New (free) tech paper

[JulianEdgar]

Originally Posted by aerohead

Customarily, the drag force is only the net, horizontal, axial force, generated by the pressure differential from front, to rear, expressed in terms of the projected frontal area. It's just dynamic pressure times CdA. ( 1/2 x rho x Cd x A x Velocity-squared )
Take the drag force, at any velocity, then divide by the frontal area, and you'll get the net force per unit area, as a function of delta-P.
Hucho felt that since bluff-body automotive drag is ruled by pressure drag, and pressure drag was a function of flow separation, then the rear base pressure would be of paramount concern as regarding drag.
I've never seen any text in which this concept wasn't promoted.
We would treat any vertical components separately under lift.

I am sorry, that doesn't seem to have answered the point that base pressures do not need to be similar to body pressures at the point of separation. Wasn't that what you were saying?

[aerohead]

Originally Posted by JulianEdgar

I am sorry, that doesn't seem to have answered the point that base pressures do not need to be similar to body pressures at the point of separation. Wasn't that what you were saying?

The base pressure is affected by local pressure along it's entire separation line defining perimeter.
If a portion of that perimeter experiences an excursion upstream ( like over the boot ), due to separation there, or upstream, then that low pressure column of air will be affecting the overall base pressure. And that column of air WILL be affected by the pressure above the separation line creating it.
Since vortex-drag is the highest form of drag, should it also be present, then it will impact the overall drag. It would be manifested in a higher Cd.

[JulianEdgar]

Originally Posted by aerohead

The base pressure is affected by local pressure along it's entire separation line defining perimeter.
If a portion of that perimeter experiences an excursion upstream ( like over the boot ), due to separation there, or upstream, then that low pressure column of air will be affecting the overall base pressure. And that column of air WILL be affected by the pressure above the separation line creating it.
Since vortex-drag is the highest form of drag, should it also be present, then it will impact the overall drag. It would be manifested in a higher Cd.

You seem to be back to talking about early separation. I am talking about cars with attached flow - that's nearly all of them.

And you've not explained how what you say is supported by the pressure plots I've given you in the above posts.

So I'll assume that this is all just another Aerohead theory.

I certainly couldn't find anything that appeared to support it in my references. (I had a long look because if this theory were correct, it would make certain things very easy. But it doesn't appear to be.)

Some cars have pressures on their panels just before separation that match wake pressures, but a lot do not.

And, can I say: why not actually look at the tech paper I've cited in Post #1 and reflect on its implications for some of your theories?

[JulianEdgar]

I've been looking further in my references for the relationship between surface body pressures near the back of the car and base (ie wake) pressures.

As I said above, most publications appear to draw no direct connection between the two. However, Aero of Road Vehicles 2nd ed (ie Aerohead's favourite) does say something about this.

By tapering the body moderately the flow is subject to a pressure increase which ensures that the pressure at the rear of the vehicle, the 'base pressure' is comparatively high, which itself then reduces overall drag. (P19)

In the 5th edition the same section re-states this, although in a slightly different form. But everywhere else in this book, base pressure is treated largely through wakeflow patterns (vortices, etc).

I have written to some professional car aerodynamicists I know, asking this:

I have a question. It's one of those deceptively simple ones that may be unanswerable!

Assuming attached flow to the rear of the car, what is the relationship between surface pressure measurement on the car's rearmost panels (eg roof and side) and base pressure?

None of my references really cover this. Some talk about pressure recovery (eg through boat-tailing) and then imply this has implications for increasing base pressure, but never spell out the connection. In my own measurements, on some cars (eg squarebacks), upper body pressures just ahead of separation are similar to base pressures, but on other cars (fastbacks) they aren't.

I always thought average base pressure was more likely to be influenced by wake flow patterns - so do increased body pressures ahead of separation result in weaker vortices in the wake... and that's the connection?

I don't ask them many questions, so hopefully they'll take the time to answer.

[JulianEdgar]

My first expert has got back to me (Professor Joseph Katz).

Maybe my first email wasn't clear because in a second email I added an image and also asked: Is there any relationship between the surface body pressures we can measure at A (top of car near rear) and B (side of car near rear) and C (base pressure)? For example if we increased pressures at A and B, would we see an increase at C?



Professor Katz said:

The pressures at C should be close to B because the separation bubble is influenced by the outer flow.

The pressure at point A depends more on the rear geometry - is there a vortex there? what is the spoiler angle, etc.

[mwebb]

....."
1. I link to a current, free and excellent paper that is relevant to anyone aerodynamically modifying their cars - no matter its shape. ".....

the paper shows methods , how the tests were put together , what equipment was used , results ,
and
conclusions
to the 3 templates tested
very nice
we could duplicate those procedures using scale models of virtual imaginary modern shapes
and
possibly arrive at the same result , or not , will this results conclusions be repeated by another test facility operating as this test facility operated , maybe .
that is the scientific method , the experiment MUST be repeatable and the results obtained MUST match the original . yes or yes ?

either way
i have read the paper and many others , but my cars do not match the templates of the virtual imaginary cars

my cars exist in this world , and at the end of the day when i change something on my car i will still need to confirm by testing with measureing tools i hAVE ACCESS TO , to confirm or deny the benefit or UN benefit of my modification operating under my consistent and repeatable test parameters , such as they are based on real world considerations .

as you have done in your previous articles , as all of us here on this forum have done .

interesting and fun
now
explain to me how this helps me

[JulianEdgar]

Originally Posted by mwebb

now
explain to me how this helps me

(1) It gives some information that generically is likely to hold true for the different basic car shapes. For example:

- Fastback shapes have highest lift; squareback shapes have lowest

- Fastback shapes are likely to have lowest drag; squareback shapes highest drag

This information supports the point that low drag cars often have a lot of lift - unless very careful treatment is made under the car or the use is made of a rear spoiler.


(2) It gives some pressure patterns that show clearly what is happening, especially on the back half of different shape cars. For example:

- there is relatively little pressure increase across the roof of a squareback

- fastback shapes have low roof pressures and an upper body pressure increase that occurs mostly towards the very end of the car

- notchback shapes have low roof pressures and can develop higher pressures across the upper rear surfaces of the car (ie trunk) than fastback shapes.

These pressure patterns therefore give a good indication, for example, of what a rear spoiler will do on each of these shapes.


(3) It gives information about the different wake patterns found on the different shape cars. For example:

- fastbacks / sedans have quite different wake patterns to wagons

- the wake pattern of each car varies substantially laterally ie what is happening behind the centreline is not what is happening closer to the edges of the car

This information supports, for example, that some underfloor treatments (eg rear diffusers) may have very different impacts on the wakes of the different shape cars.


I absolutely agree that individual modifications on individual cars should always be tested!

[Joggernot]

Originally Posted by mwebb

....."
1. I link to a current, free and excellent paper that is relevant to anyone aerodynamically modifying their cars - no matter its shape. ".....

the paper shows methods , how the tests were put together , what equipment was used , results ,
and
conclusions
to the 3 templates tested
very nice
we could duplicate those procedures using scale models of virtual imaginary modern shapes
and
possibly arrive at the same result , or not , will this results conclusions be repeated by another test facility operating as this test facility operated , maybe .
that is the scientific method , the experiment MUST be repeatable and the results obtained MUST match the original . yes or yes ?

either way
i have read the paper and many others , but my cars do not match the templates of the virtual imaginary cars

my cars exist in this world , and at the end of the day when i change something on my car i will still need to confirm by testing with measureing tools i hAVE ACCESS TO , to confirm or deny the benefit or UN benefit of my modification operating under my consistent and repeatable test parameters , such as they are based on real world considerations .

as you have done in your previous articles , as all of us here on this forum have done .

interesting and fun
now
explain to me how this helps me

Your colors might be nice for most people, but for us color-blind people it is hell. Can't read it. Black on white would be nice for me. Thanks.

[Piotrsko]

2 nd the above. Found a paragraph I didnt see once I switched to B/W screen

[Joggernot]

Originally Posted by Piotrsko

2 nd the above. Found a paragraph I didnt see once I switched to B/W screen

I'm in Windows 10. How do I switch to a B/W screen?