the surface of the model, measured as local static pressure.
5) all viscous effects are limited to the boundary layer.

Can you provide me with a reference for that? I've not seen 'inviscid' used to mean that (except in what you write).

That's an odd thing to write.

Who cares that there's a good book, written by a professional aerodynamicist, that is available cheaply - and which would help counter many of the misconceptions about car aero that have been spread in this group?

I care - and I would have thought you would too.

https://encrypted-tbn0.gstatic.com/i...AU&ec=45702845

For those interested in buying this (those who care, I suppose), it looks like there are at least three editions--1996, 2001, and 2009. I was only able to find the 3rd ed. on eBay, which I just bought, but the 1st and 2nd are available through the Abe Books link JulianEdgar posted or on Amazon.

I think you'll enjoy it.

I read the book a very long time after I first got interested in car aero, and (in the context of all the other aero books I'd already read) I thought: wow, this guy writes so clearly!

I also like the 'key points' approach that he takes at the end of each chapter.

I guess it says it all that he was my first pick to act as a tech consultant when I wrote my aero book. I was absolutely delighted when he agreed*, and I have never regretted that decision.

The feedback from other professional aerodynamicists (feedback that I have received since my book has been published) is that Dick Barnard is also well-known and well-respected in the profession.

As I said, the info in his book is much more accessible than most SAE tech papers (and books like Hucho), so in the real world, it's likely to be far more useful to people modifying their cars.

* Interesting sidelight for people interested in writing books, etc. Before Dick agreed to work with me, he requested copies of some of my published material. He read the material, then proceeded to pull it apart very precisely. (For example: a stagnation point - as I had described it - is on a car, not a point - it is a zone. Airflow on a car with a rough underbody doesn't run into things, it causes a change in behaviour that results in positive pressure. And so on. All very good!)

inviscid
 

Neither 'inviscid' nor 'ideal fluid' can be found in the INDEX, page 560, of Hucho's 2nd-Ed,
I did find inviscid used in Hucho's 2nd-Ed at:
page-49,59, 50, 51, 52.
The term ' ideal inviscid' occurs only on page 52.
p. 49, ' Provided no flow separation takes place ( the template ), the viscous effects in the fluid are restricted to a thin layer of a few millimeters thickness, called the boundary layer. Beyond this layer the flow is inviscid and its pressure is imposed on the boundary layer.' Professor Dietrich Hummel.
-------------------------------------------------------------------------------------
From Daugherty and Franzini:
* page 279, ( Re boundary layer) ' outside of this layer the fluid can be considered as frictionless or ideal... allowing that the mathematical theory of ideal fluid flow can be used to determine the streamlines in the real fluid at a short distance away from a solid wall / boundary / surface. ( qualitative! )
* page 87, Bernoulli theorem, for a frictionless incompressible fluids with good results in situations where frictional effects are very small ( template ).

odd thing
 

I actually addressed this one today on another thread, but I'll repeat what I commented.
I didn't mean no interest in the book, just not really interested in 'lift' issues.
Since no caveats or conditions were volunteered in the 5th-Ed Aerodynamics of Road Vehicles, my only explanation for this relatively new interest in lift would have to do with ( in the USA anyway) increased posted speed limits since 1995, and the plethora of sales in higher profile vehicles such as pickups, SUVs, VANs, and CUVs.
Sports cars are already compensating.
I'm pinching pennies, saving for a BEV, and home improvements . Any book purchases are off the table for me.

So, the diagram on Page 49 shows flow nothing like that on any modern car. Nor does the quote in Hucho actually mention the template, nor any shape like it. You are yet again misquoting.

You didn't bother quoting Hucho Page 52, that talks about the "considerable difference" between an idealised model using inviscid fluid and the real world.

I can't see any mention of inviscid on page 59.

Aerodynamics of Road Vehicles (5th edition) has a major section on inviscid flow - much too big to summarise here.

However, a key point made early in the section (Page 94) is that where airflow wraps around a curve ("where the streamlines are curved") a force is developed that acts outwardly (an "outward oriented centrifugal force").

So yes, I have learnt something - inviscid flow is responsible for the low pressures we can measure where attached airflow wraps around curves.

Isn't that "inviscid" airflow?

Well, I think we're all a bit right on this one.

Aerodynamics of Road Vehicles (5th ed) makes the point that inviscid flow is not real, but that using the model of inviscid flow works for flows away from boundaries, and allows us to picture pressures from streamlines.

Aerodynamics of Road Vehicles (2nd ed): Provided no flow separation takes place, the viscous effects in the fluid are restricted to a thin layer of a few millimeters thickness, called the boundary layer. Beyond this layer the flow is inviscid and its pressure is imposed on the boundary layer.

Two points:

1. I've corrected Aerohead's misquote

2. Obviously there isn't a car made where flow separation doesn't take place.

So with the very important point kept in mind that inviscid flow is just a simplified model, we can use it when considering pressures where there is attached flow.

Simplified models make the world go round. Ferinstance, the mirrored half-body.

And that's fine. But the trouble is, you refuse to acknowledge when people quote from more recent references - let alone learn from them.

So we're stuck in an era where:

• full wheel covers always gave the lowest drag
• lift was caused by separation
• airflow direction can be predicted by The Template
• rear spoilers 'reach up' to flow
• airflow separates at the end of the roof of notchbacks

.... and so on - all of which are wrong for any cars of the last 30 years.

OTOH, every house in the last 30 years is wrong, excepting the hemisphere with oculus.

https://ecomodder.com/forum/member-f...h-overview.jpg

(my parents', not the one I lived in)

Sorry, I don't really see it.

I am not saying the cars (ie the house) was/were wrong.

I am saying that analysing modern aerodynamics on the basis of car shapes that are often 60+ years old is not a good idea.

Nor is pretending we have learnt nothing about car aero since the 1930s/1950s/1960s/1980s.

It would be a bit like taking the contemporaneous 1960s state of the art analysis of the house that you show - and applying it today. So no IR temperature analysis, no air leakage tests, no modern insulating materials or selective coatings on solar heaters, etc. Let alone discussions of embodied energy, etc.

I have absolutely no issue with Aerohead choosing not to keep up with current car aero thinking. But I have major issues with his continually misleading people on the basis of his outdated ideas and understandings.

If he qualified his comments ("This is what I read 40 years ago and I am not sure it is still current") then that would be fine. But in fact he actively rejects any more recent references and/or tests.

And that isn't fine.

I understand it is an analogy. You describe an analysis i'd love to see applied.

The design is from first principles, promoting a toroidal air flow inside and low surface area outside. A 1950s design built in 1980. If I were to design it today, instead of plywood and cedar shakes, it would be stainless steel. With Gorilla glass/electrochromic windows.

Still, my parents were somewhat ahead of their peers. It had a passive solar hot water system, but their subcontractor (he drove down from Dallas, OR, and had a cowboy hat with a feather rosette on the headband. Never trust a contractor in a cowboy hat) built a flawed system and the 2nd owners took it out. You can see the shingles instead of hand-split shakes on the south-facing gable.

I am designing our solar house as we speak.

I've been interested in solar design for about 40+ years, and have been playing with solar things that long. Unfortunately, though, Covid has just absolutely killed our business (face-to-face training in high-level writing skills, but no face-to-face training now...) and so everything has been put on the back burner.

Still, we have the land (next door to where we currently live), and the concrete slab for my new workshop went in last week... so not all bad news!

Off-topic, but, here's the workshop slab:

https://i.postimg.cc/nLZpjDzS/B89404...359588-A-1.jpg

https://i.postimg.cc/d3v4YJtf/IMG-3050.jpg

nothing, actually, misquoting
 

It seems like miscomprehension on your part:
1) the figure is probably the only one in the book to address a 'streamlined ' shape and lift, of which you've expressed interest.
2) the streamlines depicted are exactly as they'd be in the real world.
3) it is the pressure distribution which would be 'qualitative', however perfectly accurate for the first 92% of body length, and within 90% of what's depicted for the last 8% of body, ads it's a 'streamlined body we're discussing.
4) there is no considerable difference between an idealized model and a streamlined body in the real world and Hucho knows that. You may come to learn that.
5) I can't address page 59 until Friday. Sorry! I left both your book and Hucho's by my front door when I left for town this morning.
6) as to 'inviscid' I re-read every word of Hucho's book since last Friday. Word for word. I'll be posting an addendum to the 'Subject index' on Friday. I found dozens of critical terms that never made it into the index. One would have to read the entire book, hoping to stumble upon a topic of particular interest. Very disappointing!
7) air is a gas and has mass. It has inertia. It has momentum when it's moving. It should come as no surprise that it would exhibit centripetal / centrifugal force when moving in any acceleration, which it would be within a 'curve,'
------------------------------------------------------------------------------------
* As to inviscid flow / pressure, you're off base. Here's a checklist:
1) the body determines the streamlines.
2) streamlines determine local velocity at any point.
3) velocity is responsible for pressure.
4) pressure determines whether the boundary layer will be attached or not.
5) even a turbulent boundary layer can sustain only so much pressure rise in the direction of flow.
6) if that threshold is crossed, the boundary layer will separate from the contour, beginning the turbulent wake.
7) pressure behind the separation line will be the pressure at which separation occurred.
8) the closer the separation line is to the suction peak, the lower the pressure over the aft-body.
9) if airflow 'wrapped' over a body, the streamlines would diverge, velocity slow, and pressure increase. You'd have the highest pressure attainable. Your Porsche 911 would have Cd 0.303, not 0.40, just by having a 'template' roofline. Dig Kamm or Fachsenfeld up and ask them. Your general theory of 'wrapped' flow is riddled with physical inconsistencies, and is not supported by scientific observation.
------------------------------------------------------------------------------------
There is a 'CONTEXT' about your 'lift' concept, and we will examine this. I'm compiling a list of spoiler'd cars which fit the conditions of your logic. You're so close to 'getting it' that I hope you'll hang on for the ride. The only hitch right now is your need of a universal, absolute with respect to lift. It doesn't exist. There's no one-size-fits-all, generalized rule governing lift. Hucho expresses this specifically.

?
 

Hang on until Friday, I'll have Hucho's book with me then, and I'll be able to pick apart the 'language.'
I hope the more recent editions have been corrected. If not, they'll be nearly impossible to navigate.:(

corrected misquote
 

1) you haven't corrected a thing.
2) 'a car made' is not germane to streamlined cars.
3) and keep in mind that 'downwash' and attached longitudinal vortices' do not constitute 'attached flow.'

All correct!

Incorrect - as about a million streamline pics show. Here's one:

https://i.pinimg.com/originals/02/82...9ba1581804.jpg

Note the airflow wrapping around those upper curves. Note how the streamlines get closer together. Note how this indicates higher airflow speed and lower pressure.

How you can seamlessly segue from correct material to absolute rubbish is beyond me.

more recent
 

* what is communicated to the viewer when, a multi-billion dollar multi-national corporation, which could possess/ construct, the best technology imaginable, choose MOON discs when they attempt land speed records.
* You've yet to master the Bernoulli theorem, so I advise you to keep your mouth shut until you do. Your way out of your depth. Book sales be damned!
* The 'template' is a known quantity. It provides for fully-attached flow.
* All the spoilers listed in Hucho's 2nd-Edition were immersed in turbulence. And yes, they extend upwards through what would otherwise be turbulence, allowing the flow to intercept a target which will force the flow to follow a path of less severe pressure rise; and allow reattachment once the severe positive pressure is abated. Page - 61, ' One way to generate negative lift is a rear spoiler, the decisive feature being the relative height of 'separation' in relation to the rest of the body.' Hucho.
Page-282, 'negative lift values of... negatively inclined wings... increase with clearance above the body surfaces as they enter the ' undisturbed air flow.' ( what do you think Huchos' talking about? )
* Since notchbacks are a mutilation to a streamlined contour, one attempts to minimize the damage with whatever will pass muster with the stylist, designer, and production engineer. ( A Cd 0.22 notchback is not the same as a Cd 0.09 half-body ).

basis of car shapes
 

' [W]ith refinements in aerodynamics progress is towards the body of revolution.' Hucho, page 107 ( or skip the refinements and just leap-frog to the body of revolution )

streamlines
 

You're looking at the wrong part of the car!
At the rear, the streamlines are diverging, velocity dropping, pressure rising.
That high pressure is associated with the high base pressure/ low pressure drag, low total drag, no vorticity. Cd 0.19.
You can't see the forest for the trees!

Never mind. I think everyone else can see the airflow wrapping over the upper surfaces, giving those low pressures where the body curves are greatest. That's what we were discussing, remember?

https://i.pinimg.com/originals/02/82...9ba1581804.jpg

I originally wrote:


So we're stuck in [Aerohead's] era where:

• full wheel covers always gave the lowest drag
• lift was caused by separation
• airflow direction can be predicted by The Template
• rear spoilers 'reach up' to flow
• airflow separates at the end of the roof of notchbacks

.... and so on - all of which are wrong for any cars of the last 30 years.

Now what points has Aerohead answered?

Aerohead isn't keeping up with the literature. A good example of where he simply refuses to be guided by more recent references.

Typical personal abuse - and note, no answer to the point. Aerohead still apparently believes that lift is caused by separation. At least 30 years out of date with cars.

No doubt. But that doesn't therefore mean it can predict flow direction, as Aerohead often uses it to. Same old logical fallacy. Dogs have four legs, but not all four-legged animals are dogs.

What was Hucho talking about? Well, self evidently, cars that mostly had a lot of separated flow! That is not the case with any car of basically the last 30 years.

Meaningless stuff, I am afraid. Just go and talk to a professional car aerodynamicist about "mutilation to a streamlined contour". They'll just say: WTF?

wrapping over the upper surface
 

1) the nose and tail are part of the upper surface.
2) in side elevation view it's impossible for the streamlines not to be crowded together, it's an element of Bernoulli's relationship between displacement, velocity, and pressure.
3) I'd be surprised if you could find a member here at the aero forum that doesn't already firmly understand that.
4) that area near the roof apex does not govern lift. It doesn't matter how low the pressure is at that location. It's as if you were saying that our Hoover Dam could never be a success due to the high hydrostatic pressure out in the middle of the span, as if ignoring the countering force where the dam is anchored to the canyon walls.
5) all the low pressure of the suction peak can be overwhelmed by the high local static pressure acting at the nose and tail.
6) and no discussion about lift can ignore the underbody pressure distribution.
--------------------------------------------------------------------------------------
My # 9 comment is scientifically correct.

last 30-years
 

I'm quite pleased to learn that every automobile sold in the last 30-years had Cd 0.09. That would be the reality of separation-free automobiles.
Hummer H-1s weren't really Cd 0.70. And Ariel Atoms weren't really Cd 0.68.
And Dodge Viper ACRs weren't really Cd 0.56. And Silverados weren't Cd 0.412. Pontiac Solstice wasn't Cd 0.45. Chevy Malibu Maxx wasn't Cd 0.37. Certainly todays' Nissan Versa notchback can also defy the laws of physics.
I'm glad also that the Tesla Model X now has 178 mpg-e at 70-mph, with a range of over 525-miles, even without the new 54% power density increase mentioned on the 22nd. ( 808-miles ).
No more dirt accumulating behind vehicles. Porsche Taycan Turbo Ss at 195-mph. We've finally achieved Hucho's limit.
Like driving on the Moon.

Cybertruck. One-dimensional apex with massive tumblehome.

Longitudinal vortexes work for free.

We can very quickly see that you have not measured aero pressures on real cars on real roads (or if you have, you've forgotten what you did) when you write stuff like that!

For example, I don't know of even one road car that has a high vertical downwards force at the nose. And as for pressures at the area of the roof apex not impacting lift, well....

a source for Aerohead's confusion
 

* September, 2020, Julian Edgar makes the comment that no ( not one! ) notchback produced since 1990 suffers separation at the end of the roof.
* In 2006, 16-years after 1990, Julian Edgar publishes, in autospeed, ' As with [many] booted sedans,... the airflow tends to separate form (sic) the body at the trailing end of the roof..... the streamline doesn't stick to the body of the roof in the roof/rear window transition but instead tends to leave at this point.'
* The statistical mean average lifespan for an automobile is 13-years.
* Statistically, 'ALL' of the 'many' notchbacks inferred by Mr. Edgar's statement would have been manufactured well beyond the 1990 cut-off date.
* Would Mr. Edgar like to address this discrepancy?