A book to buy and read

[JulianEdgar]

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:



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.

[JulianEdgar]

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.

[freebeard]

Is there anything of value in this?

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

[JulianEdgar]

Originally Posted by freebeard

Is there anything of value in this?

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.

[freebeard]

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.

[JulianEdgar]

Originally Posted by freebeard

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.

[freebeard]

^^ Makes sense. Unobtainable asymptote.

[aerohead]

Originally Posted by JulianEdgar

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:



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.

who cares?

[aerohead]

Originally Posted by JulianEdgar

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.

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.

[aerohead]

Originally Posted by freebeard

Is there anything of value in this?

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.